Sex differences in strength at the shoulder: a systematic review.

Background: Understanding differential strength capability between sexes is critical in ergonomics and task design. Variations in study designs and outcome measures generates challenges in establishing workplace guidelines for strength requirements to minimize upper extremity risk for workers. The purpose of this systematic review was to collate and summarize sex differences in strength at the shoulder across movement directions and contraction types. Methods: A total of 3,294 articles were screened from four databases (Embase, Medline, SCOPUS, and Web of Science). Eligibility criteria included observational studies, direct measurement of muscular joint, and healthy adult participants (18-65 years old). Strength outcome measures were normalized to percentages of male outputs to allow comparisons across articles. Results: A total of 63 studies were included within the final review. Majority of articles observed increased strength in males; the gap between male-female strength was greater in flexion and internal/external rotation, with females generating ~30% of male strength; scaption strength ratios were most consistent of the movement groups, with females generating 55-62% of male strength. Conclusion: Sex strength differences should be considered as an important factor for workplace task design as women are more at risk for occupational-related injuries than men in equivalent strength requirements. Differences in strength were not synonymous across motions; females demonstrated increased disparity relative to male strength in horizontal flexion/extension, forward flexion and internal/external rotation. Some movements had an extremely limited pool of available studies for examination which identified critical research gaps within the literature. Collating and quantifying strength differences is critical for effective workstation design with a range of users to mitigate potential overexertion risk and musculoskeletal injury.

Sex differences in wrist strength: a systematic review.

Sex differences in strength have been attributed to differences in body anthropometrics and composition; these factors are often ignored when generating workplace guidelines. These differences directly impact the upper extremity, leaving female workers exposed to injury risk. The wide range of tools and techniques for measuring upper extremity strength presents a challenge to ergonomists and work task designers; collating outcomes to provide a clear outlook of differences between males and females is essential and the purpose of this work. Four online databases were searched (PROSPERO ID: CRD42022339023) with a focus on articles assessing sex differences in wrist strength. A total of 2,378 articles were screened for relevancy; 25 full-text articles were included in this systematic review. Articles examined movement pairs (ulnar/radial deviation, pronation/supination, and flexion/extension), as well as contraction types (isometric and isokinetic) to observe sex differences in wrist strength. Across all articles, females produced ∼60-65% of male flexion/extension strength, ∼55-60% pronation/supination strength, and ∼60-70% ulnar/radial deviation strength. Overall, females presented lower strength-producing abilities than males, but when considering strength relative to body mass, male-female differences were less pronounced and occasionally females surpassed male strength metrics; typically, this occurred during flexion/extension, particularly in isokinetic contractions. This review has identified a scarcity of articles examining ulnar/radial deviation, pronation/supination, as well as isokinetic contractions; these are needed to supplement workplace exposure guidelines.

A comparison of isometric and isokinetic normalization methods for electromyographic data from sub-regions of supraspinatus and infraspinatus during dynamic tasks.

This study explored effects of using isometric versus isokinetic maximal voluntary contractions (MVCs) to normalize EMG data from supraspinatus and infraspinatus subregions during isokinetic tasks. Participants performed submaximal isokinetic external rotation (ER) and scaption tasks at two speeds. Three isometric MVCs were used: seated ER; side-lying scaption; side-lying abduction. Isokinetic MVCs were performed in the same position and speeds as the experimental tasks. Data were normalized using peak EMG from reference tasks: MVC which produced the greatest amplitude overall (MEA), isometric MVC with greatest amplitude (isometric best), isokinetic MVC with greatest amplitude (isokinetic best), and the greatest amplitude from the isokinetic MVC that matched the experimental task (isokinetic matched). Mean %MVC from each experimental task/ sub-region were compared by normalization method. The isokinetic matched method versus the MEA method was significantly different in all comparisons with isokinetic matched resulting in relative normalized task values up to 162% greater. The isometric best method resulted in significantly greater %MVC 37% of the time compared to the MEA method, whereas there were no differences when using isokinetic best compared to MEA. Isokinetic MVCs are less likely to overestimate %MVC than isometric and their use should be considered when normalizing data from dynamic tasks.

