The effect of elbow angle and external moment on load sharing of elbow muscles.

To study elbow muscle load sharing we investigated the effect of external flexion-extension (FE) and pronation-supination (PS) moments and elbow angle on muscle activation and oxygen consumption (V O(2)). Two data sets were obtained. First, (n=6) electromyography (EMG) of elbow flexors (long and short heads of biceps brachii, brachioradialis, brachialis) and extensors (long and short heads of triceps brachii and anconeus) was recorded in all combinations of FE and PS moments at three force levels and four elbow angles (50 degrees , 70 degrees , 90 degrees and 110 degrees ). Second, (n=4) EMG and V O(2) of three muscles (both heads of biceps and lateral head of triceps) were measured simultaneously during a subset of the above conditions. Joint angle and therefore both moment arm and muscle length influenced both EMG amplitude and the load sharing between muscles. The principles behind load sharing, however, were difficult to quantify, since it was impossible to distinguish between all individual aspects that affect muscle activity. We found a linear relationship between EMG and V O(2), while joint angle had no major effect. Although in general subjects showed comparable muscle activation patterns, there were also considerable inter-individual differences, which might be explained by the use of different optimisation strategies or differences in morphology.