Hybrid Soft-Rigid Active Prosthetics Laboratory Exercise for Hands-On Biomechanical and Biomedical Engineering Education.

This paper introduces a hands-on laboratory exercise focused on assembling and testing a hybrid soft-rigid active finger prosthetic for biomechanical and biomedical engineering (BME) education. This hands-on laboratory activity focuses on the design of a myoelectric finger prosthesis, integrating mechanical, electrical, sensor (i.e., inertial measurement units (IMUs), electromyography (EMG)), pneumatics, and embedded software concepts. We expose students to a hybrid soft-rigid robotic system, offering a flexible, modifiable lab activity that can be tailored to instructors' needs and curriculum requirements. All necessary files are made available in an open-access format for implementation. Off-the-shelf components are all purchasable through global vendors (e.g., DigiKey Electronics, McMaster-Carr, Amazon), costing approximately USD 100 per kit, largely with reusable elements. We piloted this lab with 40 undergraduate engineering students in a neural and rehabilitation engineering upper year elective course, receiving excellent positive feedback. Rooted in real-world applications, the lab is an engaging pedagogical platform, as students are eager to learn about systems with tangible impacts. Extensions to the lab, such as follow-up clinical (e.g., prosthetist) and/or technical (e.g., user-device interface design) discussion, are a natural means to deepen and promote interdisciplinary hands-on learning experiences. In conclusion, the lab session provides an engaging journey through the lifecycle of the prosthetic finger research and design process, spanning conceptualization and creation to the final assembly and testing phases.

A comparative probabilistic analysis of human and chimpanzee rotator cuff functional capacity.

Computational musculoskeletal modeling represents a valuable approach to examining biological systems in physical anthropology. Probabilistic modeling builds on computational musculoskeletal models by associating mathematical distributions of specific musculoskeletal features within known ranges of biological variability with functional outcomes. The purpose of this study was to determine if overlap in rotator cuff muscle force predictions would occur between species during the performance of an evolutionarily relevant horizontal bimanual arm suspension task. This necessitated creating novel probabilistic models of the human and chimpanzee glenohumeral joint through augmentation of previously published deterministic models. Glenohumeral musculoskeletal features of anthropological interest were probabilistically modeled to produce distributions of predicted human and chimpanzee rotator cuff muscle force that were representative of the specific anatomical manipulations. Musculoskeletal features modeled probabilistically included rotator cuff origins and deltoid insertion, glenoid inclination, and joint stability. Predicted human rotator cuff muscle force distributions were mostly limited to alternating between infraspinatus and teres minor, with both 100% and 0% muscle force predicted for both muscles. The chimpanzee model predicted low-to-moderate muscle force across all rotator cuff muscles. Rotator cuff muscle force predictions were most sensitive to changes of muscle origins and insertions. Results indicate that functional rotator cuff overlap is unlikely between chimpanzees and humans without greater modifications of the glenohumeral musculoskeletal phenotypes. The results also highlight the low efficacy of the human upper extremity in overhead, weight-bearing tasks, and propensity for rotator cuff injury.

Experimental Study of Fully Passive, Fully Active, and Active-Passive Upper-Limb Exoskeleton Efficiency: An Assessment of Lifting Tasks.

Recently, robotic exoskeletons are gaining attention for assisting industrial workers. The exoskeleton power source ranges from fully passive (FP) to fully active (FA), or a mixture of both. The objective of this experimental study was to assess the efficiency of a new active-passive (AP) shoulder exoskeleton using statistical analyses of 11 quantitative measures from surface electromyography (sEMG) and kinematic data and a user survey for weight lifting tasks. Two groups of females and males lifted heavy kettlebells, while a shoulder exoskeleton helped them in modes of fully passive (FP), fully active (FA), and active-passive (AP). The AP exoskeleton outperformed the FP and FA exoskeletons because the participants could hold the weighted object for nearly twice as long before fatigue occurred. Future developments should concentrate on developing sex-specific controllers as well as on better-fitting wearable devices for women.

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.

