Relationship between scapular initial position and scapular movement during dynamic motions.

Optimal scapular position and movement are necessary for normal function of the shoulder joint and it is essential to focus on scapula in the rehabilitation for shoulder disorders. The aim of this study was to discover the relationship between the scapular initial position and scapular movement during dynamic motions in healthy young men. Thirty-four men participated in this study. The scapular angles at initial position and in elevation and lowering during flexion and abduction were measured using an electromagnetic tracking device. The scapular movements from 30° to 120° during flexion and abduction were calculated. Spearman's rank correlation coefficients were used to analyze the relationship between the scapular initial position and scapular movements. For upward rotation and posterior tilt of the scapula, there were significant positive correlations between the scapular initial position and scapular movement during flexion and abduction. For internal rotation, there were significant positive correlations, except 90° in lowering phase and 120° in both phases. While the humeral elevation increased, the correlation coefficients tended to decrease. Except for the internal rotation our results clarified the interactions between the scapular initial position and scapular movement during dynamic motions in healthy young men. The tendency of the decrease in correlation coefficient with elevation angle was shown.

Introduction

The shoulder complex consists of the glenohumeral-, acromioclavicular-, and scapulothoracic- joints and has the largest range of motion in the body[1]. Normal shoulder function needs optimal scapular position and its movement because the scapula has an integrable role for the shoulder complex. Abnormal motion and position of the scapula are defined as scapular dyskinesis[2] and present as shoulder disorders[3-6]. Preventive strategies, treatments, and development of clinical tests for shoulder rehabilitation, therefore, should focus on scapular position and movement.

The scapular initial position is probably defined by the shape of the thorax, passive tension of the scapulothoracic muscle, and acromioclavicular articulation. Previous studies investigating the scapular initial position showed upward rotation of the scapula of 5.4°; downward rotation of 2°; internal rotation of 26.5°, 40°, and 41.1°; and anterior tilt of 2° and 13.5°[7-11]. In perspective, the scapula at initial position is positioned approximately in the middle of the upward downward rotation, 35° internal rotation, and 10° anterior tilt[11].

For the scapular movement, the scapulothoracic muscles, which include the trapezius-, serratus anterior-, pectoralis minor-, levator scapula-, and rhomboid-muscles[12], control scapular movement during elevation. In particular, the trapezius and serratus anterior muscles work in coordination as a coupled force, which is needed for optimal scapular movement[13]. Upward and internal rotation and posterior tilt of the scapula generally occur with humeral elevation in healthy people[8,14,15].

Sahrman[16] described that normal alignment at the static position (i.e. resting position) is of need for normal joint movement and asserted the importance of the relationship between alignment and joint movement. In the clinical setting, Reijneveld et al.[17] and Strufy et al.[18] advocated that the assessment of scapular position and scapular-conscious exercise were implemented to improve scapular movement during humeral elevation as part of shoulder rehabilitation. They also suggested the importance of the relationship between alignment and joint movement. One previous study, to our knowledge, developed a thorax-fixed regression model for prediction of the scapular orientation from the humeral orientation using static position of upper limb (i.e. non-dynamic motion) and fixed-thorax posture [19], which may differ from actual dynamic shoulder motion. Therefore, no studies have focused on the interaction between scapular initial position and movement during dynamic motion, although scapular initial position and its movement during elevation and lowering has been frequently measured in shoulder biomechanics research. Understanding of interaction between scapular initial position and its consequential movement is meaningful for both therapeutic rehabilitation and biomechanics research. The aim of this study was to clarify the relationship between scapular initial position and scapular movement during dynamic motions in healthy young men. We hypothesized that there is a positive correlation between the two variables in healthy young men.

Materials and methods

Participants

Thirty-four men [mean age, 22.7 (3.1) years; mean height, 170.8 (5.4) cm; mean weight, 65.6 (8.1) kg] were recruited at our university and participated in this study. The upper limb used to throw a ball was defined as the dominant limb. Participants currently with or a history of a neurological or orthopaedic disorder in their nondominant limb were excluded. Given the measurement error, we also excluded women because they would have more subcutaneous fat than men [20], which could affect to the sensor, particularly the thoracic sensor. At recruitment, three men (one with shoulder impingement syndrome and two with a history of clavicle fracture) were excluded. The aim and procedure of the study were provided to all participants, who then provided informed consent. The study protocol was approved by the ethics committee of Kyoto University Graduate School and the Faculty of Medicine (R0233) and conformed to the principles of the Declaration of Helsinki.