Adaptations in Muscular Strength for Individuals With Multiple Sclerosis Following Robotic Rehabilitation: A Scoping Review.

Muscular weakness and loss of motor function are common symptoms of multiple sclerosis. Robotic rehabilitation can improve sensorimotor function and motor control in this population. However, many studies using robotics for rehabilitation have overlooked changes in muscular strength, despite research demonstrating its utility in combating functional impairments. The purpose of this scoping review was to critically examine changes in muscular strength following robotic rehabilitation interventions for individuals with multiple sclerosis. A literature search of five databases was conducted and search terms included a combination of three primary terms: robotic rehabilitation/training, muscular strength, and multiple sclerosis. Thirty one articles were found, and following inclusion criteria, 5 remained for further investigation. Although muscular strength was not the primary targeted outcome of the training for any of the included articles, increases in muscular strength were present in most of the studies suggesting that robotic therapy with a resistive load can be an effective alternative to resistance training for increasing muscular strength. Outcome measures of isometric knee-extensor force (kg) (right: p < 0.05, left: p < 0.05), isometric knee flexion and extension torque (Nm) (p < 0.05), ankle dorsiflexion and plantarflexion torque (Nm) (all p < 0.05) and handgrip force (kg) (p < 0.05) all improved following a robotic training intervention. These adaptations occurred with sustained low resistive loads of hand grip or during gait training. This scoping review concludes that, despite a lack of studies focusing on strength, there is evidence robotics is a useful modality to improve muscular strength in combination with motor control and neuromotor improvements. A call for more studies to document changes in strength during robotic rehabilitation protocols is warranted.

Upper extremity muscle activity and joint loading changes between the standard and powerlifting bench press techniques.

The bench press is a common activity found in many exercise regimens. Powerlifters often adopt non-standard techniques to potentially enhance maximal capability. The purpose of this research was to examine muscle activation and joint loading differences between the powerlifting (Arch) and standardised techniques. Twenty experienced male lifters completed lifts at an instructed cadence in the arch and the National Strength and Conditioning Association standard techniques at 25%, 50% and 75% of their self-reported one rep maximum. The arch technique increased latissimus dorsi mean and peak activation (p < 0.0001), generating activation of approximately 13% maximal voluntary contraction, regardless of percentage of the one rep maximum lifted. The standardised technique resulted in integrated shoulder moments that were 8% larger (p < 0.0001). This latissimus dorsi activation paired with decreased shoulder loading in the arch technique likely acts to minimise the amount of time spent in the "sticking region", where most lift efforts fail. It is possible to use this technique to increase latissimus dorsi activation, without increasing overall shoulder loading. The technique-specific differences can be used in performance or rehabilitation-based programmes to increase muscular output of some muscles without increasing overall loading.

The Efficacy of Upper-Extremity Elastic Resistance Training on Shoulder Strength and Performance: A Systematic Review.

Elastic resistance exercise is a popular mode of strength training that has demonstrated positive effects on whole-body strength and performance. The purpose of this work was to identify the efficacy of elastic resistance training on improving upper limb strength and performance measures for the shoulder. Seven online databases were searched with a focus on longitudinal studies assessing shoulder elastic training strength interventions. In total, 1367 studies were initially screened for relevancy; 24 full-text articles were included for review. Exercise interventions ranged from 4-12 weeks, assessing pre-/post-strength and performance measures inclusive of isometric and isokinetic strength, 1RM strength, force-velocity tests, and throwing-velocity tests. Significant increases in various isometric strength measures (IR:11-13%, ER:11-42%, FL: 14-36%, EXT: 4-17%, ABD: 8-16%), 1RM strength (~24% in bench press), force-velocities, throwing- and serve-velocities (12%) were all observed. Elastic resistance training elicited positive effects for both strength and performance parameters regardless of intervention duration. Similar significant increases were observed in isometric strength and 1RM strength across durations. Isokinetic strength increases were variable and dependent on the joint velocity conditions. Quantifying the dosage of appropriate exercise prescription for optimal strength and performance gains is inconclusive with this study due to the heterogeneity of the intervention protocols.