Examining the Effect of Time-From-Treatment on Activities of Daily Living Kinematics in Breast Cancer Survivors.

Breast cancer affects one in 8 females with a 5-year survival rate of 89%. Up to 72% of breast cancer survivors have trouble with activities of daily living (ADL) following treatment. Increased time-from-treatment improves some measures of function, yet ADL limitations persist. Therefore, this study assessed the effect of time-from-treatment on upper extremity kinematics during ADLs in breast cancer survivors. Twenty-nine female breast cancer survivors were divided into 2 groups: <1 year (n = 12) and 1-2 years (n = 17) from treatment. Kinematics were collected during 6 ADL tasks, and humerothoracic joint angles were quantified. A 2-way mixed analysis of variance assessed the effects of time-from-treatment and arm on maximum angles for each ADL. Decreased maximum angle existed for breast cancer survivors with increased time-from-treatment during all ADLs. Breast cancer survivors in the 1-2 years group used ∼28° to 32° lower elevation, ∼14° to 28° lower axial rotation, and ∼10° to 14° lower plane of elevation range across tasks. Decreased ranges of arm movement during ADLs with increased time-from-treatment may reflect compensatory movement strategies. Recognizing this shift in strategies and accompanying underlying disease progression can help inform responses to functional performance limitations in breast cancer survivors as delayed effects are present posttreatment.

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.

Neural control of the healthy pectoralis major from low-to-moderate isometric contractions.

The pectoralis major critically enables arm movement in several directions. However, its neural control remains unknown. High-density electromyography (HD-sEMG) was acquired from the pectoralis major in two sets of experiments in healthy young adults. Participants performed ramp-and-hold isometric contractions in: adduction, internal rotation, flexion, and horizontal adduction at three force levels: 15%, 25%, and 50% scaled to task-specific maximal voluntary force (MVF). HD-sEMG signals were decomposed into motor unit spike trains using a convolutive blind source separation algorithm and matched across force levels using a motor unit matching algorithm. The mean discharge rate and coefficient of variation were quantified across the hold and compared between 15% and 25% MVF across all tasks, whereas comparisons between 25% and 50% MVF were made where available. Mean motor unit discharge rate was not significantly different between 15% and 25% MVF (all P > 0.05) across all tasks or between 25% and 50% MVF in horizontal adduction (P = 0.11), indicating an apparent saturation across force levels and the absence of rate coding. These findings suggest that the pectoralis major likely relies on motor unit recruitment to increase force, providing first-line evidence of motor unit recruitment in this muscle and paving the way for more deliberate investigations of the pectoralis major involvement in shoulder function.NEW & NOTEWORTHY This work is the first to investigate the relative contribution of rate coding and motor unit recruitment in the pectoralis major muscle in several functionally relevant tasks and across varying force levels in healthy adults. Our results demonstrate the absence of motor unit rate coding with an increase in EMG amplitude with increases in force level in all tasks examined, indicating that the pectoralis major relies on motor unit recruitment to increase force.

An Electromyographically Driven Cervical Spine Model in OpenSim.

Relatively few biomechanical models exist aimed at quantifying the mechanical risk factors associated with neck pain. In addition, there is a need to validate spinal-rhythm techniques for inverse dynamics spine models. Therefore, the present investigation was 3-fold: (1) the development of a cervical spine model in OpenSim, (2) a test of a novel spinal-rhythm technique based on minimizing the potential energy in the passive tissues, and (3) comparison of an electromyographically driven approach to estimating compression and shear to other cervical spine models. The authors developed ligament force-deflection and intervertebral joint moment-angle curves from published data. The 218 Hill-type muscle elements, representing 58 muscles, were included and their passive forces validated against in vivo data. Our novel spinal-rhythm technique, based on minimizing the potential energy in the passive tissues, disproportionately assigned motion to the upper cervical spine that was not physiological. Finally, using kinematics and electromyography collected from 8 healthy male volunteers, the authors calculated the compression at C7-T1 as a function of the head-trunk Euler angles. Differences from other models varied from 25.5 to 368.1 N. These differences in forces may result in differences in model geometry, passive components, number of degrees of freedom, or objective functions.

