Abstract
Impact of Muscle Glycogen Availability on the Capacity for Repeated Exercise in Man. Med. Sci. Sports Exerc., Vol. 48, No. 1, pp. 123- 131, 2016. Purpose: This study aims 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 experimental design, each involving an initial run to exhaustion at 70% of V. O2max ( Run 1) followed by a 4- h recovery and a subsequent run to exhaustion at 70% of V. O2max ( Run 2). A low- carbohydrate ( L- CHO; 0.3 gIkg body mass j1 Ih j1) or high- carbohydrate ( H- CHO; 1.2 gIkg body massj1 Ihj1) beverage was ingested at 30- min intervals during recovery. Muscle biopsies were taken upon cessation of Run 1, after recovery, and at exhaustion during Run 2 in L- CHO ( F2). In H- CHO, muscle biopsies were obtained after recovery, at the time point coincident with fatigue in L- CHO ( F2), and at the point of fatigue during the subsequent exercise bout ( F3). Results: Run 2 was more prolonged for participants on H- CHO ( 80 +/- 16 min) than for participants on L- CHO ( 48 +/- 11 min; P G 0.001). Muscle glycogen concentrations were higher at the end of recovery for participants on H- CHO ( 269 T 84 mmolIkg dry massj1) than for participants on L- CHO ( 157 +/- 37 mmolIkg dry mass j1; P = 0.001). The rate of muscle glycogen degradation during Run 2 was higher with H- CHO ( 3.1 +/- 1.5 mmolIkg dry massj1 Iminj1) than with L- CHO ( 1.6 +/- 1.3 mmolIkg dry massj1 Iminj1; P = 0.05). The concentration of muscle glycogen was higher with H- CHO than with L- CHO at F2 ( 123 +/- 28 mmolIkg dry massj1; P G 0.01), but no differences were observed between treatments at the respective points of exhaustion ( 78 +/- 22 mmolIkg dry mass j1 Imin j1 for H- CHO vs 72 +/- 21 mmolIkg dry mass j1 Imin j1 for L- CHO). Conclusion: Increasing carbohydrate intake during short- term recovery accelerates glycogen repletion in previously exercised muscles 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.