Muscle fatigue during assisted violin performance.

Muscle fatigue is a primary risk factor in developing musculoskeletal disorders, which affect up to 93% musicians, especially violinists. Devices providing dynamic assistive support (DAS) to the violin-holding arm can lessen fatigue. The objective was to assess DAS effects on electromyography median frequency and joint kinematics during a fatiguing violin-playing task. Fifteen university-level and professional violinists were equipped with electromyography sensors and reflective markers to record upper-body muscle activity and kinematics. They played G scales with and without DAS until exhaustion. Paired t-tests assessed DAS effects on delta (final-initial) electromyography median frequencies and joint kinematics. DAS prevented the median frequency decrease of left supraspinatus, superior trapezius, and right medial deltoid, and increases in trunk rotation, left-wrist abduction, and right arm-elevation plane. DAS effects on kinematics were marginal due to retention of musical performance despite fatigue. However, DAS reduced fatigue of several muscles, which is promising for injury prevention.Practitioner summary: Violinists are greatly affected by musculoskeletal disorders. Effects of a mobility assistive device on muscle fatigue during violin playing was investigated. The assistive technology slowed down the development of fatigue for three neck/shoulder muscles, making assisted musical performance a promising avenue to prevent violinists' injuries.

Effect of odor stimulations on physical activity: A systematic review.

Fewer and fewer people are reaching physical activity recommendations. Therefore, it seems important to make the practice of physical activity more enjoyable to increase the participation rate. Several environmental factors have been studied to see their impact on sports practice, and some studies investigated the effect of odors. This systematic review aims to provide a thorough view of the literature on the effect of different odors on physical activity. The search strategy consisted of using index terms and keywords in MEDLINE, EMBASE, EBM Reviews - Cochrane Central Register of Controlled Trials, CINAHL, SPORTDiscus, and Web of Science search engine. Data from 19 studies that included 458 participants revealed that the odors had different results on strength, cardiovascular, precision, and postural balance tasks depending on the odors' exposition. Among results, an important distinction was made between pleasant and unpleasant odors. Therefore, pleasant odors had better results on physical activity by improving participants' feeling. Even though this review clarified evidence about the effect of odors on physical activity, better methodological consistency is needed across studies such as the odor administration method to produce more meaningful results.

Effect of Robotic-Assisted Gait at Different Levels of Guidance and Body Weight Support on Lower Limb Joint Kinematics and Coordination.

The Lokomat provides task-oriented therapy for patients with gait disorders. This robotic technology drives the lower limbs in the sagittal plane. However, normative gait also involves motions in the coronal and transverse planes. This study aimed to compare the Lokomat with Treadmill gait through three-dimensional (3D)-joint kinematics and inter-joint coordination. Lower limb kinematics was recorded in 18 healthy participants who walked at 3 km/h on a Treadmill or in a Lokomat with nine combinations of Guidance (30%, 50%, 70%) and bodyweight support (30%, 50%, 70%). Compared to the Treadmill, the Lokomat altered pelvic rotation, decreased pelvis obliquity and hip adduction, and increased ankle rotation. Moreover, the Lokomat resulted in significantly slower velocity at the hip, knee, and ankle flexion compared to the treadmill condition. Moderate to strong correlations were observed between the Treadmill and Lokomat conditions in terms of inter-joint coordination between hip-knee (r = 0.67-0.91), hip-ankle (r = 0.66-0.85), and knee-ankle (r = 0.90-0.95). This study showed that some gait determinants, such as pelvis obliquity, rotation, and hip adduction, are altered when walking with Lokomat in comparison to a Treadmill. Kinematic deviations induced by the Lokomat were most prominent at high levels of bodyweight support. Interestingly, different levels of Guidance did not affect gait kinematics. The present results can help therapists to adequately select settings during Lokomat therapy.

Neuromotor variability partially explains different endurance capacities of expert pianists.

