The tremor in fatigue.

After a maximal voluntary effort made for about 2 min, tremor of the muscles concerned is increased in amplitude by up to one order of magnitude for a period of several hours afterwards. Spectral analysis reveals that all frequencies of tremor show this increase. Maximal electrical stimulation of the motor nerve to the muscle does not result in any change in tremor so it is inferred that the increase is due to the operation of spinal or supra-spinal mechanisms. A sub-maximal voluntary effort, maintained for about 1 h, leads to the development of large amplitude, low frequency tremor (4-6 Hz) in addition to increased physiological tremor (8-12 Hz). It is generally accepted that physiological tremor originates as an oscillation in the reflex arc servo loop. The 4-6 Hz slow tremor of this type of fatigue also appears to arise as servo loop oscillation, the feedback delays being longer than in physiological tremor. The spectrum of the slow tremor resembles that found in Parkinsonism and it may be useful model for the tremor of that condition.