TRAINING-INDUCED MUSCLE ADAPTATIONS - INCREASED PERFORMANCE AND OXYGEN-CONSUMPTION

An isolated perfused rat hindlimb preparation was used to study the impact of local muscle adaptations induced by endurance exercise training on muscle performance and peak muscle oxygen consumption. Rats were trained for 12-15 wk by a running program (30 m/min up a 15% grade for 1 h/day 5 days/wk) shown previously to increase muscle mitochondrial enzyme activity. Sedentary (n = 11) and trained (n = 11) hindlimbs of similar size were perfused with a similar inflow (12.1 ml/min) at a similar oxygen content (18.1 ml O2/100 ml blood). Tetanic contractions (100 ms at 100 Hz) at 4, 8, 15, 30, 45, and 60/min were elicited in consecutive order. Initial tension was better maintained by muscles of trained animals at all frequencies above 4 tetani/min (P < 0.05). Oxygen consumption (mu-mol . min-1 . g-1) increased similarly in both groups at the lower contraction frequencies but was greater (P < 0.05) in the trained [3.52 +/- 0.32 (SE)] than in the sedentary (2.44 +/- 0.31) group at 60 tetani/min. The peak oxygen consumption of the trained group (3.93 +/- 0.27) was 20% greater (P < 0.05) than that of the sedentary group (3.28 +/- 0.28) when peak values for each animal, irrespective of the contraction condition, are compared. Blood flows to the contracting muscle (approximately-100 ml . min-1 . 100 g-1) and, therefore, oxygen deliveries (mu-mol . min-1 . g-1) were not different between sedentary (7.99 +/- 0.56) and trained groups (8.35 +/- 0.61). Thus the 20% higher peak oxygen consumption was achieved by a greater oxygen extraction. The present results indicate that peripheral adaptations induced within muscle by training enhance contractile performance and peak oxygen consumption. Knowledge of which adaptation within muscle accounts for our results awaits further investigation. If this response is evident in vivo, it should contribute to an increase in whole body aerobic capacity after training by enhancing oxygen extraction across the active muscle.