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.