Effects of progressive-versus constant-intensity high-intensity functional training on health-related physical fitness in untrained female university students: A randomized controlled trial

BACKGROUND/OBJECTIVE: High-intensity functional training (HIFT) may be feasible for university physical education, but the effects of different intensity-organization strategies with a fixed session duration remain unclear. This exploratory randomized controlled trial compared progressive-intensity HIFT, constant-intensity HIFT, and moderate-intensity continuous training (MICT) in untrained female university students. METHODS: Of 75 randomized participants, 60 completed the 9-week intervention and were included in the primary analysis: progressive-intensity HIFT (n = 20), constant-intensity HIFT (n = 20), and MICT (n = 20). All groups trained three times per week with matched session duration. Training dose was monitored using %HRmax, session-RPE, and internal load. The primary outcome was the Physical Fitness Index (PFI); secondary outcomes included BMI, vital capacity, and individual physical fitness indicators. Between-group effects were assessed using ANCOVA, with bootstrap sensitivity analyses for outcomes with unequal variances. RESULTS: PFI differed significantly among groups (F(2, 56) = 79.46, p < 0.001, partial η² = 0.739), showing a graded pattern from progressive-intensity HIFT to constant-intensity HIFT and MICT. Significant group effects were also found for the 50-m sprint, standing long jump, sit-ups, and 800-m run (partial η² = 0.341-0.475, all p < 0.001). Progressive-intensity HIFT also outperformed constant-intensity HIFT in sit-ups and the 800-m run. CONCLUSION: Under time-matched class conditions, both HIFT protocols were associated with greater improvements in health-related physical fitness than MICT, while Progressive HIFT showed additional improvements over Constant HIFT. These findings should be interpreted in light of the higher cumulative internal load observed with Progressive HIFT. TRIAL REGISTRATION: Chinese Clinical Trial Registry (ChiCTR2600126609); retrospectively registered.