Quasi-static analysis of muscle forces in the shoulder mechanism during wheelchair propulsion

During wheelchair propulsion the largest net joint moments and net joint powers are generated around the shoulder. The analysis of the contribution of arm- and shoulder muscles to the joint moments could explain the low efficiency of wheelchair propulsion. Basically, it is assumed that a large magnitude of muscle activity will be needed to stabilize the shoulder. In addition, the muscular requirements for the minimization of negative power are assumed to be of importance. For such an analysis an inverse dynamic model is required. To utilize an inverse dynamic model of the shoulder mechanism, the trajectories of the upper extremity bones are needed. Since at this stage, dynamic non-invasive measurement techniques of scapular motion are not available, the aim of this study was to record the three-dimensional position of the scapula in static situations with the help of a palpation technique. Positions of the trunk, shoulder girdle and upper extremity, and the surface EMG of ten muscles were recorded simultaneously with forces on the rim on a stationary wheelchair ergometer. Four healthy male subjects participated in the experiment. Five hand positions on the rim and five different load levels per hand position were measured for each subject. A previously developed musculoskeletal model of the shoulder mechanism (Van der Helm, 1994a, J. Biomechanics 27(5) 551-569) was used to calculate muscle forces in an inverse static simulation. The measured EMG and the calculated muscle forces compared well except for three muscles. The moment balance between external sources and muscles around each joint axis of the shoulder mechanism is discussed. Results of the experiment indicate that large muscle contributions are needed for joint stabilization. The experimental results on the scapular motions will, in combination with experimental data collected under dynamic conditions, be used for application of the model to dynamic situations. It is concluded that the musculoskeletal model of the shoulder mechanism can be very useful in studies to determine the contribution of muscles and the mechanical load on morphological structures.