Normal voluntary muscular activity in man and other mammals has for a long time been known to have a superimposed rhythmical tendency, with a predominant frequency at nine or ten per second (Horsley & Schaifer, 1886; Schaifer, 1886).The electrical activity of different motor units is in most circumstances almost asynchronous, the resulting movement of the whole muscle being smoothed apart from the superimposed ripple. Under certain conditions, however, the motor unit potentials reborded from part or the whole of an active muscle may show a periodicity varying from about eight to fourteen times a second. Bigland & Lippold (1954a, b) described this phenomenon in the human calf muscles and the small muscles of the hand, but it has not yet been conclusively shown that these modulations in frequency of motor unit activity correspond with the mechanical oscillations. This paper describes experiments upon certain muscles which, during normal activity, may display both the electrical and the mechanical rhythmicity.It has been found that the frequency of these action potential bursts progressively decreases on cooling the muscle. It will be argued that this and other evidence supports the hypothesis that the rhythm of this electrical and mechanical oscillation is determined by the delay in the stretch reflex servoloop, rather than by purely intraspinal factors. METHODSThe electrical and mechanical recording techniques have already been described (Bigland & Lippold, 1954a, b). Action potentials were recorded from the calf in a bin which could be filled with water to above knee level at any desired temperature (Fig. 1). The waterproof electrodes were of 36 S.W.G. enamelled copper wire, introduced into the muscles in hypodermic needles which were subsequently withdrawn. In connexion with these, stringent safety precautions were taken

Voluntary muscle in most human subjects shows a periodicity in the discharge of groups of motor units, at a frequency about 9-10 c/s. These periodic 'bursts' of action potentials are accompanied by a corresponding mechanical oscillation. Under normal conditions this rhythmic activity is responsible for only a small fraction of the total tension being exerted by a muscle and, as shown by Lippold, Redfearn & Vuco (1957), produces a tremor which can be regarded as physiological. We argued that the effect was peripherally determined by self-oscillation in the stretch-reflex servo-loop, since the frequency of oscillation could be altered by cooling or stretching the muscle. The abolition of the rhythm by de-afferentation and by ischaemia of the muscle provides additional evidence for this hypothesis.These facts do not, however, rule out the possibility of a central contributory factor. Continuous medullary stimulation is known to produce rhythmical activity in motoneurones at about 10 c/s independently of the frequency of stimulation (Bernhard, Skoglund & Therman, 1947). The 10 c/s precentral cortical rhythm might also contribute to such a central driving tendency, although Lindqvist (1941) has shown that the slowing of the alpha rhythm as a result of overbreathing is not accompanied by any change in the frequency of tremor.The influence of any such central pace-maker upon the stretch reflex should be revealed by a study of the relationship between input to and output from the reflex centre. If there were a central rhythmical drive, then this relationship would be expected to vary in time with this rhythm.

A normal muscular contraction is never perfectly smooth, and frequency analysis of a myographic record shows that in general there is a regular rhythmical component (which in man is about 9 c/s; in the cat about 15 c/s) the mechanism of which seems to be based upon oscillation in the stretch reflex (Lippold, Redfearn & Vuco, 1957). There is also an irregular component in the frequency spectrum of muscular activity Pot dependent upon the stretch reflex, because it is still present after deafferentation (Perkins, 1945) and occurs in tabetics (Halliday & Redfearn, 1958).It became apparent, during a study of the frequency spectra of various types of tremor (e.g. thyrotoxic tremor, emotional tremor, shivering, etc.), that this random element had a variable amplitude under different circumstances although the over-all tension in the muscle remained the same. Thus the random component could make up a varying proportion of the total strength of a muscular contraction.In this paper, variability of the random element has been confirmed by comparison of the mechanical records of various types of muscular movement. These were shivering (in man and the cat), reflex contraction (in the cat), voluntary contraction (in man) and muscular activity resulting from electrical stimulation of the motor cortex of the cat. METHODS Human experiment8Production of 8hivering. Twenty-two young adult male subjects were made to shiver by stripping them naked and pouring cold water over them until they did so.Recording apparatu8. Tremor of the forefinger, and separately of the middle finger, was recorded

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Renshaw cell activity was recorded simultaneously with motoneuronal unit discharge during vibration and tetanic stimulation of triceps muscles in decerebrated cats. The experiments confirm that, in this preparation, the motoneurones are the main source of Renshaw cell firing during muscle stretch and vibration and when motoneuronal discharge was induced through the gamma loop. However they also show that a discharge of Renshaw cells, monosynaptically coupled with triceps motoneurones through their recurrent collaterals, could be elicited during contraction of the muscle at the time when the discharge of these motoneurones had been silenced. The recording of the stretch receptors and motoneuronal unit discharge during stretch, vibration, and ventral root stimulation gave evidence of the contribution of the withdrawal of excitation by primary endings to the occurrence of the silent period during tetanic contraction of the muscle. The measurements of the critical firing level in motoneuronal units responding reflexly to held stretch and vibration of the muscles, and silencing their discharge during muscle shortening, showed that these cells are amongst the lowest ranking in the pool For these reasons, these data suggest that Renshaw cell firing during vibration and tetanic contraction of the muscle cannot be attributed only to the alpha motoneurone excitation by the Ia fibres.