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In a muscle contracting voluntarily, the tension is proportional to the electrical activity, both under isometric (Lippold, 1952) and under isotonic conditions (Bigland & Lippold, 1954). The measure of this electrical activity was obtained by determining the area under the action potential curve recorded from surface electrodes over the belly of the muscle, and was considered to represent the 'excitation' in the muscle, i.e. to be a function of the number and the frequency of discharge of motor units. Such a composite measure, however, could not distinguish between the two means by which a voluntary contraction is graded (variation in the number of active units or their frequency), and it. therefore became of interest to discover how the frequency of a unit varied with changes in tension in the muscle during a voluntary contraction. This has been investigated to a limited extent previously, but only at contraction strengths which were a small percentage of maximum levels (Gilson & Mills, 1941). This was because at high levels of activity analysis of the frequency of single units became impossible through interference from adjacent units.The efficiency of muscle reaches its maximum level when the frequency of stimulation of its fibres is just sufficient to produce maximum tetanic tension; at frequencies above and below this, the muscle fibre is less efficient (Bronk, 1930). These facts might indicate that the fibres in a muscle would tend to operate near their tetanic frequency, and that changes in tension would, as. a result, be brought about mainly by means of recruiting motor units whose frequency of discharge would rapidly rise to and remain at tetanic level.We have studied the problem in two ways. First, by stimulating a human muscle artificially and determining the relation between frequency and tension produced as has been done already in anaesthetized animals (Adrian & Bronk, 1929;Brown & Burns, 1949). The expected results would be a proportionality between mean tension and frequency until a certain frequency is reached, above which no further increase in tension would occur, indicating that this represented full tetanic frequency in the muscle fibres.

The term negative work has been commonly used in the past half century in connexion with experiments in which work is done on a subject by forcibly stretching his contracting muscles. Although this term is unsatisfactory and somewhat confusing, it has been retained here in preference to the alternative expression 'eccentric work' used more recently by other workers.In previous experiments the relative oxygen consumptions required for equal amounts of positive and negative work were measured (Abbott, Bigland & Ritchie, 1952). It was found that, although the oxygen consumption in positive work increased rapidly with the rate of work, that required over the same range of negative work remained relatively constant; this has since been confirmed and extended by Asmussen (unpublished). Each active fibre within a muscle develops a tension dependent on the speed and direction of motion; this tension is considerably greater when the fibre is being lengthened (Katz, 1939; Abbott, Aubert & Hill, 1951) than when it shortens (Hill, 1938; Wilkie, 1950). The forces exerted in these experiments were not maximal and might be varied by changing the number of active fibres and their frequency of excitation. At a given speed and load, fewer fibres were needed during negative work, when the subject's muscles were being stretched than when shortening and doing positive work. The results were interpreted in terms of the forcevelocity relation for active muscle assuming that the oxygen consumption depended primarily upon the number of fibres in action.In these earlier experiments the oxygen consumption during the negative work rarely rose to more than twice the resting value. The rate of work was varied mainly by changing the speed. Although the force exerted by the two subjects was kept constant throughout each experiment, its absolute value

One of the recent developments in surgery is the performance of operations on patients whose body temperature has been lowered, but little is known of how this procedure affects the action of drugs given during operation. Holmes, Jenden & Taylor (1951), in experiments carried out on the isolated diaphragm of the rat, have demonstrated that cooling reduces the effect of tubocurarine, a substance which interrupts neuromuscular transmission by competition with acetylcholine. In view of this it was felt it might be of importance to find out how the action of depolarizing neuromuscular blocking drugs, such as suxamethonium and decamethonium, is modified by changes in skeletal muscle temperature. METHODSCats and dogs were anaesthetized with a mixture of chloralose (0.08 g/kg for cats, and 0-11 g/kg for dogs) and pentobarbitone sodium (3-6 mg/kg) injected into the subcutaneous vein of the forelimb or the internal saphenous vein. The dogs also received 2 mg/kg of morphine hydrochloride subcutaneously 30-60 min before they were anaesthetized. Hind limbs were set up horizontally on a Brown-Schuster myograph stand, shielded silver electrodes were placed on the sciatic nerve, and the nerve ligated central to the electrodes. Twitches of the tibialis muscle were elicited by square-wave pulses of 0-2 msec duration, and twice the strength required to evoke a maximal twitch. The muscle was attached to a flat steel spring, and its contractions recorded on a smoked drum.Body temperature was varied at the rate of 40 C/hr by passing hot or cold water through a thin rubber bag inserted into the abdominal cavity or by modifying the external heating. This method made it possible to produce a slow but steady fall in body temperature without perceptible shivering. In some experiments, while body temperature was kept normal the temperature of one limb was lowered by the external application of rubber bags containing crushed ice. Oral or rectal temperatures were measured by means of mercury thermometers and muscle temperature by the use of a thermistor (Bigland & Zaimis, 1958).