This study examined changes in V˙O2max and cardiac output (CO) in response to periodized HIIT.Methods
Thirty-nine active men and women (mean age and V˙O2max = 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˙O2max = 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˙O2max and maximal CO were estimated.Results
Compared with CON, significant (P < 0.001) increases in V˙O2max 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 O2 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˙O2max exhibited in response to different HIIT regimes are due to improvements in oxygen delivery.