Development of a comparative chimpanzee musculoskeletal glenohumeral model: implications for human function.

Modern human shoulder function is affected by the evolutionary adaptations that have occurred to ensure survival and prosperity of the species. Robust examination of behavioral shoulder performance and injury risk can be holistically improved through an interdisciplinary approach that integrates anthropology and biomechanics. Coordination of these fields can allow different perspectives to contribute to a more complete interpretation of biomechanics of the modern human shoulder. The purpose of this study was to develop a novel biomechanical and comparative chimpanzee glenohumeral model, designed to parallel an existing human glenohumeral model, and compare predicted musculoskeletal outputs between the two models. The chimpanzee glenohumeral model consists of three modules - an external torque module, a musculoskeletal geometric module and an internal muscle force prediction module. Together, these modules use postural kinematics, subject-specific anthropometrics, a novel shoulder rhythm, glenohumeral stability ratios, hand forces, musculoskeletal geometry and an optimization routine to estimate joint reaction forces and moments, subacromial space dimensions, and muscle and tissue forces. Using static postural data of a horizontal bimanual suspension task, predicted muscle forces and subacromial space were compared between chimpanzees and humans. Compared with chimpanzees, the human model predicted a 2 mm narrower subacromial space, deltoid muscle forces that were often double those of chimpanzees and a strong reliance on infraspinatus and teres minor (60-100% maximal force) over other rotator cuff muscles. These results agree with previous work on inter-species differences that inform basic human rotator cuff function and pathology.

The Effect of Shoulder Mobilization on Scapular and Shoulder Muscle Activity During Resisted Shoulder Abduction: A Crossover Study of Asymptomatic Individuals.

OBJECTIVE: The primary aim was to investigate the effect of inferior shoulder mobilization on scapular and shoulder muscle activity during resisted shoulder abduction in asymptomatic individuals. METHODS: This was a lab-based, repeated-measures, crossover, randomized controlled study. Twenty-two participants were recruited. The order of experimental conditions was randomized. Each participant performed 5 repetitions of resisted shoulder abduction before and after the control and mobilization (grade +IV inferior shoulder mobilization, 3 sets, 60 seconds) conditions. Surface electromyography recorded the muscle activity of anterior, middle, and posterior deltoid; supraspinatus; infraspinatus; upper and lower trapezius; serratus anterior; and latissimus dorsi muscles. RESULTS: Muscle activity levels reduced for infraspinatus (11.3% MVIC, 95% CI: 1.7-20.8), middle (22.4% MVIC, 95% CI: 15.9-28.8) and posterior deltoid (8.7 % MVIC, 95% CI: 4.6-12.9), and serratus anterior (-28.1% MVIC, 95% CI: 15.6-40.8) muscles after the mobilization condition during the eccentric phase of shoulder abduction. No carryover effects were observed, and within-session reliability was excellent (intraclass correlation coefficient scores ranging from 0.94 to 0.99). CONCLUSION: Our findings suggest that inferior glenohumeral mobilization reduces activity levels of some scapular and shoulder muscles. Given the exploratory nature of our study, changes in muscle activity levels may have been found by chance. Confirmatory studies are required.

Estimating Muscle Forces for Breast Cancer Survivors During Functional Tasks.

Breast cancer survivors have known scapular kinematic alterations that may be related to the development of secondary morbidities. A measure of muscle activation would help understand the mechanisms behind potential harmful kinematics. The purpose of this study was to define muscle force strategies in breast cancer survivors. Shoulder muscle forces during 6 functional tasks were predicted for 25 breast cancer survivors (divided by impingement pain) and 25 controls using a modified Shoulder Loading Analysis Module. Maximum forces for each muscle were calculated, and 1-way analysis of variance (P < .05) was used to identify group differences. The differences between maximum predicted forces and maximum electromyography were compared with repeated-measures analysis of variance (P < .05) to evaluate the success of the model predictions. Average differences between force predictions and electromyography ranged from 7.3% to 31.6% but were within the range of previously accepted differences. Impingement related pain in breast cancer survivors is associated with increased force of select shoulder muscles. Both pectoralis major heads, upper trapezius, and supraspinatus peak forces were higher in the pain group across all tasks. These force prediction differences are also associated with potentially harmful kinematic strategies, providing a direction for possible rehabilitation strategies.

