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 healthy, middle-aged men enrolled in a 6-month, home-based jogging program of 120 min/wk at 75% VO2max. Only 12 men showed a sufficient exercise adherence over the 6 months (> or = 60 min/wk on average) and were included into statistical analysis. Their average training activity was 105 +/- 31 min/wk. The results revealed significant increases in VO2max (+8.4%, p < 0.01) and submaximal power output (+18.1%, p < 0.01). Total mitochondrial volume density in M. vastus lateralis increased by 20% (p < 0.05) with a larger increase in subsarcolemmal volume compared to central volume (50% vs 15%). No changes in volume of intracellular lipid droplets, capillary density, capillary per fiber ratio, fiber mean cross-sectional area and muscle fiber type could be observed. Body composition analysis showed a decrease in trunk 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 training beneficially affected cardiovascular and muscle oxidation capacity as well as body composition in the trunk area. No adaptations in capillaries 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 stimulus.