The isometric action of the forearm muscles.

The isometric forces and moments generated at the hand segment by thirteen of the forearm muscles were measured on three test subjects. Small bipolar surface electrodes were used to activate each muscle separately. The direction of the force vector generated at the fingertips and the percentage torque generated about the longitudinal axis of the hand segment were measured and were modeled for one subject as a parabolic function of the position of the wrist and radioulnar joints in their ranges of motion. The results exhibited a high standard deviation. It is proposed that a significant component of this standard deviation is inherent in the biomechanics of the musculo-skeletal system. Variations in the test protocol were used to test this hypothesis. The standard deviation was found to be significantly larger in the flexor muscles than the extensors. It was concluded that the stability and elasticity of the soft tissue structures in the region of the wrist joint may significantly influence the direction of the generated joint moment vector.