High-intensity functional training (HIFT), involving strenuous exercises mimicking daily movements with short breaks, has gained popularity. Unlike unimodal training focusing on a single motor ability, HIFT aims for concurrent improvements in cardiovascular, neuromotor, and muscular functions. This is achieved through whole-body exercises maximizing oxygen consumption, fast movement execution, and optional weights. HIFT offers potential advantages, especially for inactive individuals due to its short duration (often under 30 minutes), potentially increasing adherence compared to longer conventional programs. The multidimensional nature of HIFT also contrasts with the limitations of unimodal approaches in sports, where isolated motor abilities are often weakly predictive of overall performance. This systematic review aims to quantify the chronic effects of HIFT on motor function (strength, endurance, balance) in healthy individuals, comparing it to no exercise and conventional training methods.

The introduction cites several studies supporting the potential benefits of HIFT. Reference 1 mentions HIFT as a trending fitness method. Reference 2 defines HIFT and its research implications. Reference 3 highlights the time constraint barrier for physical activity, making the short duration of HIFT workouts appealing. Reference 4 suggests stronger intrinsic motivation with HIFT compared to moderate-intensity exercise. References 5, 7, and 8 demonstrate the limited predictive validity of isolated motor function markers in team sports, further justifying the investigation into the comprehensive effects of HIFT.

This systematic review and meta-analysis followed PRISMA guidelines and ethical publishing recommendations. A literature search across MEDLINE, Cochrane Central, Web of Science, and Google Scholar (February-March 2020) identified randomized controlled trials (RCTs) with accessible full texts. Inclusion criteria included healthy participants, HIFT interventions lasting at least four weeks, assessment of strength, endurance, or balance, and control groups (inactivity, strength, endurance, or balance training). Data extraction involved two independent investigators using a standardized sheet, obtaining pre-post changes and standard deviations. Incomplete data were either requested from authors or imputed using established methods. A multilevel meta-analysis with a robust random-effects meta-regression model pooled standardized mean differences (SMDs) between HIFT and control groups (no exercise, moderate continuous aerobic training, high-intensity interval training, resistance training, balance training). Moderator analysis examined program duration, session duration, age, and sex. Methodological quality was assessed using the PEDro scale. Publication bias was checked through funnel plots and sensitivity analyses.

The search yielded 16 studies (864 participants, 302 men, 458 women; ages 11.7-72.8 years). Ten studies compared HIFT vs. no exercise; four compared HIFT vs. high-intensity interval training (HIIT); and seven compared HIFT vs. moderate continuous training (MCT). No studies compared HIFT against resistance or balance training. Compared to no exercise, HIFT showed small to moderate positive effects on endurance capacity (SMD: 0.42, 95% CI: 0.07-0.78, p=0.03) and strength (SMD: 0.60, 95% CI: 0.02-1.18, p=0.04). There was no significant effect on balance (SMD: 0.10, 95% CI: -1.13 to 0.92, p=0.42). For endurance, HIFT was non-inferior to MCT and HIIT. Moderator analysis revealed no significant influence of most effect modifiers, except for sex: females showed a stronger response to HIFT regarding endurance compared to mixed-sex samples (p<0.05). PEDro scale scores indicated moderate methodological quality (mean 5.9 ± 0.9). Funnel plots suggested potential publication bias for HIFT vs. no exercise, but sensitivity analyses did not change conclusions.

This study is the first to comprehensively analyze the long-term effects of HIFT on motor performance. The positive findings on endurance and strength suggest HIFT as a viable alternative to conventional training, particularly for individuals who dislike traditional methods or have time constraints. The cardiopulmonary demands during HIFT explain its effectiveness on endurance, while the prolonged time under tension and high metabolic stress contribute to strength gains. The lack of significant balance improvements may reflect the limited focus on balance-specific exercises in most included studies. The greater response in females to HIFT suggests that sex-specific training programs should be considered. While the study highlights HIFT's potential, future research should address its effects on other motor abilities (speed, power), include comparisons against resistance and balance training, and account for potential publication bias in a more robust manner. It is also critical to broaden study participant demographics (e.g., children and older adults).

HIFT effectively improves muscle strength and endurance, offering a time-efficient, multidimensional training approach for both active and inactive individuals. However, the study's limitations highlight the need for future research focusing on age- and sex-specific responses, inclusion of other motor abilities, and comparisons with resistance and balance training to provide a more complete picture of HIFT's effects.

The study's limitations include potential publication bias (suggested by funnel plot asymmetry), the limited number of studies comparing HIFT to resistance and balance training, and the underrepresentation of certain demographic groups (children, older adults, males only). The moderate methodological quality of some included studies, as indicated by PEDro scores, could also influence the generalizability of the results. The relatively small sample sizes in some of the included studies also affect the robustness of the findings.