High-Intensity Interval Training Increases Cardiac Output and VO2max.

Increases in maximal oxygen uptake (VO2max) frequently occur with high-intensity interval training (HIIT), yet the specific adaptation explaining this result remains elusive. PURPOSE: This study examined changes in VO2max and cardiac output (CO) in response to periodized HIIT. METHODS: Thirty nine active men and women (mean age and VO2max = 22.9 +/- 5.4 yr and 39.6 +/- 5.6 mL/kg/min) performed HIIT and 32 men and women (age and VO2max = 25.7 +/- 4.5 yr and 40.7 +/- 5.2 mL/kg/min) were non-exercising controls (CON). The first 10 sessions of HIIT required eight to ten 60 s bouts of cycling at 90 - 110 percent peak power output (PPO) interspersed with 75 s recovery, followed by randomization to one of three regimes (sprint interval training (SIT), high-volume interval training (HIITHI), or periodized interval training (PER) for the subsequent 10 sessions. Before, midway, and at the end of training, progressive cycling to exhaustion was completed during which VO2max and maximal CO (COmax) were estimated. RESULTS: Compared to CON, significant (p

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