Cardiorespiratory and metabolic responses after exercise-induced muscle damage: the influence of lowered glycogen

BACKGROUND. We examined the effect of early-onset of muscle damage and low muscle glycogen on cardiorespiratory and metabolic responses to low-intensity exercise. METHODS. Twelve men cycled for 10 min at 50% maximal oxygen uptake before, and 12 h after a morning downhill run (five, 8 min bouts at -12% gradient, with 2 min rests) under normal (NORM) and lowered glycogen (LOW) conditions, following a cross-over design with conditions separated by six weeks. Cardiorespiratory responses were recorded, with oxidation measures derived from stoichiometry equations. RESULTS. Muscle damage symptoms post-downhill (0 h) were similar between conditions. Carbon dioxide ventilatory equivalent increased 12 h post-downhill for LOW (P<0.05), but not NORM (P=0.7). A trend towards decreased respiratory exchange ratio (RER) was shown 12 h post-downhill for LOW (1.00 +/- 0.07 to 0.89 +/- 0.12, P=0.06), but not NORM (0.94 +/- 0.11 to 0.94 +/- 0.08; P=0.6). Twelve hours after LOW downhill running fat oxidation increased (0.21 +/- 0.18 g.min(-1) to 0.36 +/- 0.27 g.min(-1); P<0.05) and carbohydrate oxidation decreased (2.68 +/- 0.52 g.min(-1) to 1.98 +/- 0.75 g.min(-1); P<0.05); NORM oxidation rates were unchanged (fat: 0.26 +/- 0.18 g.min(-1) to 0.33 +/- 0.18 g.min(-1); P=0.5; carbohydrate: 2.51 +/- 0.49 g.min(-1) to 2.29 +/- 0.47 g.min(-1); P=0.3). CONCLUSIONS. Cycling at low-intensity 12 h post-downhill running with lowered muscle glycogen increased fat oxidation, decreased carbohydrate oxidation and elevated carbon dioxide ventilation. Damaging exercise with reduced glycogen availability increases fat utilization during subsequent low-intensity exercise as little as 12 h later.