Muscle glycogen synthesis in recovery from intense exercise in humans

The present study examined the role of lactate and glucose as substrates for glyconeogenesis in muscle in recovery from high-intensity exercise in humans. Seven subjects performed approximately 100 min of intense intermittent one-legged knee extensor exercise on two occasions: with [high lactate (HL)] and without [control (C)] intense arm exercise between the leg exercise bouts, leading to end exercise arterial plasma lactate concentrations of 16.0 +/- 1.6 and 9.2 +/- 1.6 mmol/l, respectively (P < 0.05). At the end of exercise, muscle lactate and glycogen were similar in HL and C (20.5 +/- 1.3 vs. 17.3 +/- 2.0 mmol/kg wet wt and 48.1 +/- 11.3 vs. 56.3 +/- 8.6 mmol/kg wet wt, respectively). Muscle glycogen increased (P < 0.05) during the first 5 min of recovery only in HL, but after 90 min of recovery the muscle glycogen concentration was the same in C and HL (61.2 +/- 12.0 vs. 71.5 +/- 10.9 mmol/kg wet wt). Muscle lactate not released to the blood could maximally account for 28 (C) and 54% (HL) of the increase in muscle glycogen during 90 min of recovery or < 10% of glycogen synthesis after full recovery. The total net glucose uptake corresponded to 84 (C) and 57% (HL) of the glycogen synthesized. Apparently, muscle glyconeogenesis may occur in humans, but the role of lactate as a substrate is minor. Instead, blood glucose appears to be the most important precursor for muscle glycogen synthesis after intense exercise.