UC Irvine

It is often assumed that the inherent peak muscle metabolic capacity scales in direct proportion to muscle cross-sectional area and is the same in small and large animals (A. V. Hill. Sci. Prog. 38: 208-230, 1950). We wondered whether this relationship between size and function was true during the period of growth and development in humans. Magnetic resonance imaging (MRI) was used to determine calf muscle cross-sectional area (CSA) in 20 children (6-11 yr old, 11 boys) and in 18 adults (23-42 yr old, 10 men). Progressive cycle ergometer exercise was performed to determine peak oxygen uptake (VO2peak) and work rate (WRpeak). The scaling factor (determined by allometric analysis) relating maximal O2 uptake (VO2max) to muscle CSA for the whole sample population was 1.04 +/- 0.12 (SE), but the scaling factor relating WRpeak to muscle CSA was significantly greater (1.37 +/- 0.12). Consistent with this, VO2max/CSA was not affected by body weight, but the WRpeak/CSA increased as a function of weight both in males (P < 0.005) and females (P < 0.05). No differences in VO2max/CSA were found between children and adults. WRpeak/CSA was significantly higher in adults compared with children (P < 0.05). It appears that the inherent peak muscle metabolic capacity is smaller in children than in adults. Moreover, the coupling of muscle capacity with whole body metabolic rate changes during growth in humans.