Experimental procedures

All procedures of this study were conducted in our laboratory. To measure the scapular initial position, participants were asked to sit on a stool in a relaxed manner with their upper limbs and the palms beside their body. No instructions were given on how to position the lower limbs, pelvis, trunk, and head so each participant could reflect his natural posture (Fig 1A). To measure scapular movement during dynamic motions i.e. elevation and lowering, the participants sitting on the stool were asked to raise and lower their nondominant arm in the sagittal (flexion) and frontal (abduction) planes. From the starting position, with the upper limb beside their body, the elbow fully extended, and the palm against the body, the participants fully elevated their upper limb in 4 s and then lowered it to the starting position in 4 s three consecutive times with the use of a metronome at 60 beats/min (Fig 1B). The participants fixed their eyes on a target placed in front of them at eye level. The scapular initial position was measured first followed by random measurements of scapular movements. The participants underwent sufficient familiarization sessions before measurements began.

Fig 1

Experimental procedures.

The scapular initial position and movement were assessed dung sitting (a) and dynamic motions (b) respectively. The individual in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish these case details.

Instrumentation

The angles of the scapula and the humerus in three dimensions were measured at rest and during flexion and abduction using a 6-degrees-of-freedom electromagnetic tracker (Liberty, Polhemus, Colchester, VT, USA) at 120 Hz on the nondominant limb. This system comprises a stylus, five sensors, and a transmitter controlled by an electronic unit. The accuracy of the sensor is 0.762 mm in terms of position and 0.15 degrees in terms of orientation. The transmitter was put on a wooden stand 30 cm behind the participants and at height of 100 cm. The transmitter emitted an electromagnetic field that detected the sensors and the stylus. The global coordinate system was used to represent the electromagnetic field, with the X-, Y-, and Z-axes being the forward, upward, and right directions, respectively, and the transmitter was the origin. The sensors were placed on the skin over the bony landmarks using adhesive tape. The thoracic sensor was attached to the sternum inferior to the jugular notch, the humeral sensor was attached to the midpoint of the humerus using a thermoplastic cuff, and the scapular sensor was attached to the plateau of the acromion. The location of these sensors formed the local coordinate system of the thoracic, humeral, and scapular segment respectively through the digitization of each bony landmark. These procedures were implemented on all participants while they sat on the stool.

Data processing

All local coordinate systems were according to the shoulder standardization proposal of the International Society of Biomechanics [21]. The rotations were represented by the distal coordinate system relative to the proximal coordinate system using the Euler angle (Fig 2). Details is written in our previous study[22]. All data were processed using MATLAB software (MathWorks, Natick, MA, USA).

Fig 2

Definition of the coordinate system.

In the local coordinate system of the humerus (left), the Xh-axis was perpendicular to the plane defined by the glenohumeral rotation centre (GH), lateral epicondyle (EL), and medial epicondyle (EM). In the local coordinate system of the scapula (right), the Xs-axis was perpendicular to the plane defined by the trigonum spina scapula (TS), acromial angle (AA), and inferior angle (AI). The Ys-axis was defined as the cross product of the Xs-axis and the Zs-axis. The Zs-axis was defined as the direction from the TS to the AA. ER, external rotation; IR, internal rotation; UR, upward rotation; DR, downward rotation; AT, anterior tilt; PT, posterior tilt.

