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1. The interphalangeal joint of the thumb was driven through sinusoidal flexion-extension movements while electromyograms were recorded from over the flexor pollicis longus muscle. 2. When the subject relaxed his thumb the movement generated no detectable e.m.g. response. When, however, he exerted a voluntary flexing force electrical activity could be recorded from the flexor pollicis longus, the amplitude of which was modulated at the frequency of the movement. 3. As the driving frequency was increased, the maximal e.m.g. activity occurred progressively later in the cycle of movement; for frequencies above about 6 Hz the timing of the averaged e.m.g. was compatible with a reflex delay of 55-65 msec. 4. The frequency-phase plot was not, however, the perfect straight line that would arise from a simple and constant reflex delay. There were some consistent departures from linearity and some random variations. In either case, the timing of the e.m.g. and the timing of the reflex force (Brown, Rack & Ross, 1982a) changed together in ways that increased confidence in each of the measurements. 5. The amplitude of the e.m.g. signal was more deeply modulated by movements at 8-14 Hz than by higher or lower frequencies, and it was concluded that the stretch reflex responds particularly readily to signals in that frequency range.