High-intensity interval training speeds the adjustment of pulmonary O2 uptake, but not muscle deoxygenation, during moderate-intensity exercise transitions initiated from low and elevated baseline metabolic rates

During step transitions in work rate (WR) within the moderate-intensity (MOD) exercise domain, pulmonary O-2 uptake ((V) over dotO(2p)) kinetics are slowed, and (V) over dotO(2p) gain (Delta(V) over dotO(2p)/Delta WR) is greater when exercise is initiated from an elevated metabolic rate. High-intensity interval training (HIT) has been shown to speed (V) over dotO(2p) kinetics when step transitions to MOD exercise are initiated from light-intensity baseline metabolic rates. The effects of HIT on step transitions initiated from elevated metabolic rates have not been established. Therefore, this study investigated the effects of HIT on (V) over dotO(2p) kinetics during transitions from low and elevated metabolic rates, within the MOD domain. Eight young, untrained men completed 12 sessions of HIT (spanning 4 wk). HIT consisted of 8-12 1-min intervals, cycling at a WR corresponding to 110% of pretraining maximal WR (WRmax). Pre-, mid- and posttraining, subjects completed a ramp-incremental test to determine maximum O-2 uptake, WRmax, and estimated lactate threshold ((theta) over cap (L)). Participants additionally completed double-step constant-load tests, consisting of step transitions from 20 W -> Delta 45% (theta) over cap (L) [lower step (LS)] and Delta 45 -> 90% (theta) over cap (L) [upper step (US)]. HIT led to increases in maximum O-2 uptake (P < 0.05) and WRmax (P < 0.01), and tau(V) over dotO(2p) of both lower and upper MOD step transitions were reduced by similar to 40% (LS: 24 s -> 15 s; US: 45 s -> 25 s) (P < 0.01). However, the time course of adjustment of local muscle deoxygenation was unchanged in the LS and US. These results suggest that speeding of (V) over dotO(2p) kinetics in both the LS and US may be due, in part, to an improved matching of muscle O-2 utilization to microvascular O-2 delivery within the working muscle following 12 sessions of HIT, although muscle metabolic adaptations cannot be discounted.