The scapular initial position was measured for 3 s while the participants sat on the stool; the mean value was calculated and used for further analysis. The scapular movements during flexion and abduction were measured while the humeral angle ranged from 30° to 120° relative to the thorax. The data from the three movements was averaged. The mean value every 30° was then calculated and used for further analysis. In addition, integral values and the coefficient of variance (CV) of scapular movement were calculated as the humerus was elevated from rest to 120° in 10° increments, which means scapular kinematic characteristic with respect to humeral movement and its’ variability among participants. The humeral angle upto 120° was chosen for scapular movement analysis because of the small effect of soft tissue on the surface measurement [23], and because the data could be collected for all participants. The intraclass correlation coefficient (1,3) calculated from the three scapular movements in ten healthy men [mean age, 25.1 (3.2) years; mean height, 171.1 (6.7) cm; mean weight, 60.5 (13.8) kg] is shown in Table 1, with a high enough reliability.

Table 1

Measurement reliability for scapular movements.

		Elevation phase				Lowering phase
Elevation angle		30°	60°	90°	120°	120°	90°	60°	30°
Flexion	Internal/External rotation	0.98(0.96–0.99)	0.98(0.95–0.99)	0.97(0.91–0.99)	0.94(0.83–0.98)	0.97(0.92–0.99)	0.98(0.94–0.99)	0.98(0.95–0.99)	0.98(0.94–0.99)
	Downward/Upward rotation	0.98(0.96–0.99)	0.98(0.94–0.99)	0.95(0.86–0.98)	0.98(0.95–0.99)	0.99(0.97–0.99)	0.98(0.94–0.99)	0.94(0.82–0.98)	0.86(0.61–0.96)
	Posterior/Anterior tilt	0.97(0.93–0.99)	0.98(0.94–0.99)	0.98(0.96–0.99)	0.98(0.94–0.99)	0.98(0.95–0.99)	0.97(0.93–0.99)	0.96(0.88–0.98)	0.91(0.75–0.97)
Abduction	Internal/External rotation	0.94(0.83–0.98)	0.97(0.93–0.99)	0.99(0.98–0.99)	0.99(0.97–0.99)	0.99(0.97–0.99)	0.99(0.98–0.99)	0.98(0.96–0.99)	0.98(0.95–0.99)
	Downward/Upward rotation	0.97(0.92–0.99)	0.99(0.97–0.99)	0.99(0.98–0.99)	0.99(0.98–0.99)	0.99(0.98–0.99)	0.99(0.98–0.99)	0.97(0.92–0.99)	0.97(0.93–0.99)
	Posterior/Anterior tilt	0.99(0.97–0.99)	0.99(0.97–0.99)	0.98(0.96–0.99)	0.99(0.97–0.99)	0.99(0.97–0.99)	0.99(0.98–0.99)	0.97(0.93–0.99)	0.99(0.97–0.99)

The values are intraclass correlation coefficients (1,3) and the range in parentheses is the 95% confidence interval.

Statistics analysis

All data were analysed using SPSS Statistics 22 software (IBM, Armonk, NY, USA). The Shapiro Wilk test was used to confirm the normality of the data; it showed non-normal distributions in some of the data. The Friedman test was used for each scapular rotation (i.e., internal external rotation, downward upward rotation, and anterior posterior tilt) to investigate whether the scapular initial position differed from scapular movement during flexion and abduction. When a significant main effect was confirmed, the Wilcoxon signed rank test with Bonferroni correction for post hoc analysis was performed to compare the scapular initial position with scapular movement at each humeral elevation. Spearman’s rank correlation coefficient was calculated to examine the relationship between the scapular initial position and scapular movement at each humeral angle for each scapular rotation. A confidence level of 0.05 was used in all of the statistical tests.

Results

All data were represented by a median value (25%, 75%) because some data had a non-normal distribution. The scapular initial position had an internal rotation of 29.7° (24.0°, 32.9°), upward rotation of 1.6° (3.4°, 4.1°), and posterior tilt of 5.2° (6.9°, 3.7°). The Friedman test indicated main effects of scapular rotation variable. Then post hoc test showed the significant differences between the scapular initial position and almost scapular movements in each rotation. No significant differences were found between the scapular initial position and the posterior tilt at 30° in lowering phase during flexion and in both phases during abduction (Figs 3 and 4). The integrated amount of change in the scapular movement and the CVs for every 30° are presented in Table 2. CVs between participants tended to be small for the upward rotation and large for the internal rotation and posterior tilt.

