Effect of order of exercise intensity upon cardiorespiratory, metabolic, and perceptual responses during exercise of mixed intensity

Exercise of mixed intensities can be of benefit in many different ways. However, whether physiological interaction exists between exercises of different intensity is questionable. As such, the primary aim of this study was to examine the effect of order of exercise intensity upon cardiorespiratory, metabolic, and perceptual responses during exercise of mixed intensity. Eight males and four females volunteered to serve as subjects for the study. They were informed of the purpose of the experiment and gave their written consent to participate. Each subject completed a peak oxygen uptake (VO₂peak) test and two submaximal exercises of mixed intensity on three separate laboratory visits. During each submaximal exercise trial, subjects performed a 15-min (high intensity) exercise at 70%VO_2peak that was followed by another 15-min (low intensity) exercise at 50%VO_2peak (high/low, H/L), or a 15-min exercise at 50%VO_2peak that was followed by another 15-min exercise at 70%VO_2peak (low/high, L/H). Oxygen uptake (̇VO₂), respiratory exchange ratio (R), expired ventilation (V_E), heart rate (HR) and ratings of perceived exertion (RPE) were measured every 5 min throughout exercise. Energy expenditure and carbohydrate and fat oxidation were calculated from VO₂ adjusted for substrate metabolism using R and then accumulated for each phase of exercise intensity as well as for the entire exercise session. VO₂ and HR were higher (P < 0.05, while R was lower (P < 0.05) at the lower intensity in H/L than in L/H. V_E and RPE were lower (P < 0.05) at the higher intensity in H/L than in L/H. While no differences in caloric expenditure and carbohydrate oxidation between the two trials were observed, fat oxidation was higher (P < 0.05) both at the lower intensity and for the entire trial in H/L than in L/H. It appears that during exercise of mixed intensity, placing some periods of moderate intensity exercise prior to a milder one is a more favorable sequence in that it can elicit a greater fat oxidation while being felt less stressful.