High-Intensity Interval Training Increases Cardiac Output and (V) over dotO2max

Increases in maximal oxygen uptake ((V) over dotO(2max)) frequently occur with high-intensity interval training (HIIT), yet the specific adaptation explaining this result remains elusive. Purpose: This study examined changes in (V) over dotO(2max) and cardiac output (CO) in response to periodized HIIT. Methods: Thirty-nine active men and women (mean age and (V) over dotO(2max) = 22.9 +/- 5.4 yr and 39.6 +/- 5.6 mL.kg(-1).min(-1)) performed HIIT and 32 men and women (age and (V) over dotO(2max) = 25.7 +/- 4.5 yr and 40.7 +/- 5.2 mL.kg(-1).min(-1)) were nonexercising controls (CON). The first 10 sessions of HIIT required eight to ten 60 s bouts of cycling at 90%-110% percent peak power output 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 (V) over dotO(2max) and maximal CO were estimated. Results: Compared with CON, significant (P < 0.001) increases in (V) over dotO(2max) in HIIT + SIT (39.8 +/- 7.3 mL.kg(-1).min(-1) to 43.6 +/- 6.1 mL.kg(-1).min(-1)), HIIT + HIITHI (41.1 +/- 4.9 mL.kg(-1).min(-1) to 44.6 +/- 7.0 mL.kg(-1).min(-1)), and HIIT + PER (39.5 +/- 5.6 mL.kg(-1).min(-1) to 44.1 +/- 5.4 mL.kg(-1).min(-1)) occurred which were mediated by significant increases in maximal CO (20.0 +/- 3.1 L.min(-1) to 21.7 +/- 3.2 L.min(-1), P = 0.04). Maximal stroke volume was increased with HIIT (P = 0.04), although there was no change in maximal HR (P = 0.88) or arteriovenous O-2 difference (P = 0.36). These CO data are accurate and represent the mean changes from pre-to post-HIIT across all three training groups. Conclusions: Increases in (V) over dotO(2max) exhibited in response to different HIIT regimes are due to improvements in oxygen delivery.