Fig 3

Scapular position and movement during flexion.

Box-and-whisker plots show internal and external rotation (top), downward and upward rotation (middle), and anterior and posterior tilt (bottom) of the scapula. The X-axis is the humeral angle and the Y-axis is the scapular angle. The asterisk represents the significant difference compared with the scapular position at rest (P < 0.05).

Fig 4

Scapular position and movement during abduction.

The descriptions are same as the Fig 3.

Table 2

Integral value of scapular movement and coefficient of variance.

			Elevation phase				Lowering phase
Elevation angle			rest to 30°	30 to 60°	60 to 90°	90 to 120°	120 to 90°	90 to 60°	60 to 30°	30° to rest
Flexion	Internal/ExternalRotation	Integral value (°)	3.1(1.8, 5.9)	4.9(4.1, 6.4)	3.8(2.2, 5.4)	3.9(2.1, 5.6)	2.7(1.5, 4.9)	2.6(1.6, 3.3)	3.8(2.3, 5.8)	4.6(2.4, 8.2)
		CV (%)	83	37	51	64	71	63	54	74
	Downward/UpwardRotation	Integral value (°)	5.9(3.3, 8.0)	11.6(10.1, 12.6)	13.6(12.2, 15.9)	9.5(5.8, 12.0)	8.5(6.0, 11.2)	14.3(12.9, 15.8)	12.2(11.2, 13.8)	4.6(1.4, 7.0)
		CV (%)	53	16	20	42	46	21	18	76
	Posterior/Anteriortilt	Integral value (°)	5.8(3.1, 9.0)	2.1(1.6, 3.7)	2.5(1.6, 3.6)	3.4(2.7, 5.5)	3.7(1.6, 6.2)	2.4(1.8, 3.4)	2.4(1.4, 3.2)	6.0(4.1, 10.0)
		CV (%)	60	52	55	52	63	57	59	63
Abduction	Internal/ExternalRotation	Integral value (°)	23.6(16.8, 29.8)	2.7(1.8, 5.5)	2.7(1.6, 4.2)	5.1(3.8, 7.7)	5.4(4.0, 7.8)	2.7(2.1, 4.3)	2.4(1.2, 4.4)	23.3(19.2, 29.3)
		CV (%)	38	76	57	44	45	45	76	33
	Downward/UpwardRotation	Integral value (°)	34.1(28.7, 37.5)	12.6(11.3, 13.8)	11.4(9.6, 13.4)	9.9(7.6, 11.9)	9.9(7.9, 11.6)	11.3(8.3, 13.1)	11.9(9.3, 13.1)	34.2(29.4, 37.0)
		CV (%)	19	15	23	29	28	31	25	20
	Posterior/Anteriortilt	Integral value (°)	6.4(3.2, 10.6)	4.4(3.2, 5.5)	4.6(2.7, 5.9)	2.7(1.9, 3.6)	2.8(1.9, 3.9)	5.0(3.7, 6.7)	4.1(3.1, 5.7)	7.3(4.5, 13.0)
		CV (%)	60	49	54	66	58	48	57	62

Values are expressed as median (25th, 75th). CV, coefficient of variance

There were significant positive correlations between the scapular initial position and scapular movements with respect to upward rotation and posterior tilt at all humeral angles in elevation and lowering phase during flexion. For internal rotation, the scapular position in elevation phase showed significant positive correlation with scapular movements at humeral elevations of 30°, 60°, and 90° even though significant positive correlations between two variables at all humeral angles were found in lowering phase during flexion. As well as the correlations during flexion, there were significant positive correlations between the scapular initial position and scapular movements with respect to upward rotation and posterior tilt at all humeral angles during abduction. For internal rotation, the scapular position showed significant positive correlations with scapular movements at all humeral elevations except 90° in lowering phase and 120° in both phases. In addition, correlation coefficients for each scapular rotation during flexion and abduction with humeral elevation tended to be small (Table 3).

Table 3

Correlation coefficient between scapular initial position and scapular movement.

