Upper body kinematic and muscular variability in response to targeted rotator cuff fatigue

Muscle coactivation changes following a fatiguing overhead drilling task: Implications for subacromial impingement syndrome

Individuals with subacromial impingement syndrome (SAIS) demonstrate altered upper limb muscle coactivation during humeral elevation and overhead tasks. This study investigated the effect of an occupationally-relevant overhead task designed to induce global shoulder muscle fatigue on scapular stabilizer and deltoid-rotator cuff muscle coactivation, and upper limb kinematics. Upper limb muscle activity and kinematics were analyzed from 33 right-handed male (n = 17) and female (n = 16) participants before and immediately following a simulated overhead drilling task. Co-activation was generally not affected by fatigue, however scapulothoracic, thoracohumeral, and elbow angles all displayed significant responses to fatigue. Muscle activity and kinematics were highly variable, with a considerable number of participants displaying muscular and kinematic adaptations that would increase SAIS risk. Disadvantageous scapular kinematics adaptations to fatigue correlated with disadvantageous scapular stabilizer coactivity when working in postures requiring more scapular internal rotation.

Variability in musculoskeletal fatigue responses associated with repeated exposure to an occupational overhead drilling task completed on successive days

Emerging research suggests that muscular and kinematic responses to overhead work display a high degree of variability in fatigue-related muscular and kinematics changes, both between and within individuals when evaluated across separate days. This study examined whether electromyographic (EMG), kinematic, and kinetic responses to an overhead drilling task performed until volitional fatigue were comparable to those of a repeated identical exposure of the task completed 1 week later. Surface EMG and intramuscular EMG, sampled from 7 shoulder muscles, and right upper limb kinematics and kinetics were analyzed from 15 male and 14 female participants. No significant day-to-day changes in EMG mean power frequency (MPF) were observed, though serratus anterior displayed significantly less fatigue-related increase in EMG root-mean-squared (RMS) signal amplitude on day 2. Unfatigued upper kinematics on day 2 featured an increase in thoracohumeral elevation, elbow flexion, and decrease in wrist ulnar deviation compared to unfatigued state on day 1. Fatigue-related changes in shoulder joint flexion moment that were present on day 1 were reduced on day 2, suggesting that a more efficient overhead work strategy was learned and preserved across successive days. Day-to-day changes in upper limb joint angle variability, quantified by median absolute deviation (MdAD), were joint dependent. Despite yielding a variable fatigue-related kinetic strategy on both days, kinematic and kinetic fatigue-related changes on a second day of completing an overhead drilling task suggested a potential kinematic learning effect.

Scapular dyskinesis after treatment of proximal humerus fracture, a 3-dimensional motion analysis and clinical outcomes

BACKGROUND

The alteration of scapular kinematics can predispose patients to shoulder pathologies and dysfunction. Previous literature has associated various types of shoulder injuries with scapular dyskinesis, but there are limited studies regarding the effect that proximal humeral fractures (PHFs) have on scapular dyskinesis. This study aims to determine the change in scapulohumeral rhythm following treatment of a proximal humerus fracture as well as differences in shoulder motion and functional outcomes among patients who presented with or without scapular dyskinesis. We hypothesized that differences in scapular kinematics would be present following treatment of a proximal humerus fracture, and patients who presented with scapular dyskinesis would subsequently have inferior functional outcome scores.

METHODS

Patients treated for a proximal humerus fracture from May 2018 to March 2021 were recruited for this study. The scapulohumeral rhythm and global shoulder motion were determined using a 3-dimensional motion analysis (3DMA) and the scapular dyskinesis test. Functional outcomes were then compared among patients with or without scapular dyskinesis, including the SICK (scapular malposition, inferomedial border prominence, coracoid pain and malposition, and dyskinesis of scapular movement) Scapula Rating Scale, the American Shoulder and Elbow Surgeons Standardized Shoulder Assessment Form (ASES), the visual analog scale (VAS) for pain, and the EuroQol-5 Dimension 5-Level questionnaire (EQ-5D-5L).

RESULTS

Twenty patients were included in this study with a mean age of 62.9 ± 11.8 years and follow-up time of 1.8 ± 0.2 years. Surgical fixation was performed in 9 of the patients (45%). Scapular dyskinesis was present in 50% of patients (n = 10). There was a significant increase in scapular protraction on the affected side of patients with scapular dyskinesis during abduction of the shoulder (P = .037). Additionally, patients with scapular dyskinesis demonstrated worse SICK scapula scores (2.4 ± 0.5 vs. 1.0 ± 0.4, P = .024) compared to those without scapular dyskinesis. The other functional outcome scores (ASES, VAS pain scores, and EQ-5D-5L) showed no significant differences among the 2 groups (P = .848, .713, and .268, respectively).

