Muscle capillarization, O2diffusion distance, and V˙o 2kinetics in old and young individuals

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Abstract
Chilibeck, P. D., D. H. Paterson, D. A. Cunningham, A. W. Taylor, and E. G. Noble. Muscle capillarization, O2 diffusion distance, andV˙o 2 kinetics in old and young individuals. J. Appl. Physiol.82(1): 63–69, 1997.—The relationships between muscle capillarization, estimated O2diffusion distance from capillary to mitochondria, and O2 uptake (V˙o 2) kinetics were studied in 11 young (mean age, 25.9 yr) and 9 old (mean age, 66.0 yr) adults. V˙o 2kinetics were determined by calculating the time constants (τ) for the phase 2V˙o 2 adjustment to and recovery from the average of 12 repeats of a 6-min, moderate-intensity plantar flexion exercise. Muscle capillarization was determined from cross sections of biopsy material taken from lateral gastrocnemius. Young and old groups had similarV˙o 2 kinetics (τV˙o 2-on = 44 vs. 48 s; τV˙o 2-off = 33 vs. 44 s, for young and old, respectively), muscle capillarization, and estimated O2 diffusion distances. Muscle capillarization, expressed as capillary density or average number of capillary contacts per fiber/average fiber area, and the estimates of diffusion distance were significantly correlated toV˙o 2-off kinetics in the young (r = −0.68 to −0.83;P < 0.05). We conclude that1) capillarization andV˙o 2 kinetics during exercise of a muscle group accustomed to everyday activity (e.g., walking) are well maintained in old individuals, and2) in the young, recovery ofV˙o 2 after exercise is faster, with a greater capillary supply over a given muscle fiber area or shorter O2 diffusion distances.