		Elevation phase				Lowering phase
Elevation angle		30°	60°	90°	120°	120°	90°	60°	30°
Flexion	Internal/External rotation	0.64** (< .001)	0.51**(.002)	0.35*(.041)	0.29(.094)	0.35*(.044)	0.37*(.034)	0.37*(.032)	0.57* (< .001)
	Downward/Upward rotation	0.78** (< .001)	0.66** (< .001)	0.57** (< .001)	0.39*(.022)	0.39*(.024)	0.34*(.048)	0.39*(.019)	0.55** (< .001)
	Posterior/Anterior tilt	0.77** (< .001)	0.76** (< .001)	0.60** (< .001)	0.43*(.011)	0.37**(.030)	0.56** (< .001)	0.72** (< .001)	0.76** (< .001)
Abduction	Internal/External rotation	0.61** (< .001)	0.49**(.003)	0.37*(.034)	0.27(.129)	0.23(.201)	0.26(.133)	0.38*(.027)	0.51**(.002)
	Downward/Upward rotation	0.76** (< .001)	0.77** (< .001)	0.69** (< .001)	0.59** (< .001)	0.56** (< .001)	0.64** (< .001)	0.64** (< .001)	0.62** (< .001)
	Posterior/Anterior tilt	0.69** (< .001)	0.53** (< .001)	0.41*(.015)	0.40*(.019)	0.39*(.022)	0.37*(.030)	0.47**(.005)	0.67** (< .001)

The values are ρ of Spearman’s rank correlation coefficient and P values are in parentheses.

*, significant correlation (P < 0.05).

**; significant correlation (P < 0.01).

Discussion

Our study investigated the relationship between the scapular initial position and scapular movement in elevation and lowering phase during flexion and abduction and showed that there were significant positive correlations between these variables except that at a few humeral angles in healthy young men. In addition, these correlation coefficients tended to decrease with humeral angle during flexion and abduction. To the best of our knowledge, this is the first study to focus the interaction between scapular initial position and its movement and to demonstrate significant correlations between them in elevation and lowering phase during flexion and abduction, i.e. dynamic motions, in healthy young men. Our results partly supported our hypothesis that the scapular initial position is related to scapular movement during dynamic motions in healthy young men.

In the present study, the three-dimensional scapular initial position represented about 30° of internal rotation, the middle of the upward/downward rotation, and about 5° of anterior tilt. The scapula rotated upward, externally, and then internally and tilted posteriorly during flexion, and rotated upward and externally and tilted posteriorly during abduction. Strufy et al.[11] demonstrated that the scapula at rest (i.e. initial position) was approximately horizontal, had an internal rotation of 35°, and an anterior tilt of 10° with some extent individual difference. Previous studies that investigated the three-dimensional scapular angle during arm elevation reported that the scapula had upward rotation, posterior tilt, and internal rotation, followed by external rotation during flexion, whereas external rotation occurred throughout the abduction phase[7,8,15]. Our study therefore most likely is in same lines of above-mentioned ones.

Our results clarified the significant positive correlations between the two variables in scapular downward upward rotation and anterior posterior tilt, except internal external rotation, during flexion and abduction. To our knowledge, there is one study to directly explore the relationship between the scapular initial position and the consequently scapular movement. de Groot and Brand[19] developed a thorax-fixed regression model for prediction of the scapular orientation from the humeral orientation, and they showed that the scapular orientations were predicted by humeral variables, external road, and scapular initial position. Our study could be the same line of this previous study even though the direct comparison between two studies is complicated because their experimental procedure constituted of static position of upper limb (i.e. non-dynamic motion) and fixed-thorax posture, which may be far from actual dynamic shoulder movement. Thus, our findings did not only support this previous study but also made the evidence for interaction between scapular initial position and scapular movement during dynamic motions.

