The Significance of Intramuscular Receptors in Load Compensation During Voluntary Contractions in Man

Abstract

Muscle spindle afferent discharge was analysed in single unit recordings from the median nerve of awake human subjects. Impulse frequency from spindle in the antebrachial finger flexor muscles was related to metacarpo-phalangeal joint angle when the muscles were relaxed, and to active torque at the same joint during isometric voluntary contractions. The steady state impulse frequency was varied linearly with joint angle within the range of study. The average position sensitivity, defined as impulses per second per degree joint movement (ips/deg), was 0.18 ips/deg for the spindle primary endings and 0.14 ips/deg for the secondaries. It was estimated that the maximal position sensitivity during muscle contraction, would be 0.9 ips/deg for the primaries and 0.7 ips/deg for the secondaries. In isometric voluntary contractions the spindle discharge increased, due to the fusimotor outflow running parallel with the skeletomotor outflow. The relation was found to be approximately linear, and the average slope was 32.8 ips/Nm (metre-Newton) for the primaries and 22.8 ips/Nm for the secondaries. From these data the muscle stiffness accounted for by the intramuscular receptors may be deduced, if some additional assumptions are adopted, the most crucial one being that the central effects of the muscular afferents are the same in isometric voluntary contractions and in load carrying contractions.