Literature DB >> 35089977

Validation of video analysis of marker-less barbell auto-tracking in weightlifting.

Hideyuki Nagao1, Daichi Yamashita2.   

Abstract

We determined the marker-less barbell auto-tracking accuracy using the Kanade-Lucas-Tomasi (KLT) algorithm in a digital video for two-dimensional analysis (2D-AT). The position coordinates of the barbell's right end during multiple loads (60%-90% of one-repetition maximum) of snatch motion in eight participants were recorded using a three-dimensional motion capture system. Simultaneously, the snatch motion was recorded by a digital camera from the right side. Based on the digital video, the center of the barbell's right end was auto-tracked using the KLT algorithm. Six barbell kinematic variables (forward, backward, and vertical displacement, peak forward, backward, and vertical velocity) were calculated. Intraclass correlation coefficient (ICC) analysis was performed to establish the agreement level between the methods. The Bland-Altman plots and regression were used to examine the agreement between the methods. ICCs of 0.999-0.971 revealed a strong agreement level between the methods. The Bland-Altman plot showed small bias (-0.001-0.001 m, -0.034 --0.005 m/s). We considered that 2D-AT could obtain barbell position coordinates with sufficient accuracy to discriminate the difference in the lifter's level and a successful or unsuccessful lift.

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Year:  2022        PMID： 35089977      PMCID： PMC8797250          DOI： 10.1371/journal.pone.0263224

Source DB:  PubMed          Journal:  PLoS One        ISSN： 1932-6203            Impact factor:   3.240


Introduction

The barbell trajectory during lifting motion needs to be analyzed to improve weightlifting performance. Previous studies have reported that the barbell forward displacement in the 2nd pull position to the most forward position of the snatch was 0.037 m, and the peak horizontal speed in the forward direction was 0.25 m/s larger in inferior lifters than that of superior lifters [1]. Furthermore, the barbell backward displacement after the 2nd pull phase was 0.056 m larger in the inferior lifter than that of the superior lifter [2]. An analysis of the barbell trajectory shows the difference between successful and unsuccessful lifts as well as the level of competition in weightlifting. Specifically, the horizontal displacement of the barbell was 0.01 m larger in a successful snatch than that in an unsuccessful snatch [3]. Non-national level lifters reported a 0.01–0.02 m decrease in the amount of the horizontal displacement of the barbell before and after the practice of snatch [4]. A previous study indicated the need for coaches and scientists to monitor barbell kinematic variables because such variables are correlated with weightlifting performance [5]. A linear position transducer (LPT) is a device used for obtaining data on barbell kinematic and kinetic variables. LPTs use a cable attached to a barbell, which measures variables using a potentiometer or rotary encoder [6]. One of the advantages of using an LPT is that the output of the variables is immediate and can be utilized by one person without taking up space. However, since the LPT is a device that measures the length of the drawn cable, the output is limited to one-dimensional data [7]. In weightlifting, the factors related to the performance and success of the trial include the peak and average vertical velocity, barbell power, and horizontal displacement and velocity of the barbell [1-3]. Hence, the information obtained from the LPT is insufficient. To utilize the scientific knowledge for weightlifting training, it is necessary to obtain data regarding the variables in the horizontal direction. Three-dimensional (3D) motion-capture (3D-MC) systems are currently the gold standard to obtain an object’s position coordinates with high accuracy. However, the 3D-MC system is very expensive, requires a large space, and takes considerable time to be prepared. Actually, the system must be simpler to analyze the barbell trajectory in training and competitions. An analysis method using digital video data taken with one digital video camera has been proposed as a simple method for analyzing the barbell trajectory during the lifting motion [8]. In this system, a light-emitting diode (LED) sensor is attached to the end of the barbell, the lifting motion is recorded with a video camera, and the position coordinates of the LED are automatically tracked based on the digital video data. The LED sensor has a very light weight (37 g) and can be easily attached to a barbell; therefore, it is expected to provide good applicability in weightlifting practice. However, there is an issue concerning durability, as LED sensors are damaged when the barbell falls to the ground. Furthermore, in weightlifting competitions, these sensors may not be attached to the barbell. The direct linear transformation method has been used in previous studies for calibration in analysis methods using a digital video camera [9,10]. Nonetheless, this method is not suitable for analysis in competitions because it is necessary to enter the movement space. Indeed, the limitations of manual digitization include time and digitization error. Moreover, the calibration method using the known barbell plate diameter (0.45 m) has been used in previous studies [3,11,12], although the accuracy remains uncertain. Therefore, we propose a marker-less auto-tracking system for barbell trajectory using the Kanade–Lucas–Tomasi (KLT) algorithm [13,14] from digital video for two-dimensional analysis (2D-AT), using the calibration from the barbell plate diameter (0.45 m) as the reference. Compared with previous methods, 2D-AT can contribute to the analysis of weightlifting performance by expanding the range of barbell position coordinates. Weightlifting is an indoor competition, and the lifters do not move too far from the spot. Therefore, since the camera can be fixed and the video can be recorded without significant changes in the light environment, the color and shape of the target to be tracked are considered to be less varied. Therefore, the position of the barbell during weightlifting is considered to be suitable for automatic digitization by the KLT algorithm. From these, we hypothesized that the KLT algorithm could quantify with sufficient accuracy the variables related to the barbell trajectory that determine differences in weightlifting performance. In particular, when the barbell trajectory during lifting can be objectively evaluated during competitions, it will provide important information for athletes and coaches, which could be used in training.

Materials and methods

Participants

Eight men participated in this investigation (height: 166.5 ± 4.0 cm, body mass: 74.3 ± 9.7 kg, snatch one-repetition maximum: 105.3 ± 6.9 kg). The participants had at least 3 years of experience in resistance training and the snatch exercise. They were not active weightlifters; however, they were weightlifting coaches. The study was approved by the Institutional Review Board of the Japan Institute of Sports Sciences (approval number: 2019034). All participants were informed of the benefits and risks of the investigation prior to signing an institutionally approved informed consent document to participate in the study.

Procedures

A barbell (Eleiko Sport AB, Halmstad, Sweden) with a length of 2.2 m and a weight of 20 kg, approved by the International Weightlifting Federation (IWF), was used for the experiments. The plates attached to the barbell (Uesaka, Tokyo, Japan) were also approved by the IWF. To verify the accuracy of the method, it was necessary to provide some variation in independent variables [15]. Therefore, the load of the snatch was set to multiple conditions of 60%, 70%, 80%, and 90% of one-repetition maximum. A total of 160 snatches were recorded, whereas every subject completed five repetitions for each load condition. Preliminary experiments showed that a load of <50% of one-repetition maximum would result in an unnatural snatch motion; therefore, loads within that range were not included in the present study. In addition, the experiment with the maximum lifting weight was not employed to reduce the risk of injury. In the experiment, the warming-up method and the rest time between lifting were decided by each participant. The position coordinates of the 14-mm-diameter reflective marker attached to the barbell during the snatch motions were recorded at 600 Hz using an eight-camera 3D-MC system (Vicon MX; Vicon Motion Systems, Oxford, UK). Two reflective markers were placed at each of the barbell’s right and left ends, resulting in a total of four markers (Fig 1). In previous studies, the sampling frequency was at 200–250 Hz in the 3D motion analysis of snatch [1,5,16]. However, the body collides with the barbell during the second pull phase of the snatch [17]. Therefore, the barbell position was measured at a higher sampling frequency in the present study (600 Hz) than those in the previous studies (200–250 Hz) to accurately measure the trajectory of the barbell, including the vibration due to the collision.
Fig 1

Placement points of the reflective markers on the barbell’s right end.

To confirm the accuracy of the 3D-MC system, the length of the barbell was calculated from the left and right points of the barbell recorded by the 3D-MC system. The distance between the midpoint of the two markers on the right end of the barbell and the midpoint of the two markers on the left end of the barbell was taken as the length of the barbell. The mean difference in the barbell length against 2.2 m was 0.41 ± 0.32 mm (maximum: 1.10 mm, minimum: <0.01 mm). The lifting motion was simultaneously recorded using a digital camera (ILCE-7M3; Sony, Tokyo, Japan) and telephoto zoom lens (SEL70200G; Sony, Tokyo, Japan) with the 3D-MC system. The accuracy was verified by comparing 2D-AT with the gold standard—that is, 3D-MC. The digital video data were recorded in color at a resolution of 1080 × 1920 (full high definition), approximately 100 Mbps, at a sampling rate of 100 Hz, using a shutter speed of 1/500 s, and using a white balance setting of 5500 K. The digital camera (image sensor) was placed orthogonal to the sagittal plane on the right side of the participant. Following the suggestion of a previous study [10], the digital camera (image sensor) was positioned at 20 m from the barbell’s right end and 0.70 m in height. The field of view of the digital video was based on the size of the weightlifting platform (width: 4 m, depth: 4 m) used in competitions organized by the IWF. The focus of the digital camera was set in a manual adjusting mode, and the focus was adjusted at the barbell’s right end. All experiments were conducted in the laboratory, not at the competition venue. A still image of a digital video taken with a digital camera is shown in Fig 2. Based on the digital video, the center of the barbell’s right end was auto-tracked using the KLT algorithm [13,14]. There are several methods for computer-based auto-tracking; nevertheless, in this study, we employed the KLT algorithm, which is the basis for these methods [18]. The KLT algorithm tracks where a point in an image has moved in the next frame based on the local color gradient in the image. There is an open-source of the KLT algorithm in numerous computer languages. We developed a custom script written in Mathematica version 11.3 (Wolfram Research Inc., Champaign, IL, USA) using the built-in function “ImageFeatureTrack” [19,20]. The start points of auto-tracking (center of the barbell’s right end) were manually selected for the digital video in each attempt. Based on a previous study [3,11,12], the barbell plate diameter (0.45 m) was used as the reference for calibrating the barbell’s real-space from the camera-space position coordinates. The diameter of the plate was converted from its width and height (pixel) as 0.45 m by manually obtaining the positional coordinates of the right and left ends of the plate and the top and bottom ends of the plate.
Fig 2

One of the video images for the marker-less auto-tracking.