The correlations found in the present study could be due to kinematic interaction between humerus and scapula during dynamic motions. For the upward rotation of the scapula, the kinematic characteristic of the humerus and the scapula probably affected the correlations. Inman et al.[24] firstly found the scapulohumeral rhythm, and then many subsequent studies validated the kinematic characteristic between humerus and scapula[8,14,25]. In other words, the angles of the humerus and the scapula constantly shifted through arm elevation. In the present study, through the humeral angles except rest to 30°, which is called setting phase[26], the integral values to some extent were constant, and the CVs among the participants tended to be small for the upward rotation of scapula. The scapula therefore probably maintains a consistent pattern from rest to the end of movement except setting phase. With respect to internal rotation and posterior tilt no studies have demonstrated a specific pattern between the humerus and the scapula, such as the scapulohumeral rhythm. Our results found that the CVs for internal rotation and posterior tilt tended to be large, indicating the individual variability of scapular movement. On the other hand, compared to that for the upward rotation of the scapula, the integral value for the internal rotation and posterior tilt of the scapula tended to be small. The scapular movement in these rotations thus would be dependent on the scapular initial position because of the small changes in scapular movement during elevation and lowering, even if the specific characteristic between the humerus and the scapula was not seen.

The present study showed the tendency of the correlation coefficient to decrease with humeral angle for each scapular rotation. Previous studies that investigated the activities of the scapular muscles during elevation and lowering showed that muscle activity around the scapula among participants during arm elevation varied widely with arm angle, although no statistical analysis was performed[27,28]. Given these previous studies, the correlation coefficients tended to be small with respect to arm angle because the individual variability of scapular movement could be large due to the various activities of scapular muscles.

Three-dimensional measurement of the scapula would require the use of a motion-capture system such as an optical camera or electromagnetic sensors. However, the use of these devices in the clinical setting and on the sports field is not practical with respect to time and cost. In addition, assessment of scapular movement during elevation is difficult and varies among investigators[29]. Our results clarified significant correlations between scapular initial position and scapular movement during flexion and abduction. In other words, it is likely that scapular movement during elevation can be estimated from the scapular initial position. From clinical perspective, although lateral scapular side test and modified one is generally used as diagnostic method to discrete people with or without shoulder dysfunction, some previous studies argued that these test don not have clinical utility because of low accuracy[30,31] and specificity[32]. Baring these clinical situations in our mind, therefore, we advocate that our findings would not be directly applicable for diagnosis of shoulder dysfunction but useful in assessing scapula movement or as criteria for effect of therapeutic exercise because the assessment of the scapular initial position is easy and reliable[33]. We conjecture that deviation of scapular motion at first would be an unneglectable sign of shoulder dysfunction, and then the deviation may alter due to pain and muscle weakness, resulting in the low accuracy and specificity of scapular-based assessments such as the lateral scapular side test. So, prospective study to discover whether the scapular initial position associates with the shoulder dysfunction is of importance.

There are some limitations to the interpretations of our findings. First, all of the participants were healthy young men, as prescribed by the exclusion criteria. Therefore, it is not clear whether the findings can be applied to women, older adults, and individuals with a shoulder disorder such as shoulder instability or rotator cuff tear. Future studies involving these participants are needed. Second, although we analyzed the scapular movement in humeral elevation up to 120° in the present study, measurement errors could not been completely excluded because the acromial method always included some errors compared with the percutaneous pinning[23] and the tripod measurement[34]. Therefore, the precise relationship between the scapular initial position and scapular movement without subcutaneous tissue remains unclear. A future study is, therefore, necessary to investigate the precise relationship between the scapular initial position and its movement using 2D/3D registration technique. Moreover, whether this relationship exists with other tasks is unclear because flexion and abduction were the only tasks used for measurement. Thus, other tasks such as the activities of daily living or overhead motion should be the focus of future studies.

Conclusions

We investigated the relationship between the scapular initial position and scapular movement in elevation and lowering phase during dynamic motions i.e. flexion and abduction in healthy young men. Our results found significant positive correlations in upward and internal rotation and in posterior tilt at all humeral angles, but not for internal rotation at a humeral elevation of 90° in lowering phase and 120° in both phases. In addition, these correlation coefficients for flexion and abduction along with humeral elevation tended to be small.