CONCLUSIONS

Scapular dyskinesis affects a significant number of patients following treatment of their PHFs. Patients presenting with scapular dyskinesis exhibit inferior SICK scapula scores and have more scapular protraction during shoulder abduction compared to patients without scapular dyskinesis.

Between Two Rocks and in a Hard Place: Reflecting on the Biomechanical Basis of Shoulder Occupational Musculoskeletal Disorders

OBJECTIVE

The aim was to review the biomechanical origins of occupational shoulder damage, while considering the complexity of shoulder mechanics and musculoskeletal consequences of diverse task demands.

BACKGROUND

Accessible measures of physical exposures are the primary focus of occupational shoulder assessments and analyses. This approach has led to guidelines and intervention strategies that are often inadequate for mitigating shoulder disorders amongst the complexity of modern workplace demands. Integration of complex shoulder mechanics into occupational assessments, analyses, and interventions is critical for reducing occupational shoulder injury risk.

METHOD

This narrative review describes shoulder biomechanics in the context of common injury mechanisms and consequent injuries, with a particular focus on subacromial impingement syndrome. Several modulators of shoulder injury risk are reviewed, including fatigue, overhead work, office ergonomics considerations, and pushing and pulling task configurations.

RESULTS

Relationships between work requirements, muscular demands, fatigue, and biomechanical tissue loads exist. This review highlights that consideration of specific workplace factors should be integrated with our knowledge of the intricate arrangement and interpersonal variability of the shoulder complex to proactively evaluate occupational shoulder demands and exposures.

CONCLUSION

A standard method for evaluating shoulder muscle exposures during workplace tasks does not exist. An integrated approach is critical for improved work design and prevention of shoulder tissue damage and accompanying disability.

APPLICATION

This review is particularly relevant for researchers and practitioners, providing guidance for work design and evaluation for shoulder injury prevention by understanding the importance of the unique and complex mechanics of the shoulder.

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.

The Effects of Fatigue on Muscle Synergies in the Shoulders of Baseball Players

BACKGROUND

Muscle synergies are defined as the central nervous system's organizational structure for movement. Muscle synergies change after muscular fatigue, with certain synergies assuming the primary role to compensate for fatigue within another muscle synergy. Owing to the high eccentric forces imposed upon the external rotators (ie, posterior rotator cuff), pitchers that continue to throw while fatigued are at a significantly higher risk of shoulder and/or elbow injury; however, the neuromuscular compensation strategies of baseball players in response to fatigue are currently unknown.

HYPOTHESIS

Players would utilize the same muscle synergy structure following external rotation (ER) fatigue; however, muscle coefficients of nonfatigued muscles would increase (ie, compensate for the external rotators) after fatigue.

STUDY DESIGN

Cross-sectional study conducted in a controlled, laboratory setting.

METHODS

Nine players from an intercollegiate competitive club baseball team voluntarily participated in this study. Surface electromyography was used on 14 muscles of the glenohumeral and scapulothoracic joints of the dominant arm during a reaching protocol. Players completed a baseline reaching protocol (prefatigue), then an ER fatigue protocol until maximum concentric ER was reduced by 40%, and finally repeated the same reaching protocol (postfatigue). Principal component analysis was used to extract muscle synergies, the variance accounted for (VAF) of each synergy, and muscle coefficients. Prefatigue was compared with postfatigue using paired t tests for all dependent variables.

RESULTS

Four muscle synergies were extracted for both pre- and postfatigue. The VAF for the ER/abduction synergy decreased significantly (prefatigue, 34.6%; postfatigue, 32.4%; P = 0.03), showing a decreased reliance on ER/abduction during the reaching task after fatigue. Within synergy 1, the pectoralis major muscle coefficient (-0.489 vs -0.552; P = 0.01; effect size = 1.68) decreased significantly from prefatigue to postfatigue, indicating that the pectoralis major assumed more of an antagonist role during ER/abduction. Within synergy 2 (forward reaching), there were no significant changes in VAF or muscle coefficients observed. For the third synergy, muscle coefficients increased for the serratus anterior (P = 0.02) and middle deltoid (P = 0.01), whereas in the fourth synergy, the pectoralis major (P = 0.01) increased and teres major (P = 0.01) and biceps brachii (P = 0.05) muscle coefficients decreased.

