Cognitive stress changes the attributes of the three heads of the triceps brachii during muscle fatigue.

INTRODUCTION: Cognitive stress (CS) changes the peripheral attributes of a muscle, but its effect on multi-head muscles has not been investigated. The objective of the current research was to investigate the impact of CS on the three heads of the triceps brachii (TB) muscle.
METHODS: Twenty-five young and healthy university students performed a triceps push-down exercise at 45% one repetition maximum (1RM) with and without CS until task failure, and the rate of fatigue (ROF), endurance time (ET) and number of repetitions (NR) for both exercises were analyzed. In addition, the first and last six repetitions of each exercise were considered non-fatiguing (NF) and fatiguing (Fa), respectively, and the root mean square (RMS), mean power frequency (MPF) and median frequency (MDF) for each exercise repetition were evaluated.
RESULTS: The lateral and long head showed significant differences (P<0.05) in the ROF between the two exercises, and all the heads showed significant (P<0.05) differences in the RMS between the two exercises under NF conditions. Only the long head showed a significant difference (P<0.05) in the MPF and MDF between the two exercises. CS increases the ET (24.74%) and NR (27%) of the exercise. The three heads showed significant differences (P<0.05) in the RMS, MPF and MDF under all exercise conditions.
CONCLUSION: A lower ROF was obtained with CS. In addition, the RMS was found to be better approximator of CS, whereas MPF and MDF were more resistant to the effect of CS. The results showed that the three heads worked independently under all conditions, and the non-synergist and synergist head pairs showed similar behavior under Fa conditions. The findings from this study provide additional insights regarding the functioning of each TB head.

Introduction

The development of muscle fatigue depends on a number of factors, including the exercise type, intensity and duration, the type and fiber composition of the muscle, the synergist pairs, the subject’s physical condition and other external factors, such as temperature [1,2,3,4]. Peripheral muscle fatigue can be defined as a decrease in the force-generating capacity of a muscle or group of muscles during or after a particular task [5]. Peripheral muscle fatigue is caused by a loss in the force-generating capacity of motor units (MUs) [6], and to continue performing a desired task, a muscle recruits newer MUs and increases the firing of already recruited MUs. The number of active MUs and the conduction velocity of muscle fibers decreases over time [7], and these effects slow the firing rate of MUs. Eventually, these events lead to the synchronization of MU firing [8], which in turn induces an increase in the surface electromyography (sEMG) root mean square (RMS) and a decrease in the median frequency (MDF). The persistence of such a phenomenon leads to task failure. A previous study revealed that peripheral muscle fatigue is addressed by different motor control strategies depending on the feedback mode [1]. Hence, the involvement of the central nervous system (CNS) in peripheral muscle fatigue cannot be overruled.

The outcome of a type of movement, such as an exercise or day-to-day activity, is affected by the psychological and cognitive demands of the body [9]. Various psychological effects, such as those of thinking about an upcoming assignment or a future meeting, might induce stress [10]. The effect of cognitive stress (CS, also called mental stress or mental load) is more prominent in skeletal muscles closer to the CNS, and the activity in these muscles exhibit a decreasing trend in the caudal direction [11,12]. As described in the literature, the strongest impact of this type of stress is observed in the neck, trapezius and shoulder muscles [11,13]. Increase in CS increases the neuro-motor (also called neural or motor) noise, and this noise propagates in both time and space [14] and might impact muscle performance due to its potential effects on kinematics and joint variability [15]. The MU coordination and control strategy might also be altered by this increased neural noise. As described in a previous study [16], muscle fatigue also increases neural noise. Hence, peripheral fatigue is expected to be affected by an increase in the mental load induced by CS. The effect of CS on fatigue is a complex issue and depends on many factors, including the level of CS. Hence, the behavior of a muscle under fatigue during CS becomes unpredictable due to the neuro-motor noise induced by fatigue and mental stress simultaneously. This variation in the properties of muscles can be observed through sEMG, as shown by a number of researchers [17,18,19].

The triceps brachii (TB), which is the only muscle in the posterior compartment of the arm, acts as an elbow extensor, a horizontal arm abductor, and an antagonist in elbow flexion [20,21]. The anatomical distance of the TB from the CNS renders it more susceptible to the effects of CS compared with distal skeletal muscles, such as the forearm and leg muscles. A previous study [11] analyzed the impact of an increase in the mental load on the TB during finger tapping while the elbow is flexed and observed no change in the activity of the TB, probably due to it being overshadowed by the biceps brachii. Within the TB, the lateral and long heads and the lateral and medial heads act as synergist pairs [22,23], but the behavior of these synergist pairs in the presence of CS has not yet been explored.

CS can be introduced in a number of forms, and its key purpose is to increase the burden on the CNS using the attention diversion technique. For example, multi-tasking is the activity undertaken by an individual who is asked to perform multiple actions at the same time, and previous studies on multi-tasking have employed tests of visual memory (the subject is asked to remember an alphabet or number sequence and reproduce them at a later time), audible memory (the subject is asked to remember a tone and reproduce it at a later time), arithmetic (the subject is asked to solve arithmetic riddles), and logic (the subject is asked to answer logical questions), among others [18,24]. In addition, social [25], work environment [18] and ethical stresses have been commonly used as stress-inducing factors. Based on electroencephalogram (EEG), Zarjam P. classified an arithmetic test into seven levels ranging from a very low to a very high mental load [26]. In the present study, an arithmetic test involving problems with medium to high complexity was used to induce CS in the subjects, and because the subjects were university students, it was expected that they would not find it difficult to answer the arithmetic riddles.

Dynamic contractions are psychologically more demanding than isometric contractions. Cognitively, isometric contractions require focus on postural control, whereas dynamic contractions require both movement and postural control. Thus, the effect of attention diversion techniques is likely to be more easily observable during dynamic contractions. Additionally, dynamic contractions are more closely related to day-to-day activities. Thus, studies on the behavior of a muscle during CS should consider dynamic instead of isometric exercise.

The effect of CS on muscle activity is obvious [27] and can thus be estimated by sEMG. The RMS, which is a temporal parameter, has been used for analyzing variations in sEMG activity due to CS [11] and has been primarily used to determine whether the sEMG activity increases or decreases due to the presence of CS. Although the RMS can be used for the analysis of both isometric [28] and dynamic contractions [29], its application for the latter is not popular [30]. Spectral parameters, such as the mean power frequency (MPF) and median frequency (MDF), have been utilized by some researchers to analyze the impact of CS on sEMG signals [19]. Furthermore, the MPF and MDF tend to decrease with the advent of muscle fatigue and their rate of decrease is termed as rate of fatigue (ROF) [30,31,32,33] and is an important parameter for analyzing the effect of exercise on muscle.

