1. This paper asks how the decline in maximal O2 uptake rate (V(O2, max)) with age is related to the properties of a key muscle group involved in physical activity - the quadriceps muscles. Maximal oxygen consumption on a cycle ergometer was examined in nine adult (mean age 38.8 years) and 39 elderly subjects (mean age 68.8 years) and compared with the oxidative capacity and volume of the quadriceps. 2. VO(2,max) declined with age between 25 and 80 years and the increment in oxygen consumption from unloaded cycling to V(O2),(max) (ΔV(O2)) in the elderly was 45% of the adult value. 3. The cross-sectional areas of the primary muscles involved in cycling - the hamstrings, gluteus maximus and quadriceps - were all lower in the elderly group. The quadriceps volume was reduced in the elderly to 67% of the adult value. Oxidative capacity per quadriceps volume was reduced to 53% of the adult value. The product of oxidative capacity and muscle volume - the quadriceps oxidative capacity - was 36% of the adult value in the elderly. 4. Quadriceps oxidative capacity was linearly correlated with ΔV(O2) among the subjects with the slope indicating that the quadriceps represented 36% of the V(O2) increase during cycling. 5. The decline in quadriceps oxidative capacity with age resulted from reductions in both muscle volume and oxidative capacity per volume in the elderly and appears to be an important determinant of the age-related reduction in ΔV(O2) and V(O2,max) found in this study.
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