CONCLUSION

The decreased VAF of the ER/abduction synergy after fatigue indicate that other muscles within that synergy could not fully compensate to maintain function. Interestingly, the changes in muscle coefficients suggest that players relied less on the internal rotation (IR) synergy and more on the cross-body synergy following fatigue. This may be due to imbalances between ER and IR while maintaining balance between cross-abduction and adduction.

CLINICAL RELEVANCE

Clinicians may consider implementing low-load, high-repetition training programs to develop posterior shoulder endurance and prolong the onset of muscular fatigue.

Effects of Vertical Lifting Distance on Upper-Body Muscle Fatigue

Manual material handling (MMH) is commonly demanded in the manufacturing industry. Occupational muscle fatigue of the arm, shoulder, and back, which arise from MMH tasks, can cause work absences and low efficiency. The available literature presents the lack of the fatigue comparison between targeted muscles, on the same part or on different parts. The main aim of the present study was to evaluate and compare the fatigue of upper-body muscles during repetitive bending tasks, an experiment involving 12 male subjects has been conducted to simulate material handling during furniture board drilling. The vertical lifting distance was chosen to be the single independent variable, and the three levels were 0, 250, and 500 mm. Surface electromyography (sEMG) was used to measure the muscle fatigue of the biceps brachii, upper trapezius, and multifidus, while the sEMG parameters, including the normalized electromyographic amplitude (Normalized EA) and mean power frequency (MPF), of the target muscles were analyzed. The experimental results reveal that during the manual handling tasks, the biceps brachii was the most relaxed muscle, contributing the least muscle tension, while the multifidus was the most easily fatigued muscle. Furthermore, the EMG MPF fatigue threshold (MPFFT) of multifidus muscle tension was tested to estimate its maximum workload in the long-term muscle contraction. In conclusion, bending angle should be maintained to a small range or bending should even be avoided during material-handling tasks.

USE OF AN UPPER EXTREMITY FUNCTIONAL TESTING ALGORITHM TO DETERMINE RETURN TO PLAY READINESS IN COLLEGIATE FOOTBALL PLAYERS: A CASE SERIES

BACKGROUND/PURPOSE

Return to play decision making for upper extremity injuries is challenging due to a lack of evidence-based protocols and testing. Current guidelines utilize tests and measures with minimal evidence on re-injury risks and prediction. The purpose of this case series is to highlight a functional testing algorithm for upper extremities injuries and the outcomes for the patients that followed it.

STUDY DESIGN

Case series.

CASE DESCRIPTIONS

Six subjects (18 - 21 years old) who underwent shoulder capsulolabral repair secondary to recurrent instability and/or unyielding pain are included. All subjects underwent a criterion-based rehabilitation program before being assessed with the authors' upper extremity functional testing algorithm. The upper extremity functional testing algorithm consists of measures of active range of motion (AROM), passive range of motion (PROM), peak isometric force, a fatigue testing battery, and the closed kinetic chain upper extremity stability test (CKCUEST) to assess readiness for return to sport.

OUTCOMES

All athletes achieved > 90% symmetry on at least two out of three tests during a fatigue testing protocol and at least 25 touches on the CKCUEST. All of the athletes returned to unrestricted football the season following surgical intervention. None of the athletes sustained an additional glenohumeral subluxation, dislocation, or upper extremity injury requiring surgical intervention for the remainder of their athletic careers (six years).

DISCUSSION

The presented cases help to illustrate the effectiveness of the upper extremity functional testing algorithm to assess return to sport readiness for male collegiate football athletes. The algorithm included testing of AROM/PROM and strength that is typically used, but also included the CKCUEST and fatigue testing to further challenge and assess the upper extremity prior to returning to sports.

LEVEL OF EVIDENCE

4.

Joint moment trade-offs across the upper extremity and trunk during repetitive work

Individuals can coordinate small kinematic changes at several degrees of freedom simultaneously in the presence of fatigue, leaving it unclear how overall biomechanical demands at each joint are altered. The purpose of this study was to evaluate trade-offs in joint moments between the trunk, shoulder, and elbow during repetitive upper extremity work. Participants performed four simulated workplace tasks cyclically until meeting fatigue termination criteria. Emergent fatigue-induced adaptations to repetitive work resulted in task-dependent trade-offs in joint moments. In general, reduced shoulder moments were compensated for by increased elbow and trunk joint moment contributions. Although mean joint moment changes were modest (range: 1-3 Nm) across participants, a wide distribution of responses was observed, with standard deviations exceeding 10 Nm. Re-distributing biomechanical demands across joints may alleviate constant tissue loads and facilitate continued task performance with fatigue but may be at the expense of increasing demands at adjacent joints.