Muscle capillarization, O2 diffusion distance, and V̇O2 kinetics in old and young individuals

Abstract

The relationships between muscle capillarization, estimated O2 diffusion distance from capillary to mitochondria, and O2 uptake (V̇O2) kinetics were studied in 11 young (mean age, 25.9 yr) and 9 old (mean age, 66.0 yr) adults. V̇O2 kinetics were determined by calculating the time constants (τ) for the phase 2 V̇O2 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 similar V̇O2 kinetics (τV̇O-on= 44 vs. 48 s; τV̇O2-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 to V̇O2-off kinetics in the young (r = -0.68 to -0.83; P < 0.05). We conclude that 1) capillarization and V̇O2 kinetics during exercise of a muscle group accustomed to everyday activity (e.g., walking) are well maintained in old individuals, and 2) in the young, recovery of V̇O2 after exercise is faster, with a greater capillary supply over a given muscle fiber area or shorter O2 diffusion distances.