Health-Related Effects of Individual and Paired Functional High-Intensity Interval Training on Body Composition, Strength and VO2max in Primary School Children

Background/Objectives: In the school setting, high-intensity interval training (HIIT) has emerged as a time-efficient strategy to improve children’s physical fitness; however, different implementation modalities have not been compared. The aim of this study was to compare the effects of an individual versus paired HIIT protocol based on functional bodyweight exercises on physical fitness-related and anthropometric outcomes in primary school children. Methods: Sixty-one children (11.6 ± 0.3 years) participated in a 10-week experimental study with three parallel groups: individual HIIT (EG1, n = 21), paired HIIT (EG2, n = 20), and a control group (CG, n = 20). Although both HIIT groups performed the same bodyweight functional exercises, in EG2 the exercises required coordinated movement between the partners. The HIIT protocol was integrated into the warm-up of Physical Education (PE) classes twice per week (Tabata-type protocol; 8 × 20 s/10 s/≤8 min per session). Body composition, muscular strength, and cardiorespiratory fitness (estimated VO2max) were assessed at pre- and post-test, along with a rating of perceived exertion (1–10 scale) and enjoyment/motivation (1–5 scale) across several sessions (1, 7 and 14). Data were analyzed using pre-post comparisons, ANOVA, and ANCOVA models adjusted for baseline values. Results: Body fat percentage decreased in all groups. The individual HIIT group showed within-group improvements in VO2max (+5.3%, p < 0.001), handgrip strength (+10.1%, p = 0.003), and standing long jump (+4.1%, p = 0.033), with moderate-to-large effect sizes, whereas the paired HIIT group showed smaller and statistically non-significant changes. Between-group comparisons suggested a tendency toward greater improvements in VO2max and handgrip strength in the individual HIIT group compared with the paired group, although the overall ANOVA for VO2max was not statistically significant. Perceived exertion declined over time in the paired group but remained relatively stable in the individual group. Conclusions: A low-volume HIIT program performed individually was associated with improvements in several physical fitness outcomes in schoolchildren. In contrast, paired execution showed smaller and mostly non-significant changes, together with a progressive reduction in perceived intensity.