Increased resting discharge of human spindle afferents following voluntary contractions.

1. The aim of this study was to assess the incidence of lasting alterations in discharge rate of muscle spindle afferents innervating human ankle and toe dorsiflexor muscles following isometric contractions. 2. The subjects performed controlled isometric ankle dorsiflexions maintained for approximately 5 s. During the contraction the discharge of all but one spindle afferent increased above the precontraction level. After complete relaxation, there was prolonged enhancement of the discharge rate of nineteen of fifty-five muscle spindle afferents and none of three Golgi tendon organ afferents. Ten of the nineteen spindle afferents had been silent prior to the contraction. For the population of fifty-five spindle afferents, the mean 'postcontraction' discharge rate was 65% higher than the mean precontraction discharge rate, with the mean rate increasing from 2.3 to 3.9 Hz (P < 0.001). The mean duration of the enhanced postcontraction discharge was 52 s (range, 8-240 s). 3. Stretch applied to the tendon of the receptor-bearing muscle in twelve of fourteen spindle afferents with an enhanced postcontraction discharge rate eliminated or reduced the enhanced discharge rate. 4. The high incidence of an enhanced spindle discharge after voluntary contraction (35% of spindle afferents) suggests that muscle 'history' should be taken into account when interpreting changes in spindle discharge rates. The enhanced discharge rates following contraction probably reflect a long-lasting effect of the contraction-associated increase in fusimotor drive on intrafusal stiffness, rather than the persistence of fusimotor drive following relaxation.