Chronic effects of high-intensity functional training on motor function: a systematic review with multilevel meta-analysis

High-intensity functional training (HIFT) has become a popular method in the sports and fitness sector. In contrast to unimodal approaches such as strength or endurance training, it has been hypothesized to induce concurrent adaptations in multiple markers of motor function. However, to date, the effectiveness of HIFT in this regard has not been studied. The present systematic review quantified the chronic effects of HIFT on motor function in healthy individuals. A multilevel meta-analysis with a robust random effects meta-regession model was used to pool the standardized mean differences (SMD) between (a) HIFT and (b) no-exercise (NEX) as well as conventional endurance, resistance and balance training for outcomes of muscle strength, endurance capacity and balance. The influence of possible effect modifiers such as program duration, session duration, age or sex was examined in a moderator analysis. Seventeen papers with moderate to high methodological quality (PEDro scale) were identified. Compared to NEX, HIFT had small to moderate positive effects on endurance capacity (SMD: 0.42, 95% CI 0.07-0.78, p=0.03) and strength (0.60, 95% CI 0.02-1.18, p=0.04) but no effect on balance (SMD: - 0.10, 95% CI - 1.13 to 0.92, p=0.42). Regarding endurance, HIFT showed similar effectiveness as moderate-intensity endurance training (SMD: - 0.11, 95% CI - 1.17 to 0.95, p=0.75) and high-intensity interval endurance training (SMD: - 0.15, 95% CI - 1.4 to 1.1, p=0.66). No comparisons of HIFT vs. classical resistance or balance training were found. Moderator analyses revealed no influence of most effect modifiers. However, regarding endurance, females seemed to respond more strongly to HIFT in the comparison to NEX (p<.05). HIFT appears to represent an appropriate method to induce chronic improvements in motor function. While being superior to NEX and non-inferior to endurance training, current evidence does not allow a comparison against resistance and balance training. The impact of possible effect moderators should be further elucidated in future research.