Effects of high-intensity interval training on aerobic capacity and muscle adaptation: a systematic review and meta-analysis

Background and purpose High-intensity interval training (HIIT) is widely implemented in sport and exercise settings to enhance aerobic capacity and induce muscular adaptations. However, findings are dispersed across diverse protocols and populations, and the relative effectiveness of different HIIT configurations remains unclear. This systematic review and meta-analysis aimed to synthesise experimental evidence on the effects of HIIT on (a) aerobic capacity and (b) indicators of muscle adaptation in adult sport, exercise, and clinical populations, and to examine how protocol characteristics and participant profiles moderate these effects. Material and methods Major databases were systematically searched in accordance with PRISMA guidelines for controlled trials examining HIIT interventions in adults (≥18 years) reporting outcomes related to aerobic fitness (e.g., VO₂max/VO₂peak, lactate threshold) and muscular adaptation (e.g., oxidative enzyme activity, muscle fibre characteristics, hypertrophy). Risk of bias and methodological quality were appraised using established scales, and random-effects meta-analyses were conducted where outcomes were sufficiently homogeneous. Subgroup and exploratory moderator analyses considered training status, setting (sport, recreational, clinical), interval structure, intensity, and intervention duration. Results Twenty-four experimental studies met the inclusion criteria and were included in the quantitative synthesis. Across studies, HIIT produced consistent, moderate improvements in aerobic capacity compared with non-exercising controls, with effects that were comparable to or greater than those observed with traditional moderate-intensity continuous training. Evidence for muscular adaptations was more heterogeneous but generally supported beneficial changes in oxidative capacity and, in some protocols, muscle hypertrophy. Interventions prescribing intensities at or above ~90% of maximal heart rate and lasting at least six weeks tended to elicit the most pronounced improvements, although estimates were based on a limited number of studies and heterogeneous protocols. Conclusions HIIT appears to be an efficient strategy for improving aerobic capacity and supporting muscular adaptations in adults across sport, recreational, and selected clinical contexts. Nevertheless, conclusions are tempered by short intervention durations, variability in protocol design, and under-representation of female and older participants. Future trials should prioritise standardised reporting of HIIT parameters, longer-term follow-up, and more diverse samples to refine dose–response recommendations for different populations and settings.