During fatiguing piano tasks, muscle fatigue develops differently between expert pianists. Differences in neuromotor strategies employed could explain a slower rate of fatigue development. The objective was to compare muscle activation and kinematic variabilities between ShortDuration (i.e., pianists with less endurance) and LongDuration groups. Results from 49 pianists showed that EMG activation variability of most shoulder and upper limbs muscles was greater for the ShortDuration group with time during two piano fatiguing tasks, namely Digital and Chord tasks. Segment acceleration variability, assessed using inertial measurement units, was also greater with time for the ShortDuration group at the right arm during the Digital task, and at the thorax and head during the Chord task. Finally, thorax lateroflexion variability increased with time for the LongDuration group (but not the ShortDuration group) during the Digital task. During the Chord task, wrist flexion variability was higher for the LongDuration group compared to the ShortDuration group. These results showed a direct effect of time on the pianists' acceleration variability and EMG activation variability. In contrast, a protective effect of fatigue development could be attributed to kinematic variability. Results also suggest a higher risk of injury among pianists in the ShortDuration group.

The effects of hand dominance, fatigue, and sex on muscle activation during a repetitive overhead fatiguing task.

Previous studies have shown that the dominant arm is generally stronger and more resistant to fatigue. However, whether there are side differences in shoulder muscle activation during a fatiguing upper limb task, and whether this varies according to sex, is unknown. Thirty right-handed adults (15 females) were recruited to complete two sessions of an overhead repetitive fatiguing task (shoulder flexion between 90 and 135° at 1 Hz), performed in two separate sessions with their dominant arm (DA) and non-dominant arm (NDA) until exhaustion. Electromyographic (EMG) data was collected from 11 shoulder muscles of the moving arm, and their activation amplitude (RMS) and activation variability (SD) were assessed. Results show that time to exhaustion was not affected by arm or by sex. There were some main arm effects on EMG activity amplitude, with higher activity on the DA's pectoralis major (p < 0.001), and on the NDA's middle (p = 0.009) and posterior deltoid (p = 0.001) and infraspinatus (p < 0.001). The pectoralis major was affected by arm and fatigue mostly in males. Their DA's pectoralis major activity amplitude was higher, and the amplitude variability was lower, compared to the NDA, with both parameters showing fatigue-dependent decreases at the NDA only (arm x sex x fatigue: RMS: p = 0.007; SD: p = 0.001). As for females, the DA variability of their lower trapezius was smaller, and that of their subscapularis was higher, compared to the NDA (sex x arm, p = 0.028, p = 0.05). There was also more EMG variability on the supraspinatus' dominant side, and on the posterior deltoid and infraspinatus ND side. Results show an overhead shoulder flexion task dependency on pectoralis major control in males, and on lower trapezius and shoulder girdle stabilizers in females, which could be related to both sex- and gender-based factors. This knowledge can help identify side-specific injury risk factors due to overhead work in males and females, and help determine the appropriateness of implementing sex-specific workplace protocols, including alternating arms as fatigue compensatory and recovery strategies.

Does Cardiorespiratory Fitness Protect Memory from Sleep Deprivation?

INTRODUCTION: Animal studies have demonstrated that physical exercise can protect memory from the effects of sleep deprivation (SD). We examined whether having a high cardiorespiratory fitness (V̇O 2peak ) is associated with an enhanced capacity to encode episodic memory after one night of SD. METHODS: Twenty-nine healthy young participants were allocated into either an SD group ( n = 19) that underwent 30 h of uninterrupted wakefulness, or a sleep control (SC) group ( n = 10) that followed a regular sleep routine. Following either the SD or SC period, participants were asked to view 150 images as the encoding part of the episodic memory task. Ninety-six hours after viewing the images, participants returned to the laboratory to perform the recognition part of the episodic memory task, which required the visual discrimination of the 150 images previously presented from 75 new images introduced as distractors. Cardiorespiratory fitness (V̇O 2peak ) was assessed with a bike ergometer graded exercise test. Group differences in memory performance were assessed with independent t tests and associations between V̇O 2peak and memory with multiple linear regression. RESULTS: The SD group showed a significant increase in subjective fatigue (mean difference [MD] [standard error {SE}] = 38.94 [8.82]; P = 0.0001) and a worse capacity to identify the original 150 images (MD [SE] = -0.18 [0.06]; P = 0.005) and discriminate them from distractors (MD [SE] = -0.78 [0.21] P = 0.001). When adjusted for fatigue, higher V̇O 2peak was significantly associated with better memory scores in the SD (R 2 = 0.41; ß [SE] = 0.03 [0.01]; P = 0.015) but not in the SC group ( R2 = 0.23; ß [SE] = 0.02 [0.03]; P = 0.408). CONCLUSIONS: These results confirm that SD before encoding impairs the capacity to create robust episodic memories and provide preliminary support to the hypothesis that maintaining high levels of cardiorespiratory fitness could have a protective effect against the disruptive effects of sleep loss on memory.

