Influence of voluntary intent on the human long-latency stretch reflex

Abstract

In man, sudden stretch of an actively contracting muscle evokes a classical monosynaptic spinal reflex followed by an automatic ‘long-latency’ response, both evident as bursts of activity in the electromyogram (EMG)¹. This long-latency response, extensively investigated in the human long thumb flexor, but present in many other muscles², occurs too early to be voluntary and much indirect evidence suggests it represents the operation of a ‘long-loop’, perhaps transcortical, stretch reflex mechanism^3,4. Some investigators have found that the long-latency responses may be modified substantially by the subject's intent, possibly by pre-setting of excitability levels within the central nervous system thereby influencing the ‘long-loop’ mechanism^5–9. However, others have observed little effect of voluntary set on the automatic long-latency stretch response^10,11. It is notable that these studies, which have yielded conflicting results, have involved different muscles in a variety of experimental conditions. In some studies a warning signal preceded muscular stretch, while in others control trials were interspersed with stretches. To resolve the dilemma we have examined the influence of these variables, and found that the automatic long-latency response can be modified most strongly in conditions when the voluntary reaction time is shortest, for example, when the timing of the stimulus can be predicted accurately. We suggest that modification of a long-loop stretch response is not due to some central pre-setting process, but merely represents interaction between a reflex of long latency and a subsequent very rapid voluntary event, occurring early because of predictability of the stimulus.