Probability of peripheral interaction between motor units and implications for motor control

Abstract

Potential and importance of mechanical interactions between motor units are examined. Studies were conducted on simple physical models of systems of motor units assembled from separate muscles and driven with electrical stimulus. Two separate muscles were connected to move a common load to represent mechanically coupled motor units while avoiding other interactions present between natural units. Force, velocity, length, power, and work outputs of one unit were measured with and without stimulus to the other unit. Excitation of one unit modified all response measures in the other. The basis for these interactions appears equally applicable to real motor units. Consequently, unqualified use of such terms, which imply independence, as quantal, summation, and average unit response is not acceptable without qualification when referring to activity of motor units. It is argued that the effects of force-velicty and length-tension relationships will cause appreciable mechanical interaction between motor units. Therefore, central nervous system strategies for organization of motor control cannot depend on unchanging response of individual units, and the principle of superposition should not be assumed in analyses of motor activities. The nature of the interactions suggests that the total effect of a unit response may include a "negative force" phase, and also energy exchanges can be expected between motor units in some configurations.