How Do Violinists Adapt to Dynamic Assistive Support? A Study Focusing on Kinematics, Muscle Activity, and Musical Performance.

OBJECTIVE: Assessing violinists' motor and musical performance adaptations to dynamic assistive support (DAS) provided by a passive device, using a force-field adaptation paradigm. BACKGROUND: Up to 93% of instrumentalists are affected by musculoskeletal injuries and particularly violinists. The repetitive nature of their work may lead to muscle fatigue, an injury risk factor. DAS has been used in occupational settings to minimize muscle activations and limit fatigue accumulation. DAS may however affect motor and musical performance. METHOD: Fifteen expert violinists were equipped with reflective markers and surface and intramuscular electromyography (EMG) sensors. Movements, muscle activations, and sound were recorded while participants completed three experimental conditions for which they continuously played a 13-s musical excerpt: Control (no DAS), Adaptation (DAS), and Washout (no DAS). DAS was applied at the left elbow (violin-holding side). Conditions were repeated 1 week later. Participants later listened to their own audio recordings playing with and without DAS and blindly assessed their performances. Linear mixed models were used to compare DAS and no-DAS conditions' kinematic, EMG, and musical performance data. RESULTS: DAS perturbed user kinematics but reduced mean activations of left medial deltoid and superior trapezius. Joint kinematic and muscle activation patterns between DAS and no DAS conditions however remained similar. Musical performance was unchanged with DAS. CONCLUSION: Though DAS modified violinists' upper-limb configurations, resulting kinematics were not detrimental to musical performance. Reduced muscle activations with DAS could contribute to lessening muscle fatigue. APPLICATION: Although its effect on muscle fatigue should be further investigated, DAS might be useful in preventing violinists' injuries.

Power Spectrum of Acceleration and Angular Velocity Signals as Indicators of Muscle Fatigue during Upper Limb Low-Load Repetitive Tasks.

Muscle fatigue is a risk factor for developing musculoskeletal disorders during low-load repetitive tasks. The objective of this study was to assess the effect of muscle fatigue on power spectrum changes of upper limb and trunk acceleration and angular velocity during a repetitive pointing task (RPT) and a work task. Twenty-four participants equipped with 11 inertial measurement units, that include acceleration and gyroscope sensors, performed a tea bag filling work task before and immediately after a fatiguing RPT. During the RPT, the power spectrum of acceleration and angular velocity increased in the movement and in 6-12 Hz frequency bands for sensors positioned on the head, sternum, and pelvis. Alternatively, for the sensor positioned on the hand, the power spectrum of acceleration and angular velocity decreased in the movement frequency band. During the work task, following the performance of the fatiguing RPT, the power spectrum of acceleration and angular velocity increased in the movement frequency band for sensors positioned on the head, sternum, pelvis, and arm. Interestingly, for both the RPT and work task, Cohens' d effect sizes were systematically larger for results extracted from angular velocity than acceleration. Although fatigue-related changes were task-specific between the RPT and the work task, fatigue systematically increased the power spectrum in the movement frequency band for the head, sternum, pelvis, which highlights the relevance of this indicator for assessing fatigue. Angular velocity may be more efficient to assess fatigue than acceleration. The use of low cost, wearable, and uncalibrated sensors, such as acceleration and gyroscope, in industrial settings is promising to assess muscle fatigue in workers assigned to upper limb repetitive tasks.

Effect of Expertise on Shoulder and Upper Limb Kinematics, Electromyography, and Estimated Muscle Forces During a Lifting Task.

