The effectiveness of polarized versus time-matched high-intensity interval training in untrained individuals

Training intensity distribution (TID) refers to the structured allocation of training volume across the moderate, heavy, and severe intensity zones (Rosenblat et al., 2025). Various TIDs have been shown to differentially alter mitochondrial adaptation (Granata et al., 2018), possibly even influenced by training status (Rosenblat et al., 2025). We sought to test the differential adaptive effects of polarized (POL) and high-intensity interval (HIIT) training in sedentary and recreationally active individuals attempting to meet the WHO physical activity minimum guidelines of approximately 150 minutes/week (WHO, 2010). To date, 12 sedentary and recreationally active individuals were randomly allocated to six weeks of either HIIT (four HIIT sessions per week) or POL (two HIIT sessions and two zone two sessions per week) cycling. Participants were stratified by age (18-35 or 36-60), sex (male or female), and training status (tier 0 or 1; McKay et al, 2021). Each HIIT session consisted of four intervals of four minutes at 80% work rate peak (WRpeak), spaced with three minutes of unloaded cycling between each interval. Zone two sessions consisted of 60 minutes of continuous cycling at 95% of the first lactate threshold work rate (LT). Muscle biopsies were taken pre and post training, while performance testing (WRpeak, LT test) took place pre, mid (week 3), and post training. At week 3 in the HIIT group change in both WRpeak (HIIT: 22.883 ∓ 6.309; POL: 22.880 ∓ 7.891 W) and LT (HIIT: 18.67 ∓ 27.558 W; POL: 6.40 ∓ 8.764 W) was higher, but these differences were not significant (p = 0.999, p = 0.368). Findings from the complete dataset and muscle biopsy analysis (primary outcome, ongoing) will inform how individuals seeking to meet WHO physical activity guidelines should structure their weekly training to optimize aerobic and mitochondrial adaptations.