The mid-point between the position coordinates of the two markers on the right end of the bar was obtained with 3D-MC and was used as the barbell position coordinates. The 2D plane in the participant’s sagittal plane was analyzed. The x-axis indicated the front-rear direction, whereas the y-axis indicated the vertical direction. The direction of the lifter’s line of sight was set in a positive direction, and the definition of the coordinate system was the same for the data obtained with 2D-AT. The position coordinates of the barbell’s right end obtained with 3D-MC and 2D-AT were smoothed using a fourth-order Butterworth low-pass filter, with the optimum cut-off frequency calculated using the residual analysis method [21]. The cut-off frequency for the x-axis and y-axis of 3D-MC were 46.0 ± 5.2 Hz and 50.7 ± 5.0 Hz, respectively, whereas the corresponding values of 2D-AT were 13.6 ± 1.0 Hz and 11.6 ± 0.5 Hz, respectively. Six barbell kinematic variables used in previous studies [1-3] for the analysis of the snatch were calculated from the smoothed 2D position coordinates of the barbell’s right end obtained with 3D-MC and 2D-AT. Forward displacement (DxF), backward displacement (DxB), maximum vertical height (DyMH), maximum horizontal linear velocity in the forward direction (pVxF), maximum horizontal linear velocity in the backward direction (pVxB), and maximum vertical linear velocity (pVy) were defined and calculated for each lift. A central difference approximation was used to calculate the velocity. The “start position” and “catch position” (analysis range) were defined based on the methods of a previous study [3]. The displacement and velocity in the horizontal and vertical directions, which are widely used in the biomechanical analysis of snatches, were compared between the methods. All analyses were performed using a personal computer (MacBook Pro with macOS Mojave, running on 2.5 GHz Intel Core i7 and 16 GB 2133 MHz LPDDR3; Apple Inc., Cupertino, CA, USA) and Mathematica version 11.3 (Wolfram Research Inc., Champaign, IL, USA).

Statistical analyses

Regression analysis with Pearson’s product-moment correlation coefficient (r) was performed to investigate the linear relationship between the barbell kinematic variables calculated from 3D-MC and 2D-AT. Intraclass correlation coefficient (ICC, 2.1) analysis was employed to examine the level of agreement between methods. Bland–Altman plots were used to identify potential systematic bias and standard deviations as indicators of precision. Precision is independent of the true value and is a measure of the degree of statistical variance among the measured values. The statistical significance level was set at p <0.05. All statistical analyses were performed using Mathematica version 11.3 (Wolfram Research Inc., Champaign, IL, USA).

Results

Representative examples of the barbell’s 2D trajectory, position coordinates, and velocity obtained with 3D-MC and 2D-AT are presented in Fig 3. The trajectory of the barbell in the snatch usually has an S-shaped pattern [22,23]. The participants in this study also showed an S-shaped pattern. Therefore, it is considered that the participants had the essential skills of the snatch. The time required for 2D-AT per trial was approximately 30 s (0.15 s per frame), and it took 60–90 s per trial to copy the digital video data to a personal computer, auto-track, and calculate each variable.
Fig 3

A representative bar trajectory of snatch (a) and horizontal and vertical position coordinates and velocity (b) The thick line represents the data from the 3D-MC system, whereas the circle indicates the data from 2D-AT. The direction of the lifter’s line of sight was set in the forward direction 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis.

A representative bar trajectory of snatch (a) and horizontal and vertical position coordinates and velocity (b) The thick line represents the data from the 3D-MC system, whereas the circle indicates the data from 2D-AT. The direction of the lifter’s line of sight was set in the forward direction 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis. The results of the 3D-MC and 2D-AT correlation analysis for six variables calculated from 160 trials are presented in Fig 4. If the plotted point is above the identity line, the 2D-AT indicates that the variable has been overestimated. In all variables, a significant positive relationship between the 3D-MC and 2D-AT was observed. Furthermore, there was a very high agreement between the two methods and the variables, as revealed by the ICC (DxF: 0.997, 95% CI 0.994–0.998; DxB: 0.993, 95% CI 0.990–0.995; DyMH: 0.999, 95% CI 0.999–0.999; pVxF: 0.944, 95% CI 0. 894–0. 967; pVxB: 0.995, 95% CI 0.987–0.997; pVy: 0.995, 95% CI 0.993–0.997).
Fig 4

Linear relationship between the data from the 3D-MC and 2D-AT methods.

The dashed and the thick lines correspond to the linear regression and identity lines, respectively 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis. DxF: The displacement of forward direction, DxB: The displacement of backward direction, DyMH: The maximum vertical height, pVxF: The maximum horizontal linear velocity in the forward direction, pVxB: The maximum horizontal linear velocity in the backward direction, pVy: Maximum vertical linear velocity.

Linear relationship between the data from the 3D-MC and 2D-AT methods.

The dashed and the thick lines correspond to the linear regression and identity lines, respectively 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis. DxF: The displacement of forward direction, DxB: The displacement of backward direction, DyMH: The maximum vertical height, pVxF: The maximum horizontal linear velocity in the forward direction, pVxB: The maximum horizontal linear velocity in the backward direction, pVy: Maximum vertical linear velocity. Table 1 presents the systematic errors by the Bland–Altman analysis. The precision was almost the same among all the displacement-related variables (DxF, DxB, and DyMH); however, among the velocity-related variables, pVxF showed a relatively larger value than pVxB and pVy. This indicates that pVxF has a relatively larger statistical variability in the measurement results among the variables of velocity in the 2D-AT method. The line of equality was within the agreement limits (Fig 5). Concerning the limits of agreement (LoA in Fig 5), the method of this study showed better results than those of previous studies that used other methods (smartphone app, LPT) to obtain barbell velocity and displacement [24,25]. The limits of agreement are for visual judgment of how well two methods of measurement agree. The smaller the range between these two limits, the better the agreement is.
Table 1

Mean and standard deviation (std) values of variables, bias, and precision between the two methods in the Bland–Altman analysis.

variableunitmean ± std 3D-MCmean ± std 2D-ATbias (3D-MC–2D-AT)precision
DxF[m]0.121 ± 0.0410.122 ± 0.041-0.0010.003
DxB[m]0.155 ± 0.0370.155 ± 0.0380.0010.004
DyMH[m]1.180 ± 0.1101.178 ± 0.1120.0010.005
pVxF[m/s]1.023 ± 0.2241.057 ± 0.238-0.0340.071
pVxB[m/s]-0.568 ± 0.101-0.562 ± 0.103-0.0050.009
pVy[m/s]2.266 ± 0.1902.272 ± 0.191-0.0060.018

3D-MC: Data from 3D motion capture system, 2D-AT: Data from out-tracing with the video movie.

Fig 5

The Bland–Altman plot of the mean differences across the 3D-MC and 2D-AT methods.

The data are plotted against the mean value for both methods (dot-dashed line), with the upper and lower 95% LoA shown as dotted lines 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis, LoA: Limits of agreement. DxF: The displacement of forward direction, DxB: The displacement of backward direction, DyMH: The maximum vertical height, pVxF: The maximum horizontal linear velocity in the forward direction, pVxB: The maximum horizontal linear velocity in the backward direction, pVy: Maximum vertical linear velocity.

The Bland–Altman plot of the mean differences across the 3D-MC and 2D-AT methods.

The data are plotted against the mean value for both methods (dot-dashed line), with the upper and lower 95% LoA shown as dotted lines 3D-MC: Three-dimensional motion capture system, 2D-AT: Two-dimensional analysis, LoA: Limits of agreement. DxF: The displacement of forward direction, DxB: The displacement of backward direction, DyMH: The maximum vertical height, pVxF: The maximum horizontal linear velocity in the forward direction, pVxB: The maximum horizontal linear velocity in the backward direction, pVy: Maximum vertical linear velocity. 3D-MC: Data from 3D motion capture system, 2D-AT: Data from out-tracing with the video movie.

Discussion

This study was designed to validate kinematic data derived from 2D-AT in comparison with 3D-MC. The primary finding was that the analysis derived from the KLT algorithm showed high concurrent validity when compared with laboratory-based instrumentation. The results of the correlation analysis for the 3D-MC and 2D-AT showed significant correlations for all variables (r > 0.90). Furthermore, the line of equality was within the agreement limits in the Bland–Altman plot. For weightlifting analysis, the 3D-MC system is costly and requires a large space to place several cameras. It was considered that by employing the 2D-AT system that needs only one digital camera, it was possible to obtain the barbell trajectory at a lower cost than the 3D-MC system, and without the limitations of location, while maintaining the accuracy of the measurement. A previous study reported a significant difference of 0.056 m for the barbell backward displacement (DxB in this report) and of approximately 0.1 m (0.086–0.109 m) for the maximum height (DyMH) in the snatch between the superior and inferior lifters [2]. Furthermore, DxB is significantly greater by 0.01 m for successful attempts than that for failed attempts at snatch [3]. Using the auto-tracking by the KLT algorithm shown in this study, the bias of the barbell displacement variable was lower than 0.01 m for DxF, DxB, and DyMH (Table 1). The bias of the variables for barbell displacement presented in this study was smaller than the differences in barbell variables between athletes with different lifting performances and between successful and unsuccessful snatches presented in previous studies. Therefore, it is considered that 2D-AT can obtain the barbell position coordinates with sufficient accuracy to quantify differences in the barbell displacement between successful and unsuccessful lifts as well as between different levels of lifters. With respect to the variables related to barbell velocity, it was shown that the value of pVxF (barbell maximum forward velocity) was significantly lower by 0.25 m/s in the lifter who lifted a higher weight in the competition than that of the lifter who lifted a lower weight [1]. A previous study pointed out that the allowable error for barbell velocity in weightlifting analysis is 0.03 m/s [26]. In this study, the bias of pVxF and pVxB was -0.034 m/s and 0.005 m/s, respectively (Table 1), which was sufficiently smaller than the magnitude of the velocity difference attributable to the competition level. However, the absolute value of the bias of pVxF was >0.03 m/s. Furthermore, the range of LoA of pVxF in the Brand-Altman plot was ± 0.138 m/s (-0.173 to 0.103 m/s), which is approximately 10 times larger than that of the other variables. From these results, it is considered that 2D-AT can obtain the pVxB with sufficient accuracy for weightlifting analysis. However, barbell forward velocity may not provide data with sufficient accuracy for the analysis of weightlifting. With respect to pVy, no significant difference between the performance level of lifting and success or failure was noted [1-3]; however, similar to the other variables, the magnitude of bias was <1% of the mean value of the variables (-0.006 m/s). The coefficient of determination of pVxF was relatively small compared with those of other variables (Fig 4). The precision in the Bland–Altman analysis also indicated a relatively large value (Table 1). In snatch, the bar comes into full contact with the body at the crease of the hips at approximately the time when the trunk reaches at a vertical orientation. When the barbell contacts the body significantly earlier, it will gain excessive horizontal force [17]. As pVxF appears immediately after the barbell contacts the body, the magnitude of pVxF is considered to include the low-frequency component of vibration in the x-axis component of the barbell position coordinates. Therefore, the difference in sampling frequency may have affected the magnitude of the first-order differentiated value. A previous study [8] proposed an auto-tracking method by attaching a LED to the end of the barbell and increasing the contrast with the background of the barbell end on the image. However, with the 2D-AT method shown in this study, it is not necessary to attach a sensor or device to the barbell, and the lifter’s motion can be analyzed in an environment where nothing changes from the usual. However, our study had some limitations. Specifically, we used a barbell that had a dark blue end (Fig 1). Interestingly, the barbell designs and colors vary according to the manufacturer. The KLT algorithm tracks a target point based on the local color gradient in the image. Therefore, the accuracy of tracking could have been slightly different in the case of different color barbells. In particular, barbells used during women’s competitions almost always have a yellow end. Those barbells have lower contrast than the barbell used during men’s competitions that was employed in this study, which has a dark blue end. The effect of the barbell’s condition on the auto-tracking with the KLT algorithm, including color differences and the presence or absence of reflective markers, needs to be investigated in the future. In this study, the camera was placed orthogonal to the sagittal plane of the lifter and 20 m from the end of the barbell. However, this condition may not be replicated depending on the environment of the competition venue or training space. The effect of the distance and angle of the camera placement from the object being measured on the accuracy of the auto-tracking with the KLT algorithm of the barbell is a topic for future research. In addition, if it becomes clear how the resolution and sampling frequency of the video data affect the accuracy of the auto-tracking with the KLT algorithm of the barbell, it is expected that the application range of this method will be further expanded.

