LOAD-SHARING PATTERNS IN THE SHOULDER DURING ISOMETRIC FLEXION TASKS

Patterns of load-sharing between the shoulder muscles during isometric flexion tasks were studied by using both a biomechanical shoulder mod el and electromyographic (EMG) recordings of ten subjects. The effect of changes in several model parameters and shoulder stiffness constrai nts on the predicted load-sharing patterns were studied, while the arm position, hand load and precision requirements of the tasks were vari ed. The results calculated using the model were, when compared to the EMG recordings, plausible predicting a high level of synergistic contr action of muscles of the shoulder muscles during flexion tasks. The tr ends of the model-predicted muscle forces corresponded well to the EMG recordings. At low hand load levels the increasing of the shoulder st iffness strongly increased the muscle force levels, thus increasing al so the level of synergistic contraction of muscles. At higher load lev els the increase in the muscle forces was not so high, because the mod el predicted a high level of simultaneous contraction of muscles alrea dy at a low level of shoulder stiffness. Cluster analysis of the EMG r ecordings revealed large inter-individual differences in the load dist ribution patterns during flexion tasks. The constraint angle of the gl enohumeral joint contact force direction was found to be an important model parameter affecting both the predicted forces and the maximum Fo rce production ability of the shoulder.