Effect of varying exercise intensity on glycogen depletion in human muscle fibres

Glycogen depletion of muscle fibre types I, IIA, IIAB and IIB was studied during bicycle exercise at 43% (n = 5), 61% (n = 7) and 91% (n = 5) of VO2max. Glycogen content in individual fibres from vastus lateralis muscles was quantified as optical density of periodic acid-Schiff (PAS) stain. After 60 min at the lowest intensity, glycogen depletion was observed in almost all type I fibres and in about 20% of type IIA fibres. After 60 min exercise at 61% of VO2max, glycogen breakdown was observed in all type I fibres and in about 65% of type IIA fibres. During the first part of exercise at 91% of VO2max, glycogen breakdown was observed in all type I and IIA and in about 50% of type IIAB and IIB fibres. Muscle lactate concentration increased during the the first 5 min of exercise at 91% of VO2max to 15 mmol kg-1 (w/w) and remained thereafter at this level. From start of exercise the average rates of glycogen depletion in type I fibres were about 1.0, 2.0 and 4.3 mmol glucosyl units kg-1 (w/w) min-1 at 43%, 61% and 91% of VO2max. The depletion rates were almost constant with time at the two lower intensities. The results indicate that the number of fibres activated from the start increase gradually in response to increased exercise intensity. The rates of glycogen depletion in type I fibres suggest a progressive tension output of these fibres with increasing intensity.