Algorithmically detectable directional changes in upper extremity motion indicate substantial myoelectric shoulder muscle fatigue during a repetitive manual task.

Repetitive workplace tasks are associated with fatigue-induced changes to shoulder muscular strategies, potentially altering kinematics and elevating susceptibility to tissue overexposures. Accessible and reliable methods to detect shoulder muscle fatigue in the workplace are therefore valuable. Detectable changes in joint motion may provide a plausible fatigue identification method. In this investigation, the onset of the first kinematic changes, as identified by a symbolic motion representation (SMSR) algorithm, and the onset of substantial surface electromyography (sEMG) mean power frequency (MPF) fatigue were not significantly different, both occurring around 10% of task duration. This highlights the potential utility of SMSR identified directional changes in joint motion during repetitive tasks as a cue of substantial muscle fatigue, enabling ergonomics responses that can mitigate shoulder muscular fatigue accumulation and its associated deleterious physical effects. Practitioner Summary: The onset of substantial muscle fatigue during a repetitive dynamic task was assessed using kinematics and myoelectric-based techniques. Algorithmically detectable directional changes in upper extremity joint motion occurred with the onset of substantial muscle fatigue, highlighting the potential of this as a useful approach for workplace fatigue identification.

An evaluation of off-axis manual forces and upper extremity joint moments during unilateral pushing and pulling exertions.

This study quantified changes in off-axis manual force production and upper extremity joint moments during sub-maximal one-handed push and pull tasks. Off-axis forces in the up/down and left/right directions were quantified in the presence or absence of constraints placed upon the direction of manual force application and/or arm posture. Resultant off-axis forces of 13.1% and 9.4% were produced for pulls and pushes, respectively. Off-axis forces during pulling were oriented downwards and to the right and were associated with a decreased should flexion moment when posture was constrained. Off-axis forces in the up/down direction were minimized with increased on-axis force level. Off-axis forces during pushing tended to be oriented to the left and were associated with increased elbow flexion moment when off-axis forces were allowed. By not accounting for these off-axis forces, we may not be accurately reflecting actionable muscle- and joint-level loading characteristics derived from biomechanically-based proactive ergonomics assessment approaches. Practitioner Summary: Constrained arm postures and directions of manual force application influence the production of off-axis forces. As inaccurate estimation of true manual forces can markedly influence actionable outcomes of proactive ergonomic assessments, this study suggests that simplification of these estimates is insufficient and potentially misleading.

The effect of aging and contextual information on manual asymmetry in tool use.

Healthy aging affects manual asymmetries in simple motor tasks, such as unilateral reaching and aiming. The effects of aging on manual asymmetries in the performance of a complex, naturalistic task are unknown, but are relevant for investigating the praxis system. This study examined how aging influences manual asymmetry in different contexts in a tool manipulation task. Fifty healthy, right-hand-dominant young (N = 29; 21.41 ± 2.87 years), and elderly (N = 21; mean: 74.14 ± 6.64 years) participants performed a 'slicing' gesture in response to a verbal command in two contexts: with (tool) and without the tool (pantomime). For interjoint relationships between shoulder plane of elevation and elbow flexion, a HAND × AGE × CONTEXT interaction existed (F1,43 = 4.746, p = 0.035). In pantomime, interjoint control deviated more in the left (non-dominant) than the right (dominant) limb in the elderly adult group (Wilcoxon, p = 0.010). No such differences existed in the young adult group (Wilcoxon, p = 0.471). Furthermore, contextual information reduced interjoint deviation in young adults when the task was performed with the right (dominant) hand (Wilcoxon, p = 0.001) and in the elderly adults when the task was performed with the left (non-dominant) hand (Wilcoxon, p = 0.012). The presence of the tool did not reduce interjoint deviation for the right hand in the elderly group (Wilcoxon, p = 0.064) or the left hand in the young group (Wilcoxon, p = 0.044). Deviation within trials (i.e., intrasubject deviation) in elbow flexion was higher in the elderly relative to the young adult group (p = 0.003). Finally, resultant peak velocities were smaller (p = 0.002) and cycle duration longer (p < 0.0001) in the elderly adult group. This study provides novel evidence that aging affects manual asymmetries and sensorimotor control in a naturalistic task and warrants that aging research considers the context in which the task is performed.

