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Department of Physiology, St Thomas's Hospital Medical School, London.

1. Frequency-modulated vibration was applied to the elbow flexor and extensor tendons to produce reflex movements of the forearm in normal subjects. The modulating (command) signal caused equal and opposite deviations from the 40 Hz carrier frequency so that when flexor vibratory frequency increased, extensor frequency decreased, and vice versa. 2. It is argued that the movements resulted largely from the reflex action of muscle spindle primary afferents whose firing frequency had been 'taken over' and modulated by the vibration. 3. Bode plots relating forearm movements to command signal (modulating) frequency showed the transfer function of the Ia afferent-CNS-muscle-load system to have a low-pass filter characteristic. The phase lag of movement on command increased progressively with command signal frequency, exceeding 180 deg at 3-4 Hz. 4. The transduced forearm movements were fed back to provide the command signal to the vibrators (and thus indirectly to the spindle afferents) via a filter mimicking the dynamic responsiveness of muscle spindle primary endings. Our aim was to 'break into' the reflex arc, and re-route it so that we could artificially vary the gain without significantly altering the dynamics of the pathway. 5. Nearly all subjects developed forearm oscillations (tremor) when the gain exceeded a threshold value. Subjects varied widely in their threshold, though for a given subject the threshold remained fairly constant from day to day. The results suggest that reflexly active individuals may not have a large safety margin with respect to forearm instability. 6. The frequency range of the oscillations observed in seven subjects was 3-8 Hz. The frequencies depended upon the level of flexor-extensor co-contraction, and increased from 3 to 5 Hz at 10% co-contraction to 5-8 Hz at 100% co-contraction. An analysis of the mechanical impedance of the arm provided estimates of tremor frequencies consistent with these results. 7. These unexpectedly low tremor frequencies led us to propose that it may be erroneous to expect stretch reflexes to contribute to forearm tremor in the range 8-12 Hz (e.g. physiological and 'enhanced' physiological tremors). Rather, their contributions should be sought in the range 3-8 Hz (e.g. pathological tremors such as those of Parkinson's and cerebellar disease).

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