OBJECTIVE: To highlight the working strategies used by expert manual handlers compared with novice manual handlers, based on recordings of shoulder and upper limb kinematics, electromyography (EMG), and estimated muscle forces during a lifting task. BACKGROUND: Novice workers involved in assembly, manual handling, and personal assistance tasks are at a higher risk of upper limb musculoskeletal disorders (MSDs). However, few studies have investigated the effect of expertise on upper limb exposure during workplace tasks. METHOD: Sixteen experts in manual handling and sixteen novices were equipped with 10 electromyographic electrodes to record shoulder muscle activity during a manual handling task consisting of lifting a box (8 or 12 kg), instrumented with three six-axis force sensors, from hip to eye level. Three-dimensional trunk and upper limb kinematics, hand-to-box contact forces, and EMG were recorded. Then, joint contributions, activation levels, and muscle forces were calculated and compared between groups. RESULTS: Sternoclavicular-acromioclavicular joint contributions were higher in experts at the beginning of the movement, and in novices at the end, whereas the opposite was observed for the glenohumeral joint. EMG activation levels were 37% higher for novices but predicted muscle forces were higher in experts. CONCLUSION: This study highlights significant differences between experts and novices in shoulder kinematics, EMG, and muscle forces; hence, providing effective work guidelines to ensure the development of a safe handling strategy is important. APPLICATION: Shoulder kinematics, EMG, and muscle forces could be used as ergonomic tools to identify inappropriate techniques that could increase the prevalence of shoulder injuries.

Shoulder electromyography-based indicators to assess manifestation of muscle fatigue during laboratory-simulated manual handling task.

Muscle fatigue is a risk factor for developing shoulder musculoskeletal disorders. The aim of this study was to identify shoulder electromyographic indicators that are most indicative of muscle fatigue during a laboratory simulated manual handling task. Thirty-two participants were equipped with electromyographic electrodes on 10 shoulder muscles and moved boxes for 45-minutes. The modified rate of perceived exertion (mRPE) was assessed every 5-minutes and multivariate linear regressions were performed between myoelectric manifestation of fatigue (MMF) and the mRPE scores. During a manual handling task representative of industry working conditions, spectral entropy, median frequency, and mobility were the electromyographic indicators that explained the largest percentage of the mRPE. Overall, the deltoids, biceps and upper trapezius were the muscles that most often showed significant changes over time in their electromyographic indicators. The combination of these three indicators may improve the accuracy for the assessment of MMF during manual handling. Practitioner Summary: To date, muscle fatigue has primarily been assessed during tasks done to exhaustion, which are not representative of typical working conditions. During a manual handling task representative of industry working conditions, EMG-derived spectral entropy, and median frequency, both extracted from time-frequency analysis, and mobility extracted from time domain, were the best indicators of the manifestation of muscle fatigue.

Effects of body weight support and guidance force settings on muscle synergy during Lokomat walking.

BACKGROUND: The Lokomat is a robotic device that has been suggested to make gait therapy easier, more comfortable, and more efficient. In this study, we asked whether the Lokomat promotes physiological muscle activation patterns, a fundamental question when considering motor learning and adaptation. METHODS: We investigated lower limb muscles coordination in terms of muscle activity level, muscle activity pattern similarity, and muscle synergy in 15 healthy participants walking at 3 km/h on either a treadmill or in a Lokomat at various guidance forces (GF: 30, 50 or 70%) and body weight supports (BWS: 30, 50 or 70% of participant's body weight). RESULTS: Walking in the Lokomat was associated with a greater activation level of the rectus femoris and vastus medialis (×2-3) compared to treadmill walking. The level of activity tended to be diminished in gastrocnemius and semi-tendinosus, which particularly affected the similarity with treadmill walking (normalized scalar product NSP = 0.7-0.8). GF and BWS independently altered the muscle activation pattern in terms of amplitude and shape. Increasing BWS decreased the level of activity in all but one muscle (the soleus). Increasing GF slightly improved the similarity with treadmill walking for the tibialis anterior and vastus medialis muscles. The muscle synergies (N = 4) were similar (NSP = 0.93-0.97), but a cross-validation procedure revealed an alteration by the Lokomat. The activation of these synergies differed (NSP = 0.74-0.82). CONCLUSION: The effects of GF and BWS are modest compared to the effect of the Lokomat itself, suggesting that Lokomat design should be improved to promote more typical muscle activity patterns.

Exhausting repetitive piano tasks lead to local forearm manifestation of muscle fatigue and negatively affect musical parameters.

