Lactate elimination and glycogen resynthesis after intense bicycling

Muscles break down glycogen to lactate during intense exercise, and in the recovery period, glycogen reappears while lactate disappears. The purpose of this study was to examine to what extent lactate is resynthesized to glycogen within the formerly active muscles themselves in man.

Fifteen healthy young men cycled for 2 min to exhaustion. Muscle biopsies were taken from the knee extensor muscle before the exercise, just after the ride, and again after 45 min of recovery. In addition, blood samples were taken from the femoral artery and vein, and the leg blood flow was measured using the ultrasound Doppler technique. The muscle biopsies were analysed for glycogen, lactate and other metabolites, and the blood samples were analysed for lactate and glucose. The exchanges of lactate and glucose of the leg were assessed by multiplying the measured arterio-venous (a-v) differences by the blood flow.

During the exercise the muscles broke down 20+/-4 mmol glycogen kg(-1) wet muscle mass and produced 26+/-1 mmol lactate kg(-1). In the recovery period after 24+/-1 mmol lactate kg(-1) had disappeared, of which 48 % was released to the blood, 52 % disappeared within the muscle. An R-value of 0.62 across the leg suggests that none of the lactate was oxidized. Altogether, 10+/-3 mmol glycogen kg(-1) reappeared during recovery. Glucose uptake accounted for 2 mmol kg(-1) and glycolytic intermediates (G-6-P and free glucose) accounted for 4 mmol kg(-1); 4 mmol glycogen kg(-1) (42 %) reappeared from unknown sources.

The present data are compatible with the idea that around half of the lactate produced during intense bicycling is resynthesized to glycogen within the working muscles themselves in the recovery period after the bicycling.