The adaptation of muscle structure, power output, and mass-specific rate of maximal O2 consumption (VO2max/Mb) with endurance training on bicycle ergometers was studied for five male and five female subjects. Biopsies of vastus lateralis muscle and VO2max determinations were made at the start and end of 6 wk of training. The power output maintained on the ergometer daily for 30 min was adjusted to achieve a heart rate exceeding 85% of the maximum for two-thirds of the training session. It is proposed that the observed preferential proliferation of subsarcolemmal vs. interfibrillar mitochondria and the increase in intracellular lipid deposits are two possible mechanisms by which muscle cells adapt to an increased use of fat as a fuel. The relative increase of VO2max/Mb (14%) with training was found to be smaller by more than twofold than the relative increase in maximal maintained power (33%) and the relative change in the volume density of total mitochondria (+40%). However, the calculated VO2 required at an efficiency of 0.25 to produce the observed mass-specific increase in maximal maintained power matched the actual increase in VO2max/Mb (8.0 and 6.5 ml O2 X min-1 X kg-1, respectively). These results indicate that despite disparate relative changes the absolute change in aerobic capacity at the local level (maintained power) can account for the increase in aerobic capacity observed at the general level (VO2max).

Quadriceps muscle and fibre cross-sectional areas (CSA), torque and neural activation were studied in seven healthy males during 6 months of weight training on alternate days with six series of eight unilateral leg extensions at 80% of one repetition maximum. After training, the quadriceps cross-sectional area increased by 18.8 +/- 7.2% (P < 0.001) and 19.3 +/- 6.7% (P < 0.001) in the distal and proximal regions respectively, and by 13.0 +/- 7.2% (P < 0.001) in the central region of the muscle. Hypertrophy was significantly different between and within the four constituents of the quadriceps. Biopsies of the vastus lateralis at mid-thigh did not show any increase in mean fibre cross-sectional area. Maximum isometric voluntary torque increased by 29.6 +/- 7.9%-21.1 +/- 8.6% (P < 0.01-0.05) between 100 degrees and 160 degrees of knee extension, but no change in the optimum angle (110 degrees-120 degrees) for torque generation was found. A 12.0 +/- 10.8% (P < 0.02) increase in torque per unit area together with a right shift in the IEMG-torque relation and no change in maximum IEMG were observed. Time to peak isometric torque decreased by 45.8% (P < 0.03) but no change in time to maximum IEMG was observed. In conclusion, strength training of the quadriceps results in a variable hypertrophy of its components without affecting its angle-torque relation. The increase in torque per unit area, in the absence of changes in IEMG, may indicate changes in muscle architecture. An increase in muscle-tendon stiffness may account for the decrease in time to peak torque.

To investigate changes in the ultrastructure of the different muscle fiber types induced by endurance training ten sedentary subjects (five women and five men) were exercised on bicycle ergometers 5 times a week for 30 min. After 6 weeks of training there were significant changes in VO2max (+14%), in the percentage of type I (+12%) and type IIB fibers (-24%) as well as in the volume densities of mitochondria. The latter increased 35% in type I, 55% in type IIA and 35% in type IIB fibers. The relative increase in subsarcolemmal mitochondria was larger than in interfibrillar mitochondria in all fiber types. There was also a significant increase in the volume density of intracellular lipid in type II fibres. It is concluded that high intensity endurance training leads to an enhancement of the oxidative capacity in all muscle fiber types.

Muscle structural changes during typical mountaineering expeditions to the Himalayas were assessed by taking muscle biopsies from 14 mountaineers before and after their sojourn at high altitude (greater than 5000 m for over 8 weeks). M. vastus lateralis samples were analyzed morphometrically from electron micrographs. A significant reduction (-10%) of muscle cross-sectional area was found on CT scans of the thigh. Morphologically this loss in muscle mass appeared as a decrease in muscle fiber size mainly due to a loss of myofibrillar proteins. A loss of muscle oxidative capacity was also evident, as indicated by a decrease in the volume of muscle mitochondria (-25%). In contrast, the capillary network was mostly spared from catabolism. It is therefore concluded that oxygen availability to muscle mitochondria after prolonged high-altitude exposure in humans is improved due to an unchanged capillary network, supplying a reduced muscle oxidative capacity.

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