Impact of weekly frequency of high-intensity interval training on cardiorespiratory, metabolic, and performance...

This exploratory study examined the effects of varying frequencies of 4 × 4-min high-intensity interval training (HIIT) sessions (once, twice, or thrice weekly) on cardiorespiratory, metabolic, perceptual, and performance variables in recreationally active individuals. Twenty-six participants (VO_2max: 51.3 ± 7.1 mL·kg^-1·min^-1) completed a six-week intervention, utilizing individualized HIIT protocols based on maximal treadmill speed and heart rate. Pre- and post-assessments involved an incremental treadmill test evaluating maximal oxygen uptake (VO_2max), time to exhaustion (TTE), ventilatory threshold (VT₁), submaximal heart rate, and running economy (i.e., submaximum oxygen uptake). Adherence rates ranged from 91% to 100%. Significant improvements in VO_2max and TTE were observed across all training frequencies (p < 0.05), with moderate-to-large effect sizes (Cohen's d [95% CI] for VO_2max = 0.09-0.61 [-0.56 to 1.42]; TTE: 0.02-0.77 [-0.65 to 1.61]). The largest effects (d > 0.5) were found for VO_2max and TTE in the groups training two or three times per week. 2-3 weekly sessions of 4 × 4-min HIIT may be effective for improving VO_2max and TTE in recreationally active individuals, with no clear additional benefit from increasing the frequency to three sessions per week. These preliminary findings highlight the potential of twice-weekly HIIT as a practical and time-efficient training strategy, while acknowledging limited precision.