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

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Abstract
Glycogen depletion of muscle fibre types I, IIA, IIAB and IIB was studied using a histochemical method to quantify glycogen content in individual fibres. 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 mmole glucosyl units. kg‐1 wet weight) a linear relationship (r=0.93) was found between the two methods for quantification of muscle glycogen. Glycogen depletion patterns in type I, IIA, IIAB and IIB fibres were studied in 5 subjects during exhaustive bicycle exercise at 75% of VO2 max. 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 IIA.Glycogen content of Type IIAB and IIB was unchanged during the first part of exercise. Later a decrease was observed, first in type IIAB and finally in IIB, suggesting a decrease in threshold force of these fibre types. The results indicate physiological differences between the 3 subgroups of type II fibres in man, whereas at the present exercise intensity type I and IIA fibres were recruited simultaneously from start.

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