Mechanical and metabolic responses with exercise and dietary carbohydrate manipulation.

ABSTRACT

PURPOSE: To investigate the effects of altered muscle glycogen content on the mechanical and metabolic responses to prolonged exercise of moderate intensity. METHODS: Eight volunteers (.VO2peak = 49.3 +/- 1.2 mL.kg(-1).min(-1)) cycled to fatigue on two occasions after a 3-d low-carbohydrate diet (Lo CHO), which had been preceded by glycogen-depleting exercise, and then after a 3-d high-carbohydrate diet (Hi CHO). Metabolic and mechanical properties were assessed at both Lo CHO and Hi CHO before exercise (Pre), at 30 min of exercise (30 min), at fatigue in Lo CHO (Post 1), and again at fatigue after a brief rest (Post 2). RESULTS: For the Lo CHO cycle, time to fatigue averaged 66.7 +/- 4.5 and 9.5 +/- 1.7 min for Post 1 and Post 2, respectively. For Hi CHO, Post 2 time to fatigue was 64.9 +/- 6.3 min. Muscle glycogen was elevated (P < 0.05) by approximately 40% in Hi CHO compared with Lo CHO. Phosphocreatine, although higher (P < 0.05) by approximately 25% during exercise in Hi CHO, was not different at Pre. Similar but reciprocal effects (P < 0.05) were observed for inorganic phosphate and creatine. Force at low frequencies of stimulation was maximally reduced (P < 0.05) by approximately 26-38% by 30 min of exercise, regardless of condition. CONCLUSION: A 7-d exercise-dietary protocol leads to both an elevation in muscle glycogen and improved energy homeostasis during exercise. Although these adaptations may explain the improved cycle performance, they are not related to the progression of muscle fatigue as assessed statically at low frequencies of stimulation.