Muscle activation changes associated with loads at adjacent joints - or - why muscle moment arms are not sufficient to predict functional role

An examination is made of the activation of muscles that contribute to elbow flexion/extension and forearm supination/pronation during various static postures. Intramuscular and surface EMGs for five muscles and force in three dimensions have been simultaneously recorded while subjects produce proscribed steady-state forces at the forearm. Tests were done with subjects at three different angles of forearm supination/pronation. The activation levels of some muscles were found to be significantly modulated during the generation of torques at joints which the muscles could not possibly contribute. For example, during isometric elbow extension with superimposed pro-supination loads of varying magnitudes, EMGs from the triceps brachii were observed to increase with supination forearm loads compared to EMGs at neutral forearm loads.