Muscle Glycogen Utilization and the Expression of Relative Exercise Intensity

Abstract

Previous research has shown that the rate of muscle glycogen utilization is related to exercise intensity expressed relative to maximal aerobic power (%V̇O₂max). The purpose of this study was to compare the relationship between glycogen utilization and %V̇O₂max to that between glycogen utilization and intensity expressed relative to the onset of blood lactate accumulation (%OBLA) during cycle exercise. It was hypothesized that the rate of glycogen utilization would be related more closely to intensity expressed as %OBLA than to intensity expressed as %V̇O₂max. Nineteen subjects (15 males and 4 females) performed two separate tests to determine V̇O₂max and OBLA during continuous incremental exercise. On a third occasion biopsies were taken from the m. vastus lateralis before and after 30 min of exercise at randomly assigned intensities ranging from 50-80% V̇O₂max, corresponding to 67-117% OBLA. There was a large inter-subject variation in aerobic fitness with V̇O₂max ranging from 34 to 66 mL·kg^-1·min^-1and OBLA ranging from 64-84% V̇O₂max. Absolute V̇O₂max and the V̇O₂at OBLA were correlated strongly (r = 0.90). The change in glycogen concentration during the 30-min exercise bout ranged from an increase of 58 to a depletion of 200 mmol glucose units·kg ^-1dry muscle weight. Neither absolute nor relative glycogen utilization was significantly related to the exercise intensity expressed as either %V̇O₂max or %OBLA. Stepwise multiple regression was used to identify variables which could account for the variation in glycogen depletion. From among indicators of aerobic fitness (V̇O₂max, OBLA) and exercise intensity (%V̇O₂max, %OBLA, absolute V̇O₂during exercise), V̇O₂max (mL·kg^-1· min ^-1) was the only variable which accounted for a significant amount of the intersubject variation in glycogen depletion (r= -0.56). The hypothesis was not accepted and the results suggest that the extent of muscle glycogen depletion among subjects of different sex and varied fitness levels cannot be predicted accurately from the relative expression of exercise intensity.

Key words

Onset of blood lactate accumulation - maximal oxygen consumption - individual variation - carbohydrate - fitness