Effect of short-term high-intensity interval training vs. continuous training on O2 uptake kinetics, muscle deoxygenation, and exercise performance

The early time course of adaptation of pulmonary O (2) uptake (Vo (2) (p) ) (reflecting muscle O (2) consumption) and muscle deoxygenation kinetics (reflecting the rate of O (2) extraction) were examined during high-intensity interval (HIT) and lower-intensity continuous endurance (END) training. Twelve male volunteers underwent eight sessions of either HIT (8-12 x 1-min intervals at 120% maximal O (2) uptake separated by 1 min of rest) or END (90-120 min at 65% maximal O (2) uptake). Subjects completed step transitions to a moderate-intensity work rate (approximately 90% estimated lactate threshold) on five occasions throughout training, and ramp incremental and constant-load performance tests were conducted at pre-, mid-, and posttraining periods. Vo (2) (p) was measured breath-by-breath by mass spectrometry and volume turbine. Deoxygenation (change in deoxygenated hemoglobin concentration; DeltaHHb) of the vastus lateralis muscle was monitored by near-infrared spectroscopy. The fundamental phase II time constants for Vo (2) (p) (tauVo (2) ) and deoxygenation kinetics effective time constant, tau' = (time delay + tau), DeltaHHb during moderate-intensity exercise were estimated using nonlinear least-squares regression techniques. The tauVo (2) was reduced by approximately 20% (P < 0. 05) after only two training sessions and by approximately 40% (P < 0. 05) after eight training sessions (i. e. , posttraining), with no differences between HIT and END. The tau'DeltaHHb (approximately 20 s) did not change over the course of eight training sessions. These data suggest that faster activation of muscle O (2) utilization is an early adaptive response to both HIT and lower-intensity END training. That DeltaHHb kinetics (a measure of fractional O (2) extraction) did not change despite faster Vo (2) (p) kinetics suggests that faster kinetics of muscle O (2) utilization were accompanied by adaptations in local muscle (microvascular) blood flow and O (2) delivery, resulting in a similar "matching" of blood flow to O (2) utilization. Thus faster kinetics of Vo (2) (p) during the transition to moderate-intensity exercise occurs after only 2 days HIT and END training and without changes to muscle deoxygenation kinetics, suggesting concurrent adaptations to microvascular perfusion.