9 Aug 2019

PONE-D-19-19999

Relationship between scapular initial position and scapular movement during dynamic motions

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Reviewer #1: Yes

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5. Review Comments to the Author

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Reviewer #1: The authors investigated the interaction between the scapular initial position and the scapular motions using the motion trackers. Although their findings that the scapular initial positions mostly correlated to the scapular movement were considered important, there are some comments for this study

#1

First, they adopted three motion trackers and secure them to the skin using the tapes. We should carefully consider how accurate they detect the motions. Especially the rotations, the authors demonstrated tiny differences during the motions. It would be helpful for us to understand the outcomes more clearly if the authors could demonstrate the accuracy of the device.

In addition, how did they assess the effect of subcutaneous fat on the outcomes. We can assume the authors selected young males without excessive obesity from the overall data. Please mention these parts more carefully at the Materials and Methods.

#2

They described their hyposthesis for this study: the initial position might correlate to the motions. The major problem of this study was the participants’ characteristics. They only assessed “young” “male” subjects. If the authors would like to prove it, I strongly recommend them to assess the participates including elder people and female people as well.

#3

The authors assessed three times for each motion. Regarding the methodology, please mention how the authors calculate each motions from triplicated data.

In addition, the authors mentioned the reliability of the measurement “the ICC(1,1) calculated from ten healthy men …”. Regarding the reliability, please explain the detail about the values for all motions and rotations, because these information would be helpful to understand the reliability of the methodology using the motion trackers with adhesive tapes.

#4

Regarding my understanding, the measurements using the surface motion trackers have disadvantage for the accuracy even they selected limited motion from 30 to 120 degrees. Some researchers used the percutaneous pinning or fluoroscopic measurement to address to the scapula. I suppose that only the upward/downward rotation of the scapula could be assessed with this measurement tool. I recommend the authors to explain the comparison of various techniques to assess the scapular and/or humerus motions with the accuracy values.

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Reviewer #1: No

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23 Aug 2019

We thank the reviewer for their constructive comments on our manuscript, as well as for the suggestions provided. Our responses to your comments have been uploaded as the file labeled 'Response to Reviewers'. Please find the file.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

16 Oct 2019

PONE-D-19-19999R1

Relationship between scapular initial position and scapular movement during dynamic motions

PLOS ONE

Dear Mr Umehara,

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.

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Chunfeng Zhao, MD

Academic Editor

PLOS ONE

[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: (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: Partly

**********

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. 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

**********

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

**********

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: There are some comments for the revised manuscript.

Comment to Response #1

Page 7 Lines 111-112

Instead of using inches, I think the authors should use the unit of mm, cm, since they have used cm in the next sentence.

Page 5 Lines 79-80

If the authors described the sentence regarding the exclusion criteria for women, they should demonstrate scientific evidence that women have more measurement error because of the subcutaneous fat.

Comment to Response #2

Again, I recommend the authors to include others with additional characteristics such as elder populations, instead of adding the description of “healthy young men”.

Comment to Response #3

The authors modified the description regarding the methodology and its reliability.

Comment to Response #4

They modified the sentence regarding the limitation. I suppose they should have descripted how they justified the internal/external rotation as well as anterior/posterior tilt with scientific values in advance to this study.

**********

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Reviewer #1: 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 to be viewed.]

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21 Oct 2019

Response to Reviewer

Itemized responses to the reviewer’s comments have been listed below in reference to our manuscript entitled “Relationship between scapular initial position and scapular movement during dynamic motions”.

Reviewer #1

We thank the reviewer for their constructive comments on our manuscript, as well as for the suggestions provided.

Our responses to your comments are listed below. For clarity, the parts quoted from the manuscript are in red, and the implemented changes are highlighted in yellow.

Comment #1

Page 7 Lines 111-112

Instead of using inches, I think the authors should use the unit of mm, cm, since they have used cm in the next sentence.

Response

We have changed the unit from inches to mm.

Revised manuscript

The accuracy of the sensor is 0.762 mm …

Comment #2

Page 5 Lines 79-80

If the authors described the sentence regarding the exclusion criteria for women, they should demonstrate scientific evidence that women have more measurement error because of the subcutaneous fat.

