GLYCOGEN BREAKDOWN AND LACTATE ACCUMULATION DURING HIGH-INTENSITY CYCLING

High-intensity exercise results in a large breakdown of glycogen. The glycogen lost may reappear as hexose phosphates, lactate, or it may be fully oxidized. Part of the lactate produced may be transferred from muscle to blood. There is, however, incomplete information on the rela tive importance of each endpoint of glycogen breakdown during high int ensity exercise. Therefore, 16 healthy men cycled for between 30 s and 3 min until exhaustion. Muscle biopsies were taken from m. vastus lat eralis before and immediately after exercise and analysed for glycogen , glucose, glucose-6-phosphate and lactate. In addition the blood lact ate concentration was measured at exhaustion, and the O2 uptake was me asured throughout the exercise for calculation of glycogen oxidation. The muscle glycogen concentration fell by 17-24 mmol kg-1 wet wt muscl e, the muscle glucose and G-6-P concentrations rose by 1 and 4 mmol kg -1 respectively, and the muscle lactate concentration rose by 20-30 mm ol kg-1. The blood lactate concentration at exhaustion was 4-9 mmol l- 1 above pre-exercise value. Consequently, 60% of the glycogen lost rea ppeared as lactate within the working muscle, another 20-25% was found as other glycolytic intermediates, 4-13% of the glycogen loss could b e accounted for by oxidation. Lactate released to blood could account for almost-equal-to 10% of all lactate produced. Therefore, when large muscles are heavily engaged, as during high intensity cycling, most o f the glycogen broken down appears as lactate within the working muscl e.