The objective of the current study was to analyze the effects of CS on the three heads of the TB under both non-fatiguing (NF) and fatiguing (Fa) conditions. Because the CS increases neuro-motor noise, it can be hypothesized that the presence of CS would increase muscle activity and that the increases in the activities of synergist muscles could be comparable. Subsequently, it is further hypothesized that the presence of CS would result in a relatively greater decrease in the MPF and MDF.

Materials and methods

Participants

Twenty-five young, healthy, recreationally active male university students were recruited for this study. The recruited subjects had no history or on-going diagnosis of neuromuscular disorder in the upper arms. The age of the subjects was 23.8(3.6) years, and their height and weight were 169.1(5.5) cm and 71.2(11.2) kg, respectively. The experimental protocol was approved by the Medical Research and Ethics Committee of Malaysia and is in accordance with the Declaration of Helsinki. The subjects who agreed to participate were informed prior to the experiment and provided written consent. Further, the individual who appears in this manuscript has given written informed consent (as outlined in PLOS consent form) to publish his/her case details. The experiment was conducted at the university gymnasium, and a physician was available to handle any emergency and aid the researchers.

Experimental setup

The three heads of the TB were observed via disposable pre-gelled bipolar sEMG electrodes (Kendall™ 100 MediTrace®, Tyco Healthcare Group, USA). The heads were identified with the aid of a physician as described by [34], and based on the SENIAM recommendations, the electrodes were placed on the belly of each head in line with the muscle fibers. The inter-electrode distance was 20 mm, and the skin was shaved, abraded and cleaned prior to electrode application.

A Shimmer 2.0r Model SH-SHIM-KIT-004 instrument (Realtime Technologies Ltd., Ireland) was used to record the sEMG signals. The Shimmer board has built-in filters that capture sEMG data from 5–322 Hz. Each shimmer board was connected to one of the heads of the TB muscle through the corresponding electrode. The reference electrode was placed over an electrically neutral area in the vicinity of the muscle, i.e., lateral epicondyle and olecranon of the shoulder and elbow.

The muscle identification and electrode placements were validated by a physician present on site. The Shimmer system was interfaced with a computer via class 2 Bluetooth®. The raw EMG signals were recorded at a sampling rate of 1 kHz. The computer was placed at a distance of 2 to 3 meters from the subject, and a line of sight was maintained between the subject and computer. The Shimmer Sensing LabVIEW program, accompanying the device was used to store the obtained data on the computer.

Experimental procedure

When the subject arrived at the site, it was ensured through verbal communication and visual feedback by the physician that the subject was not under any form of CS. The electrodes were placed on the dominant arm of the subject prior to the familiarization session. After placement of the electrodes and sEMG equipment, the subject was asked to warm-up through upper body stretching and exercises consisting of five to eight repetitions using light weights. Subsequently, a resting period of approximately 2 minutes was given.

The subject then stood in-front of triceps push-down machine and held the bar with both hands in the pronated position at shoulder width. During this exercise, the subject kept his arms close to, but not touching, his body and perpendicular to the ground and his torso slightly leaned forward, as shown in Fig 1. The torso was leaned so that hands did not touch the body during full elbow extension. The subject then moved his forearm toward the ground while maintaining the above-described posture until his forearms were fully extended and then slowly moved his forearm back to their starting position (full range of motion (ROM)), and this movement was considered one repetition. The maximal load that each subject held while performing one repetition successfully was termed as one repetition maximum (1RM). It was ensured that the subjects were in the neutral psychological state and any forms of adverse distractions were absent during data collection. Further, the duration of 1RM was conducted at the subject’s ease to the same effect. The subject was allowed a 15-minute rest after determination of the 1RM.

Fig 1

Posture of the subjects during the triceps push-down exercise.

Following 1RM test, subjects were asked to perform the TB push-down exercise at 45% 1RM until task failure, under no-cognitive stress (NCS) and cognitive stress (CS). The exercise was performed until exhaustion and the exercise was terminated if the subject could not, control the speed of the bar during the eccentric phase or maintain balance between their dominant and non-dominant arms for two consecutive repetitions. The two exercises were randomly assigned to the subject on arrival at the experiment location. For CS, the subject was to solve mathematical additions, while performing the exercise simultaneously, which induce CS in the subject [15,35]. The complexity of the mathematical riddles ranged from medium to high on the scale introduced by [26]. Specifically, the subject was shown two numbers (each consisting two to three digits depending on the complexity of the problem) on a computer screen (17″ LCD) that was placed such that the subject could clearly see the numbers. The numbers were displayed in Times New Roman font in the middle of the screen for 3 seconds, and the subject was audio alerted prior to their display. The numbers were then removed for a gap time of 2 seconds to test the subject’s retention, and a multiple-choice menu with five choices–a, b, c, d and e, was then displayed for 2 seconds. The subject announced his choice, and 2 seconds later, the next question appeared. For NCS condition, subject simply performed the TB push-down exercise at 45% 1RM without performing any other cognitive activity. A rest period of around 15 minutes was taken between the two exercises. Fig 2 demonstrates the temporal experimental procedure. We note that performing these exercises at 45% 1RM may incur low frequency fatigue (LFF) in the subjects as they may get 55% fatigued from their base value. Although [36] suggests that subjects who get fatigued at 35% or greater from their base value could induce LFF, which could delay the recovery process, the authors reported that after 5 minutes of rest from isometric contractions, muscles regained about 85% of their strength. In addition, [37] commented that LFF is more prominent in eccentric and isometric exercises as compared to dynamic exercises. Hence, we believe that the rest period of about 15 minutes observed in our experiment is enough for the subjects to recover.

Fig 2

Temporal experimental procedure for the experiment.

If a subject was frequently unable to maintain correct posture i.e. torso leaned too much, subject’s torso got straightened or arm abduction varied, a new subject was recruited as a replacement. Our pilot study showed that it is very difficult for a subject to maintain a predetermined exercise tempo, by either following metronome beeps or visual feedback, and answer the questions simultaneously. Hence, the tempo of the exercise was maintained at subject’s ease. Subjects were not allowed to pause between transitions from the eccentric to concentric or concentric to eccentric phases. A custom-made program in LabVIEW measured the duration of a complete rep from the real-time sEMG data, and it was ensured that all repetitions were within ±15% of this duration. During the experiment, the subject was continuously given verbal encouragement (not amounting to a form of distraction) to exert maximal effort and maintain the tempo. Posture (including all appropriate joint angles) throughout the ROM and exercises was monitored by a dedicated assistant present on site and it was ensured that the subject did not use his body weight to move the bar. Both the exercises were performed on the same day as psychological conditions of subjects may change over different days. The experiment was designed to vary the cognitive stress while keeping all other conditions same.