Regional activation of supraspinatus and infraspinatus sub-regions during dynamic tasks performed with free weights.

Growing evidence supports the existence of distinct anatomical sub-regions within supraspinatus and infraspinatus, but only recently has attention turned to exploring their potential functional differences. Using indwelling fine-wire electromyography, muscle activity was investigated from these sub-regions in 15 participants (mean 34 yr, 170 cm, 71.9 kg) during dynamic external rotation (ER), abduction, flexion, and scaption tasks with and without free weights corresponding to 50% and 75% of the participant's five repetition maximum. Electromyography data were normalized to isometric and isokinetic maximal voluntary contractions and activation ratios for each sub-region compared. Differences in mean regional activation ratios for supraspinatus and infraspinatus varied by arm posture, but were not influenced by load. Relative activation of posterior supraspinatus was greater during an ER task performed in side lying compared to an ER task performed with 90° of humeral elevation in seated and prone postures. Relative activation of superior infraspinatus was greater during an ER task in prone and side lying postures compared to flexion and scaption. Similar results were found when comparing regional muscle activation ratios for infraspinatus between tasks regardless of normalization method employed. These findings may impact exercise selection in the non-operative management of rotator cuff tears.

Instructional Cueing Alters Upper Limb Muscle Activity and Kinematics During Elastic Resistance Exercise.

OBJECTIVE: The purpose of this study was to quantify the differences in upper limb muscle activity and kinematics when performing shoulder elastic resistance exercises with no cue, slouched posture, and corrected posture. DESIGN: Fifteen healthy participants completed four shoulder elastic resistance exercises (unilateral flexion, bilateral flexion, external rotation, and external rotation with towel) across three simulated body postures (no cue, corrected posture, and slouched posture). Surface electromyography was measured on 16 upper limb muscles and kinematics were collected. Two-way repeated-measures analyses of variance examined differences in muscle activation and kinematics across postures and exercises. RESULTS: Interactions between exercise and posture were found for most muscles. Muscle activity interactions existed in 14 of the 16 muscles examined, with 8 muscles having the greatest activity in the unilateral flexion, slouched condition (P < 0.0001). The slouched posture generated activity up to 88.4 ± 5.1 %MVC in the cervical extensors. Completing flexion or external rotation exercises with a slouched posture led to increased glenohumeral range of motion (P < 0.0001), but these differences were less than 5 degrees between the greatest and smallest ranges of motion (85.8 vs. 81.0 degrees). CONCLUSION: Posture influenced muscle activation and kinematics, with slouched postures increasing muscle activity and range of motion. There was little to no difference between the no cue and corrected cue conditions, suggesting that perhaps a clinician's time may be better spent focusing on avoiding slouched postures rather than ensuring mastering technique.

Quantifying supraspinatus tendon responses to exposures emulative of human physiological levels in an animal model.

Rotator cuff pathology typically originates in the supraspinatus tendon, but uncertainty exists on how combinations of glenohumeral elevation angle and load intensity influence responses of the intact, functional supraspinatus unit. This study exposed the supraspinatus tendon to mechanical loading scenarios emulative of derived muscle force and postural conditions measured in vivo to document its responses. Right shoulders from 48 Sprague-Dawley rats were placed into one of eight testing groups combining glenohumeral elevation angles (0/30/60/75°) and a high or low load intensity for 1500 cycles at 0.25 Hz using a custom mounting apparatus attached to a tensile testing system. Load intensities were derived from in vivo human partitional muscular activation levels collected previously and scaled to the animal model. Mechanical response variables examined included tangent stiffness and hysteresis, in addition to localized surface stretch ratios calculated via virtual tracking points. A significant three-way interaction (p = 0.0009) between elevation angle, load magnitude and cycle number occurred for tangent stiffness, with increasing angles, loads and cycles increasing stiffness by up to 49%. Longitudinal stretch ratios had significant interactions (p = 0.0396) with increasing elevation angles, load intensities and cycle numbers, and differences existed between the articular and bursal sides of the tendon. Complex interactions between angle, load and cycle number suggest higher abduction angles, increased load magnitude and higher loading cycles increase tangent stiffness, stretch ratios and hysteresis within the tendon.

