Muscle glycogen accumulation after a marathon: roles of fiber type and pro- and macroglycogen

Muscle glycogen remains subnormal several days after muscle damaging exercise. The aims of this study were to investigate how muscle acid-soluble macroglycogen (MG) and acid-insoluble proglycogen (PG) pools are restored after a competitive marathon and also to determine whether glycogen accumulates differently in the various muscle fiber types. Six well-trained marathon runners participated in the study, and muscle biopsies were obtained from the vastus lateralis of the quadriceps muscle before, immediately after, and 1, 2, and 7 days (days 1, 2, and 7, respectively) after the marathon. During the race, 56 +/- 3.8% of muscle glycogen was utilized, and a greater fraction of MG (72 +/- 3.7%) was utilized compared with PG (34 +/- 6.5%). On day 2, muscle glycogen and MG values remained lower than prerace values, despite a carbohydrate-rich diet, but they had both returned to prerace levels on day 7. The PG concentration was lower on day 1 compared with before the race, whereas there were no significant differences between the prerace PG concentration and the concentrations on days 2 and 7. On day 2 the glycogen concentration was particularly low in the type I fibers, indicating that local processes are important for the accumulation pattern. We conclude that a greater fraction of human muscle MG than of PG is utilized during a marathon and that accumulation of MG is particularly delayed after the prolonged exercise bout. Furthermore, factors produced locally appear important for the glycogen accumulation pattern.