Servo action in the human thumb.

Abstract

The servo-like properties of muscle in healthy human subjects were studied by interfering unexpectedly with flexion movements of the top joint of the thumb. This movement is carried out by the flexor pollicis longus muscle only. The movements were standardized in rate by giving the subject a tracking task. They started off against a constant torque load offered by an electric motor. In some movements the load remained constant, but in others, in mid-course, perturbations were introduced at random. Either the movement was halted, or released and allowed to accelerate by reducing the load, or reversed by suddenly increasing the current in the motor, so stretching the muscle. Usually 8 or 16 responses to each kind of perturbation and a similar number of controls against a constant load were averaged. Muscle activity was recorded as the electromyogram from surface electrodes over the belly of the long flexor in the lower forearm. Action potentials were usually full-wave rectified and integrated. About 50 ms after a perturbation the muscle''s activity altered in such a sense as to tend to compensate for the perturbation, i.e., it increased after a halt or a stretch and decreased after a release. The latency was similar in each case. These responses are interpreted as manifestations of automatic servo action based on the stretch reflex. They are considered to be too early to be voluntary. This interpretation was supported by measuring voluntary reaction times to perturbations under tracking conditions. They were 90 ms or longer. When the initial load was increased by a factor of 10, the servo responses were all scaled up likewise. Thus to a 1st approximation the gain of the servo was proportional to initial load. It follows that in relaxed muscle the gain should be zero. This was confirmed by showing that stretching a relaxed muscle gave no reflex, or only a small one. Gain appeared to be determined by the level of muscle activation as determined by the effort made by the subject, rather than by the actual pressure exerted by the thumb. Thus in fatigued muscle gain was boosted as the muscle had to be activated more strongly to keep up the same force output. The net effect was to compensate for fatigue and maintain the performance of the servo. The implications of gain control in the servo were discussed. If the gain of the stretch reflex arc is zero in relaxed muscle, contractions cannot be initiated via the stretch reflex by simply causing the spindles to contract, as proposed on the original follow-up servo theory.