Examining Muscle Activity Differences During Single and Dual Vector Elastic Resistance Exercises.

BACKGROUND: Elastic resistance exercise is a common part of rehabilitation programs. While these exercises are highly prevalent, little information exists on how adding an additional resistance vector with a different direction from the primary vector alters muscle activity of the upper extremity. PURPOSE: The purpose of this study was to examine the effects of dual vector exercises on torso and upper extremity muscle activity in comparison to traditional single vector techniques. STUDY DESIGN: Repeated measures design. METHODS: Sixteen healthy university-aged males completed four common shoulder exercises against elastic resistance (abduction, flexion, internal rotation, external rotation) while using a single or dual elastic vector at a fixed cadence and standardized elastic elongation. Surface electromyography was collected from 16 muscles of the right upper extremity. Mean, peak and integrated activity were extracted from linear enveloped and normalized data and a 2-way repeated measures ANOVA examined differences between conditions. RESULTS: All independent variables differentially influenced activation. Interactions between single/dual vectors and exercise type affected mean activation in 11/16 muscles, while interactions in peak activation existed in 7/16 muscles. Adding a secondary vector increased activation predominantly in flexion or abduction exercises; little changes existed when adding a second vector in internal and external rotation exercises. The dual vector exercise in abduction significantly increased mean activation in lower trapezius by 25.6 ± 8.11 %MVC and peak activation in supraspinatus by 29.4 ± 5.94 %MVC (p<0.01). Interactions between single/dual vectors and exercise type affected integrated electromyography for most muscles; the majority of these muscles had the highest integrated electromyography in the dual vector abduction condition. CONCLUSION: Muscle activity often increased with a second resistance vector added; however, the magnitude was exercise-dependent. The majority of these changes existed in the flexion and abduction exercises, with little differences in the internal or external rotation exercises. LEVEL OF EVIDENCE: 3b.

The rate of tendon failure in a collagen fibre recruitment-based model.

Accurate characterization of the mechanical response of collagenous tissues is critical for investigations into mechanisms of soft tissue injury. These tissues are inherently viscoelastic, exhibiting strain-rate dependent stiffnesses, creep, and stress-relaxation. The strain-rate features of the failure portion of the stress-strain curve are less well developed. Collagen-distribution based models are improving and capable of reproducing the non-linear aspects of the elastic response of soft tissues, but still require parameterization of failure regions. Therefore, the purpose of this investigation, was to determine whether the parameters characterizing the rate of damage accumulation in a collagen-distribution model are proportional to strain rate. Fifty rat tail tendons were subjected to one of five different strain rates (0.01, 0.05, 0.1, 0.15, 0.20 s-1) until failure in an uni-axial strain test. To test the hypothesis that the parameters associated with damage rate are proportional to strain rate, a collagen distribution model was employed with the parameters describing the rate of fibre damage being obtained by least-squares and regressed against the strain rate. The breaking function was found to be proportional to strain rate, with a proportionality constant of 60.7 s-1. Properties characterizing the failure portion of the stress-strain curves for rat tail tendons are also reported. The Young's Modulus did not vary with strain rate and was found to be 103.3 ± 49.5 MPa. Similarly, failure stresses and strains did not vary across the strain rates tested, and were 15.6 ± 6.1 MPa and 32.2 ± 9.1%, respectively.

The ability of surface electromyography to represent supraspinatus anterior and posterior partition activity depends on elevation angle, hand load and plane of elevation.