Muscle fatigue is considered as a risk factor for developing playing-related muscular disorders among professional pianists and could affect musical performance. This study investigated in 50 pianists the effect of fatiguing repetitive piano sequences on the development of forearm muscle fatigue and on piano performance parameters. Results showed signs of myoelectric manifestation of fatigue in the 42-electromyographic bipolar electrodes positioned on the forearm to record finger and wrist flexor and extensor muscles, through a significant non-constant decrease of instantaneous median frequency during two repetitive Digital (right-hand 16-tones sequence) and Chord (right-hand chords sequence) excerpts, with extensor muscles showing greater signs of fatigue than flexor muscles. In addition, muscle fatigue negatively affected key velocity, a central feature of piano sound intensity, in both Digital and Chord excerpts, and note-events, a fundamental aspect of musicians' performance parameter, in the Chord excerpt only. This result highlights that muscle fatigue may alter differently pianists' musical performance according to the characteristics of the piece played.

Short Walking Exercise Leads to Gait Changes and Muscle Fatigue in Children With Cerebral Palsy Who Walk With Jump Gait.

OBJECTIVE: The aim of this study was to evaluate kinematic changes and muscle fatigue in jump gait during a walking exercise and the relationship between kinematic changes and muscle fatigue and strength. DESIGN: This preliminary study included 10 children with cerebral palsy who walk with jump gait. Hip and knee maximal isometric muscle strength were measured using a dynamometer. Then, lower-limb kinematics and electromyography were collected while children walked continuously for 6 min at their self-selected speed. Electromyography median frequency and lower-limb joint angles were compared between the first and the sixth minutes of the walking exercise using t test and Wilcoxon rank test. The relationship between kinematic changes and muscle strength and changes in electromyography median frequency were assessed using correlation analyses. RESULTS: During stance, maximal knee flexion significantly increased at the sixth minute (P = 0.01) and was associated with knee extensor muscle weakness (ρ = -0.504, P = 0.03). Muscle fatigue was observed only in the gluteus medius muscle (P = 0.01). CONCLUSIONS: Children with cerebral palsy who walked with jump gait and who had knee extensor weakness were more prone to an increase in knee flexion during a continuous walk. The fatigue in the gluteus medius muscle suggests that physical intervention should target the endurance of this muscle to improve jump gait.

Effect of low dose robotic-gait training on walking capacity in children and adolescents with cerebral palsy.

OBJECTIVE: Robotic gait training presents a promising training modality. Nevertheless, evidence supporting the efficacy of such therapy in children with cerebral palsy remains insufficient. This study aimed to assess the effect of robotic gait training in children/adolescents with cerebral palsy. METHODS: Twenty-four children/adolescents with bilateral cerebral palsy (12 female, 10.1 ±â€¯3.1 years, Gross Motor Function Classification System II to IV) took part in this study. They received two 30-45 min sessions/week of Lokomat training for 12-weeks. Muscle strengths, 6-min walk exercise and gait parameters were evaluated pre- and post-training and at 6-months-follow-up. Training effect according to the level of impairment severity (moderate vs severe) was analyzed using a change from the baseline procedure. RESULTS: A significant increase in muscle strength was observed after training (p ≤ 0.01). Hip flexors and knee extensors strength changes were maintained or improved at follow-up (p < 0.05). Comfortable walking speed was significantly increased by +20% after training with a slight reduction at follow-up compared to post-training condition (-2.7%, p < 0.05). A significant step length increase was observed after training (14%, p ≤ 0.001). The distance covered in 6 min was higher in post-training (+24%, p ≤ 0.001) and maintained at follow-up compared to pre-training conditions. No significant changes in kinematic patterns were observed. The analysis by subgroup showed that both groups of children (with moderate and severe impairments) improved muscle strength and walking capacities after Lokomat training. CONCLUSION: The suggested Lokomat training induced improvement in walking capacity of children/adolescents with cerebral palsy whatever the level of severity. Hence, Lokomat training could be viewed as a valuable training modality in this population.

Unfolding the Effects of Acute Cardiovascular Exercise on Neural Correlates of Motor Learning Using Convolutional Neural Networks.

