Oxygen uptake kinetics during moderate, heavy and severe intensity 'submaximal' exercise in humans: the influence of muscle fibre type and capillarisation

The purpose of the present study was to test the hypothesis that muscle fibre type influences the oxygen uptake ((V)over dotO(2)) on-kinetic response (primary time constant; primary and slow component amplitudes) during moderate, heavy and severe intensity sub-maximal cycle exercise. Fourteen subjects [10 males, mean (SD) age 25 (4) years; mass 72.6 (3.9) kg; (V)over dotO(2peak) 47.9 (2.3) ml kg(-1) min(-1)] volunteered to participate in this study. The subjects underwent a muscle biopsy of the vastus lateralis for histochemical determination of muscle fibre type.. and completed repeat 'square-wave' transitions from unloaded cycling to power outputs corresponding to 80% of the ventilatory threshold (VT; moderate exercise), 50% (heavy exercise) and 70% (severe exercise) of the difference between the VT and (V)over dotO(2peak). Pulmonary (V)over dotO(2) was measured breath-by-breath. The percentage of type I fibres was significantly correlated with the time constant of the primary (V)over dotO(2) response for heavy exercise (r = -0.68). Furthermore, the percentage of type I muscle fibres was significantly correlated with the gain of the (V)over dotO(2) primary component for moderate (r = 0.65), heavy (r = 0.57) and severe (r = 0.57) exercise, and with the relative amplitude of the (V)over dotO(2) Slow component for heavy (r = -0.74) and severe (r = -0:64) exercise. The influence of muscle fibre type on the (V)over dotO(2) on-kinetic response persisted when differences in aerobic fitness and muscle capillarity were accounted for. This study demonstrates that muscle fibre type is significantly related to both the speed and the amplitudes of the (V)over dotO(2) response at the onset of constant-load sub-maximal exercise. Differences in contraction efficiency and oxidative enzyme activity between type I and type II muscle fibres may be responsible for the differences observed.