A Simple Model for Fast Planar Arm Movements; Optimising Mechanical Activation and Moment-arms of Uniarticular and Biarticular Arm Muscles

In this study a simple model of a two segmented arm with only two muscles is used to study an underarm throw and an overarm stroke. There is a uniarticular shoulder muscle and an elbow muscle with facultatively an additional moment arm on the shoulder. The effect of changing the shoulder moment of the latter muscle, and of changing its mechanical delay is measured. The effect of tendons is assessed, and of throwing or striking using heavy weights. Some of the trials are analysed in detail to answer the question why some results are better than others. With uniarticular muscles we always found a proximal-distal sequence in muscle activity, but with a biarticular muscle sometimes this sequence can be absent. Tendons and the weight that is held do influence the optimal delay, but not the optimal moment are. The model predicts a different optimal technique for a stroke with a light or a heavy tool.