Conclusions

When the position coordinates of the barbell trajectory can be obtained, the lifting techniques can be objectively evaluated by the kinematic variables during weightlifting practice or competition. By employing the marker-less auto-tracking shown in this study, it was possible to analyze the barbell trajectory with high accuracy. However, there are some limitations in the accuracy of the barbell forward velocity. This method is low in cost and has few restrictions on tools and places; hence, it can be used to analyze weightlifting in various places, such as competition venues and practice areas. The method proposed in this study has the potential to be applied in sagittal plane exercises using barbells, such as squats.

Barbell kinematics data.

This is the file containing the data used for the statistical analyses in this paper. (CSV) Click here for additional data file. 5 Mar 2021 PONE-D-20-38952 Validation of Video Analysis of Marker-Less Barbell Auto-Tracking in Weightlifting PLOS ONE Dear Dr. Nagao, Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process. Please submit your revised manuscript by Apr 19 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. When you're ready to submit your revision, log on to https://www.editorialmanager.com/pone/ and select the 'Submissions Needing Revision' folder to locate your manuscript file. 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In statistical methods, please refer to any post-hoc corrections to correct for multiple comparisons during your statistical analyses. If these were not performed please justify the reasons. Please refer to our statistical reporting guidelines for assistance (https://journals.plos.org/plosone/s/submission-guidelines.#loc-statistical-reporting). 4. In your statistical analyses, please state whether you accounted for repeated measurements per participant. 5. Thank you for stating the following in the Acknowledgments Section of your manuscript: "The authors wish to thank the International Weightlifting Federation and the Japan Weightlifting Association for their support of this study." We note that you have provided funding information that is not currently declared in your Funding Statement. However, funding information should not appear in the Acknowledgments section or other areas of your manuscript. 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Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. Reviewer #1: Partly Reviewer #2: Yes ********** 2. Has the statistical analysis been performed appropriately and rigorously? Reviewer #1: N/A Reviewer #2: Yes ********** 3. Have the authors made all data underlying the findings in their manuscript fully available? The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified. Reviewer #1: Yes Reviewer #2: Yes ********** 4. Is the manuscript presented in an intelligible fashion and written in standard English? PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here. Reviewer #1: Yes Reviewer #2: No ********** 5. Review Comments to the Author Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters) Reviewer #1: I would like to compliment the authors on their research paper. In my opinion, the methodological approach is described transparently, designed thoroughly and it arises logically from the discussed rationale of simplifying video analysis in weightlifting. The analytical approach is similar to recently published validation studies on technology for kinematic assessment. However, a few points need to be considered to ensure a high academic quality of the paper: - Line 8: Please reconsider the spelling of the word “peek”. Do you mean “peak” (maximum)? - Line 24: For clarity, I suggest specifying whether the reported change was an increase or decrease in horizontal displacement. - 32-33: Currently, there is a number of LPT systems available that simultaneously assesses the angle of the drawn cable, allowing to calculate the tether position either in 2 dimensions (e.g. GymAware, Kinetic Performance Technology) or 3 dimensions (e.g. RepOne, Red Matter Labs, Inc). Hence, I suggest removing or reformulating this sentence. - Line 77-78: I suggest reformulating the sentence: “[…] 3 years of experience in resistance training and the snatch exercise.” - Line 155: Apparently, statistical analysis was performed on a pooled data set (i.e. all 160 repetitions were included at once). As stated by Orange et al. (2019) this approach may be common in validation studies, yet it violates the assumption of independence of data. This should particularly be considered since the present study only features eight subjects. To assure that the nested (pooled) structure of data does not influence the results, I suggest recalculating the main statistics using a multilevel approach treating subjects as s random variable. If this multilevel approach does not substantially change the statistics, the presentation of your current results (using the pooled data) would be inherently justified as an equivalently valuable, but simpler model. - Line 161: Although it is widely applied in validation research, I recommend against interpreting the proportionality effect in Bland-Altman analyses. It has been suggested that Bland-Altman analysis succumbs artifactual bias, in that it shows systematic proportional bias in controlled simulations, where no systematic bias is prevalent (Hopkins, 2004). A better solution would be to interpret the slope coefficient of your regression analysis as a measure of proportional error. If it is significantly different from 1 (not from zero!) this could be interpreted as a systematic proportional error. However, this issue may not be too problematic in the present investigation, as there seems to be only little random error. Hence, the artifactual bias from Bland-Altman analysis can probably be neglected. - Line 205 (Table 1): Please specify for the reader what the statistic “precision” means. Also, I wonder if you applied any form of statistical correction to your calculation of bias precision? If I recalculate them as the simple mean of differences and the standard deviation of differences between 3D-MC and 2D-AT using your provided data file, my result for bias in pVxF (-0.034 m/s) and my result for precision in pVxB (0.009 m/s) differ slightly from your reported values. Interestingly, only those two variables show noticeable differences from your values in my calculations. I may be wrong, but please check the calculation on those statistics, as any error might influence your interpretation. - Line 241-245: You interpret your result according to a sample mean bias. For pVxF, the magnitude is indeed below 0.03 m/s, but your LOAs range from -0.173 to 0.103 m/s. This random variability in accuracy should not be neglected in your interpretation! Here is why: According to a simple descriptive calculation I did on you provided data file, only about 40% of your sample’s pVxF values actually achieve an accuracy that falls within your proposed range of +/- 0.03 m/s. If you want to include magnitude thresholds like 0.03 m/s in your paper, this should be included in your statistical analysis, e.g. by applying equivalence testing (Lakens, 2017). However, there are various ways to analytically include uncertainty. At the very least, you should consider confidence intervals in the interpretation. - Line 254: Please maintain a consistent style of referencing according to the journal’s guidelines. Hopkins, W.G. (2004) Bias in Bland-Altman but not Regression Validity Analyses. Sportscience 8, 42-46. (sportsci.org/jour/04/wghbias.htm) Lakens D. Equivalence Tests: A Practical Primer for t Tests, Correlations, and Meta-Analyses. Soc Psychol Personal Sci. 2017 May;8(4):355-362. doi: 10.1177/1948550617697177 Orange ST, Metcalfe JW, Liefeith A, Marshall P, Madden LA, Fewster CR, Vince RV. Validity and Reliability of a Wearable Inertial Sensor to Measure Velocity and Power in the Back Squat and Bench Press. J Strength Cond Res. 2019 Sep;33(9):2398-2408. doi: 10.1519/JSC.0000000000002574 Reviewer #2: Ref #: PONE-D-20-38952. TITLE: VALIDATION OF VIDEO ANALYSIS OF MARKER-LESS BARBELL AUTO-TRACKING IN WEIGHTLIFTING. SUMMARY: This is a topical study that is of interest to Plos One readership however the paper lacks clarity in key areas. The practical relevance of this work is not clear. The authors are urged to rewrite the Introduction to provide greater clarity as to the practical relevance of their work. The authors should discuss technical determinants of successful performance in weightlifting. For instance, why is it important to track barbell trajectory in training / competition? What are the major issues with current methods? These points should be clarified to the reader. The Introduction contains detail that is relevant to the Methods section. This should be addressed to aid interpretation. Research Methods and statistical tests used are appropriate to answer the research question proposed. Greater clarity is required in parts to facilitate reproducibility of the study. ABSTRACT: Page 2, Line 8; Replace period with comma after displacement. Page 2, Line 13; The authors should clarify better the practical relevance of their findings. The authors state that 2D-AT could obtain barbell position coordinates with sufficient accuracy to discriminate the difference in barbell velocity due to the lifting performance difference. The meaning of this point is unclear. Are they referring to differences between analysis methods, between exercises or between levels of lifters? INTRODUCTION: Page 3, Line 19; The authors provide data illustrating differences between inferior and superior lifters. The authors should be clear throughout when making such comparisons. Did superior or inferior lifters exhibit more or less horizontal displacement? This section would benefit from a sentence that clarifies technical determinants of successful performance in weightlifting. Page 3, Line 21; The authors state that there was a 0.25 m/s difference in bar linear velocity. Between which groups? Which population exhibited greater velocities? Page 3, Line 23; Correct 0.0244 to two decimal places. Page 3, Line 25; Why should analysis of barbell trajectory during lifting enhance performance? What is the basis for this? Page 3, Line 29; Check wording here. Consider rewording to something like ‘LPTs use a cable attached to a barbell which measures variables using a potentiometer or rotary encoder.’ Page 3, Line 34; The authors state determinants of success in weightlifting performance. This should be stated earlier in the introduction to give context to discussions on the importance of measuring bar trajectory. Page 3, Line 39; Change ‘system’ to ‘systems’ and replace ‘is’ with ‘are’. Page 4 Line 49; Replace ‘by’ with ‘when’. Page 4, Line 59; The authors state that “The accuracy was verified by comparing 2D-AT with the gold standard—that is, 3D-MC”. Are they referring to the current study or to previous studies? If referring to previous studies, please include relevant citations. If referring to the current study, please save this information for the Methods section. Page 4, Line 61 – 68; This detail should be removed from the Introduction and placed in the Methods section. METHODS: Page 5, Line 77; replace ‘year’ with ‘years’. Since the snatch is a technical lift, the level of participants’ proficiency with this exercise should be clarified for the reader. Did participants have three years of experience in the performance of the snatch? Were participants competitive lifters or did they perform the snatch in training on occasion? Page 5, Line 79; Please clarify if all participants were informed of the benefits of the study as opposed to just a single participant. Reword this sentence to clarify. Page 5, Line 84; The authors state the barbell used was dedicated for men. Please classify the bar based on its weight and dimensions rather than gender. Page 5, Line 88; Greater clarity required here. How many repetitions did participants perform at each load? How much recovery did participants receive between repetitions within and between loads? Why did the authors choose the loads presented? Considering the intended application of the current data to competition and the use of maximum loads during competition, is the data valid under one-repetition maximum conditions? Please clarify. Page 5, Line 89; What is meant by the “The warming-up method and the rest time between lifting were free”? Did participants complete their own desired warm-up routine? Please reword. Page 6, Line 102; The authors state that two reflective markers were placed at the end of the barbell. This information is also stated in line 93. Please clarify how many markers were attached the barbell and digitized? This should be stated once only. Page 6, Line 103; It is stated that barbell length was regulated for competition. Please clarify the location of data collection. Specifically, were data collected in a lab or at a competition venue? Page 6, Line 104; Change 2,200 mm to 2.2-m. Page 6, Line 113; Insert ‘the’ before ‘weightlifting platform’. Page 7, Line 135; Insert ‘was’ after ‘bar’. Page 7, Line 144; Insert ‘the’ after ‘of. RESULTS: Page 8, Line 167; Insert ‘the’ before ‘snatch’. Page 8, Line 168; Replace ‘participant’ with ‘participants’. Page 8, Line 169; Replace ‘participant’ with ‘participants’. Page 8, Line 183; The authors present ICC data describing the level of agreement between the two methods. Please include also the 95% confidence interval of the ICC to identify the upper and lower bound intervals that describe the ICC. Page 9, Line 196; The authors state that “pVxF showed a relatively larger value than pVxB and pVy”. Please simplify this statement to enhance clarity for the reader. Is it meant that the 2D-AT method exhibited greater error for velocity measures relative to the 3D-MC method? Page 9, Line 199; Please insert a sentence that clarifies the specific nature of the limit-of-agreement to suggest that data showed good agreement between the 2D-AT and 3D-MC methods for kinematic measures. DISCUSSION: Page 10, Line 218; Delete “measures obtained by”. Page 10, Line 225; What is meant by having a few restrictions on tools and places? Is it meant that the 2D-AT method can be used with limited equipment in a variety of environmental settings? Page 10, Line 233; The meaning of this sentence is unclear. Consider rewording. Page 10, Line 236; Check wording here. Are the authors referring to differences between successful and unsuccessful lifts? Page 11, Line 239; The authors are urged to be specific throughout to aid clarity. What is meant by high and low competition levels? Page 11, Line 248; The meaning of this sentence is unclear. Consider rewording. Page 11, Line 255; Insert ‘magnitude of’ before pVxF and delete ‘value magnitude’. Page 12, Line 267; Please change the reference to ‘women’s barbells’ to ‘barbells used during women’s competitions’. Please also refer to ‘barbell’s used during men’s competitions’. ********** 6. