1. The contractile properties of the triceps surae were evaluated in 11 patients after unilateral fracture of the lower leg and subsequent immobilization for 135 ± 68 days. Calf muscle cross-sectional area (plus bone: CSA) was assessed from anthropometric measurement. 2. It was shown that the injured leg had a faster time to peak tension and increased half-relaxation time (1/2RT); twitch force ( P t ) was reduced by 25%. Evoked maximal tetanic tensions ( P 0 ) at 10 and 20 Hz were reduced by 51% and 46% respectively compared with the uninjured leg. The force of a maximal voluntary contraction (MVC) was also reduced, by 50%, but calf circumference and CSA were only 5% and 16% respectively lower in the injured leg. 3. It was concluded that the changes in contractile speed may indicate a relatively greater atrophy of slow (type I) muscle fibres. 4. The relationship between CSA and tension generation in the injured limb was shown to be poor after immobilization and during recovery. Anthropometric estimation of CSA does not appear to reflect the degree of muscle wasting, as indicated by reduced tension development after immobilization.

1. We have investigated the effects of moderate long-term exercise on protein turnover in fed man by measuring the extent of whole-body nitrogen production, the labelling of urinary ammonia from ingested [ 15 N]glycine and plasma, muscle and urine free amino acid concentrations. 2. Judged both from nitrogen production, and from the extent of 13 CO 2 production from ingested l -[l- 13 C]leucine, exercise causes a substantial rise in amino acid catabolism. 3. Amino acids catabolized during exercise appear to become available through a fall in whole-body protein synthesis and a rise in whole-body protein breakdown. After exercise, protein balance becomes positive through a rise in the rate of whole-body synthesis in excess of breakdown. 4. Studies of free 3-methylhistidine in muscle, plasma and urine samples suggest that exercise decreases the fractional rate of myofibrillar protein breakdown, in contrast with the apparent rise in whole-body breakdown.

1. Six patients were studied after prolonged immobilization of an injured leg resulting in muscle atrophy. 2. The forces exerted by the atrophied and normal legs during continuous dynamic exercise (one- and two-leg cycling) were examined by a specially adapted ergometer. 3. In one-leg cycling the peak force exerted on the crank at a given work rate, the net work rate performed on the crank, and the proportion of work rate performed in leg extension and flexion phases of the cycle were the same whether the atrophied or normal limb was used. 4. Despite these similarities there was an unexplained reduction in efficiency when using the atrophied leg to perform one-leg cycling. 5. In two-leg cycling the peak force exerted at a given work rate by the atrophied leg was reduced by about 40% as compared with the normal leg, which reflected a similar reduction in the contribution of that leg to the total net work rate. Possible reasons and implications for this disproportionate sharing of work between the normal and atrophied leg are discussed.

1. Seven patients who had suffered unilateral leg fracture were studied after removal of immobilizing plaster casts. 2. Leg volume measured anthropometrically was reduced by 12% in the injured leg (5·68 ± 1·05 litres) compared with the uninjured (6·43 ± 0·87 litres). Associated with this loss was a similar reduction in the net maximum oxygen uptake achieved in one-leg cycling, from 1·89 ± 0·21 l/min in the uninjured leg to 1·57±0·18 l/min in the injured. 3. Measured by a percutaneous needle biopsy technique, a reduction of 42% was found in the cross-sectional area of the muscle fibres sampled from the vastus lateralis of the injured compared with the uninjured leg. 4. Staining for myosin adenosine triphosphatase activity showed that both type I and II fibres were affected, being reduced respectively from 3410 to 1840 μm 2 and from 3810 to 2390 μm 2 cross-sectional area. 5. Possible reasons and implications are discussed for the discrepancy between the magnitude of the difference observed in the gross measurement of leg function (maximum oxygen uptake) and structure (leg volume) as compared with the cellular level (cross-sectional fibre area).

1. Eight patients who had suffered a fracture of one leg were studied before and after a 7 weeks period of rehabilitation during work with one leg and both legs on a bicycle ergometer. 2. In submaximal exercise minute ventilation for a given carbon dioxide output and tidal volume at a given minute ventilation remained unchanged throughout the period of therapy for both one- and two-leg exercise: oxygen intake for a given work output and cardiac frequency for a given oxygen intake decreased in both the injured and uninjured limb during one-leg work, although in two-leg exercise there was no significant change. 3. Oxygen intake at zero load was subtracted from the maximum oxygen intake measured during loaded exercise to give net values for each limb exercised separately or both legs exercised together. The net maximum oxygen intake thus calculated increased 8.9% (+ 0.17 l/min) in the uninjured leg and 17.4% (+0.29 l/min) in the injured leg during one-leg exercise. In two-leg exercise the increase was 17.2% (+ 0.43 l/min), which approximately equals the increase in the two legs measured separately. 4. In both legs there was an increase in leg muscle (plus bone) volume although this was significant in the injured leg only. 5. The maximum oxygen intake attained in two-leg exercise for a given leg volume in the patients at discharge was not significantly different from that found previously in a cross-sectional survey of young healthy (naval) servicemen. Thus the rehabilitation programme investigated appears to be effective, although the spontaneous recovery without a rehabilitation programme is unknown.