This study examined relationships between electromyography recorded from indwelling electrodes of the anterior and posterior supraspinatus and a surface supraspinatus electrode. Twenty male and twenty female participants completed full range humeral elevations in three planes of elevation (0/40/90°) and three hand loads (unloaded/20%/40% of maximal elevation strength). EMG activation was combined with motion capture to determine activation at instantaneous activation angles, and linear regressions of anterior and posterior indwelling electrodes relative to the surface electrode determined relationships between these signals. Regressions between surface and indwelling signals were affected by plane of elevation, elevation angle, load intensity and participant sex, but no interactions existed. Surface signals underestimated activation at low elevation angles for both regions, and up to 45% in the anterior supraspinatus (p < 0.01), then overestimated activation at higher elevation angles. Surface EMG underestimated indwelling signals by up to 15% in unloaded conditions, while overestimating the posterior region by up to 17% in the 40% load condition (p < 0.01). Sex effects showed increased overestimation by surface signals in the posterior region in males by 21% (p < 0.01). Better agreement existed for the anterior region than the posterior region across postures, but postural relationships should be considered when choosing electrodes for this muscle.

Examining upper extremity muscle demand during selected push-up variants.

Push-ups are regularly adapted for specific muscular demands. The push-up plus (PUP) has been used to emphasize serratus anterior activation. Alterations in body posture have been suggested for targeted activation of muscles surrounding the shoulder and rotator cuff, but little data exists to inform these changes. The purpose of this research was to examine upper extremity muscle activity changes when varying aspects of the PUP. Healthy male participants (n = 20) performed PUP trials using combinations of hand contact area (knuckles/palms), humeral rotation (neutral/60° internal/60° external) and hand location (shoulder height/±30% arm length superior/inferior) at a fixed cadence. Electromyographic (EMG) activation from 14 muscles of the right upper extremity during each trial were examined, as well as ratios of activation for 7 muscle pairs. Palm contact and a superior hand location increased serratus activation, but absolute EMG changes between these effects were marginal (standard error = 3.8). Each independent variable altered mean EMG for most muscles as a main effect, with the largest changes in infraspinatus, upper and lower trapezius, and anterior deltoid. Altering the standard PUP does not seem to enhance serratus activation, but diverse responses in other muscles have training implications. These changes in muscle strategies can tailor the use of modified PUP exercises to target individual muscles or groups.

Regional activation of anterior and posterior supraspinatus differs by plane of elevation, hand load and elevation angle.

The supraspinatus is one of the muscles of the rotator cuff, and growing research on fibre type composition and mechanical advantages in specific postures suggest this muscle may have distinct anterior and posterior regions. Activation differences between these regions may identify important functional differences. This research quantified muscular activation of these regions throughout a range of motion with differing hand loads. Forty participants completed paced humeral elevations in 7 planes of elevation (0/15/30/40/60/75/90°) using 3 hand loads (unloaded arm/20%/40% maximal elevation strength). Indwelling electromyography collected muscle activity of the anterior and posterior supraspinatus. Hand load and elevation angle interacted to affect activity of the anterior supraspinatus in most planes of elevation - by up to 41 %MVC (p < 0.01), but in few planes for the posterior region. Plane of elevation influenced anterior and posterior region activation by up to 17 %MVC and 13 %MVC, respectively (p < 0.01). Increasing hand loads increased activation in both regions (p < 0.01), but more so for the anterior region. These differences may indicate differences in function between the two regions. The sustained activation in the smaller posterior supraspinatus may indicate this region as primarily a glenohumeral stabilizer, while the larger anterior region acts to achieve glenohumeral motion.

Female maximal push/pull strength capabilities by humeral abduction angle in bilateral exertions.