Data analyses

The collected data were stored in a computer system for further analysis. Custom-written programs in MATLAB 17 (MathWorks Inc., USA) were used to filter, normalize and evaluate the parameters. A fourth-order bandpass Butterworth filter (cut-off frequencies of 5–450 Hz) was used for filtration. A 512-point Short-Time Fourier Transform (STFT) computed with 50% window overlap was used to estimate MPF and MDF. The filtered and rectified sEMG signals from each exercise and subject were used to extract segments corresponding to the active phase (concentric and eccentric). The active phases were identified by using a 256 ms moving window to obtain the mean of the signal, with a threshold set at 15% of the maximum value in the entire recording, as shown in Fig 3. The RMS, MPF and MDF were calculated for each active phase. Subsequently, the RMS was normalized against a dynamic contraction rather than using a maximal voluntary contraction (MVC), by considering the average RMS amplitude from 1RM repetition. This was done due to the difficulty in finding the optimal joint angle that provides maximal output effort for all three heads. A similar approach was also observed to be used by [38]. The time from the start of the exercise until task failure (endurance time (ET)) and the total number of segments (number of repetitions (NR)) were compared. The first six (non-fatigue) and last six segments (fatigue) were identified, and the MPF, MDF and normalized RMS (onwards RMS) were calculated for all three heads and for all segments. The ROF was calculated from the slop of MPF using regression analysis as suggested by [30,31,32,33].

Fig 3

Filtered and rectified sEMG signal from the lateral head of the TB of a subject.

The active phase, NF and Fa regions are shown.

While both MPF and MDF have been used to explain similar interpretations of physiological aspects of exercises, in this work, both MPF and MDF have been used. This is because several researchers found that there may be potential differences between the two parameters [39,40]. It was further stated by [41] that “certain factors have a differential influence on the different spectral statistics”. Hence, we preferred to use both spectral indices.

Statistical analyses

For each subject, the ROF, ET and NR were obtained for both NCS and CS exercises for all the heads. Subsequently, the RMS, MPF and MDF values were obtained from the active phase under non-fatigue (NF) and fatigue (Fa) conditions in all the heads for each exercise. All data were tested for normality using the Shapiro-Wilks test and found normally distributed. Descriptive statistics was then used to compare ET and NR for the two exercises. One-way repeated measures ANOVA was employed to make comparisons for, ROF between the two exercises in a particular head and ROF between three heads in a particular exercise. Two-way repeated measures ANOVA was employed to analyze the main effects of fatigue-cognitive stress interaction for each head of the TB. For the cases that violated the sphericity assumption, Greenhouse-Geisser corrections were used. Bonferroni adjustments were applied for the post hoc analysis to observe the behavior of parameters among pairs of TB heads. A difference was considered significant if P<0.05, and thus, a confidence level of 95% was considered. The statistical analyses were performed using IBM SPSS 20.0 (SPSS Inc., USA).

Results

All subjects, with the exception of one, showed significant increase in ET from 58.33(10.89) sec to 82.8(14.83) sec and NR from 21.39(4.54) to 29.67(5.31), in the CS exercise compared with the NCS exercise. The increment was 24.74% and 27% for ET and NR respectively.

Overall, the two exercises showed significant difference (P<0.05) for ROF with mean ROF higher in the NCS exercise in all three heads when compared to the CS exercise. Comparison among the exercises revealed ROF to be statistically significant (P<0.05) for the lateral and long heads only. Fig 4 presents the mean ROF of the three heads.

Fig 4

ROF in μ(SD) of all the subjects during exercise with and without cognitive stress.

Table 1 and Fig 5 summarizes the statistical results (one-way ANOVA) of RMS, MPF and MDF under NF and Fa conditions between NCS and CS exercises. Under the NF condition, all three heads showed higher RMS (P<0.05) in CS, while only the long head showed higher MPF and MDF (P<0.05) in NCS. In the Fa condition, both the lateral and long heads showed higher MPF and MDF (P<0.05) in CS. Comparisons between NF and Fa conditions were significantly different (P<0.05) for both exercises in all the heads for all observed parameters, with the exception of RMS in the long head during NCS. Overall, RMS was observed to be higher in Fa condition whereas MPF and MDF were higher in NF conditions.

Table 1

Results of one-way ANOVA for RMS, MPF and MDF between different exercise conditions in each head (P-values).

RMS Lat	RMS Lo	RMS Med	MPF Lat	MPF Lo	MPF Med	MDF Lat	MDF Lo	MDF Med
Between NF-NCS and NF-CS
<0.001	<0.001	<0.001	0.34	0.004	0.057	0.92	0.002	0.17
Between Fa-NCS and Fa-CS
0.27	0.58	0.041	<0.001	<0.001	0.14	0.006	<0.001	0.32
Between NF-NCS and Fa-NCS
<0.001	0.08	0.02	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
Between NF-CS and Fa-CS
<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001

*Bold font indicates statistically significant. Lat–Lateral, Med–Medial and Lo–Long.

Fig 5

RMS, MPF and MDF of the three heads of the TB under NF and Fa conditions during exercise with and without cognitive stress.

Table 2 summarizes the statistical results (one-way ANOVA) between the three heads for RMS, MPF and MDF for all four conditions, i.e., NF-NCS, Fa-NCS, NF-CS and Fa-CS. The three heads were statistically significant (P<0.05) for all conditions and parameters with the exception of RMS in NF-NCS. The lateral head produced the highest mean MPF, MDF and RMS in both exercises. The post hoc analysis revealed that only the lateral-long and lateral-medial synergist pairs observed statistical significance under all conditions and parameters (P<0.05). Nevertheless, the long-medial muscle pair showed statistical significance (P<0.05) for MPF and MDF during Fa condition for both exercises.

Table 2

Results of one-way ANOVA among the three heads under different conditions for RMS, MPF and MDF (P-values) and post hoc tests (a—lateral & long, b—long & medial, c—lateral & medial).

