AJP - Regulatory, Integrative and Comparative Physiology, Vol 238, Issue 1 119-R137, Copyright © 1980 by American Physiological Society
Probability of peripheral interaction between motor units and implications for motor control
H. N. Demieville and L. D. Partridge
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.
