Muscle capillarization, O-2 diffusion distance, and Vo(2) kinetics in old and young individuals

The relationships between muscle capillarization, estimated O-2 diffusion distance from capillary to mitochondria, and O-2 uptake (VO2) kinetics were studied in 11 young (mean age, 25.9 yr) and 9 old (mean age, 66.0 yr) adults. VO2 kinetics were determined by calculating the time constants (tau) for the phase 2 VO2 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 VO2 kinetics (tau VO2-on 44 vs. 48 s; tau VO2-off = 33 vs. 44 s, for young and old, respectively), muscle capillarization, and estimated O-2 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 VO2-off kinetics in the young (r = -0.68 to -0.83; P < 0.05). We conclude that I) capillarization and VO2 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 VO2 after exercise is faster, with a greater capillary supply over a given muscle fiber area or shorter O-2 diffusion distances.