RMS 1	RMS 2	RMS 3	RMS 4	MPF 1	MPF 2	MPF 3	MPF 4	MDF 1	MDF 2	MDF 3	MDF 4
0.27	<0.001a,c	<0.001 b	0.002 a,c	<0.001 a,c	<0.001 a,b,c	<0.001 a,c	<0.001 a,b,c	<0.001 a,c	<0.001 a,b,c	<0.001 a,c	<0.001 a,b,c

*Bold font indicates statistically significant. 1: NF-NCS; 2: Fa-NCS; 3: NF-CS; 4: Fa-CS.

Interaction results (two-way ANOVA) between fatiguing condition and cognitive stress in all the heads for RMS, MPF and MDF are presented in Table 3. The main effects of CS were found significant in all heads for RMS only, while the fatiguing condition was significant in all the heads for RMS, MPF and MDF. The interaction between cognitive stress and fatigue was significant in all the heads for RMS and MPF.

Table 3

P-values for the main effects of cognitive stress and fatigue and their interaction on RMS, MPF and MDF for the three heads of the TB (n = 25).

	Lat			Lo			Med
	RMS	MPF	MDF	RMS	MPF	MDF	RMS	MPF	MDF
EXER	0.003	0.084	0.148	0.008	0.897	0.456	0.003	0.824	0.755
COND	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001	<0.001
EXER × COND	<0.001	0.047	0.061	0.003	<0.001	<0.001	<0.001	0.012	0.078

*Bold font indicates statistically significant.

Table 4 presents the percentage changes in the RMS, MPF and MDF from NF to Fa conditions in both exercises. Percentage change in RMS was significantly higher in the CS exercise. However, the spectral parameters showed comparable percentage change from NF to Fa conditions in both exercises. The absolute changes in the RMS, MPF and MDF from NCS to CS exercises under NF and Fa conditions are summarized in Table 5. The difference in RMS was more pronounced under the NF condition while the differences in MPF and MDF were comparable in both conditions.

Table 4

ΔRMS(%), ΔMPF(%) and ΔMDF(%) from NF to Fa conditions, μ(SD).

	NCS			CS
	Lat	Lo	Med	Lat	Lo	Med
%ΔRMS	11.01(20.6)	3.35(30.07)	6.28(25.25)	22.33(26.81)	8.63(28.07)	21.11(17.33)
%ΔMPF	-28.22(17.76)	-35.64(15.34)	-28.2(14.86)	-30.98(17.8)	-44.3(19.53)	-34.25(23.7)
%ΔMDF	-29.34(21.2)	-33.72(19.3)	-26.6(17.95)	-33.16(21.5)	-44.79(24.8)	-32.3(26.3)

Table 5

ΔRMS, ΔMPF and ΔMDF from NCS to CS exercise, μ(SD).

	NF			Fa
	Lat	Lo	Med	Lat	Lo	Med
ΔRMS (mV)	0.11(0.19)	0.07(0.2)	0.11(0.18)	-0.02(0.22)	0.02(0.25)	-0.03(0.19)
ΔMPF (Hz)	1.11(8.7)	-1.79(6.98)	-0.99(6.8)	2.26(6.18)	1.83(5.45)	1.3(8.1)
ΔMDF (Hz)	0.3(9.93)	-2.17(7.7)	-0.74(8.5)	1.94(7.16)	1.55(4.68)	1.25(8.02)

Discussion

This study aimed to investigate the effect of CS on peripheral fatigue in the three heads of the TB. Global fatiguing effects were observed through the ET and NR, and the ROF, RMS, MPF and MDF for the three heads of the TB during the active phase under the different conditions were calculated and statistically compared. The RMS for all three heads increased in the presence of CS under NF conditions only, and the observed increase in the lateral and medial heads were comparable, confirming our hypothesis that CS would induce comparable increases in the RMS in synergist muscles. A higher percent decrease in the MPF and MDF for all three heads was observed in the presence of CS, which confirmed our second hypothesis.

The key findings of the current study indicate the functional variations in the sEMG patterns of the three heads of the TB in the presence of CS. Most previous studies examined the effect of CS on the trapezius muscle, which is most prone to be affected by CS [42,43,44,45]. In addition, some studies also included the arm, shoulder and neck muscles in their analysis of the effects of additional mental processing [11,12,27,46,47]. All of these studies have shown that the presence of CS increases neural and thus muscle activity compared with that found under neutral or non-mental-stress conditions. A previous study [48] investigated the deltoid and upper trapezius during isometric tasks with CS and found that CS reduced the endurance performance at 35% MVC but had no effect at 55% MVC. As observed in previous studies [32,48], CS might improve the endurance performance within a certain intensity range, but at other intensities, CS might have no or even a negative effect on exercise. At low exercise intensities, the extended time to task failure might cause central fatigue [49] and might thus negatively affect the ET, NR and ROF.

Attention diversion might partly explain the higher ET and NR observed during exercise with CS. As noted in a previous study [50], the ability of the CNS to decide whether to stop an exercise due to excessive load is reduced in the presence of CS, and this reduction causes the subject to work approximately 20% more until the CNS receives a sufficiently severe alert to stop the exercise. The dissociative focus observed in the presence of CS makes an individual more resistant to fatigue due to a reduction in fatigue perception because the individual is distracted from sensing fatigue [51]. The lower ROF observed in the current study might be responsible for the higher ET during the CS exercise, which concurs with a previous observation [32] of a higher ET during isometric elbow flexion performed at 50% MVC in the presence of CS. Taken together, it appears that the findings of this study concur with those in literature, suggesting similar behavior of ROF for both isometric and dynamic contractions during CS.

A higher RMS was found for all three heads during exercise in the presence of CS compared with NCS. Lateral and medial heads showed greater change in the RMS between the two exercises than for the long head, and this difference might be due to the different biomechanical roles of the TB heads. The long head, which is bi-articular, is more activated than other two mono-articular heads at a shoulder elevation angle of 0° [22], and maintained a constant force generating capacity across a wide range of elbow angles [20,21]. Because the exercises were performed at a shoulder elevation of 0°, the long head is likely to show greater activity during both exercises, which would reduce the possibility of further changes in the activation levels due to the presence of CS. Two previous studies [52,53] revealed that the activation levels of the two other heads were higher but comparable. These observations appear to indicate that from a biomechanical viewpoint, the effects of CS in the lateral and medial heads are more pronounced than that in the long head. In addition, for all three heads, the change in the RMS between the NCS and CS exercises was more negligible under Fa compared with NF conditions. This observation could be due to the negligible effect of neural noise on muscles during fatigue, as observed in a previous study [16], and notes that a muscle exhibits a similar state of fatigue irrespective of the psychological state, as observed in another previous study [48].

