Reflex gain of muscle spindle pathways during fatigue.

There are conflicting observations of the effects of fatigue on the sensitivity of large diameter Ia afferents. Our goal was to characterize any fatigue-related changes in the spinal reflex pathways during fatigue. Manipulation of the Ia afferent response by vibration and tendon tap, in which the motor neuron pool is modulated by both short- and long-loop activation from muscle spindles, were elicited before and after a fatigue task. The fatigue task consisted of intermittent submaximal and maximal voluntary contractions (MVCs). Percent voluntary activation fell from 98.75% MVC to 80.92% MVC following the fatigue task as measured by the twitch interpolation technique. Voluntary contractions of the same force profile as the force produced by 30 s of vibration were produced by having participants (n = 10) follow the trajectory on a computer monitor, before and after the fatigue task. Recruitment thresholds (RTs) of voluntarily activated units showed no change during fatigue; however, units activated via the reflex pathway were recruited approximately 30% sooner during fatigue (P < 0.05). The ratio of the electrical-to-mechanical response of the tendon tap increased significantly with fatigue. Our findings of decreased RTs in response to vibration and increased EMG activity during the tendon tap following the fatigue task indicate that Ia afferent input to the motoneuron pool was increased. The decrease in MVC force indicates that during this time the descending drive was compromised. These results provide evidence that the gain of the gamma loop is increased during fatigue, indicating possible peripheral neural compensation to the motor neuron pool in order to preserve force output.