Does concurrent high intensity interval training and resistance training impede resistance training induced hypertrophy and strength?

![CDATA[Introduction: Coaches often prescribe high-intensity interval training (HIIT) and resistance training (RT) in programs to improve athletic performance. Previous reviews have indicated that when combined with RT, aerobic exercise can impede muscular strength and/or hypertrophy. Whether these findings are replicated when HIIT is combined with RT remains unknown. Aim: The purpose of this systematic review and meta-analysis is to compare HIIT+RT to RT alone on measures of strength and hypertrophy. Methods: Five electronic databases were searched using terms related to HIIT, RT, and concurrent training. Inclusion criteria were: randomised comparative studies; healthy adults; compared HIIT+RT group to a RT alone group; duration ≥ 4 weeks; measured dynamic muscular strength and/or muscle hypertrophy changes. Effect size (ES) was used to examine the effect of HIIT+RT compared to RT alone on muscle strength and/or muscle hypertrophy. Results: 13 studies were included involving 246 participants. Compared to RT alone, HIIT+RT led to similar changes in muscle hypertrophy and upper body strength. However, greater lower body strength was found for RT alone when compared to HIIT+RT (ES = -0.29, p = 0.03). Sub analyses showed greater lower body strength gains in RT alone vs cycling-based HIIT+RT (ES = -0.53, p = 0.02) but not compared to running-based HIIT+RT (ES = -0.16, p = 0.33). No difference was observed between HIIT+RT with >24h intermodal rest and RT alone on lower body strength (ES = -0.06, p = 0.82). Conclusion: The data suggests that HIIT+RT does not negatively impact hypertrophy or upper body strength, and that the small negative effect on lower body strength observed may be ameliorated by the incorporation of running based HIIT and longer inter-modal rest periods.]]