Shoulder muscular activity in individuals with low back pain and spinal cord injury during seated manual load transfer tasks.

This study aimed to compare the activity of four shoulder muscles in individuals with low back pain (LBP), spinal cord injuries (SCI) and a control group, during one-handed load transfer trials. Nine individuals with minimum one-year of LBP, eleven with thoracic/lumbar SCI and nine healthy controls participated in this study. The activations of anterior deltoid, upper trapezius, infraspinatus and pectoralis major were recorded by surface EMG during one-handed transferring of a cylinder from a home shelve to six spatially distributed target shelves. The integrated EMG values were compared using repeated measure ANOVA. Both LBPs and SCIs had higher anterior deltoid activation and LBPs required more upper trapezius activation than controls (p < 0.05). The spatial position of the targets also significantly influenced demands for these two muscles. The anterior deltoid and upper trapezius in LBP and SCI individuals are under higher demand during occupational load transfer tasks. Practitioner Summary: This study aimed to compare the activation of four shoulder muscles in individuals with low back pain, spinal cord injuries and healthy condition. EMG analysis showed that the injured groups required more upper trapezius and anterior deltoid activation during load transfer tasks, which may predispose them to muscle overexertion.

Scapular Kinematics by Sex Across Elevation Planes.

Scapular kinematics are important indicators of dyskinesis, often suggesting underlying shoulder pathology, but the influence of sex is unknown. This study's objective was to examine scapular kinematics in healthy males and females. Positions of surface-mounted reflective markers were tracked during arm elevation movements in 0°/30°/40°/60°/90°/120° planes. Scapulothoracic rotations (protraction/retraction, medial/lateral rotation, posterior/anterior tilt) were calculated. ANOVA analysis evaluated main and interaction effects of sex, plane, phase, and elevation angle. Males and females had similar protraction/retraction and medial/lateral rotation kinematics; mean sex-related peak angle differences were 2.5°, 1.8° (raise [concentric]), respectively, and 2.9°, 2.7° (lower [eccentric]), respectively. Largest sex differences for mean peak angle occurred for posterior/anterior tilt at higher elevation angles (raise, 8.4°; lower, 8.5°). Elevation, plane, and phase were main effects for all scapular rotations (P < .001). Sex was not a main effect for any rotations. Sex × elevation interactions influenced protraction/retraction (P < .001) and posterior/anterior tilt (P < .001). Sex × plane (P ≤ .01) and sex × phase (P ≤ .002) interactions influenced all rotations. Lower posterior tilt for females compared to males at higher elevation angles could relate to higher female shoulder pathology incidence. Sex, plane, and phase are necessary components of uninjured scapular kinematics. Sex-specific differences provide insight into potential shoulder pathology etiology. These data provide a benchmark to assess pathological populations.

Older females in the workforce - the effects of age on psychophysical estimates of maximum acceptable lifting loads.

The number of older workers in the workforce is increasing substantially, and advanced age is associated with factors that could influence musculoskeletal injury risk and work capacity. This study's goals were to test whether psychophysical estimates of maximum acceptable weight of lift (liftmax) differed between younger and older workers, and to examine potential explanatory factors. Twenty-four female workers (half 50 + years; half 20-32 years) self-adjusted a box's mass to their perceived liftmax during four lifting tasks. Older workers' liftmax values were significantly lower (by approximately 24%) than their younger counterparts. There were no age-related differences in resting heart rate, or peak joint angles and final heart rate during the lifting trials. However, the older group demonstrated lower grip strength (by 24%), and lower heart rate reserve during the trials (by 18%). These results question whether current maximum acceptable lifting weights based on psychophysical information are appropriately protective for female workers greater than 50 years of age. Practitioner Summary: This psychophysical study demonstrated that older female workers (aged 50-63 years) selected maximum acceptable lift masses that were (on average) 24% lower than younger workers (aged 20-32 years), which corresponded with lower grip strength and heart rate reserve. Current maximum acceptable lifting weights based on psychophysical information may not protect female workers greater than 50 years of age.

