Effects of muscle glycogen depletion on some metabolic and physiological responses to submaximal treadmill exercise

The aim of this study was to investigate the effects of reduced muscle glyc ogen concentration on some physiological and metabolic responses during mod erate intensity treadmill exercise in horses. Six Thoroughbred geldings wer e randomly allocated to 2 treatments (protocols A and B) or control in a 3 x 3 replicated Latin square design, In protocol A, horses performed low int ensity exercise while horses in protocol B performed short bursts of high i ntensity exercise. Protocol A was designed to induce glycogen depletion mai nly of slow twitch muscle fibers while protocol B aimed to deplete mainly f ast twitch muscle fibers. Horses in the control group did not undergo exerc ise prior to the exercise test. Five hours after glycogen depletion, horses performed treadmill exercise at 60% (V)over dot (2max) at a treadmill slop e of 10% until fatigue (20-30 min). The induced glycogen depletion prior to exercise had no significant effect on plasma glucose, insulin, or lactate concentrations during the exercise test, and there was no effect on glycoge n utilization rate, although respiratory exchange ratios were lower in the glycogen-depleted groups. The (V)over dot(2), heart rate and central blood temperature did not vary significantly between the protocols A and B and co ntrol throughout the exercise test. It was concluded that 20-30% depletion of glycogen concentration in the middle gluteal muscle resulted in a shift towards fat metabolism, but does not significantly affect heart rate, oxyge n uptake, or concentrations of plasma glucose and lactate during moderate i ntensity exercise.