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

The study addresses whether high-intensity functional training (HIFT)—characterized by strenuous, functional whole-body movements performed with brief rests—can produce chronic, concurrent improvements in multiple motor abilities (strength, endurance, balance) in healthy individuals. HIFT contrasts with unimodal high-intensity protocols focused on a single motor ability. Its time-efficient nature (often <30 minutes) and reported effects on motivation and adherence suggest practical benefits for inactive individuals, while athletes may benefit because sports performance depends on multiple motor functions rather than isolated capacities. Despite increasing popularity, the chronic effectiveness of HIFT across key motor outcomes had not been systematically quantified, motivating this review and meta-analysis.

Background literature indicates HIFT integrates cardiovascular, neuromotor, and muscular demands using whole-body exercises and scalable loads. Prior work highlights time efficiency as a facilitator of physical activity uptake and suggests higher intrinsic motivation and enjoyment compared with moderate-intensity continuous exercise. In athletic contexts, isolated motor markers (e.g., strength or endurance alone) weakly predict performance, supporting interest in multimodal training approaches like HIFT. However, before this review, no comprehensive synthesis of HIFT’s chronic effects on strength, endurance, and balance existed.

Design: Systematic review with multilevel meta-analysis and robust random-effects meta-regression, reported per PRISMA and registered in PROSPERO (CRD42020170412). Search: MEDLINE (PubMed), Cochrane Central, Web of Science, and Google Scholar searched (Feb–Mar 2020). Example PubMed query: ‘high-intensity AND (functional OR body weight) AND (exercise OR training OR workout OR circuit OR conditioning) NOT acute.’ Reference lists of included studies were also screened. Inclusion criteria: Randomized controlled trials (parallel-group or crossover), full text available, healthy participants, HIFT intervention at high relative intensity targeting multiple motor functions with multiple functional whole-body movements, chronic training (≥4 weeks), outcomes in strength, endurance, or balance, comparator of no-exercise (NEX), endurance, resistance, or balance training, English, peer-reviewed. Exclusions: acute effects only, combined/other treatments not meeting HIFT criteria, inadequate control groups, clinical populations. Data extraction: Two reviewers independently extracted design, sample, intervention characteristics, outcomes, and pre–post changes with SDs. If multiple measures existed within a domain (strength, endurance, balance), all effect sizes were extracted. Data synthesis and statistics: For parallel-group trials, pre–post mean changes and SDs were used; missing SDs were requested or derived from figures/statistics, or imputed via SD_change = sqrt(SD_baseline^2 + SD_post^2 − 2 × Corr × SD_baseline × SD_post) with Corr=0.7. For crossover trials, SD of differences and correlations were calculated; if unavailable, Corr=0.5 was assumed. Standardized mean differences (SMDs) and SEs were computed using appropriate formulae for combining parallel and crossover studies. A multilevel meta-analysis with study as a random factor accounted for dependent effect sizes. Between-study variance (Tau²) and within-study variance for dependent effects (ω²) were estimated. Potential moderators (program duration, session duration, total volume, interval/rest durations, age, sex) were dichotomized and tested in separate models via 95% CI inspection and level comparisons. Effect size interpretation: small 0.2, moderate 0.5, large 0.8; p<0.05 significant. Software: R with meta and robumeta packages. Risk of bias and quality: Publication bias assessed via funnel plots with sensitivity analyses when ≥10 effect sizes were available. Methodological quality was rated using the PEDro scale by two independent reviewers; disagreements were resolved by discussion.

Study/sample: Sixteen RCTs (15 parallel-group, 1 crossover) with 864 participants (302 men, 458 women; two studies with unknown sex distribution), ages 11.7–72.8 years. Comparisons included HIFT vs NEX (10 papers), HIFT vs HIIT (4), HIFT vs MCT (7); none compared HIFT to classic resistance or balance training. HIFT vs no exercise (NEX): - Endurance capacity: SMD 0.42 (95% CI 0.07 to 0.78), p=0.03; Tau² 0.19, ω² 0; 8 studies, 18 ES. - Muscle strength: SMD 0.60 (95% CI 0.02 to 1.18), p=0.04; Tau² 0.50, ω² 0; 7 studies, 44 ES. - Balance: SMD 0.10 (95% CI −1.13 to 0.92), p=0.42; Tau² 0, ω² 0.01; 2 studies, 3 ES. Moderators (HIFT vs NEX): Most moderators showed no significant effects. Sex moderated endurance responses: females appeared to respond more strongly than mixed samples (p=0.02). Insufficient data for male-only comparisons and for balance moderators. HIFT vs endurance training: - Versus moderate continuous training (MCT): SMD −0.11 (95% CI −1.17 to 0.95), p=0.75; Tau² 0.29, ω² 0.06; 4 studies, 9 ES. - Versus high-intensity interval training (HIIT): SMD 0.15 (95% CI −1.4 to 1.1), p=0.66; Tau² 0.048, ω² 0; 3 studies, 5 ES. Risk of bias/quality: PEDro scores ranged 4–7/10 (mean 5.9±0.9; moderate). Funnel plots suggested possible reporting bias for HIFT vs NEX in endurance and strength due to a few small, imprecise studies; sensitivity analyses excluding outliers did not change conclusions. Reporting bias could not be assessed for HIFT vs MCT/HIIT due to <10 ES.

The findings indicate HIFT produces small-to-moderate chronic improvements in endurance and muscle strength relative to no exercise, with no clear effect on balance. For endurance outcomes, HIFT is non-inferior to both moderate continuous training and HIIT, supporting its use as an alternative training modality. Potential mechanisms include cardiopulmonary demands during HIFT comparable to endurance training (high heart rates and lactate), substantial time under tension from high repetitions with short rests promoting protein synthesis, and high metabolic stress and inflammatory signaling (e.g., IL-6) conducive to muscular adaptations. Practically, HIFT’s time efficiency and multimodal stimulus make it attractive for inactive individuals facing time barriers and for practitioners seeking comprehensive fitness improvements. The identified sex-related moderator suggests women may achieve greater endurance benefits compared with mixed samples in HIFT vs NEX comparisons, although data are limited. Overall, the results address the research question by showing HIFT can elicit concurrent improvements in key motor domains and perform comparably to traditional endurance training for endurance outcomes.

HIFT is an effective, time-efficient modality to improve muscle strength and endurance capacity in healthy individuals. It is superior to no exercise and non-inferior to traditional endurance training (MCT and HIIT) for endurance outcomes. Evidence is currently insufficient to compare HIFT directly with classical resistance or balance training, and effects on balance remain unclear. Future research should: (1) include direct comparisons with resistance and balance training; (2) examine additional performance domains (e.g., speed, power); (3) clarify moderators such as age and sex; (4) include pediatric and elderly populations; and (5) address potential reporting biases and methodological limitations with larger, rigorously designed trials.

Potential publication/reporting bias indicated by funnel plot asymmetry for HIFT vs NEX in endurance and strength (though sensitivity analyses did not alter conclusions). Limited number of studies and effect sizes for certain comparisons (HIFT vs MCT/HIIT) precluded formal bias assessments and moderator analyses. Few trials assessed balance outcomes, and none compared HIFT directly to resistance or balance training. Participant samples were heterogeneous, with few studies in older adults or children and limited sex-specific analyses. Methodological quality was moderate (PEDro), with common limitations including lack of concealed allocation and blinding. Some outcome variances were imputed due to incomplete reporting.