Grade-IV inferior glenohumeral mobilization does not immediately alter shoulder and scapular muscle activity: a repeated-measures study in asymptomatic individuals.

OBJECTIVES: To assess: (1) the presence of any carry-over effect between interventions; (2) the immediate effects of inferior shoulder mobilization on shoulder and scapular muscle activity; and (3) to compare muscle activity response between the control and mobilization conditions. Repeated measures, cross-over, pre-post intervention study with sample of convenience. METHODS: Twenty-two asymptomatic individuals performed 10 repetitions of shoulder abduction before and after the control and mobilization, with a dosage of three sets of 30-s duration, with grade-IV. The order of intervention was randomized. Surface electromyography was used for recording activity of upper and lower trapezius; anterior, middle and posterior deltoids; supraspinatus; infraspinatus; and serratus anterior. Repeated measures mixed-model analysis of variance was used to assess immediate changes in muscle activity levels following inferior shoulder mobilization. Statistical parametric mapping (SPM) was used for comparing muscle activity waveforms between control and mobilization conditions throughout the range of motion. RESULTS: No systematic changes in muscle activity levels were found between: (1) baseline and follow-up for each condition, at the concentric and eccentric phases of shoulder abduction; (2) control and mobilization conditions during the concentric and eccentric phases of shoulder abduction. SPM results suggested no differences in muscle activity pattern between conditions. CONCLUSIONS: Inferior shoulder mobilization did not produce immediate effects on shoulder and scapular muscle activity. It is possible that the dose used was insufficient to generate an immediate neuromuscular response to the mobilization.

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.

Distribution of bone and tissue morphological properties related to subacromial space geometry in a young, healthy male population.

PURPOSE: Particular bone and tissue morphological features of the scapula and humerus often exist disproportionately in persons with subacromial impingement syndrome (SAIS) and/or rotator cuff pathology. However, the origins of morphological variation, genetic or mechanistic, remain unclear. This research evaluated the distribution of and correlation between several bone and tissue characteristics associated with these pathologies amongst a baseline cohort population consisting of young, healthy, males. As well, the predisposition to SAIS was estimated by calculating the ratio (occupation ratio) of subacromial tissue thickness to minimum subacromial space width (SAS). METHODS: Anterior-posterior and trans-scapular radiographs and musculoskeletal ultrasound were used to measure morphological characteristics related to the subacromial space. Each bone morphological characteristic was classified as healthy or unhealthy based on previous definitions. Supraspinatus tendon and subacromial bursa thicknesses were used to calculate the occupation ratio from both radiographic and ultrasonic measures of the SAS. RESULTS: Each characteristic demonstrated considerable variability, with some participants having 'unhealthy' variants for each bone characteristic examined. The percentage of the population with bone characteristics classified as "unhealthy" ranged from 15 to 55 % across characteristics evaluated. The strongest correlation existed between the acromion index and the minimum subacromial space width (-0.59) suggesting that a larger lateral extension of the acromion may predispose an individual to SAIS. The average occupation ratio was 65.3 % with a 1-99 % confidence interval ranging from 21.6 to 108.9 %. CONCLUSIONS: The distributions of both morphological characteristics and occupation ratios indicate that individuals within this healthy, baseline population have a highly differential predisposition for subacromial tissue compression solely based on inherent morphological variation. This suggests that while mechanistic and/or age-related degenerative changes may contribute to SAIS and eventual rotator cuff pathology, intrinsic predisposing geometry should not be discounted.