Impact of weekly frequency of high-intensity interval training on cardiorespiratory, metabolic, and performance measures in recreational runners - An exploratory study.

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 (VO2max: 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 (VO2max), 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 VO2max and TTE were observed across all training frequencies (p < 0.05), with moderate-to-large effect sizes (Cohen's d [95% CI] for VO2max = 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 VO2max 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 VO2max 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.