The temporal and spectral parameters obtained for all three heads during exercise showed greater differences between NF and Fa conditions in the presence of CS, and this finding can be attributed to the increase in the exercise duration in the presence of CS. It is interesting to note that although the drop in MPF and MDF is higher under CS, their rates of drop (ROF) was found lower, albeit the higher energy cost during task execution under CS, as indicated by the higher sEMG activity. The reason for this could be the increased ET observed during the CS exercise. In addition, CS is known to induce neuro-motor noise in subjects, and an increase in neuro-motor noise causes an increase in kinematic variability. To suppress this variability, muscles tend to stiffen through agonist-antagonist co-activation. Thus, the observed increase in activity could be attributed to increased CS [18]. Although this finding was obtained for all three heads in our study, another research group [11] previously observed no change in the behavior of the lateral head of the TB with an increase in CS during finger tapping with a flexed elbow. These researchers claimed that their finding might be due to the co-contraction activity of the biceps brachii, which they found was more pronounced in the presence of CS. Although significant interactions between the two exercises and conditions were found for the RMS and MPF, the changes in the MPF and MDF obtained for both exercises between NF and Fa conditions were rather small, whereas the RMS exhibited greater changes. Hence, spectral parameters are less susceptible to cognitive stress than temporal parameters and are thus better approximators of peripheral muscle fatigue at different psychological states.

Comparable RMS values for the three heads were only obtained during the NCS exercise under NF conditions. In contrast, the three heads showed significant differences in the RMS, MPF and MDF under all other conditions, further emphasizing the individual roles of each head. Collectively, the three heads continue to work individually in both conditions, NF and Fa. The analysis of synergist pairs revealed that the lateral-long and lateral-medial pairs showed consistent behavior in terms of the spectral parameters among all four exercise-condition cases, namely, NF-NCS, Fa-NCS, NF-CS and Fa-CS. Although the responses of the long-medial head pair during both NCS and CS exercises under Fa conditions were consistent with those of the two other synergist pairs, the opposite behavior was observed under NF conditions. This finding could be related to the fact that the long head, which is bi-articular, experiences increased changes in length and joint angles compared with its mono-articular counterparts, the lateral and medial heads [52]. The behavior of the lateral-medial synergist pair was consistent because both heads are mono-articular, and hence, external and internal factors exert similar effects on these heads. In contrast, the lateral-long synergist pair function well in unison because both are large muscles with similar numbers of muscle fibers [54]. However, the behavior of the long-medial pair cannot be ascertained because the two muscles belonging to this pair do not share such characteristics (i.e., one of the heads is not mono-articular, and the two heads have different muscle sizes).

Muscle fatigue consists of two components, peripheral and central fatigue. Because the central mechanisms were not examined in tandem with neuromuscular behavior, the possibility that it plays a role in the manifestation of peripheral fatigue cannot be excluded. The inclusion of such an analysis might provide more convincing and accurate findings that provide further insights on the mechanisms and relationships between CS and peripheral fatigue. The effects of crosstalk among the three heads of the TB, particularly between the medial and lateral heads cannot be outright neglected. By adhering to the recommendations in [55] precautionary measures were taken, and the researchers are confident that the effect of crosstalk is negligible.

Conclusion

The presence of CS decreases the ROF and increases the ET and NR. For all three heads, larger differences were observed in the normalized RMS between the NCS and CS exercises under NF compared with Fa conditions. Temporal parameters were found to be better approximators of CS, whereas spectral parameters were more resistant to the effect of CS. In addition to previous results for individual heads, the findings obtained in this study further affirm that the three TB heads work independently under fatiguing conditions and in the presence of CS. However, an analysis of the various pairs of TB heads revealed that the behavior of the non-synergist head pair under fatiguing conditions was similar to those of the synergist head pairs. These results could help researchers obtain a more in-depth understanding on the functioning of the TB and thus can potentially be used in clinical applications for prosthetic control or targeted sports training. Furthermore, the effects of CS on peripheral muscle fatigue can improve the understanding of the condition of an individual during training or rehabilitation. Future work may also consider using the attention diversion technique to induce CS in athletes, who are usually more focused during task execution.

5 Aug 2019

PONE-D-19-18253

Cognitive Stress Changes the Attributes of the Three Heads of the Triceps Brachii during Fatigue

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

1. 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: Partly

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Reviewer #1: Yes

Reviewer #2: Yes

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Reviewer #1: Yes

Reviewer #2: No

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Reviewer #1: Yes

Reviewer #2: Yes

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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: The purpose of the study brings up an interesting question. By assessing EMG from each individual head from the triceps brachii, the current study was wanting to assess peripheral fatigue, and muscle firing patterns of each of the individual muscles with two different treatment conditions. The present study was trying to assess individual specific muscle responses with and without cognitive stresses. While the research question is relevant, interesting, and could add to the literature (specifically including synergist muscles), there are few clarifications required prior to publication of the article.

General comments:

Please remain consistent with the terminology throughout the article, such as cognitive stress and mental load were used interchangeably. It would be better if the authors replace ML (for mental load) with CS (cognitive stress) as CS is appearing in the title. Though on P3L7, authors mentioned “… mental load induced by cognitive stress.”, the use of CS is recommended for this particular work.

Further, peripheral fatigue has been closely related to a slowing of the sarcolemma and t-tubule conduction velocity (Zhou 1996 and Fitts 2006). By having participants perform dynamic contractions at 45% 1-RM to failure, it is plausible that the participant may get 55% fatigued compared to their baseline strength. Iguchi et al. 2008, suggests that fatigue that induces at least 35% or greater could induce low frequency fatigue (LFF), which could delay recovery responses for a prolonged period of time.

Particular comments:

P2L20,” …trapezius, neck and shoulder muscles…” may be replaced by “…neck, trapezius and shoulder muscles…” (note the order of the muscles).

P3L2, “amount of cognitive stress.” This phrase suggests that cognitive stress is quantifiable! Is it the case? Please clarify.

P3L4, mental may be replaced by cognitive.

P3L6, It should be specified that TB is involved in horizontal arm abduction.

P3L7-9, why TB activity was overshadowed? Due to task performed? Please clarify in the manuscript.

P3L12, why there is the word “additional”. Does the subjects were already suffering from cognitive stress?

P5L1,” …23.8±3.6…” may be replaced by 23.8 (3.6) etc.

P5L10, The skin should always be shaved (as per SENIAM recommendations). Why was the skin only shaved as needed? How was “as needed” defined?

P5L9, How the authors identified medial head of the TB? There is no recommendation regarding medial head in SENIAM!