Pushing or pulling an object constitutes the majority of manual materials handling tasks. Anthropometric differences between workers alter pushing and pulling strategies at fixed heights, potentially modifying exposures and causing overexertion. Capability normalized to arm posture rather than work height remains unknown, and the purpose of this research was to quantify maximal pushing and pulling strength and upper extremity joint moments using fixed humeral abduction angles and a flexed arm position. Twenty university-aged females completed maximal pushes and pulls at 0°, 30°, 60°, 90°, and 120° of humeral abduction with an elbow to 90°. Abduction angle was the largest modifier of both push and pull force generating capability (p < 0.01), with increasing abduction reducing force capability by up to 30%. While push exertions on average exceeded pulls by 17 N (p < 0.01), the difference is much smaller than previous reports. Ergonomists should consider humeral angle of the worker rather than work heights, as individuals working in increased abduction decreases capability and increases potential overexertion injury risk.

Examining upper limb kinematics and dysfunction of breast cancer survivors in functional dynamic tasks.

BACKGROUND: Comorbidities within the breast cancer population can reduce quality of life. Current breast cancer survivor upper limb kinematic strategies unfortunately lack robust connection with performing important activities of daily living. METHODS: Accordingly, fifty breast cancer survivors performed 88 dynamic tasks (divided into range of motion-reach, range of motion-rotate, activity of daily living, and work tasks). Humerothoracic and scapulothoracic angles were extracted from motion capture data. Bilateral differences existed for range of motion, and maximal and minimal scapulothoracic and humerothoracic angles. FINDINGS: Generally, the affected side used less range of motion across task types. Humerothoracic angles on the affected side experienced 6.7° less range of motion in plane of elevation in range of motion-reach (p < 0.01), 2.3° less elevation angle range of motion in range of motion-rotate (p = 0.01), and 7.1° more internal rotation range of motion in range of motion-rotate (p < 0.01). Scapulothoracic angles on the affected side had 2° more anterior/posterior tilt range of motion in work tasks (p = 0.03), 3.4° less maximal protraction in activity of daily living tasks (p = 0.01), and 3.5° less minimum downward rotation in range of motion-rotate (p < 0.01). INTERPRETATION: A reduced range of motion on the affected side suggests the breast cancer population had less varied movement strategies, keeping movements in narrower ranges to avoid disability, pain, or subacromial impingement. This investigation produced an unprecedentedly diverse collection of three-dimensional humerothoracic and scapulothoracic kinematics for a breast cancer population. Documentation of physical capability, dysfunction, and adaptive strategies is a crucial step towards developing targeted strategies for enhancing functional recovery in breast cancer survivors.

Quantification of upper limb electromyographic measures and dysfunction of breast cancer survivors during performance of functional dynamic tasks.

BACKGROUND: Upper limb morbidities within the breast cancer population can interfere with completing daily life activities. Current knowledge of upper limb capabilities is limited; previous increases in muscle activation on the affected cancer side suggest this population works at a higher fraction of their capability. The purposes of this study were to describe upper limb capabilities and dysfunction of breast cancer survivors through muscle activation monitoring via surface electromyography and muscle-specific strength tests during functional tasks. METHODS: Fifty survivors performed 88 dynamic tasks (divided into range of motion-reach or rotate, activities of daily life and work tasks). Muscle activation was examined for functional and strength testing tasks. FINDINGS: Total muscle effort (summation of integrated electromyography across measured muscles) was up to 5.1% greater on the affected side during work tasks (p=0.0258). Increased activations existed in posterior deltoid, supraspinatus, upper trapezius and serratus anterior (p<0.05) for several tasks, including daily living tasks. Reduced activation occurred in affected pectoralis major sternal during all tasks (p<0.0001-0.0032), and affected infraspinatus in all but daily living tasks (p=0.0002-0.0328). The affected side infraspinatus, supraspinatus and upper trapezius muscles demonstrated significant reductions in targeted strength testing (p=0.0001-0.0057). INTERPRETATION: Both primary and secondary muscles (outside surgery and radiation fields) were affected. In general, this population works at higher levels of muscle effort for the affected side yet demonstrates weakness in strength testing, which may reflect tissue damage. Strengthening exercises for the posterior rotator cuff and upper trapezius may be the most beneficial.

The Influence of Hand Location and Force Direction on Shoulder Muscular Activity in Females During Nonsagittal Multidirectional Overhead Exertions.

