ABsTRAcr Maximal respiratory pressures at the mouth (PEm. and PI,M.) have been measured in 370 normal caucasian children and adults. Age, height, and weight were recorded for all subjects and incorporated in a stepwise multiple regression analysis to determine prediction equations for the maximal respiratory pressures in the children and adults for both sexes. In men Pi,. and PEmax were significantly correlated only with age (p < 0-001 and < 0-035 respectively), whereas in women they were correlated with height (p < 0-035 and < 0-03). In both boys and girls PN., was related to weight (p < 0-0001 and <0-01 respectively) and PEma. to age (p < 0-001 for both). The values for PImax and PEmax in adults were lower than in previously reported series, but in children the values obtained were similar to those reported for several smaller series.In recent years interest has been rekindled in methods for measurement of respiratory muscle function in patients with neuromuscular disease. One of the simplest non-invasive measurements is that of maximal pressures, generated at the mouth, after full inspiration and full expiration-that is, maximal expiratory pressure (PE,,,C) and maximal inspiratory pressure (PI n).

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1. Normal subjects performed a step test in which the quadriceps of one leg contracted concentrically while the contralateral muscle contracted eccentrically. 2. Maximal voluntary force and the force: frequency relationship were altered bilaterally as a result of the exercise, the changes being greater in the muscle which had contracted eccentrically. Recovery occurred over 24 h. 3. Electromyographic studies using three sites on each muscle showed an increase in electrical activation during the exercise only in the muscle which was contracting eccentrically. Recovery followed a time course similar to that of the contractile properties. 4. Pain and tenderness developed only in the muscle which had contracted eccentrically. Pain was first noted approximately 8 h after exercise and was maximal at approximately 48 h after exercise, at which time force generation and electrical activation had returned to pre-exercise values. 5. Eccentric contractions cause more profound changes in some aspects of muscle function than concentric contractions. These changes cannot be explained in simple metabolic terms, and it is suggested that they are the result of mechanical trauma caused by the high tension generated in relatively few active fibres during eccentric contractions.

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SUMMARY1. The biochemical basis of the slowing of relaxation seen in fatigue has been examined using an isolated mouse soleus preparation.2. Slowing of relaxation occurred during prolonged tetani under anaerobic conditions when ATP and PC fell and lactate accumulated.3. Slowing of relaxation was also demonstrated with muscles poisoned with cyanide and iodoacetic acid when there was a fall in ATP and PC but no accumulation of lactate. During a period of anaerobic recovery following a fatiguing tetanus, relaxation became faster at a time when lactate was accumulating in the muscle.4. It is concluded that the slowing of relaxation in fatigue is not a consequence of lactate accumulation, and a relationship is demonstrated between the ATP content of the muscle and the rate of relaxation in muscles fatigued by prolonged stimulation.5. Rates of ATP turn-over in fresh muscle, and at intervals throughout a tetanus are consistent with the suggestion that the rate limiting step for myofibrillar ATPase may be directly related to the rate limiting step for the decay of tension during relaxation.