P5L18, This sampling rate was on the low end for surface EMG. It is important that you state the specifications of the Shimmer system. What were the analog high and low pass cutoff frequencies? If the high pass was greater than 499 Hz, it is possible that you got some aliasing in your signal due to the relatively low sampling rate. I can't find any information on this amplifier, even on the RealTime website. I did find information for the Shimmer 3.0, but it does not indicate the analog filtering used, only that the bandwidth is 8.5 kHz

Methodology section: At which speed 1RM test and rest of the experiment was performed? Did you measure the actual angular velocity during the contractions?

P6L10, How the proper posture was ensured? Was an assistant dedicated for this purpose or was that done through some equipment?

Was control of movement only experimenter’s confirmation? Did authors measure joint angles by electrical goniometers or motion capture?

Joint angles when surface EMG was sampled were not controlled.

P6L12, between which trials? It may be written as “between 1RM trials”.

As suggested in the general comments, NML and ML may be replaced with NCS and CS.

P5L22, Why was the study all done on one day? Why was the study not spread out over 3 days? Day 1 – familiarization and 1-RM, day 2 one condition, day 3 the other condition.

P7L8,9, the sentence, “The tempo of the exercise was maintained at subject’s ease.” is not enough. Authors should mention WHY the tempo of exercise was maintained at subject’s ease.

P7L19, What did you use to normalize the RMS. This is not clear from the description at Line P7L19. How exactly did you determine the MVC from the 1 RM contraction? Was it the highest value? The average?

P7L21-24, please clarify the sentence. Some information seems overwritten. Please clarify the methodology for the detection of active phase.

P8L2, Please correct. It seems “for all the segments” is missing.

As you used parametric analyses, did your data satisfy the assumption of a normal distribution?

The reasons that effects of mental stress are different among the synergistic muscles are not stated. Please explain them with references.

Authors should state how the EMG data was sampled for further analysis (data window) more than authors described in P7 L19-24 and Fig. 2. During dynamic movements, surface EMG signal is strongly influenced by non-physiological factors, i.e., distance between electrode and innervation zone etc. So, we need to control the joint angle when surface EMG is sampled during dynamic movement. Only visual inspection, the data from non-fatigue and fatigue periods or the data from with and without mental stress may be sampled from different joint angle range. If you sampled surface EMG from different joint range, this difference in surface EMG cannot be explained by the effect of fatigue or mental stress.

What are the units of rate of fatigue?

what was the purpose of calculating both MPF and MDF? Was there unique information you expected to uncover from each? It does not appear so from the Results and Discussion and you should consider only presenting one, or justifying why it is important to present both.

Although the result section is written fairly detailed, it needs some modifications. The independent and dependent variables need to be identified and reported properly. For example, intensity may have a significant effect on RMS, but it is not RMS that is significant.

I am not sure it is necessary to be so specific with p values that are so far below zero. (eg. 8.15E-41). It just clutters up the table and the specificity does not add anything. Consider replacing these with <0.001 to be consistent with the 3 decimal places for the other p values.

Reviewer #2: Cognitive Stress Changes the Attributes of the Three Heads of the Triceps Brachii during Fatigue

Manuscript Number: PONE-D-19-18253

This study aims to examine the effect of cognitive stress on the activity behavior of the three heads of the triceps brachii (TB) during dynamic fatiguing exercise (triceps push-down exercise at 45% of 1RM). The authors found that in the presence of cognitive stress, the rate of fatigue (ROF) for all the three heads of the TB demonstrated no significant differences, whereas endurance time and number of repetitions increased. Significant differences were found in the RMS of the EMG activity of all three heads of TB between the two exercises (with and w/o the cognitive load) under no muscle fatigue condition, whereas no differences were found under fatigue condition. Only the long head of TB demonstrated sig. differences of MPF and MDF between the exercises with and without mental load, regardless of fatigue condition. The authors concluded that spectral parameters acquired from the three heads of TB were found to be more robust towards the effect of cognitive stress on the fatiguing TB.

The MS in general is well-written, but there are parts that need improvement so as to become more clear and more easily understood, e.g. the results sections of abstract and MS (see also specific comments below). It is an interesting topic, but I have concerns whether the used methodology is appropriate for drawing concrete conclusions about neural noise and peripheral fatigue.

Specific comments:

Abstract:

Page 8, line 19: Please rephrase the sentence in order to become easier to follow: “Similarly, the RMS was statistically significant between the two exercises under NF conditions (P<0.05), whereas it was non-significant (P>0.05) under Fa conditions”

Introduction:

Page 10, Line 7: Please specify compared to what is the TB “more susceptible to the effects of cognitive stress”.

Page 10, Line 24: Please explain, why isometric contractions require focus on postural control. For example, when the isometric contraction is performed at a sitting position and on an isokinetic dynamometer, on which the person is well secured, no focus on postural control is required. Maybe the authors just want to justify why they preferred dynamic instead of isometric contraction, so it would be better to rephrase accordingly.

Methodology:

Experimental procedure:

• A figure, depicting the experimental procedure would be helpful for understanding the experiment set up.

• Have you checked if the mental load exercise has an effect on the execution of 1RM? In other words, whether the maximum lifted weight is affected by mental loading? It is possible that mental load may influence the force generation capacity, and thus although the fatiguing tasks are performed at the same weight-lift, this same weight-lift may correspond to different work load. Please comment on that.

Page 13/ Line 17: While you define task failure later in the MS, it would be better for the reader to provide the definition already at this point.

Page 14/ Line 4: Have you checked the recovery? Had the subjects the same 1RM before starting the second fatiguing task?

Page 14/ Line 10: Have you checked range of motion during the repetitions?

Statistical analysis:

• It would be interesting to see if there is any interaction between the effect of mental load and fatigue condition. This could be examined by using statistical tests like two-way ANOVA, with the one factor being the mental load and the second fatigue condition.

• What statistical test was used for checking the differences of the ROF, endurance time and number of repetitions between the two fatiguing exercises?

Page 15/ Line 11: Please change “spehricity” into “sphericity”

Results:

Page 15/ Line 22: Please address also the mean and SD values of the endurance time and number of reps.

Figure 3: The statistically significant differences are not shown in the figure. This would make the results demonstration easier to understand. Furthermore, why is there a shift between ML and NML in figure?

The results session is complicatedly demonstrated. It should be made more clear to the reader which are the most important results.

Discussion:

Page 18/ Line 6: Since the central mechanisms were not examined, but only the neuromuscular behavior by means of EMG, the authors should explain the accuracy of their conclusions on peripheral fatigue. E.g. how can it be excluded that no central mechanisms were involved in the changes of neuromuscular activity?