Response

In accordance with your comment, we have cited a previous study investigating sex differences in skeletal muscle, subcutaneous adipose tissue, bone, and so with use of magnetic resonance imaging (Abe et al., 2003). The study included Japanese healthy young men and women as subjects and showed women had higher percentage fat and fat mass than men. This result allows us to assume that women have more measurement error than men because of the subcutaneous fat.

Revised manuscript

…we also excluded women because they would have more subcutaneous fat than men [20], …

Comment #3

Again, I recommend the authors to include others with additional characteristics such as elder populations, instead of adding the description of “healthy young men”.

Response

Thank you for your suggestion. You recommend to include population(s) with additional characteristics such as women and the elderly. However, we beg to disagree that this is necessary. Herein there are two reasons.

1. Populations with additional characteristics such as older and women, to our knowledge, have a possibility to disturb our result, interpretation and conclusion. Previous studies showed the differences in scapular kinematics between asymptomatic older and young population (Saker, 2014) even when the comparison between men and women (Schwartz et al., 2016). These characteristics dependent on the population are due to several factor. For instance, muscle strength loss is inevitable with age and older people have less muscle strength than young (Larsson et al., 1979; Murray et al., 1985). Anthropometric parameter (Winter, 2009), bone shape (Paraskevas et al., 2008), and neuromuscular function (Anders et al., 2004) are different between gender. Namely, additional population in the current study done not make sense.

2. Inclusion of populations such as the elderly and women may create large measurement error between bone and skin due to the subcutaneous tissues. As described in response to your comment#2, women have higher percentage fat and fat mass than men (Abe et al., 2003) and older groups show thicker fat than young groups (Kanehisa et al., 2004).

Bearing in mind these difficulties, we decided to limit the population to healthy young men. As described in our limitations, it is not clear whether the findings can be applied to women, older adults, and individuals with shoulder disorders. However, the lack of inclusion of other populations does not imply that our data do not have scientific value. In the future we plan to conduct these experiment including populations such as older adults and women. In short, we believe that excluding such populations does not make our results invalid.

Since this is the second time you have raised this point, we understand that you feel it is important. However, you have not provided any scientific rationale to convince us of that importance. Moreover, thousands of studies have been published which consider just one population. Population comparisons is therefore clearly not a prerequisite for publication in general. While we agree that a population comparison would be useful, and would also be necessary to demonstrate general validity, we believe that these are separate issues that are tangential to our paper’s message.

If you still feel that it is necessary to add additional populations, we respectfully request that you clarify the scientific rationale for your recommendation.

Comment #4

They modified the sentence regarding the limitation. I suppose they should have descripted how they justified the internal/external rotation as well as anterior/posterior tilt with scientific values in advance to this study.

Response

We apologize, but we do not understand this comment. Please allow us to rephrase your comment as follows: “Authors should use a priori rational to explain why they used internal/external rotation and anterior/posterior tilt.”

If this is what your comment means, we believe that our Introduction and Methods already sufficiently detail the rationale for choosing these tasks.

If this is not what your comment means, we respectfully request clarification.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

17 Dec 2019

Relationship between scapular initial position and scapular movement during dynamic motions

PONE-D-19-19999R2

Dear Dr. Umehara,

We are pleased to inform you that your manuscript has been judged scientifically suitable for publication and will be formally accepted for publication once it complies with all outstanding technical requirements.

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With kind regards,

Chunfeng Zhao, MD

Academic Editor

PLOS ONE

Additional Editor Comments (optional):

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 #2: 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 #2: Yes

**********

3. Has the statistical analysis been performed appropriately and rigorously?

Reviewer #2: Yes

**********

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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 #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 #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 #2: The authors have addressed the comments raised in the previous review. I think that this manuscript is now acceptable for publication.

**********

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Reviewer #2: No

19 Dec 2019

PONE-D-19-19999R2

Relationship between scapular initial position and scapular movement during dynamic motions

Dear Dr. Umehara:

I am 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 notify them about your upcoming paper at this point, to enable them to help maximize its impact. If they will be preparing press materials for this manuscript, 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.

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on behalf of

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Academic Editor

PLOS ONE