1. The physiological responses to exercise of two groups of industrial workers with moderate and severe anaemia were measured and compared with a group of matched controls, working and living under similar conditions. Exercise was performed on a stationary bicycle ergometer at five consecutive work loads. During the final minute of each exercise periods, oxygen intake, minute ventilation volume, and cardiac and respiratory frequencies were measured. On a separate occasion, blood haemoglobin concentration was measured at rest and cardiac output and blood and plasma volumes were estimated while subjects exercised at a fixed percentage of the maximum aerobic power for 6 min. 2. Oxygen intake for a given work load and ventilation rate was similar in all subjects studied but exercise cardiac output was elevated in the anaemic group. The increased cardiac output was due to an increased heart rate, the stroke volume being similar in the anaemic and control subjects. There was a marked decrement (∼34%) in maximum aerobic power and some evidence of a raised plasma volume in the severely anaemic group. 3. These results suggest that anaemia impairs performance during moderate and near maximum exercise. There is an associated rise of cardiac output and plasma volume.

1. An extensive study of the functional and dimensional components of the oxygen-transporting system in relation to age and the indirect estimation of maximum aerobic power (V̇o 2max. ) has been conducted on eighty healthy male subjects aged 20–50 years. Measurements were made of ventilation, cardiac frequency and blood lactic acid expressed in terms of oxygen intake of 1·51/min (V̇e, 1·5; fh, 1·5 and LA, 1·5 respectively) maximum blood lactic acid, ventilation and oxygen intake, thigh (TM) and calf (CM) and total body muscle estimated from soft tissue radiographs, whole body potassium ( 40 K), blood volume (BV), heart volume (HV), and tests of lung function. 2. In the younger subjects it was shown that the functional and dimensional capacities of the O 2 -transporting system were closely integrated and the performance of organs in relation to size was essentially similar. In older subjects there was a gradual decline in the overall capacity to transport oxygen which was closely associated with a decrease in the functional rather than the dimensional aspects of the cardiovascular system. In terms of a given body weight the dimensional capacities of the older subjects, BV, THb (total body haemoglobin), 40 K, TM CM, were shown to be independent of age. 3. The best single guide to V̇o 2max. was given by BV in subjects under 35 years of age and by HV in older subjects. By using a multiple regression on either V̇e, 1·5, fh, 1·5 and leg muscle or FVC, HV, TM and age, V̇o 2max. could be estimated with coefficient of variation of ∼9%. This is ∼2% improvement on conventional methods of determining V̇o 2max. from measurements of fh and V̇o 2 taken at four levels of work and in addition when using the latter equation there is no need for the subject to exercise. The use of these equations and their clinical application to the testing of V̇o 2max. in large community studies of physical working capacity are discussed.

1. The results of studies during simple progressive exercise to exhaustion and steady-state submaximal exercise in 117 boys and girls aged 6–16 years are presented. 2. In the simple progressive exercise test, the highest work load achieved and the submaximal heart rate were related to size and sex. The maximum heart rate and submaximal ventilation were largely independent of size and sex. 3. Steady-state exercise was performed at one-third and two-thirds of the maximum work load achieved in the simple progressive test. The Indirect (CO 2 ) Fick method was used to measure cardiac output. 4. At any given level of steady-state work, tidal volume, dead space, heart rate and stroke volume were closely related to size, with girls having higher heart rates and smaller stroke volumes than boys. Minute ventilation and cardiac output were virtually independent of size and sex. The cardiac output in children was the same as that in the adult for any given oxygen consumption. Blood lactate was related to size at any given work load, but was independent of size at any given fraction of the maximum working capacity.

1. A study of exercise physiology in 117 children aged 6–16 is described including the measurement of cardiac output by the Indirect (CO 2 ) Fick Principle. 2. Computer produced graphs enabled various alternative values for arterial P co 2 ( P a,co 2 ) to be inserted simultaneously into the Fick equation for cardiac output and the Bohr equation for dead space. 3. Dead space could be estimated with reasonable accuracy using end-tidal P co 2 . 4. Cardiac output determination was unreliable at rest due to the small venoarterial P co 2 difference but it was very good on exercise when this difference is much larger. 5. The P a,co 2 used for the calculation of cardiac output could be derived either from ear lobe P co 2 or end-tidal P co 2 . However, the best estimate of cardiac output was that using the P a,co 2 implied by assuming a normal dead space.

1. The physiological responses to repeated continuous (progressive) exercise together with the relationship between continuous and discontinuous work tests on an upright bicycle ergometer have been studied in healthy male subjects. 2. The results show a marked decline in mean cardiac frequency (f H ) from 145 beats/min to 124 beats/min during the first four experiments at a given submaximal oxygen intake of 1·5 1/min and thereafter a smaller decline to reach 118 beats/min on the final (sixteenth) day of the investigation. This latter decrease was associated with a small but significant rise in maximum oxygen intake (V̇O 2 max) which occurred from Day 8 onwards. Pulmonary minute ventilation at an oxygen intake of 1·5 1/min (V̇ E 1·5 ) decreased by about 5 1/min immediately following the first occasion of submaximal work but thereafter remained unchanged. There was however no decrease in O 2 cost of exercise, as V̇O 2 at a fixed rate of external work remained unchanged throughout the investigation. 3. At maximal effort there was no significant decrease in either V̇ E max or f H max, but the f H at which the V̇O 2 max was reached declined significantly from Day 8. Thus the asymptotic nature of the f H /V̇O 2 curve which was very pronounced on Day 1 virtually disappeared following the third visit to the laboratory. The accuracy of predicting V̇O 2 max from f H and V̇O 2 increased noticeably from − 15·4 ± 8·9% on Day 1 to −6·5 ± 10·5% on Day 7 and −0·9 ± 3·4% on Day 15. The possible physiological basis and implications of the results are discussed. 4. No significant differences were found between continuous and discontinuous work. Thus in large scale population studies of work capacity or in the evaluation of training programmes in rehabilitation studies, a continuous test may be used but attention must be paid to familiarization procedures. At least three preliminary test periods are necessary before the results become reproducible and reliable.