Muscle glycogen depletion patterns in type I and subgroups of type II fibres during prolonged severe exercise in man

Abstract

Glycogen depletion of muscle fiber types I, II A, II AB and II B was studied using a histochemical method to quantify glycogen content in individual fibers. The reliability was examined in 29 muscle biopsies, in which total glycogen content was compared to average periodic acid Schiff (PAS) stain intensity in sections from the same samples. Over a wide range of glycogen content (1-252 mmol glucosyl units .cntdot. kg-1 wet wt) a linear relationship (r = 0.93) was found between the 2 methods for quantification of muscle glycogen. Glycogen depletion patterns in type I, II A, II AB and II B fibers were studied in 5 subjects during exhaustive bicycle exercise at 75% of VO2max. At rest before exercise glycogen content was 16% higher in type II subgroups than in type I (P < 0.05). From start of exercise the same glycogen depletion rate was observed in type I and II A. Glycogen content of type II AB and II B was unchanged during the 1st part of exercise. Later a decrease was observed, 1st in type II AB and finally in II B, suggesting a decrease in threshold force of these fiber types. Physiological differences are indicated between the 3 subgroups of type II fibers in man, whereas at the present exercise intensity type I and II A fibers were recruited simultaneously from start.