OBJECTIVE: We examined interactions of overhead work location and direction of force application on shoulder muscular activity. BACKGROUND: Overhead work tasks are common occupational stressors. Previous research has quantified influences of overhead work spatial placement and different force application directions but typically separately or exclusively for tasks done in the median plane. METHOD: Twenty female participants exerted 40 N of force in six directions (forward/backward, upward/downward, left/right) 150 cm off the floor while seated. An asymmetric pattern of 14 work locations spaced 15 cm centered directly overhead were evaluated. RESULTS: Force direction and work location strongly influenced mean muscle activity (F = 559, p < .01). Interaction effects existed between force direction and hand location in the transverse plane (F = 21, p < .01), with increases as high as 49% in normalized mean muscle activity. CONCLUSION: Backward exertions produced the highest mean overall muscle activity across hand force directions, exceeding 30% maximum voluntary isometric exertion (MVE) across work locations, with higher activation of anterior deltoid, biceps, infraspinatus, supraspinatus, and upper and lower trapezius. Downward exertions had the lowest mean overall activity, with <10% MVE across work locations. Altered (up to 47%) muscular activity occurred as exertions moved laterally from the origin, and increasingly forward hand positions generally yielded decreased mean overall activity for most force directions. APPLICATION: This study provides previously unavailable submaximal shoulder muscular activity data for a wide range of overhead tasks. As such, it enables novel work design considerations that include modifying existing overhead elements to reduce or redistribute associated muscular demands.

Effects of overhead work configuration on muscle activity during a simulated drilling task.

Overhead work is a known catalyst for occupational shoulder injury. Industrial workers must often adopt awkward overhead postures and loading profiles to complete required tasks, potentially elevating injury risk. This research examined the combined influence of multiple overhead working parameters on upper extremity muscular demands for an industrial drilling application. Twenty-two right-handed males completed 24 unilateral and bilateral overhead work exertions stratified by direction (upward, forward), point of force application (15, 30 and 45 cm in front of the body), and whole-body posture (seated, standing). The dependency of electromyographic (EMG) activity on several factors was established. Significant two-way interactions existed between point of force application and direction (p < 0.0001) and direction and whole body posture (p < 0.0001). An average increase in muscular activity of 6.5% maximal voluntary contraction (MVC) occurred for the contralateral limb when the bilateral task was completed, compared to unilateral tasks, with less than a 1% MVC increase for the active limb. These findings assist evidence-based approaches to overhead tasks, specifically in the construction industry. A bilateral task configuration is recommended to reduce glenohumeral stability demands. As well, particularly for tasks with a far reach distance, design tasks to promote a forward directed exertion. The considerable inter-subject variability suggests that fixed heights are not ideal, and should be avoided, and where this is not possible reaches should be reduced.

The influence of cycle time on shoulder fatigue responses for a fixed total overhead workload.

The relationship between overhead work and musculoskeletal health depends on multiple task and individual factors. Knowledge gaps persist, despite examination of many of these factors individually and in combination. This investigation targeted task variation, as parameterized by cycle time within a fixed overall workload. Participants performed an intermittent overhead pressing task with four different cycle time conditions while overall workload and duty cycle was held constant. Several manifestations of fatigue were monitored during task performance. Endurance time was influenced by cycle time with shorter cycle times having endurance times up to 25% higher than longer cycle times. Surface electromyography (sEMG) results were mixed, with two muscles demonstrating amplitude increases (middle deltoid and upper trapezius) that varied with cycle time. sEMG frequency was not influenced by cycle time for any muscle monitored, despite decreases for several cycle times. Trends existed for the influence of cycle time on time-varying reported discomfort (p=0.056) and static strength (p=0.055); large effect sizes were present (ηp(2)=0.31 and 0.27, respectively). The equivocal association of fatigue indicators and cycle time is analogous to the influence of other factors implicated in overhead work musculoskeletal risk, and probabilistic modeling offers a compelling avenue for integration of the known variation in the many factors that combine to inform this risk.