Page 19/Line 6: The authors state that: “The ROF of all three heads was higher in the NML exercise compared with the ML exercise”, however no statistically significant differences were found. Please rephrase.

Page 19/Line 11: The authors address that RMS showed greater variability in the ML exercises between NF and Fa conditions, and this variability can be explained by neural noise. Nevertheless, neither in methodology session nor in the results session there is any reference on the RMS variability; i.e. how was variability assessed and what were its values (only RMS differences between trials and conditions are shown). Furthermore, please provide reference on how RMS variability is related to neural noise.

Page 20, Line 1: It is not so clear how from the references [7,40] the addressed conclusion is made, i.e. that larger muscles show decreased variability in joint movements in the presence of cognitive stress, which indicates that these muscles can more effectively handle neuro-motor noise than smaller muscles. Firstly [40], in their study do not examine cognitive stress. There is no reference on cognitive stress in their paper. Secondly, [7] report as possible explanations for the differences in muscle activity following mental load (in terms of amplified memory processing) between triceps and biceps brachii, their anatomical difference (biarticular vs, monoarticular) and functional proximity to the shoulder, but not their size. Please provide appropriate reference to support the finding that larger muscles show decreased variability in joint movements in the presence of cognitive stress.

Page 20/Line 18: I would like a better justification, on how the results are related to neuro-motor noise. How can conclusions be made on the role of neuro-motor noise, from the differences between the RMS values?

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Reviewer #1: Yes: Rajkumar Palaniappan

Reviewer #2: No

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14 Sep 2019

The manuscript has been formatted as per PLOS ONE’s style requirements. About the availability of the data, it was not mentioned explicitly in our ethical approval document that the data can be shared publicly. The data may be shared upon request to the corresponding author. We have added a statement in the revised manuscript regarding Figure 1.

Detailed response to the reviewers' comments has been attached.

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

9 Oct 2019

PONE-D-19-18253R1

Cognitive stress changes the attributes of the three heads of the triceps brachii during fatigue

PLOS ONE

Dear Mr. Hussain,

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.

We would appreciate receiving your revised manuscript by Nov 23 2019 11:59PM. When you are 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.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

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We look forward to receiving your revised manuscript.

Kind regards,

Nizam Uddin Ahamed, PhD

Academic Editor

PLOS ONE

Additional Editor Comments (if provided):

Thank you for carefully revised the manuscript based on the reviewer comments and addressed all issues raised with respect to the first version in a satisfactory way. However, please have the entire manuscript edited for English expression by a native speaker of English who has knowledge of your field. Language imprudent is required; avoid the first-person writing style.

There are some comments those need to be addressed before further processing:

1. One important key word “Muscle” is missing in the title, therefore I would recommend putting this word either with triceps brachii “....triceps brachii muscle...” or just before the word fatigue, “…..triceps brachii during Muscle fatigue”

2. Define the acronym(s), first time that they are used; such as in Abstract, iRM, RMS, MPF and MDF. Please check the entire manuscript.

3. The keywords need to arrange based on alphabetical order.

4. In the Introduction, first paragraph: only two references (one is very old), are not enough to support the statement. Specially the external factor “Temperature”,. Please add relevant and recent articles. In the same paragraph, instead of writing “[1] revealed that…”, please put the author name(s). Similarly, please add reference(s) to support your first two statements in the second paragraph.

5. In the Introduction, “The triceps brachii, as the largest arm muscle…’, is there any right reference other than your own article to support this? Because, Biceps Brachii muscle is recognized as largest and right place to place EMG electrode as per SENIAM.

6. Please revise the sentence “Because the TB is close to the CNS..”, because, facial muscle, eye muscle etc. are the close to CNS.

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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)

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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: Partly

**********

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: No

**********

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: "Cognitive stress changes the attributes of the three heads of the triceps brachii during fatigue "

The authors have addressed all the comments in the revised manuscript. The manuscript can be accepted in the present form.

Reviewer #2: Cognitive Stress Changes the Attributes of the Three Heads of the Triceps Brachii during Fatigue

Manuscript Number: PONE-D-19-18253

The authors replied and tried to address all my comments. However, I still have some concerns, especially on the fact on how the results are related to neuro-motor noise. When conclusions for motor units behavior are drawn from bipolar surface EMG, this should be done very carefully. The conclusion that the greater RMS increase during the CS exercise compared to the NCS could be due to neuro-motor noise, still needs justification. It is possible that an increase in EMG activity, can be explained due to the increased antagonist-agonist co-contraction, which can be an indication of higher neuro-motor noise in order to counteract the kinematic variability [as stated by “The effects of workplace stressors on muscle activity in the neck shoulder and forearm muscles during computer work: a systematic review and meta-analysis”], but there are many other factors as well. For example, it is possible that the greater duration of the exercise under CS may explain the greater RMS increase. Furthermore, the authors have not investigated neither co-contraction nor kinematics variability, which could be indicators for motor noise existence, so that they can argue that motor noise is the factor explaining RMS increase.

Specific comments:

Methods:

1. In the previous review I commented that a figure depicting the experimental procedure would be helpful, meaning the temporal set up, e.g. first warming up then 2 min rest, 3 RM (the between intervals), etc. I believe that the words “experimental set up” confused the authors, and added a figure depicting the spatial set up of the experiment. Nevertheless, this is also described in figure 1, so a suggestion could be to keep either figure 1 or figure 2 and add another figure depicting the temporal experimental procedure.

2. Page 7/Line 16-18: At the present study EMG parameters (RMS, MDF and MPF) were compared between different conditions. In order to investigate the effect of the different conditions, motion should be the same, i.e. constant ROM at constant velocity, since these two parameters influence the EMG signal. Thus, I disagree with the statement that the present study did not require the joint angles to be measured or controlled during sEMG data acquisition as data was required throughout the entire ROM.

3. Page 9/Line 8-10: I have some concerns on how accurate the duration of one repetition can be defined from EMG signal. There is an activation of TB during elbow flexion (eccentric phase) and also during elbow extension (concentric phase) with a small silent window at the end of each repetition. The duration of this silent time window could be fatigue depended.

Results:

4. Results session continuous to be complicatedly demonstrated. For example, it is mentioned that “Under the NF condition, all three heads were significantly different (p<0.05) between the NCS and CS exercises for RMS, while only the long head was statistically significant for MPF and MDF.” But there is no information on which variable is higher. Furthermore, in figure 5, 6 and 7 the statistically significant differences are not indicated.

