Availability of glycogen and plasma FFA for substrate utilization in leg muscle of man during exercise

Summary. Six men performed two‐legged cycle ergometer exercise at loads demanding 2–4 and 3–4 litres O 2 min ‐1 (62 and 84% of Vo 2 max) for 20 min each or to exhaustion twice with 1 h rest between. An initial glycogen difference of 28 mmol glucose units kg ‐1 of thigh muscle between the two legs was produced by one‐legged exercise on a previous day followed by the consumption of a low carbohydrate diet. During the 1 h rest nicotinic acid (NA) was administered to inhibit lipolysis. Total body Fo 2 was unchanged by the NA administration. Work done by each leg, indicated by force on the pedals, was equal. RQ indicated a larger oxidation of fat in the leg with low glycogen. Muscle glycogen was 15 and 10 mmol kg ‐1 in the normal and low glycogen leg at the end of the first exercise bouts and 3–8 mmol kg ‐1 in both legs at exhaustion. The low glycogen leg extracted lactate from the blood whereas the normal leg released lactate and the uptake of glucose from the blood was greater by the low glycogen leg. These differences between the low glycogen and control legs did not persist during the NA condition when muscle glycogen content was equal in both legs. Further, the leg glucose uptake in the control and the NA conditions was positively related to the percentage of glycogen‐empty muscle fibres and inversely to the glucose‐6‐P0 4 concentration. Thus the magnitude of the local glycogen stores of muscle influences the uptake and use of blood‐borne substrates as well as determining endurance capacity during moderate to high intensity exercise.