Increased cardiac output and maximal oxygen uptake in response to ten sessions of high intensity interval training

BACKGROUND : Increases in maximal oxygen uptake (VO2max) are widely reported in response to completion of high intensity interval training (HIIT), yet the mechanism explaining this result is poorly understood. This study examined changes in VO2max and cardiac output (CO) in response to 10 sessions of low-volume HIIT. METHODS: Participants included 30 active men and women (mean age and VO2max = 22.9 +/- 5.4 years and 39.6 +/- 5.6 mL/kg/min) who performed HIIT and 30 men and women (age and VO2max = 25.7 +/- 4.5 years and 40.7 +/- 5.2 mL/kg/min) who served as non-exercising controls (CON). High intensity interval training consisted of 6-10 s bouts of cycling per session at 90-110 percent peak power output (PPO) interspersed with 75 s recovery. Before and after training, progressive cycling to exhaustion was completed during which CO, stroke volume (SV), and heart rate (HR) were estimated using thoracic impedance. To confirm VO2max attainment, a verification test was completed after progressive cycling at a work rate equal to 110% PPO. RESULTS: Data demonstrated significant improvements in VO2max (2.71 +/- 0.63 L/min to 2.86 +/- 0.63 L/min, P<0.001) and CO max (20.0 +/- 3.1 L/min to 21.7 +/- 3.2 L/min, P=0.04) via HIIT that were not exhibited in CON. Maximal SV was increased in HIIT (P=0.04) although there was no change in maximal HR (P=0.57). CONCLUSIONS: The increase in VO2max seen in response to ten sessions of HIIT is due to improvements in oxygen delivery.