Cardiovascular exercise is known to promote the consolidation of newly acquired motor skills. Previous studies seeking to understand the neural correlates underlying motor memory consolidation that is modulated by exercise, have relied so far on using traditional statistical approaches for a priori selected features from neuroimaging data, including EEG. With recent advances in machine learning, data-driven techniques such as deep learning have shown great potential for EEG data decoding for brain-computer interfaces, but have not been explored in the context of exercise. Here, we present a novel Convolutional Neural Network (CNN)-based pipeline for analysis of EEG data to study the brain areas and spectral EEG measures modulated by exercise. To the best of our knowledge, this work is the first one to demonstrate the ability of CNNs to be trained in a limited sample size setting. Our approach revealed discriminative spectral features within a refined frequency band (27-29 Hz) as compared to the wider beta bandwidth (15-30 Hz), which is commonly used in data analyses, as well as corresponding brain regions that were modulated by exercise. These results indicate the presence of finer EEG spectral features that could have been overlooked using conventional hypothesis-driven statistical approaches. Our study thus demonstrates the feasibility of using deep network architectures for neuroimaging analysis, even in small-scale studies, to identify robust brain biomarkers and investigate neuroscience-based questions.

Muscle fatigue during a short walking exercise in children with cerebral palsy who walk in a crouch gait.

BACKGROUND: A deterioration of crouch gait was found in a group of children with cerebral palsy (CP) after a short walking exercise. The increased knee flexion reported after a continuous walk could be related with muscle fatigue and muscle strength. AIM: Does muscle fatigue appears at the end of a walking exercise in children with CP who walk in a crouch gait? METHODS: Eleven children with cerebral palsy (GMFCS I to III) who walk in a crouch gait were included. Isometric muscle strength was assessed using a handheld dynamometer. Children were asked to walk for 6 min at comfortable speed. Spatio-temporal, kinematic and electromyographic (EMG) measurements were recorded at the first and the last minute of the 6-minute walking exercise. Muscle fatigue was evaluated using the shift of EMG signals median frequency. RESULTS: There was no significant difference in walking speed, cadence, and step length at the end of the 6mwe. Maximal and mean anterior pelvic tilt decreased and knee flexion increased (p < 0.05). Rectus femoris EMG median frequency decreased (p < 0.05). The median frequency in other muscles did not decrease significantly. Greater hip extensor strength was associated with lesser knee flexion at the end of the 6-minute walking exercise (p < 0.05). SIGNIFICANCE: The increase in knee flexion at the end of the 6-minute walking exercise can be explained by muscle fatigue found in rectus femoris. Hip extensor strength can limit the deterioration of crouch gait after a 6-minute walking exercise representative of daily activities.

Acute cardiovascular exercise promotes functional changes in cortico-motor networks during the early stages of motor memory consolidation.

A single bout of cardiovascular exercise performed immediately after practicing a visuo-motor tracking task has been shown to improve the long-term retention of this motor skill through an optimization of the memory consolidation process. The mechanisms underlying the time-dependent effects of acute cardiovascular exercise on motor memory consolidation, however, remain poorly understood. In this study, we sought to determine the impact of a single bout of cardiovascular exercise performed immediately after motor skill practice on those mechanisms using electroencephalography (EEG) and electromyography (EMG). Specifically, we assessed exercise-induced changes in the activity and connectivity of cortico-motor networks during early consolidation and the impact of these changes on skill retention. Participants practiced a visuo-motor tracking task followed by either a short bout of intense exercise or a rest period. EEG along with EMG data of hand muscles were collected during the production of low-force isometric contractions. Event-related desynchronization, functional connectivity and corticomuscular coherence were measured at baseline, 30, 60 and 90 min after the bout of exercise or the rest period. Improvements in motor memory were inferred via retention tests of the motor skill performed 8 and 24 h after motor practice. We found that participants who performed the single bout of exercise showed better motor skill retention 24 h after motor practice. This improvement in skill retention in the exercise group was associated with significant decreases in beta-band event-related desynchronization in EEG electrodes located over the left sensorimotor areas. We also found that after exercise, alpha-, and even more significantly, beta-band functional connectivity, increased between EEG electrodes located over left and right sensorimotor areas. The exercise group also showed greater beta-band corticomuscular coherence but only in a small number of electrodes. Neither functional connectivity nor corticomuscular coherence measures correlated with skill retention scores. This is the first study exploring brain mechanisms underlying the summative effects of motor learning and cardiovascular exercise on motor memory consolidation. We have identified potential neural substrates through which a single bout of acute exercise, when performed in close temporal proximity to motor practice, strengthens motor memories. Our findings provide new mechanistic insights into a better understanding of the complex temporal relationship existing between cardiovascular exercise and motor memory consolidation.

