Musculoskeletal modelling in the evaluation of shoulder muscle and joint function

Abstract

Evaluation of the muscle forces generated during shoulder motion is critical in our understanding of shoulder disease and injury prevention, prosthesis design and rehabilitation. Since non-invasive muscle force measurement strategies are not readily available, computational simulations are widely adopted. This study presented a novel subject-specific experimental testing and modelling framework to evaluate shoulder muscle and joint function. Currently, upper limb muscle force estimations using Hill-type muscle models depend on musculotendon parameter values, which cannot be easily measured non-invasively. Generic and scaled-generic parameters may be quickly and easily employed, but these approaches do not account for an individual subject's joint torque capacity. The first aim of this thesis was therefore to assess the capacity of generic and scaled-generic musculotendon parameters to predict muscle and joint function using the subject-specific modelling framework. Models employing subject-specific, scaled-generic and generic musculotendon parameters were used to calculate shoulder muscle and joint loading in healthy subjects during activities of daily living. Muscle and joint forces calculated using generic and scaled-generic models were significantly different to those of subject-specific models (p < 0.05), and task dependent; however, scaled-generic model calculations of shoulder glenohumeral joint force demonstrated better agreement with those of subject-specific models during abduction and flexion. The findings suggest that generic and scaled-generic musculotendon parameters may not provide sufficient accuracy in prediction of shoulder muscle and joint loading when compared to models that employ subject-specific parameter-estimation approaches.

Kinematics of the shoulder girdle obtained from non-invasive measurement systems such as video motion analysis, accelerometers and magnetic tracking sensors has been shown to be adversely affected by instrumentation measurement errors and skin motion artefact, yet the degree to which musculoskeletal model calculations of shoulder muscle and joint loading are influenced by variations in joint kinematics is not well understood. The second aim of this thesis was to use a subject-specific modelling framework to evaluate the sensitivity of shoulder muscle and joint force calculations to changes in bone kinematics. Monte-Carlo analyses were performed by randomly perturbing scapular and humeral joint coordinates during abduction and flexion, and the effect on muscle and joint force calculations quantified. The findings suggest that musculoskeletal model sensitivity to changes in kinematics is task-specific, and varies depending on the plane of motion. Calculations of shoulder muscle and joint function depend on accurate humeral and scapular motion data, particularly that of humeral elevation and scapula medial-lateral rotation.

Robotic exoskeletons enable frequent repetitive movements without the presence of a full-time therapist; however, the way in which exoskeletons interact with the upper limb to modulate muscle and joint function is not well understood. The third and final aim of this thesis was to quantify the use of a commercially available robotic assistive exoskeleton in modulating shoulder muscle and joint force using the musculoskeletal models. Healthy subjects were asked to perform activities of daily living under three conditions: free motion (no exoskeleton), motion using an exoskeleton with upper limb weight compensation (weightlessness), and motion using the exoskeleton with negligible upper limb weight compensation. Muscle EMG, joint kinematics, and joint torques were simultaneously recorded, and shoulder muscle and joint forces calculated using subject-specific musculoskeletal modelling. The robotic exoskeleton reduced the peak joint torque, muscle forces and joint loading, with the degree of load attenuation strongly task dependent. Exoskeleton assistance significantly reduced the muscle force and EMG of the prime movers during upper limb motion, particularly those of the deltoid sub-regions, while the axiohumeral muscles were less affected. This thesis provides a subject-specific experimental testing and modelling framework for evaluating shoulder muscle and joint function, and uses this framework to provide insights into how an assistive exoskeleton influences upper limb muscle and joint behaviour, as well as the sensitivity of the model calculations to changes in joint kinematics. The outcomes of this thesis will be useful for future development of upper limb musculoskeletal models and their applications in rehabilitation, physiotherapy, surgical planning and sports biomechanics.