Effects of endurance training on oxidative capacity and structural composition of human arm and leg muscles.

Six healthy subjects performed endurance training of the same duration with legs and arms consecutively. Performance and muscle structure were measured before and after training in lower and upper limbs. Training induced similar increases in maximal oxygen consumption (6 +/- 1 vs. 7 +/- 2 mL min-1 kg-1: legs vs. arms, P > 0.05) and mitochondrial volume in leg and arm muscles (42 +/- 12 vs. 31 +/- 11%: legs vs. arms, P > 0.05). The gain in mitochondrial volume after training was achieved solely by increasing the fraction of mitochondria (+40 +/- 11%, P < 0.05) in the same muscle volume (+2 +/- 2%, P > 0.05) in the legs. In contrast, increased muscle volume (+14 +/- 3%, P < 0.05), in addition to a tendency for an increase in mitochondrial fraction (+16 +/- 11%, P > 0.05), occurred in the arms after training. Thus, similar improvements in muscle oxidative capacity in upper and lower limbs were brought about by different mechanisms. It is suggested that due to infrequent use and a lack of load-bearing function, arm muscle volume is underdeveloped in untrained, sedentary or detrained/injured subjects and that the mode of endurance training used in this study is sufficient to enlarge arm muscle volume as well as aerobic capacity.