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Department of Physiological Sciences, University of Manchester.

1. The electromyographic (EMG) reflexes evoked in the wrist flexor muscle, flexor carpi radialis (FCR), by percutaneous extension of its tendon and by forcible extension of the wrist joint have been studied. Reflexes were elicited during steadily maintained voluntary flexor contraction of 10% of each subject's maximum. 2. Tendon extension, using 'ramp and hold' displacements, evoked fairly prolonged (ca 50 ms) increases in EMG activity. These responses were usually subdivided into two main excitatory peaks of respectively short (SL, ca 20 ms) and long (LL, ca 45 ms) latency. This pattern contrasted with that observed following brief tendon taps when only a single, SL peak was elicited. 3. 'Stretch' reflexes evoked by 'ramp and hold' wrist extensions, as has been noted by numerous earlier investigators, were also protracted and comprised two main excitatory components. These responses resembled those produced by tendon extension both in their general form and in their behaviour upon altering the velocity of mechanical stimuli. Quantitatively, however, two main differences were evident. The reflexes evoked by wrist extension, including their SL and LL peaks, were generally somewhat larger. Additionally, when parameters of the two modes of stimulation were adjusted to elicit SL responses of equivalent amplitude, the LL responses elicited by tendon extension were regularly smaller and of shorter duration than those elicited by wrist extension. 4. Termination of the two forms of mechanical stimulation, by releasing tendon or wrist extension, each elicited a SL reduction in EMG activity. Such troughs were more pronounced and more consistently observed upon release of wrist extension. 5. Neither local anaesthesia of the skin overlying the flexor tendons at the wrist nor ischaemia of the hand and lower forearm produced any systematic modification of reflex response patterns. 6. It is concluded that intramuscular receptors (presumably muscle spindles) in FCR mediate both the SL and LL reflexes evoked in this muscle by extension of its tendon. Intramuscular receptors also seem certain to be very largely responsible for the EMG responses generated in this muscle by wrist extension.