RELATIVE ACTIVITY OF WRIST MOVING MUSCLES IN STATIC SUPPORT OF THE WRIST JOINT: AN ELECTROMYOGRAPHIC STUDY

Abstract

Electromyograms from each of 10 regions of the forearm muscle mass were made with surface electrodes while the hand, held stable and horizontal, supported known loads. Potentials from each of the 10 regions reflected more or less closely the activity of specific forearm muscles underlying the electrodes. A statistical measure of range applied to records of potentials showed increasing values with increased loading of the hand, the latter factor was expressed as multiples of the hand torque. Since loads were counterpoised by a summation of muscle tensions, potentials over a specific muscle reflected more or less the participation of this muscle in the total tension pattern. When the forearm and hand were rotated so that gravity acted at different angles to the plane of action of a given test muscle, potentials from the region overlying the muscle varied with the angular position. Typically, potentials were maximal when a muscle underlying the electrodes was in the direct agonist position; then the activity of the muscle was expended in directly counteracting gravity. The tension of a muscle (as reflected in potentials) gradually decreased to about half maximum as the agonist function gave way to lateral stabilizing; in this position, the test muscle acted in the same horizontal plane as the wrist pivot. With further hand and forearm rotation, the test muscle became an antagonist, working with gravity to stabilize the carpus, and activity, as measured by potentials, was a third to a quarter of the maximum agonist value for a given loading.