Impact of muscle glycogen availability on the capacity for repeated exercise in man

Purpose: To examine whether muscle glycogen availability is associated with fatigue in a repeated exercise bout following short-term recovery. Methods: Ten endurance-trained individuals underwent two trials in a repeated measures design, each involving an initial run to exhaustion at 70% (Run-1) followed by a 4-h recovery and a subsequent run to exhaustion at 70% (Run-2). A low-carbohydrate (L-CHO; 0.3 g·kg BM-1·h-1) or high-carbohydrate (H-CHO; 1.2 g·kg BM-1·h-1) beverage was ingested at 30-min intervals during recovery. Muscle biopsies were taken upon cessation of Run-1, post-recovery and fatigue during Run-2 in L-CHO (F2). In H-CHO, the muscle biopsies were obtained post-recovery, the time point coincident with fatigue in L-CHO (F2) and the point of fatigue during the subsequent exercise bout (F3).Results: Run-2 was more prolonged for every participant in H-CHO (80±16 min) than L-CHO (48±11 min; p< 0.001). Muscle glycogen concentrations were higher at the end of recovery in H-CHO (269±84 mmol·kg dm-1) versus L-CHO (157±37 mmol·kg dm-1; p= 0.001). The rate of muscle glycogen degradation during Run-2 was higher in H-CHO (3.1±1.5 mmol·kg dm-1·min-1) than L-CHO (1.6±1.3 mmol·kg dm-1·min-1; p= 0.05). The concentration of muscle glycogen was higher in H-CHO than L-CHO at F2 (123±28 mmol·kg dm-1; p< 0.01) but no differences were observed between treatments at the respective points of exhaustion (78±22 versus 72±21 mmol·kg dm-1·min-1; H-CHO and L-CHO, respectively). Conclusion: Increasing carbohydrate intake during short-term recovery accelerates glycogen repletion in previously exercised muscle and thus improves the capacity for repeated exercise. The availability of skeletal muscle glycogen is therefore an important factor in the restoration of endurance capacity because fatigue during repeated exercise is associated with a critically low absolute muscle glycogen concentration.