ULTRASTRUCTURAL MODIFICATION OF HUMAN SKELETAL-MUSCLE TISSUE WITH 6-MONTH MODERATE-INTENSITY EXERCISE TRAINING

The study was aimed at investigating if endurance training of moderate intensity and longer duration, intended to promote health rather than performance, evokes ultrastructural changes in skeletal muscle tissue comparable to those observed after high-intensity protocols. Twenty h ealthy, middle-aged men enrolled in a 6-month, home-based jogging prog ram of 120 min/wk at 75% VO(2)max. Only 12 men showed a sufficient exe rcise adherence over the 6 months (greater than or equal to 60 min/wk on average) and were included into statistical analysis. Their average training activity was 105+/-31 min/wk. The results revealed significa nt increases in VO(2)max (+8.4%, p < 0.01) and submaximal power output (+18.1%, p < 0.01). Total mitochondrial volume density in M. vastus l ateralis increased by 20% (p < 0.05) with a larger increase in subsarc olemmal volume compared to central volume (50% vs 15%). No changes in volume of intracellular lipid droplets, capillary density, capillary p er fiber ratio, fiber mean cross-sectional area and muscle fiber type could be observed. Body composition analysis showed a decrease in trun k fat mass (-7.3%, p < 0.05) and an increase in trunk lean mass (+1.7% , p < 0.05), while changes in the legs were not significant. It can be concluded that a moderate-intensity, health oriented endurance traini ng beneficially affected cardiovascular and muscle oxidation capacity as well as body composition in the trunk area. No adaptations in capil laries or lipid metabolism could be demonstrated. The results support the hypothesis of thresholds for induction of adaptation processes in muscle skeletal tissue depending on the intensity of the exercise stim ulus.