Responses to load disturbances in human shoulder muscles: The hypothesis that one component is a pulse test information signal

Summary

Human motor control has been investigated by applying displacements acting to rotate the shoulder while the subject was endeavouring to maintain a constant position against a pre-existing force delivered by a system of finite stiffness. Four separate stages of the force response were distinguished. First, for the initial 100 msec, an increase in force which was attributed to the visco-elastic properties of activated muscle. Second, after approximately 100 msec, a “medium latency” increase in force accompanied by an increase in EMG activity. Third, a “long latency” increase in force consistent with voluntary action restored the arm to its original position. Fourth, some 500 msec thereafter and dependent upon the final steady force level, a tremor might develop. No changes of force were seen that were of sufficiently short latency to be attributed either to Ia mono-synaptic action or immediately following (within 15 msec) polysynaptic action of muscle mechano-receptors. Even the “medium latency” response was too weak to make an appreciable contribution to restoring the arm to its original position; at the best it provided only 15% of the force required. This response is often considered as a “stretch reflex” responsible for maintaining posture in its own right; for example, by means of a servo-assisted transcortical loop (Marsden et al., 1972). Instead, it is now suggested that it might be a test signal designed to inform the central nervous system of the current loading on the muscle and thus permit the CNS to select an appropriate pre-programmed response from its repertoire of motor actions.