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files. If you choose “no”, your identity will remain anonymous but your review may still be made public. Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy. Reviewer #1: Yes: Benedikt Mitter Reviewer #2: No [NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.] While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step. 19 May 2021 ========================================= Response to Reviewer #1 Comment 1: Line 8: Please reconsider the spelling of the word “peek”. Do you mean “peak” (maximum)? Response: We apologize for the spelling error. The original manuscript has been revised as per the reviewers’ comment. Abstract, Page 2, Line 8 peek → peak Comment 2: Line 24: For clarity, I suggest specifying whether the reported change was an increase or decrease in horizontal displacement. Response: Thank you for your helpful comments. Based on the reference, the manuscript has been revised as follows: Introduction, Page 3, Line 24 Added: Non-national level lifters reported a 0.01-0.02 m decrease ... Comment 3: 32-33: Currently, there is a number of LPT systems available that simultaneously assesses the angle of the drawn cable, allowing to calculate the tether position either in 2 dimensions (e.g. GymAware, Kinetic Performance Technology) or 3 dimensions (e.g. RepOne, Red Matter Labs, Inc). Hence, I suggest removing or reformulating this sentence. Response: Based on the latest information about the available LPT systems and the reviewers' suggestions, some parts of the manuscript have been deleted. Introduction, Page 3 Deleted: Therefore, the output is limited to the peak and average velocity and the barbell vertical component power. Comment 4: Line 77-78: I suggest reformulating the sentence: “[…] 3 years of experience in resistance training and the snatch exercise.” Response: Thank you for the suggestion. The manuscript has been revised as per the reviewer’s suggestion. Materials and Methods, Page 5, Line 74 Before: The participants had at least 3 year of resistance training and snatch. After: The participants had at least 3 years of experience in resistance training and the snatch exercise. Comment 5: Line 155: Apparently, statistical analysis was performed on a pooled data set (i.e. all 160 repetitions were included at once). As stated by Orange et al. (2019) this approach may be common in validation studies, yet it violates the assumption of independence of data. This should particularly be considered since the present study only features eight subjects. To assure that the nested (pooled) structure of data does not influence the results, I suggest recalculating the main statistics using a multilevel approach treating subjects as s random variable. If this multilevel approach does not substantially change the statistics, the presentation of your current results (using the pooled data) would be inherently justified as an equivalently valuable, but simpler model. Response: In this study, the eight subjects are not independent variables. Studies examining the accuracy of several training-related measures have also shown correlation and Bland-Altman analyses based on multiple data from a single subject [A–D]. Therefore, the issue of sample independence, which is a concern of the reviewers, does not seem to arise. A) Balsalobre-Fernández, C., Marchante, D., Muñoz-López, M., and Jiménez, S. L. Validity and reliability of a novel iPhone app for the measurement of barbell velocity and 1RM on the bench-press exercise. J. Sports Sci, 2017, 18, 1–7. B) Balsalobre-Fernández C, Geiser G, Krzykowski J, Kipp K. Validity and reliability of a computer-vision-based smartphone app for measuring barbell trajectory during the snatch. J Sports Sci. 2020, 38, 710–6. C) Pérez-Castilla A, Piepoli A, Delgado-García G, Garrido-Blanca G, García-Ramos A. Reliability and concurrent validity of seven commercially available devices for the assessment of movement velocity at different intensities during the bench press. J Strength Cond Res. 2019, 33: 1258–1265. D) Rider BC, Conger SA, Ditzenberger GL, Besteman SS, Bouret CM, Coughlin AM. Examining the accuracy of the Polar A360 monitor. J Strength Cond Res. 2020. doi: 10.1519/JSC.0000000000003136. Comment 6: Line 161: Although it is widely applied in validation research, I recommend against interpreting the proportionality effect in Bland-Altman analyses. It has been suggested that Bland-Altman analysis succumbs artifactual bias, in that it shows systematic proportional bias in controlled simulations, where no systematic bias is prevalent (Hopkins, 2004). A better solution would be to interpret the slope coefficient of your regression analysis as a measure of proportional error. If it is significantly different from 1 (not from zero!) this could be interpreted as a systematic proportional error. However, this issue may not be too problematic in the present investigation, as there seems to be only little random error. Hence, the artifactual bias from Bland-Altman analysis can probably be neglected. Response: Thank you for your constructive comment. In line with the reviewer’s comments, the description of the proportional error of the Bland–Altman analysis has been deleted from the entire manuscript. In addition, a regression equation has been added to Figure 1. Figure 1 Materials and Methods, Page 8 Line 166 Deleted: … and the coefficient of determination (R2) of the regression line on the Bland–Altman plots was calculated to identify the proportionality effect. Results, Page 9, Table 1 Deleted: “R2 (p-value)” in Table 1 and “coefficient of determination (R2) with p-values” in description of Table 1. Results, Page 9, legend of Table 1 Deleted: and proportional Results, Page 9 Line 203 Deleted: …coefficient of determination of the regression line on the Bland–Altman plots for all variables was not significant (Table 1), and the… Results, Page 11 Line 240 Deleted: Additionally, no significant correlation (a proportional error) between the variable size and the difference between the methods was found. Comment 7: Line 205 (Table 1): Please specify for the reader what the statistic “precision” means. Also, I wonder if you applied any form of statistical correction to your calculation of bias precision? If I recalculate them as the simple mean of differences and the standard deviation of differences between 3D-MC and 2D-AT using your provided data file, my result for bias in pVxF (-0.034 m/s) and my result for precision in pVxB (0.009 m/s) differ slightly from your reported values. Interestingly, only those two variables show noticeable differences from your values in my calculations. I may be wrong, but please check the calculation on those statistics, as any error might influence your interpretation. Response: The meaning of "precision" has been added in the Materials and Methods section as follows: Materials and Methods, Page 8, Lines 166-167 Added: ... as precision. Precision is independent of the true value and is a measure of the degree of statistical variance of among the measured values. As the reviewer has pointed out, there were some errors in the values shown in the original manuscript. However, there are no errors in the raw data spreadsheet submitted with the manuscript. Therefore, the values in Table 1 and related parts of the manuscript have been revised. The recalculation results regarding the bias of pVxF was -0.034 m/s. This value exceeds the error in velocity allowed in the analysis of barbells in weightlifting (0.03 m/s), as pointed out in a previous study [26]. In this relation, some parts of the manuscript have been revised and some additions were made. Results, Page 9 Table 1 Discussion, Page 11, Lines 253-255 Added: However, the absolute value of the bias of pVxF was more than 0.03 m/s. Furthermore, the range of LoA of pVxF in the Brand-Altman plot was ±0.138 m/s (-0.173 to 0.103), which is about 10 times larger than the other variables. Discussion, Page 11, Lines 255-267 Before: From these results, it is considered that 2D-AT can obtain the barbell horizontal velocity with sufficient accuracy to discriminate the difference in lifting performance. After: From these results, it is considered that 2D-AT can obtain the pVxB with sufficient accuracy for weightlifting analysis. However, barbell forward velocity may not provide data with sufficient accuracy for the analysis of weightlifting. Conclusions, Page 12, Lines 294-295 Added: However, there are some limitations in the accuracy of the barbell forward velocity. Comment 8: Line 241-245: You interpret your result according to a sample mean bias. For pVxF, the magnitude is indeed below 0.03 m/s, but your LOAs range from -0.173 to 0.103 m/s. This random variability in accuracy should not be neglected in your interpretation! Here is why: According to a simple descriptive calculation I did on you provided data file, only about 40% of your sample’s pVxF values actually achieve an accuracy that falls within your proposed range of +/- 0.03 m/s. If you want to include magnitude thresholds like 0.03 m/s in your paper, this should be included in your statistical analysis, e.g. by applying equivalence testing (Lakens, 2017). However, there are various ways to analytically include uncertainty. At the very least, you should consider confidence intervals in the interpretation. Response: Thank you for your constructive comment. According to the reviewer’s comments, we have added a consideration of LoA with respect to the bias of pVxF. This revision is also related to Comment 7. Discussion, Page 11, Lines 255-257 Added: However, the absolute value of the bias of pVxF was more than 0.03 m/s. Furthermore, the range of LoA of pVxF in the Brand-Altman plot was ±0.138 m/s (-0.173 to 0.103 m/s), which is about 10 times larger than the other variables. Comment 9: Line 254: Please maintain a consistent style of referencing according to the journal’s guidelines. Response: The reference citation has now been revised according to the journal's guidelines. Discussion, Page 11, Line 265 Before: (Everett et al., 2008) After: [17] ========================================= Response to Reviewer #2 Comment 1: Page 2, Line 8; Replace period with comma after displacement. Response: Thank you for pointing this out. We have revised this as suggested. Comment 2: Page 2, Line 13; The authors should clarify better the practical relevance of their findings. The authors state that 2D-AT could obtain barbell position coordinates with sufficient accuracy to discriminate the difference in barbell velocity due to the lifting performance difference. The meaning of this point is unclear. Are they referring to differences between analysis methods, between exercises or between levels of lifters? Response: We have revised the sentences which the reviewer pointed out as unclear as follows: Abstract, Page 2, Lines 13-14 Before: We considered that 2D-AT could obtain barbell position coordinates with sufficient accuracy to discriminate the difference in the barbell velocity due to the lifting performance difference. After: We considered that 2D-AT could obtain barbell position coordinates with sufficient accuracy to discriminate the difference in the lifter's level and a successful or unsuccessful lift. Comment 3: Page 3, Line 19; The authors provide data illustrating differences between inferior and superior lifters. The authors should be clear throughout when making such comparisons. Did superior or inferior lifters exhibit more or less horizontal displacement? This section would benefit from a sentence that clarifies technical determinants of successful performance in weightlifting. Response: Thank you for your comment. We have added some information about inferior and superior lifters and successful performance in weightlifting. Introduction, Page 3, Lines 18-23: Added: Previous studies have reported that the barbell backward displacement in the first pull phase was 0.037 m and the peak horizontal velocity in the forward direction was 0.35 m/s larger in superior lifters compared to inferior lifters [1]. Furthermore, the barbell backward displacement after the second pull phase was 0.056 m larger in the inferior lifter than the superior lifter [2]. An analysis of the barbell trajectory shows the difference between successful and unsuccessful lifts, as well as the level of competition in weightlifting. Introduction, Page 3, Lines 24-26: Added: Non-national level lifters reported a 0.01-0.02 m decrease in the amount of the horizontal displacement of the barbell before and after the practice of snatch [4] Comment 4: Page 3, Line 21; The authors state that there was a 0.25 m/s difference in bar linear velocity. Between which groups? Which population exhibited greater velocities? Response: Based on the reviewer’s comments, we have added details on what variables differed between the groups and how. The order of the references has also been changed. Introduction, Page 3, Lines 18-21 Before: Previous studies reported a 0.023-0.056 m difference in the amount of horizontal displacement of the barbell during the snatch between an inferior and a superior lifter in national level athletes [1, 2]. After: Previous studies have reported that the barbell backward displacement in 1st pull phase was 0.037 m and the peak horizontal velocity in the forward direction was 0.35 m/s larger in superior lifters compared to inferior lifters [1]. Furthermore, the barbell backward displacement after the seconnd pull phase was 0.056 m larger in the inferior lifter than the superior lifter [2]. Reference, Page 15. and all relevant parts of the manuscript. [1] → [2], [2] → [1] Comment 5: Page 3, Line 23; Correct 0.0244 to two decimal places. Response: The manuscript was revised according to the reviewer’s suggestion. Introduction, Page 3, Line 24 0.0244 → 0.02 Comment 6: Page 3, Line 25; Why should analysis of barbell trajectory during lifting enhance performance? What is the basis for this? Response: Since the reference did not clearly include evidence as to why the analysis of barbell trajectory during lifting enhances performance, the text was revised as follows: Introduction, Page 3, Lines 26-27 Before: Interestingly, the analysis of the kinematic and kinetic variables of the barbell trajectory during the lifting motion in competitions and training, and their application in coaching can aid in improving the weightlifting performance [5]. After: A previous study indicated the need for coaches and scientists to monitor barbell kinematic variables because such variables are correlated with weightlifting performance [5]. Comment 7: Page 3, Line 29; Check wording here. Consider rewording to something like ‘LPTs use a cable attached to a barbell which measures variables using a potentiometer or rotary encoder.’ Response: The manuscript was revised according to the reviewer’s suggestion. Introduction, Page 3, Lines 29-30 Before: In LPT, a cable is attached to a barbell, and various variables are calculated from a potentiometer or rotary encoder [6]. After: LPTs use a cable attached to a barbell which measures variables using a potentiometer or rotary encoder [6]. Comment 8: Page 3, Line 34; The authors state determinants of success in weightlifting performance. This should be stated earlier in the introduction to give context to discussions on the importance of measuring bar trajectory. Response: Based on the reviewer's suggestion, a statement about the difference between successful and unsuccessful lifts has been added to the first paragraph of the Introduction. The parts of Line 34 in the original manuscript have not been changed in order to maintain the context of the sentence. Introduction, Page 3, Lines 21-22 Added: An analysis of the barbell trajectory shows the difference between successful and unsuccessful lifts, as well as the level of competition in weightlifting. Specifically, … Revised: Furthermore → Specifically Comment 9: Page 3, Line 39; Change ‘system’ to ‘systems’ and replace ‘is’ with ‘are’. Response: The manuscript was revised according to the reviewer’s suggestion. Comment 10: Page 4 Line 49; Replace ‘by’ with ‘when’. Response: The manuscript was revised according to the reviewer’s suggestion. Comment 11: Page 4, Line 59; The authors state that “The accuracy was verified by comparing 2D-AT with the gold standard—that is, 3D-MC”. Are they referring to the current study or to previous studies? If referring to previous studies, please include relevant citations. If referring to the current study, please save this information for the Methods section. Response: The sentences that were pointed out by the reviewer have been moved to the Materials and Methods section. Also, some sentences were deleted to keep the context after the changes. Introduction, Page 3 → Materials and Methods, Page 6, Line 111 Moved: The accuracy was verified by comparing 2D-AT with the gold standard—that is, 3D-MC. Deleted: This study aimed to clarify the accuracy of 2D-AT. Comment 12: Page 4, Line 61 – 68; This detail should be removed from the Introduction and placed in the Methods section. Response: The sentences that were pointed out by the reviewer have been moved to the Materials and Methods section. Introduction, Page 3 → Materials and Methods, Page 7, Lines 155-157 Moved: The displacement and velocity in the horizontal and vertical directions, widely used in the biomechanical analysis of snatches, were compared between the methods. Comment 13: Page 5, Line 77; replace ‘year’ with ‘years’. Since the snatch is a technical lift, the level of participants’ proficiency with this exercise should be clarified for the reader. Did participants have three years of experience in the performance of the snatch? Were participants competitive lifters or did they perform the snatch in training on occasion? Response: Thank you for your helpful comments. Based on the comments of several reviewers, the sentence has been revised as follows: Materials and Methods, Page 5, Lines 73-75 Before: The participants had at least 3 year of resistance training and snatch. After: The participants had at least 3 years of experience in resistance training and the snatch exercise. They were not active weightlifters, but they were weightlifting coaches. Comment 14: Page 5, Line 79; Please clarify if all participants were informed of the benefits of the study as opposed to just a single participant. Reword this sentence to clarify. Response: Based on the comments of several reviewers, the sentence has been revised as follows: Materials and Methods, Page 5, Line 76 Before: The participant was informed… After: All participants were informed… Comment 15: Page 5, Line 84; The authors state the barbell used was dedicated for men. Please classify the bar based on its weight and dimensions rather than gender. Response: Thank you for your helpful comments. Based on the reviewer’s comments, the manuscript has been revised as follows: Materials and Methods, Page 5, Lines 81-82 Before: A barbell (Eleiko Sport AB, Halmstad, Sweden) dedicated for men and approved by the International Weightlifting Federation (IWF) was used for the experiments. After: A barbell (Eleiko Sport AB, Halmstad, Sweden) with a length of 2.2 m and a weight of 20 kg, approved by the International Weightlifting Federation (IWF), was used for the experiments. Comment 16 and 17: Page 5, Line 88; Greater clarity required here. How many repetitions did participants perform at each load? How much recovery did participants receive between repetitions within and between loads? Why did the authors choose the loads presented? Considering the intended application of the current data to competition and the use of maximum loads during competition, is the data valid under one-repetition maximum conditions? Please clarify. Page 5, Line 89; What is meant by the “The warming-up method and the rest time between lifting were free”? Did participants complete their own desired warm-up routine? Please reword. Response: We apologize for the lack of clarity in the sentence of the manuscript. Criteria for selecting loads for experiments and rest methods have been added and revised to the manuscript as follows: Materials and Methods, Page 5, Lines 87-90 Added: Preliminary experiments showed that a load of less than 50% of one-repetition maximum would result in an unnatural snatch motion due to low-load, therefore it was excluded from the measurement conditions. In addition, the experiment with the maximum lifting weight was not employed to avoid risks. In the experiment, a maximum of five consecutive snatches was allowed. Materials and Methods, Page 5, Lines 90-91 Before: The warming-up method and the rest time between lifting were free. After: The warming-up method and the rest time between lifting were decided by each participant. Comment 18: Page 6, Line 102; The authors state that two reflective markers were placed at the end of the barbell. This information is also stated in line 93. Please clarify how many markers were attached the barbell and digitized? This should be stated once only. Response: Thank you for your helpful comments. Based on the reviewer’s comments, the manuscript has been revised as follows: Materials and Methods, Page 5, Lines 94-95 Before: Two reflective markers were placed at the barbell’s right end (Fig 1). After: Two reflective markers were placed at each of the barbell’s right and left ends, resulting in a total of four markers (Fig 1). Materials and Methods, Page 6, Lines 104-107 Deleted: … two reflective markers recorded at 600 Hz were also attached to the barbell’s left end. Added: …the length of the barbell was calculated from the left and right points of the barbell recorded by the 3D-MC system. The distance between the midpoint of the two markers on the right end of the barbell and the midpoint of the two markers on the left end of the barbell was taken as the length of the barbell. Materials and Methods, Page 6, Lines 107-108 Before: The mean difference in the distance between the left and right markers (barbell length) against 2,200 mm… After: The mean difference in the barbell length against 2.2 m… Comment 19: Page 6, Line 103; It is stated that barbell length was regulated for competition. Please clarify the location of data collection. Specifically, were data collected in a lab or at a competition venue? Response: In accordance with the revision of Comment 15, the part pointed out by the reviewer above has been deleted from the manuscript because its contents were overlapping. In addition, a sentence has been added about the place (laboratory) where the measurement was performed. Materials and Methods, Page 6, Lines 107 Deleted: The length of the barbell for the competition was regulated at 2,200 mm. Materials and Methods, Page 6, Lines 119-120 Added: All experiments were conducted in the laboratory, not at the competition venue. Comment 20: Page 6, Line 104; Change 2,200 mm to 2.2-m. Comment 21: Page 6, Line 113; Insert ‘the’ before ‘weightlifting platform’ Comment 22: Page 7, Line 135; Insert ‘was’ after ‘bar’. Comment 23: Page 7, Line 144; Insert ‘the’ after ‘of. Comment 24: Page 8, Line 167; Insert ‘the’ before ‘snatch’. Comment 25 and 26: Page 8, Line 168 and 169; Replace ‘participant’ with ‘participants’. Response: The manuscript was revised according to the reviewer’s suggestions in Comments 20-26. Comment 27: Page 8, Line 183; The authors present ICC data describing the level of agreement between the two methods. Please include also the 95% confidence interval of the ICC to identify the upper and lower bound intervals that describe the ICC. Response: Thank you for your helpful comments. We have added the 95% confidence interval of the ICCs. Results, Page 9, Lines 189-191 Added: ICC (DxF: 0.997, 95%CI 0.994–0.998, DxB: 0.993, 95%CI 0.990–0.995, DyMH: 0.999, 95%CI 0.999–0.999, pVxF: 0.944, 95%CI 0. 894–0. 967, pVxB: 0.995, 95%CI 0.987–0.997, pVy: 0.995, 95%CI 0.993–0.997). Comment 28: Page 9, Line 196; The authors state that “pVxF showed a relatively larger value than pVxB and pVy”. Please simplify this statement to enhance clarity for the reader. Is it meant that the 2D-AT method exhibited greater error for velocity measures relative to the 3D-MC method? Response: Thank you for your constructive comment. According to reviewer’s comments, a description of the precision of pVxF has been added to the manuscript. Results, Page 9, Lines 203-204 Added: This indicates that the precision of pVxF is lower than that of pVxB and pVy within the 2D-AT method. Comment 29: Page 9, Line 199; Please insert a sentence that clarifies the specific nature of the limit-of-agreement to suggest that data showed good agreement between the 2D-AT and 3D-MC methods for kinematic measures. Response: Thank you for your constructive comment. According to the reviewer’s comments, a description of the LoA has been added to the manuscript. Results, Page 9, Lines 207-209 Added: The limits of agreement are for visual judgement of how well two methods of measurement agree. The smaller the range between these two limits the better the agreement is. Comment 30: Page 10, Line 218; Delete “measures obtained by”. Response: The manuscript was revised according to the reviewer’s suggestion. Comment 31: Page 10, Line 225; What is meant by having a few restrictions on tools and places? Is it meant that the 2D-AT method can be used with limited equipment in a variety of environmental settings? Response: Thank you for your constructive comment. In order to avoid ambiguity in the sentences, the manuscript was revised as follows: Discussion, Pages 10-11, Lines 230-234 Before: It was considered that by employing the 2D-AT system, it was possible to analyze the barbell trajectory with high accuracy, low cost, and with only a few restrictions on tools and places. After: For weightlifting analysis, the 3D-MC system is costly and requires a large space to place many cameras. It was considered that by employing the 2D-AT system that needs only one digital camera, it was possible to obtain the barbell trajectory at a lower cost than the 3D-MC system, and without the limitations of location, while maintaining the accuracy of the measurement. Comment 32: Page 10, Line 233; The meaning of this sentence is unclear. Consider rewording. Response: The sentences have been revised for clarity as follows: Discussion, Page 11, Lines 240-243 Before: The magnitude of this bias is considered to be sufficiently smaller than the corresponding of the variable difference indicating the snatch technique, which has been clarified in the previous study. After: The bias of the variables for barbell displacement presented in this study was smaller than the differences in barbell variables between athletes with different lifting performances and between successful and unsuccessful snatches presented in previous studies. Comment 33: Page 10, Line 236; Check wording here. Are the authors referring to differences between successful and unsuccessful lifts? Response: In order to avoid ambiguity in the sentences, the manuscript was revised as follows: Discussion, Page 11, Lines 245-246 Before: …with different performances. After: …with differences between the performance level of lifting and success or failure. Comment 34: Page 11, Line 239; The authors are urged to be specific throughout to aid clarity. What is meant by high and low competition levels? Response: For the reader who is not so knowledgeable about weightlifting, the manuscript has been revised as follows: Discussion, Page 11, Lines 247-249 Before: …previous studies reported that the pVxF (barbell maximum forward velocity) was significantly lower by 0.25 m/s in athletes with high than in those with low competition levels [2]. After: … it was shown that the value of pVxF (barbell maximum forward velocity) was significantly lower by 0.35 m/s in the lifter who lifted a higher weight in the competition than the lifter who lifted a lower weight [1]. Comment 35: Page 11, Line 248; The meaning of this sentence is unclear. Consider rewording. Response: Due to other revisions that increased the word count of the manuscript, unclear sentences that had been pointed out as low priority were deleted from the manuscript. Discussion, Page 11, Line 260 Deleted: As multiple loads of snatches have been analyzed and samples of variables were widely obtained, the results of this study could be applied even if a lifting technique were to be developed in the future. Comment 36: Page 11, Line 255; Insert ‘magnitude of’ before pVxF and delete ‘value magnitude’. Response: The manuscript was revised according to the reviewer’s suggestion. Comment 37: Page 12, Line 267; Please change the reference to ‘women’s barbells’ to ‘barbells used during women’s competitions’. Please also refer to ‘barbell’s used during men’s competitions’. Response: The manuscript was revised according to the reviewer’s suggestion as follows: Discussion, Page 12, Lines 277-279 Before: In particular, women's barbells for competition almost always have a yellow end. This is a lower contrast than the dark blue (men's barbells for competition). After: In particular, barbells used during women’s competitions almost always have a yellow end. This is a lower contrast than the barbell used during men’s competitions that was employed in this study, which has a dark blue end. Submitted filename: 06_Plo_#2Response-to-reviwers_Ver02.0.doc Click here for additional data file. 24 Aug 2021 PONE-D-20-38952R1 Validation of Video Analysis of Marker-Less Barbell Auto-Tracking in Weightlifting PLOS ONE Dear Dr. Nagao, Thank you for submitting your manuscript to PLOS ONE. After careful consideration, we feel that it has merit but does not fully meet PLOS ONE’s publication criteria as it currently stands. Therefore, we invite you to submit a revised version of the manuscript that addresses the points raised during the review process. Please submit your revised manuscript by Oct 08 2021 11:59PM. If you will need more time than this to complete your revisions, please reply to this message or contact the journal office at plosone@plos.org. 