A raised metabolic rate slows pulmonary O2 uptake kinetics on transition to moderate-intensity exercise in humans independently of work rate

During exercise below the lactate threshold (LT), the rate of adjustment (tau) of pulmonary O-2 uptake ((V)over dotO(2)) is slowed when initiated from a raised work rate. Whether this is consequent to the intrinsic properties of newly recruited muscle fibres, slowed circulatory dynamics or the effects of a raised metabolism is not clear. We aimed to determine the influence of these factors on tau(V)over dotO(2) using combined in vivo and in silico approaches. Fifteen healthy men performed repeated 6 min bouts on a cycle ergometer with work rates residing between 20 W and 90% LT, consisting of the following: (1) two step increments in work rate (S1 and S2), one followed immediately by the other, equally bisecting 20W to 90% LT; (2) two 20W to 90% LT bouts separated by 30 s at 20W to raise muscle oxygenation and pretransition metabolism (R1 and R2); and (3) two 20W to 90% LT bouts separated by 12 min at 20W allowing full recovery (F1 and F2). Pulmonary O-2 uptake was measured breath by breath by mass spectrometry and turbinometry, and quadriceps oxygenation using near-infrared spectroscopy. The influence of circulatory dynamics on the coupling of muscle and lung tau(V)over dotO(2) was assessed by computer simulations. The tau(V)over dotO(2) in R2 (32 +/- 9 s) was not different (P>0.05) from S2 (30 +/- 10 s), but both were greater (P<0.05) than S1 (20 +/- 10 s) and the F control bouts (26 +/- 10 s). The slowed (V)over dotO(2) kinetics in R2 occurred despite muscle oxygenation being raised throughout, and could not be explained by slowed circulatory dynamics (tau(V)over dotO(2) predicted by simulations: S1=R2<S2). These data therefore suggest that the dynamics of muscle O-2 consumption are slowed when exercise is initiated from a less favourable energetic state.