Discussion:

5. It would be easier for the reader to follow and understand the main findings of the study, if at the beginning of the discussion the main findings are described and whether these findings confirm or reject the hypotheses (a. that the RMS activity of the lateral and long heads would increase in the presence of cognitive stress because these are larger muscles, whereas that of medial head would remain and b. that the values of MPF and MDF for the lateral and long heads would experience a relatively higher decrease in the presence of cognitive stress compared with those for the medial head). Furthermore, at the introduction is stated that the rate of fatigue (ROF) is expected to be higher in the presence of cognitive stress than under normal conditions. However, this expectation was not confirmed from the results. A restructure of the discussion where the justification, of the confirmation or rejection of the hypotheses should be done in order to improve discussion clarity.

6. There is a significant interaction found between mental load and fatigue development in the RMS and in all the heads of TB for RMS and MPF, which is not discussed in the discussion part.

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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 to be viewed.]

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7 Nov 2019

Response to Editor and Reviewer comments are in the file "Response to Reviewers.docx"

Submitted filename: Response to Reviewers.docx

Click here for additional data file.

21 Nov 2019

PONE-D-19-18253R2

Cognitive stress changes the attributes of the three heads of the triceps brachii during muscle fatigue

PLOS ONE

Dear Mr. Hussain,

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.

We would appreciate receiving your revised manuscript by Jan 05 2020 11:59PM. When you are 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.

If you would like to make changes to your financial disclosure, please include your updated statement in your cover letter.

To enhance the reproducibility of your results, we recommend that if applicable you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. For instructions see: http://journals.plos.org/plosone/s/submission-guidelines#loc-laboratory-protocols

Please include the following items when submitting your revised manuscript:

A rebuttal letter that responds to each point raised by the academic editor and reviewer(s). This letter should be uploaded as separate file and labeled 'Response to Reviewers'.

A marked-up copy of your manuscript that highlights changes made to the original version. This file should be uploaded as separate file and labeled 'Revised Manuscript with Track Changes'.

An unmarked version of your revised paper without tracked changes. This file should be uploaded as separate file and labeled 'Manuscript'.

Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

We look forward to receiving your revised manuscript.

Kind regards,

Nizam Uddin Ahamed, PhD

Academic Editor

PLOS ONE

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Reviewers' comments:

Reviewer's Responses to Questions

Comments to the Author

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Reviewer #2: (No Response)

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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

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

Reviewer #2: Yes

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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 #2: No

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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

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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: Cognitive Stress Changes the Attributes of the Three Heads of the Triceps Brachii during Fatigue

Manuscript Number: PONE-D-19-18253

The authors replied and tried to address all my comments. However, I still have some concerns mainly in the discussion part.

• In the revised discussion the authors added that “spectral parameters are less susceptible to neuro-motor noise than temporal parameters and are thus better approximators of peripheral muscle fatigue at different psychological states.” Please elaborate on how this conclusion can be drawn from the results of the present study since the presented methodology does not measure neural noise (cf. relevant comment of previous review).

• In the present study, the two hypotheses are confirmed while the third is rejected. In the introduction part the third hypothesis is based on the second: “…the presence of CS likely induces a relatively greater decrease in the MPF and MDF and thus a greater ROF than that observed under normal condition”. Please elaborate why, while the second hypothesis is confirmed, the third in rejected, clearly in the discussion part.

Specific comments

Discussion:

• Page 14/Line15-17: Please provide a literature reference for the statement that: “At low exercise intensities, the extended time to task failure might cause central fatigue and might thus negatively affect the ET, NR and ROF.”

Conclusion:

• Page 17/Line 11: Please change the word “but” to “and”, since it is reasonable a reduced ROF to lead to an increase in the ET and NR.

• Page 17/Line 11: Please cite on what the impact of CS was greater under NF conditions.

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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 to be viewed.]

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25 Nov 2019

A file has been attached containing our response to the comments of the reviewer.

Submitted filename: Response to reviewers.docx

Click here for additional data file.

10 Dec 2019

PONE-D-19-18253R3

Cognitive stress changes the attributes of the three heads of the triceps brachii during muscle fatigue

PLOS ONE

Dear Mr. Hussain,

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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Nizam Uddin Ahamed, PhD

Academic Editor

PLOS ONE

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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: (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 #2: Yes

**********

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

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 #2: (No Response)

**********

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 replied and tried to address all my comments. However, in the revised manuscript new issues in the discussion part arise.

• In the introduction part the second hypothesis, i.e. the presence of CS would induce a relatively greater decrease in the MPF and MDF is based on the assumption that there would be greater muscle activity during exercise. Indeed, the authors found greater RMS values under CS. Interestingly the ROF is lower and the duration is greater under CS, albeit the higher energy cost during task execution under CS, as indicated by the higher EMG activity. Is it possible that the differences in task duration may have influenced the MPF and MDF reduction between the different conditions (CS vs NCS)? This issue should be explained/commented in the discussion section of the paper.

• Page 14/ Line 23-25: The authors state that “The higher ET observed in our study might also be responsible for the lower ROF calculated during exercise with CS”. As the higher ET is the result of the art that the neuromuscular system is operated and not vice versa, it is more reasonable to write that the lower ROF observed in the study might be responsible for the higher ET observed during exercise with CS. Please revise the discussion accordingly.

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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.

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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 to be viewed.]

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 us at figures@plos.org. Please note that Supporting Information files do not need this step.

18 Dec 2019

Response to reviewer's comments is attached.

Submitted filename: Response to reviewers.docx

Click here for additional data file.

8 Jan 2020

Cognitive stress changes the attributes of the three heads of the triceps brachii during muscle fatigue

PONE-D-19-18253R4

Dear Dr. Hussain,

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.

Within one week, you will receive an e-mail containing information on the amendments required prior to publication. When all required modifications have been addressed, you will receive a formal acceptance letter and your manuscript will proceed to our production department and be scheduled for publication.

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With kind regards,

Nizam Uddin Ahamed, PhD

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: (No Response)

**********

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

Reviewer #2: (No Response)

**********

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 #2: (No Response)

**********

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: (No Response)

**********

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: (No Response)

**********

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.

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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 #2: No

15 Jan 2020

PONE-D-19-18253R4

Cognitive stress changes the attributes of the three heads of the triceps brachii during muscle fatigue

Dear Dr. Hussain:

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.

For any other questions or concerns, please email plosone@plos.org.

Thank you for submitting your work to PLOS ONE.

With kind regards,

PLOS ONE Editorial Office Staff

on behalf of

Dr. Nizam Uddin Ahamed

Academic Editor

PLOS ONE