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.

The validation of a low-cost inertial measurement unit system to quantify simple and complex upper-limb joint angles.

The purpose of this research was to validate the use of a low-cost IMU system to measure upper-limb joint angles by comparing it to passive optical motion capture measures. Fifteen participants (five females; 25.9 ± 4.7 years) completed one trial of four simple range of motion (ROM) movements (elbow flexion/extension, shoulder abduction/adduction, shoulder flexion/extension, and shoulder internal/external rotation), and three complex functional daily tasks [hand to: back pocket (HBP), contralateral shoulder (HCS), head (HTH)]. Movements were measured, simultaneously, using fourteen OptiTrack cameras and five Notch® IMUs. The mean joint angle difference between devices ranged from 0.10° ± 3.11° for the HBP shoulder internal/external movement to 44.95° ± 3.50° for the simple ROM shoulder internal/external rotation movement. Nine of sixteen movement and plane comparisons showed significant differences between the device-specific movement cycle waveforms. Eleven of the comparisons showed either fixed and/or proportional biases (fixed only: 9; proportional only: 1; both fixed and proportional: 1). Due to multiple movements having large amplitude errors, low waveform similarities, and/or statistically significant mean differences between the Notch® IMUs and the gold-standard motion capture devices, we cannot recommend that Notch® IMUs are valid devices for measuring upper-limb joint angles during simple ROM and complex functional daily tasks.

The role of at home workstation ergonomics and gender on musculoskeletal pain.

BACKGROUND: The recent mandate for university faculty and staff to work-from-home (WFH) during the COVID-19 pandemic has forced employees to work with sub-optimal ergonomic workstations that may change their musculoskeletal discomfort and pain. As women report more work-related musculoskeletal discomfort (WMSD), this effect may be exacerbated in women. OBJECTIVE: The purpose of this study was to describe university employee at-home office workstations, and explore if at-home workstation design mediates the effect of gender on musculoskeletal pain. METHODS: University employees completed a survey that focused on the WFH environment, at home workstation design and musculoskeletal pain. Descriptive statistics and regression analysis were used to analyze the responses. RESULTS: 61% of respondents reported an increase in musculoskeletal pain, with the neck, shoulders and lower back being reported most frequently. Women reported significantly greater musculoskeletal pain, but this relationship was significantly mediated by poor ergonomic design of the home workstation. Improper seat-height and monitor distance were statistically associated with total-body WMSD. CONCLUSIONS: WFH has worsened employee musculoskeletal health and the ergonomic gap between women and men in the workspace has persisted in the WFH environment, with seat height and monitor distance being identified as significant predictors of discomfort/pain.

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.

Kinematic and EMG analysis of horizontal bimanual climbing in humans.

Climbing is an increasingly popular recreational and competitive behavior, engaged in a variety of environments and styles. However, injury rates are high in climbing populations, especially in the upper extremity and shoulder. Despite likely arising from an arboreal, climbing ancestor and being closely related to primates that are highly proficient climbers, the modern human shoulder has devolved a capacity for climbing. Limited biomechanical research exists on manual climbing performance. This study assessed kinematic and muscular demands during a bimanual climbing task that mimicked previous work on climbing primates. Thirty participants were recruited - 15 experienced and 15 inexperienced climbers. Motion capture and electromyography (EMG) measured elbow, thoracohumeral and trunk angles, and activity of twelve shoulder muscles, respectively, of the right-side while participants traversed across a horizontal climbing apparatus. Statistical parametric mapping was used to detect differences between groups in kinematics and muscle activity. Experienced climbers presented different joint motions that more closely mimicked the kinematics of climbing primates, including more elbow flexion (p = 0.0045) and internal rotation (p = 0.021), and less thoracohumeral elevation (p = 0.046). Similarly, like climbing primates, experienced climbers generally activated the shoulder musculature at a lower percentage of maximum, particularly during the exchange from support to swing and swing to support phase. However, high muscle activity was recorded in all muscles in both participant groups. Climbing experience coincided with a positive training effect, but not enough to overcome the high muscular workload of bimanual climbing. Owing to the evolved primary usage of the upper extremity for low-force, below shoulder-height tasks, bimanual climbing may induce high risk of fatigue-related musculoskeletal disorders.

Three-dimensional comparison of static and dynamic scapular motion tracking techniques.

The shoulder is complex and comprised of many moving parts. Accurately measuring shoulder rhythm is difficult. To classify shoulder rhythm and identify pathological movement, static measures have been the preferred method. However, dynamic measures are also used and can be less burdensome to obtain. The purpose of this paper was to determine how closely dynamic measures represent static measures using the same acromion marker cluster scapular tracking technique. Five shoulder angles were assessed for 24 participants using dynamic and static tracking techniques during humeral elevation in three planes (frontal, scapular, sagittal). ANOVAs were used to identify where significant differences existed for the factors of plane, elevation angle, and tracking technique (static, dynamic raising, dynamic lowering). All factors were significantly different for all shoulder angles (p<0.001), except for elevation plane in scapulothoracic protraction/retraction (p=0.955). Tracking techniques were influential (p<0.001), but the grouped mean differences fell below a clinically relevant 5° benchmark. There was large variation in mean differences of the techniques across individuals. While population averages are similar, individual static and dynamic shoulder assessments may be different. Caution should be taken when dynamic shoulder assessments are performed on individuals, as they may not reflect those obtained in static scapular motion tracking.