Muscle Contributions to Upper-Extremity Movement and Work From a Musculoskeletal Model of the Human Shoulder.

Ajay Seth,Scott Delp,Ricardo Matias,Meilin Dong

doi:10.3389/fnbot.2019.00090

Abstract

Musculoskeletal models enable movement scientists to examine muscle function by computing the mechanical work done by muscles during motor tasks. To estimate muscle work accurately requires a model that is physiologically plausible. Previous models of the human shoulder have coupled scapula movement to humeral movement. While coupled movement produces a stereotypical scapulohumeral rhythm, it cannot model shrugging or independent movement of the scapula and humerus. The artificial coupling of humeral elevation to scapular rotation permits muscles that cross the glenohumeral joint, such as the rotator-cuff muscles and deltoids, to do implausible work to elevate and rotate the scapula. In reality, the motion of the scapula is controlled by thoracoscapular muscles, yet the roles of these muscles in shoulder function remains unclear. To elucidate the roles of the thoracoscapular muscles, we developed a shoulder model with an accurate scapulothoracic joint and includes scapular muscles to drive its motion. We used the model to compute the work done by the thoracoscapular muscles during shrugging and arm elevation. We found that the bulk of the work done in upper-extremity tasks is performed by the largest muscles of the shoulder: trapezius, deltoids, pectoralis major, and serratus-anterior. Trapezius and serratus anterior prove to be important synergists in performing upward-rotation of the scapula. We show that the large thoracoscapular muscles do more work than glenohumeral muscles during arm-elevation tasks. The model, experimental data and simulation results are freely available on SimTK.org to enable anyone to explore our results and to perform further studies in OpenSim 4.0.

Highlights

Abnormal scapular movement is indicative of shoulder dysfunction, such as subacromial impingement, rotator-cuff tears, and other injuries (Struyf et al, 2011)
Muscle activations from muscle-driven simulations of the shoulder model were compared to the EMG for the same tasks, which yielded an average mean-absolute error (MAE) of 0.06, with the vast majority of measured muscles below 0.1 (Table 3)
Our second aim was to answer what motions of the scapula are controlled by large thoracoscapular muscles such as trapezius and serratus anterior during these shoulder tasks? We addressed this question by analyzing which thoracoscapular muscles perform work on the scapula during shoulder tasks

Summary

Introduction

Abnormal scapular movement is indicative of shoulder dysfunction, such as subacromial impingement, rotator-cuff tears, and other injuries (Struyf et al, 2011). Models designed to understand glenohumeral injury and rehabilitation (Garner and Pandy, 2001; Holzbaur et al, 2005; Dickerson et al, 2007; Chadwick et al, 2009; Bolsterlee et al, 2013; Saul et al, 2015) ignore muscle actions of the largest thoracoscapular muscles: trapezius, rhomboids, and serratus-anterior (Rockwood, 2009). These muscles likely play important roles in human upper-extremity movements given their size and force-generation capacity. We can only assume that coupling scapular kinematics to humeral rotation yields the perplexing results that the rotator-cuff muscles generate the largest forces during the recovery phase of wheel-chair propulsion, while the larger superior trapezius, rhomboids, anterior deltoid, and pectoralis major muscles produced virtually no force throughout the movement (Odle et al, 2019)

Methods

Results

Discussion

Conclusion