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If you have cited papers that have been retracted, please include the rationale for doing so in the manuscript text, or remove these references and replace them with relevant current references. Any changes to the reference list should be mentioned in the rebuttal letter that accompanies your revised manuscript. If you need to cite a retracted article, indicate the article’s retracted status in the References list and also include a citation and full reference for the retraction notice. Additional Editor Comments (if provided): Some minor revisions are still needed before acceptance. [Note: HTML markup is below. Please do not edit.] Reviewers' comments: Reviewer's Responses to Questions Comments to the Author 1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation. Reviewer #1: All comments have been addressed Reviewer #2: (No Response) ********** 2. Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. Reviewer #1: Yes Reviewer #2: Yes ********** 3. Has the statistical analysis been performed appropriately and rigorously? Reviewer #1: Yes Reviewer #2: Yes ********** 4. Have the authors made all data underlying the findings in their manuscript fully available? The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. If there are restrictions on publicly sharing data—e.g. participant privacy or use of data from a third party—those must be specified. Reviewer #1: Yes Reviewer #2: Yes ********** 5. Is the manuscript presented in an intelligible fashion and written in standard English? PLOS ONE does not copyedit accepted manuscripts, so the language in submitted articles must be clear, correct, and unambiguous. Any typographical or grammatical errors should be corrected at revision, so please note any specific errors here. Reviewer #1: Yes Reviewer #2: Yes ********** 6. Review Comments to the Author Please use the space provided to explain your answers to the questions above. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. (Please upload your review as an attachment if it exceeds 20,000 characters) Reviewer #1: I believe that by considering the reviewers’ suggestions, the authors managed to eliminate or explain unclear statements and therefore substantially improved the comprehensibility of their article for future readers. That said, I still noticed a few points that I would like to address in order to have the article fulfill a high scientific standard. Line 18-20: I’d like to encourage the authors to change the wording and ensure that the numbers from the addressed study (Ikeda et al., 2012 [1]) are reported correctly. First, I suggest providing information on what exercise you are addressing (the snatch). Second, in terms of rewording, it is not perfectly clear if the reported 0.037 m of barbell backward displacement refers to superior lifters, inferior lifters, the difference between superior and inferior lifters or lifters in general. Also, the statement “[…] peak horizontal velocity in the forward direction was 0.35 m/s larger in superior lifters compared to inferior lifters” could be misleading to some readers, since it may suggest higher horizontal movement speed in superior lifters. That, however, would not be true since the mean values reported in the referenced paper of Ikeda et al. (2012) were -0.38 m/s for “Best Lifter” and -0.63 m/s for “Japanese Lifter” (p. 1287, Table 4), so the “Best Lifter” group showed a lower magnitude of speed. Knowing that the paper expresses velocity in forward direction in negative values is therefore crucial to interpret your statement correctly. I suggest using a much simpler approach and directly address “speed” (a scalar, hence omitting information on direction) rather than “velocity” (a vector) in your statement. Consider using something like “[…] the peak horizontal speed in the forward direction was … m/s higher in inferior lifters compared to superior lifters”. Third, in terms of revising the numbers of your statement: I could not find the reported values of 0.037 m and 0.35 m/s in the paper of Ikeda et al. (2012) for the associated variables in your manuscript. A value of 0.037 m would fit the difference of group means for the variable Dx3 (0.040 and 0.077), but according to the paper, this variable represents the horizontal displacement in the interval “Second pull position to the most forward position”, not the first pull phase. Wouldn’t the first pull phase be equivalent to the variable Dx1 from the referenced paper? Concerning peak horizontal velocity in forward direction, the difference in group means (I already stated the numbers reported in the paper above) does not yield 0.35, but 0.25 m/s. Please ensure that the numbers you are referencing (also from other papers) are adopted correctly, especially when you include them in the interpretation of your results (e.g. in the discussion, Line 246-248). Line 47: Consider rewording: “[…] it is expected to provide good applicability in weightlifting practice” rather than “[…] it is expected to be used in weightlifting practice situations” Line 82-83: Consider rewording: “The plates attached […] were also approved […]” rather than “The plate attached […] was also approved […]” Line 86: The sentence “A total of 160 snatches were recorded 5 times for each load” is a bit irritating. Consider rewording to something like “A total of 160 snatches were recorded, whereas every subject completed 5 repetitions for each load condition.” Line 86-88: Consider deleting “due to low load” and rewording to “[…], therefore loads within that range were not included in the present study” Line 89: Consider rewording: “[…] to reduce the risk of injury” rather than “[…] to avoid risks” Line 89-90: The expression “[…], a maximum of five consecutive snatches was allowed” is not clear to me. Did participants have the option to perform less than 5 snatches per load? If are you addressing the time in between lifts at the same load, I’m not sure if this information is necessary, given that you state in the following sentence that participants were free to choose their rest time. Line 211-213 (Table 1): Thank you for correcting the values on the Bland-Altman analysis! The values for bias now match my calculations. However, considering that you interpret “precision” as the standard deviation of differences between the two methods, the precision value for pVxB is still different in my calculations. My result would yield a precision for pVxB of 0.009 (in comparison you report a precision of 0.001). Again, I may be wrong, but given that our calculations match for all the other statistics, I suggest you should revise your calculation in this specific case. Line 269: Please change “[…] attaching an LED […]” to “[…] attaching a LED […]” Reviewer #2: SUMMARY: The authors have addressed the majority of my comments. Some minor issues remain with wording in parts. Please see specific comments below. METHODS: Page 5, Line 86; check wording of this sentence. Is it true that participants performed 160 snatches at each load? The authors should reword to clarify the specific number of repetitions completed by each participant at each load. Page 5, Line 93; insert ‘system’ after 3D-MC. Page 5, Line 97; the authors state that barbell position was measured at “a higher sampling frequency”. Please clarify the exact sampling frequency to aid repeatability of the study. Page 5, Line 113; insert ‘at a’ before ‘sampling’. Insert ‘using a’ before ‘shutter’. Reword ‘and white balance’ with ‘using a white balance setting’. RESULTS Page 9, Line 203; The authors state that “pVxF showed a relatively larger value than pVxB and pVy”. Please simplify this statement to enhance clarity for the reader. Is it meant that the 2D-AT method exhibited greater error for velocity measures relative to the 3D-MC method? This has not been addressed since the original review. DISCUSSION: Page 11, Line 242; the authors should be more concise. Consider rewording to something like ‘Therefore, it is considered that 2D-AT can obtain the barbell position coordinates with sufficient accuracy to quantify differences in the barbell displacement between successful and unsuccessful lifts as well as between different levels of athlete’. ********** 7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files. If you choose “no”, your identity will remain anonymous but your review may still be made public. Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy. Reviewer #1: Yes: Benedikt Mitter Reviewer #2: No [NOTE: If reviewer comments were submitted as an attachment file, they will be attached to this email and accessible via the submission site. Please log into your account, locate the manuscript record, and check for the action link "View Attachments". If this link does not appear, there are no attachment files.] While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com/. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Registration is free. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email PLOS at figures@plos.org. Please note that Supporting Information files do not need this step. 2 Sep 2021 Response to Reviewer #1 Comment 1: Line 18-20: I’d like to encourage the authors to change the wording and ensure that the numbers from the addressed study (Ikeda et al., 2012 [1]) are reported correctly. First, I suggest providing information on what exercise you are addressing (the snatch). Second, in terms of rewording, it is not perfectly clear if the reported 0.037 m of barbell backward displacement refers to superior lifters, inferior lifters, the difference between superior and inferior lifters or lifters in general. Also, the statement “[…] peak horizontal velocity in the forward direction was 0.35 m/s larger in superior lifters compared to inferior lifters” could be misleading to some readers, since it may suggest higher horizontal movement speed in superior lifters. That, however, would not be true since the mean values reported in the referenced paper of Ikeda et al. (2012) were -0.38 m/s for “Best Lifter” and -0.63 m/s for “Japanese Lifter” (p. 1287, Table 4), so the “Best Lifter” group showed a lower magnitude of speed. Knowing that the paper expresses velocity in forward direction in negative values is therefore crucial to interpret your statement correctly. I suggest using a much simpler approach and directly address “speed” (a scalar, hence omitting information on direction) rather than “velocity” (a vector) in your statement. Consider using something like “[…] the peak horizontal speed in the forward direction was … m/s higher in inferior lifters compared to superior lifters”. Third, in terms of revising the numbers of your statement: I could not find the reported values of 0.037 m and 0.35 m/s in the paper of Ikeda et al. (2012) for the associated variables in your manuscript. A value of 0.037 m would fit the difference of group means for the variable Dx3 (0.040 and 0.077), but according to the paper, this variable represents the horizontal displacement in the interval “Second pull position to the most forward position”, not the first pull phase. Wouldn’t the first pull phase be equivalent to the variable Dx1 from the referenced paper? Concerning peak horizontal velocity in forward direction, the difference in group means (I already stated the numbers reported in the paper above) does not yield 0.35, but 0.25 m/s. Please ensure that the numbers you are referencing (also from other papers) are adopted correctly, especially when you include them in the interpretation of your results (e.g. in the discussion, Line 246-248). Response: We thank the reviewer for these very insightful comments. We apologize for the incorrect interpretation of the variable Dx3 (the forward barbell displacement in the 2nd second pull position to the most forward position, in Ikeda 2012). This has been revised following the reference. The difference between the Japanese lifter (JL, inferior lifter) and the best lifter (BL, superior lifter) in the peak horizontal barbell velocity in the forward direction has been revised to the correct value (BL: -0.38 vs. JL: -0.63, Absolute value [speed] of JL is greater [0.25] than that of BL). In addition, we have added the information that these variables are related to the snatch. Introduction, Page 3, Lines 19–20 Before: Previous studies have reported that the barbell backward displacement in the 1st pull phase was 0.037 m and the peak horizontal velocity in the forward direction was 0.35 m/s larger in superior lifters compared to inferior lifters [1]. After: Previous studies have reported that the barbell forward