QUASI-STATIC ANALYSIS OF MUSCLE FORCES IN THE SHOULDER MECHANISM DURING WHEELCHAIR PROPULSION

During wheelchair propulsion the largest net joint moments and net joi nt powers are generated around the shoulder. The analysis of the contr ibution of arm- and shoulder muscles to the joint moments could explai n the low efficiency of wheelchair propulsion. Basically, it is assume d that a large magnitude of muscle activity will be needed to stabiliz e the shoulder. In addition, the muscular requirements for the minimiz ation of negative power are assumed to be of importance. For such an a nalysis an inverse dynamic model is required. To utilize an inverse dy namic model of the shoulder mechanism, the trajectories of the upper e xtremity bones are needed. Since at this stage, dynamic non-invasive m easurement techniques of scapular motion are not available, the aim of this study was to record the three-dimensional position of the scapul a in static situations with the help of a palpation technique. Positio ns 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 part icipated in the experiment. Five hand positions on the rim and five di fferent 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 calc ulate muscle forces in an inverse static simulation. The measured EMG and the calculated muscle forces compared well except for three muscle s. The moment balance between external sources and muscles around each joint axis of the shoulder mechanism is discussed. Results of the exp eriment 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 conditio ns, 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 an d the mechanical load on morphological structures.