Is moderate intensity exercise training combined with high intensity interval training more effective at improving cardiorespiratory fitness than moderate intensity exercise training alone?

The purpose of this study was to compare the effectiveness of either continuous moderate intensity exercise training (CMIET) alone vs. CMIET combined with a single weekly bout of high intensity interval training (HIIT) on cardiorespiratory fitness. Twenty nine sedentary participants (36.3 ± 6.9 yrs) at moderate risk of cardiovascular disease were recruited for 12 weeks of exercise training on a treadmill and cycle ergometer. Participants were randomised into three groups: CMIET + HIIT (n = 7; 8-12 x 60 sec at 100% VO_2max, 150 sec active recovery), CMIET (n = 6; 30 min at 45-60% oxygen consumption reserve (VO₂R)) and a sedentary control group (n = 7). Participants in the CMIET + HIIT group performed a single weekly bout of HIIT and four weekly sessions of CMIET, whilst the CMIET group performed five weekly CMIET sessions. Probabilistic magnitude-based inferences were determined to assess the likelihood that the true value of the effect represents substantial change. Relative VO_2max increased by 10.1% (benefit possible relative to control) in in the CMIET + HIIT group (32.7 ± 9.2 to 36.0 ± 11.5 mL·kg^-1·min^-1) and 3.9% (benefit possible relative to control) in the CMIET group (33.2 ± 4.0 to 34.5 ± 6.1 mL·kg^-1·min^-1), whilst there was a 5.7% decrease in the control group (30.0 ± 4.6 to 28.3 ± 6.5 mL·kg^-1·min^-1). It was 'unclear' if a clinically significant difference existed between the effect of CMIET + HIIT and CMIET on the change in VO_2max. Both exercising groups showed clinically meaningful improvements in VO_2max. Nevertheless, it remains 'unclear' whether one type of exercise training regimen elicits a superior improvement in cardiorespiratory fitness relative to its counterpart.