displacement in the 2nd pull position to the most forward position of the snatch was 0.037 m, and the peak horizontal speed in the forward direction was 0.25 m/s larger in inferior lifters than that of superior lifters [1]. Discussion, Page 11, Line 249 Before: … pVxF (barbell maximum forward velocity) was significantly lower by 0.35 m/s in the … After: … pVxF (barbell maximum forward velocity) was significantly lower by 0.25 m/s in the … Comment 2: Line 47: Consider rewording: “[…] it is expected to provide good applicability in weightlifting practice” rather than “[…] it is expected to be used in weightlifting practice situations” Response: We thank the reviewer for this constructive comment. We have revised the manuscript following your comment. Introduction Page 4, Line 48 Before: it is expected to be used in weightlifting practice situations. After: it is expected to provide good applicability in weightlifting practice. Comment 3: Line 82-83: Consider rewording: “The plates attached […] were also approved […]” rather than “The plate attached […] was also approved […]” Response: We thank the reviewer for this important suggestion. We have revised the manuscript following your suggestion. Materials and Methods Page5, Line 84 Before: The plate attached […] was also approved […] After: The plates attached […] were also approved […] Comment 4: Line 86: The sentence “A total of 160 snatches were recorded 5 times for each load” is a bit irritating. Consider rewording to something like “A total of 160 snatches were recorded, whereas every subject completed 5 repetitions for each load condition.” Response: We thank the reviewer for this valuable suggestion. We have revised the manuscript according to your suggestion. Materials and Methods Page 5, Lines 87–88 Before: A total of 160 snatches were recorded 5 times for each load. After: A total of 160 snatches were recorded, whereas every subject completed five repetitions for each load condition. Comment 5: Line 86-88: Consider deleting “due to low load” and rewording to “[…], therefore loads within that range were not included in the present study” Response: We thank the reviewer for this pertinent suggestion. We have revised the manuscript according to your suggestion. Materials and Methods Page 5, Lines 89–90 Before: … snatch motion due to low-load, therefore it was excluded from the measurement conditions. After: … snatch motion; therefore, loads within that range were not included in the present study. Comment 6: Line 89: Consider rewording: “[…] to reduce the risk of injury” rather than “[…] to avoid risks” Response: We thank the reviewer for pointing this out. We have revised the manuscript according to your comment. Materials and Methods Page 5, Line 91 Before: … not employed to avoid risks. After: … not employed to reduce the risk of injury. Comment 7: Line 89-90: The expression “[…], a maximum of five consecutive snatches was allowed” is not clear to me. Did participants have the option to perform less than 5 snatches per load? If are you addressing the time in between lifts at the same load, I’m not sure if this information is necessary, given that you state in the following sentence that participants were free to choose their rest time. Response: We thank the reviewer for this very valuable comment. Following your comment, we have revised the manuscript to delete unnecessary sentences. Materials and Methods Page 5, Lines 91–92 Before: In the experiment, a maximum of five consecutive snatches was allowed. The warming-up method and the rest time between lifting were decided by each participant. After: In the experiment, the warming-up method and the rest time between lifting were decided by each participant. Comment 8: Line 211-213 (Table 1): Thank you for correcting the values on the Bland-Altman analysis! The values for bias now match my calculations. However, considering that you interpret “precision” as the standard deviation of differences between the two methods, the precision value for pVxB is still different in my calculations. My result would yield a precision for pVxB of 0.009 (in comparison you report a precision of 0.001). Again, I may be wrong, but given that our calculations match for all the other statistics, I suggest you should revise your calculation in this specific case. Response: We thank the reviewer for this very pertinent suggestion. We had misjudged the calculation result by one digit and rounded it off. The correct figure was 0.009, as pointed out by the reviewer. There are no errors in the figures in the attached spreadsheet (CSV). Thank you for checking every detail. Results Page 10, Table 1 precision of pVxB: 0.001 → 0.009 Comment 9: Line 269: Please change “[…] attaching an LED […]” to “[…] attaching a LED […]” Response: We thank the reviewer for this important comment. We have revised the manuscript following your comment. Discussion Page 12, Line 270 Before: … attaching an LED … After: … attaching a LED … ＊ The manuscript was checked by a native English speaker, and some revisions were made to the entire manuscript, such as with regard to articles ("the") and comma (", "). ------------------------------------------------------------------------------------------------ Response to Reviewer #2 Comment 1: Page 5, Line 86; check wording of this sentence. Is it true that participants performed 160 snatches at each load? The authors should reword to clarify the specific number of repetitions completed by each participant at each load. Response: We thank the reviewer for this pertinent suggestion. We apologize for the inadequacies of the text, as pointed out by the reviewers. We have revised the manuscript accordingly: Materials and Methods, Page 5, Lines 87–88 Before: A total of 160 snatches were recorded 5 times for each load. After: A total of 160 snatches were recorded, whereas every subject completed five repetitions for each load condition. Comment 2: Page 5, Line 93; insert ‘system’ after 3D-MC. Response: We thank the reviewer for this constructive comment. We have revised the manuscript following your comment. Materials and Methods, Page 5, Line 94 Before: … 3D-MC (Vicon MX; Vicon Motion Systems, Oxford, UK). After: … 3D-MC system (Vicon MX; Vicon Motion Systems, Oxford, UK). Comment 3: Page 5, Line 97; the authors state that barbell position was measured at “a higher sampling frequency”. Please clarify the exact sampling frequency to aid repeatability of the study. Response: We thank the reviewer for pointing this out. We have revised the sentences accordingly: Materials and Methods, Pages 5–6, Lines 99–100 Before: Therefore, the barbell position was measured at a higher sampling frequency to accurately measure the trajectory of the barbell including the vibration due to the collision. After: Therefore, the barbell position was measured at a higher sampling frequency in the present study (600 Hz) than those in the previous studies (200–250 Hz) to accurately measure the trajectory of the barbell, including the vibration due to the collision. Comment 4: Page 5, Line 113; insert ‘at a’ before ‘sampling’. Insert ‘using a’ before ‘shutter’. Reword ‘and white balance’ with ‘using a white balance setting’. Response: We thank the reviewer for this pertinent comment. We have revised the manuscript according to your comment. Materials and Methods, Page 6, Lines 115–116 Before: … sampling rate of 100 Hz, shutter speed of 1/500 s, and white balance of 5500 K. After: … at a sampling rate of 100 Hz, using a shutter speed of 1/500 s, and using a white balance setting of 5500 K. Comment 5: Page 9, Line 203; The authors state that “pVxF showed a relatively larger value than pVxB and pVy”. Please simplify this statement to enhance clarity for the reader. Is it meant that the 2D-AT method exhibited greater error for velocity measures relative to the 3D-MC method? This has not been addressed since the original review. Response: We thank the reviewer for this very important suggestion. We have revised the manuscript following your suggestion. Materials and Methods, Page 9, Lines 205–207 Before: … however, among the velocity-related variables, pVxF showed a relatively larger value than pVxB and pVy. This indicates that the precision of pVxF is lower than that of pVxB and pVy within the 2D-AT method. After: … however, among the velocity-related variables, pVxF showed a relatively larger value than pVxB and pVy. This indicates that pVxF has a relatively larger statistical variability in the measurement results among the variables of velocity in the 2D-AT method. Comment 6: Page 11, Line 242; the authors should be more concise. Consider rewording to something like ‘Therefore, it is considered that 2D-AT can obtain the barbell position coordinates with sufficient accuracy to quantify differences in the barbell displacement between successful and unsuccessful lifts as well as between different levels of athlete’. Response: We thank the reviewer for this valuable comment. We have revised the manuscript following your comment. Discussion, Page 11, Lines 246–247 Before: Therefore, it is considered that 2D-AT can obtain the barbell position coordinates with sufficient accuracy to distinguish the characteristic of the difference in the barbell displacement amount between liftings with differences between the performance level of lifting and success or failure. After: Therefore, it is considered that 2D-AT can obtain the barbell position coordinates with sufficient accuracy to quantify differences in the barbell displacement between successful and unsuccessful lifts as well as between different levels of lifters. ＊ The manuscript was checked by a native English speaker, and some revisions were made to the entire manuscript, such as with regard to articles ("the") and comma (", "). Submitted filename: 33_Plo_2Response-to-reviwers_Ver02.0.doc Click here for additional data file. 17 Jan 2022 Validation of Video Analysis of Marker-Less Barbell Auto-Tracking in Weightlifting PONE-D-20-38952R2 Dear Dr. Nagao, We’re pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it meets all outstanding technical requirements. Within one week, you’ll receive an e-mail detailing the required amendments. When these have been addressed, you’ll receive a formal acceptance letter and your manuscript will be scheduled for publication. An invoice for payment will follow shortly after the formal acceptance. To ensure an efficient process, please log into Editorial Manager at http://www.editorialmanager.com/pone/, click the 'Update My Information' link at the top of the page, and double check that your user information is up-to-date. If you have any billing related questions, please contact our Author Billing department directly at authorbilling@plos.org. If your institution or institutions have a press office, please notify them about your upcoming paper to help maximize its impact. If they’ll be preparing press materials, please inform our press team as soon as possible -- no later than 48 hours after receiving the formal acceptance. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information, please contact onepress@plos.org. Kind regards, Yumeng Li Academic Editor PLOS ONE Additional Editor Comments (optional): The authors have successfully addressed reviewers' comments. The paper is accepted in its current form. Reviewers' comments: Reviewer's Responses to Questions Comments to the Author 1. If the authors have adequately addressed your comments raised in a previous round of review and you feel that this manuscript is now acceptable for publication, you may indicate that here to bypass the “Comments to the Author” section, enter your conflict of interest statement in the “Confidential to Editor” section, and submit your "Accept" recommendation. Reviewer #1: All comments have been addressed ********** 2. Is the manuscript technically sound, and do the data support the conclusions? The manuscript must describe a technically sound piece of scientific research with data that supports the conclusions. Experiments must have been conducted rigorously, with appropriate controls, replication, and sample sizes. The conclusions must be drawn appropriately based on the data presented. Reviewer #1: Yes ********** 3. Has the statistical analysis been performed appropriately and rigorously? Reviewer #1: Yes ********** 4. Have the authors made all data underlying the findings in their manuscript fully available? The PLOS Data policy requires authors to make all data underlying the findings described in their manuscript fully available without restriction, with rare exception (please refer to the Data Availability Statement in the manuscript PDF file). The data should be provided as part of the manuscript or its supporting information, or deposited to a public repository. For example, in addition to summary statistics, the data points behind means, medians and variance measures should be available. 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They adressed my suggestions thoroughly and presented a final draft of the manuscript that does not raise any further questions from my end. I believe the article will be well received and complement the existing literature on automated barbell tracking. ********** 7. PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files. If you choose “no”, your identity will remain anonymous but your review may still be made public. Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy. Reviewer #1: Yes: Benedikt Mitter 20 Jan 2022 PONE-D-20-38952R2 Validation of Video Analysis of Marker-Less Barbell Auto-Tracking in Weightlifting Dear Dr. Nagao: I'm pleased to inform you that your manuscript has been deemed suitable for publication in PLOS ONE. Congratulations! Your manuscript is now with our production department. If your institution or institutions have a press office, please let them know about your upcoming paper now to help maximize its impact. If they'll be preparing press materials, please inform our press team within the next 48 hours. Your manuscript will remain under strict press embargo until 2 pm Eastern Time on the date of publication. For more information please contact onepress@plos.org. If we can help with anything else, please email us at plosone@plos.org. Thank you for submitting your work to PLOS ONE and supporting open access. Kind regards, PLOS ONE Editorial Office Staff on behalf of Dr. Yumeng Li Academic Editor PLOS ONE
  16 in total