Evolution of muscular fatigue in periscapular and rotator cuff muscles during isokinetic shoulder rotations.

To this day, how shoulder muscles react to a strong fatigue stimulus during dynamic shoulder rotations remains unknown. The aims of this study were to assess the effect of repeated maximal internal-external isokinetic shoulder rotations on shoulder strength and muscle activity. Twenty-four individuals completed a 50-repetition fatiguing isokinetic protocol while electromyography was recorded on eleven muscles of the shoulder girdle. Time-frequency transformation and an ANOVA model using statistical parametric mapping methods were used to analyze shifts in instantaneous median frequency (MDF) between each 10-repetition Blocks. Peak torques decreased in both internal and external rotation (P < 0.01) by 24.8% on average which indicated the presence of fatigue. Significant decrease in MDF (P < 0.01) was observed for pectoralis, middle deltoid, upper, middle and lower trapezius, infraspinatus and subscapularis muscles. The observed fatigue to the periscapular and rotator cuff muscles suggests that shoulder stability could be compromised during repeated shoulder rotations, which could underlie the increased risk of scapular dyskinesis and shoulder impingement during fatiguing tasks. The present study provides a deeper understanding on the manifestations of fatigue within muscles of the shoulder girdle and the results could be applied toward improvements in athlete shoulder injury prevention and rehabilitation programs.

Which data should be tracked in forward-dynamic optimisation to best predict muscle forces in a pathological co-contraction case?

The choice of the cost-function for predicting muscle forces during a movement remains a challenge, especially in patients with neuromuscular disorders. Forward dynamics-based optimisations mainly track joint kinematics or torques, combined with a least-excitation criterion. Tracking marker trajectories and/or electromyography (EMG) has rarely been proposed. Our objective was to determine the best tracking objective-function to accurately predict the upper-limb muscle forces. A musculoskeletal model was created and EMG was simulated to obtain a reference movement - a shoulder abduction. A Gaussian noise (mean = 0; standard deviation = 15%) was added to the simulated EMG. Another noise - corresponding to the actual soft tissue artefacts (STA) of experimental shoulder abduction movements - was added to the trajectories of the markers placed on the model. Muscle forces were estimated from these noisy data, using forward dynamics assisted by six non-linear least-squared objective-functions. These functions involved the tracking of marker trajectories, joint angles or torques, with and without EMG-tracking. All six approaches used the same musculoskeletal model and were solved using a direct multiple shooting algorithm. Finally, the predicted joint angles, muscle forces and activations were compared to the reference values, using root-mean-square errors (RMSe) and biases. The force RMSe of the approach tracking both marker trajectories and EMG (18.45 ±â€¯12.60 N) was almost five times lower than the one of the approach tracking only joint angles (82.37 ±â€¯66.26 N) or torques (85.10 ±â€¯116.40 N). Therefore, using EMG as a complementary tracking-data in forward dynamics seems to be promising for the estimation of muscle forces.

Muscle function in glenohumeral joint stability during lifting task.

Ensuring glenohumeral stability during repetitive lifting tasks is a key factor to reduce the risk of shoulder injuries. Nevertheless, the literature reveals some lack concerning the assessment of the muscles that ensure glenohumeral stability during specific lifting tasks. Therefore, the purpose of this study was to assess the stabilization function of shoulder muscles during a lifting task. Kinematics and muscle electromyograms (n = 9) were recorded from 13 healthy adults during a bi-manual lifting task performed from the hip to the shoulder level. A generic upper-limb OpenSim model was implemented to simulate glenohumeral stability and instability by performing static optimizations with and without glenohumeral stability constraints. This procedure enabled to compute the level of shoulder muscle activity and forces in the two conditions. Without the stability constraint, the simulated movement was unstable during 74%±16% of the time. The force of the supraspinatus was significantly increased of 107% (p<0.002) when the glenohumeral stability constraint was implemented. The increased supraspinatus force led to greater compressive force (p<0.001) and smaller shear force (p<0.001), which contributed to improved glenohumeral stability. It was concluded that the supraspinatus may be the main contributor to glenohumeral stability during lifting task.