Muscle glycogen accumulation in trained and untrained individuals

Muscle glycogen accumulation was compared in six trained cyclists (T) and six untrained subjects (UT) following cycling exercise at 75% VO₂peak for 2 hr, terminating with 5 1-min sprints. Subjects ate 10g carbohydrate kg^-1 day^-1 for 48 hr post-exercise. Muscle glycogen accumulation averaged 71±9 μmol/g (T) and 36±9 μmol/g (UT) during the first 6 hr post-exercise (mean±SEM; p<0.01), and 79±22 μmol/g (T) and 60±9 μmol/g (UT) between 6 and 48 hr post-exercise (n.s.). Muscle glycogen concentration was 164±21 μmol/g (T) and 99±16 μmol/g (UT) 48 hr post-exercise (p<0.05). Glycogen synthase %I was 2.5±0.5, 3.3±0.5, and 1.0±0.3 μmol g^-1 · min^-1 in T at 0, 6, and 48 hr post-exercise, respectively; corresponding values were 1.2±0.3, 2.7±0.5, and 1.6±0.3μmol g^-1 min^-1 in UT (p<0.05 at 0 hr). Muscle GLUT-4 content was 3-fold higher in T than UT (p<0.05), and correlated with glycogen accumulation rates (r=0.63, p<0.05). Plasma insulin, plasma C-peptide, and area under the glucose curve were lower in T than UT over the first 6 hr post-exercise (p<0.05). Plasma creatine kinase concentrations (CK) were 125±25 IU/L (T) and 91±9 IU/L (UT) at 0 hr and 112±14 IU/L (T) and 144±22 IU/L (UT) at 48 hr post-exercise (normal: 30-200 IU/L). We conclude that endurance exercise training results in an increased ability to accumulate muscle glycogen after exercise.