1.  Snatch technique of collegiate national level weightlifters.

Authors:  Brian K Schilling; Michael H Stone; Harold S O'Bryant; Andrew C Fry; Robert H Coglianese; Kyle C Pierce
Journal:  J Strength Cond Res       Date:  2002-11       Impact factor: 3.775

2.  Comparison of the snatch technique for female weightlifters at the 2008 Asian championships.

Authors:  Yusuke Ikeda; Tsutomu Jinji; Takeo Matsubayashi; Akifumi Matsuo; Eiichi Inagaki; Toshiro Takemata; Miyoji Kikuta
Journal:  J Strength Cond Res       Date:  2012-05       Impact factor: 3.775

3.  The three-dimensional kinematics of a barbell during the snatch of Taiwanese weightlifters.

Authors:  Hung-Ta Chiu; Chih-Hung Wang; Kuangyou B Cheng
Journal:  J Strength Cond Res       Date:  2010-06       Impact factor: 3.775

4.  A measure of confidence in Bland-Altman analysis for the interchangeability of two methods of measurement.

Authors:  David Preiss; Joseph Fisher
Journal:  J Clin Monit Comput       Date:  2008-07-08       Impact factor: 2.502

Review 5.  Reviewing current knowledge in snatch performance and technique: the need for future directions in applied research.

Authors:  Lester K W Ho; Christian Lorenzen; Cameron J Wilson; John E Saunders; Morgan D Williams
Journal:  J Strength Cond Res       Date:  2014-02       Impact factor: 3.775

6.  A biomechanical comparison of successful and unsuccessful power clean attempts.

Authors:  Kristof Kipp; Carolyn Meinerz
Journal:  Sports Biomech       Date:  2017-03-02       Impact factor: 2.832

7.  The effects of two different correction strategies on the snatch technique in weightlifting.

Authors:  Chiara Milanese; Valentina Cavedon; Stefano Corte; Tiziano Agostini
Journal:  J Sports Sci       Date:  2016-04-12       Impact factor: 3.337

8.  Comparison of the FitroDyne and GymAware Rotary Encoders for Quantifying Peak and Mean Velocity During Traditional Multijointed Exercises.

Authors:  John F T Fernandes; Kevin L Lamb; Cain C T Clark; Jason Moran; Ben Drury; Amador Garcia-Ramos; Craig Twist
Journal:  J Strength Cond Res       Date:  2021-06-01       Impact factor: 4.415

9.  A Biomechanical Comparison of Successful and Unsuccessful Snatch Attempts among Elite Male Weightlifters.

Authors:  Hideyuki Nagao; Yasuyuki Kubo; Tempei Tsuno; Sho Kurosaka; Masato Muto
Journal:  Sports (Basel)       Date:  2019-06-21

10.  Comparative 3-dimensional kinematic analysis of snatch technique between top-elite and sub-elite male weightlifters in 69-kg category.

Authors:  Gongju Liu; Gusztáv Fekete; Hongchun Yang; Jing Ma; Dong Sun; Qichang Mei; Yaodong Gu
Journal:  Heliyon       Date:  2018-07-17
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