Intrinsic and afferent components in apparent muscle stiffness in man.

The subject was asked to bring his lower arm to a prescribed position against an applied torque. He was instructed not to intervene when the torque was unexpectedly changed and this resulted in a movement to a new position. During these experiments motor unit activity of m. biceps or triceps brachii was recorded. It was found that changes in the torque are compensated by reflex-induced recruitment of motor units and variations in their firing frequency. Recordings of motor unit activity revealed that in the range investigated the firing frequency of the motoneurones varied approximately linearly with the applied torque. From the results, it is concluded that afferent input plays a more important role in position control than intrinsic muscle properties. It is argued that the afferent feedback is controlled by central activation of the gamma-motoneurones.