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 relative importance of each endpoint of glycogen breakdown during high intensity exercise. Therefore, 16 healthy men cycled for between 30 s and 3 min until exhaustion. Muscle biopsies were taken from m. vastus lateralis before and immediately after exercise and analysed for glycogen, glucose, glucose-6-phosphate and lactate. In addition the blood lactate concentration was measured at exhaustion, and the O2 uptake was measured throughout the exercise for calculation of glycogen oxidation. The muscle glycogen concentration fell by 17-24 mmol kg-1 wet wt muscle, 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 mmol kg-1. The blood lactate concentration at exhaustion was 4-9 mmol l-1 above pre-exercise value. Consequently, 60% of the glycogen lost reappeared as lactate within the working muscle, another 20-25% was found as other glycolytic intermediates, 4-13% of the glycogen loss could be accounted for by oxidation. Lactate released to blood could account for approximately 10% of all lactate produced. Therefore, when large muscles are heavily engaged, as during high intensity cycling, most of the glycogen broken down appears as lactate within the working muscle.