HIIT alongside lifting doesn't blunt muscle growth — but watch lower-body strength

Many people want both cardiovascular fitness and muscle, but a long-standing worry — the interference effect — holds that endurance work done alongside lifting can dampen strength and size gains. High-intensity interval training has become a popular, time-efficient way to train the cardiovascular system, which raises a practical question the older interference literature could not answer: do short, hard intervals carry the same cost as traditional endurance training when layered onto a resistance programme? This review set out to test exactly that, and to ask whether the type of interval work and the spacing between sessions change the answer.

The authors systematically searched five electronic databases using terms related to HIIT, resistance training and concurrent training. Eligible studies compared programmes combining HIIT with resistance training against resistance training alone. Effect sizes were calculated as standardised differences in means for muscle strength and hypertrophy. The team then ran sub-analyses to probe region-specific outcomes in the upper versus lower body, the HIIT modality used — cycling versus running — and how the rest period between the two types of training session influenced the results.

Concurrent HIIT and resistance training produced changes in muscle hypertrophy and upper-body strength that were similar to resistance training alone. The exception was the lower body, where concurrent training led to a smaller increase in strength (effect size -0.248, p = 0.049). Sub-analyses suggested the penalty may hinge on modality: there was a trend for cycling-based HIIT to negatively affect lower-body strength (effect size -0.377, p = 0.074) that was not evident for running (effect size -0.176, p = 0.261). The authors suggest longer rest between the two kinds of session may also help.

The interference effect was originally described in the context of traditional, longer-duration endurance training, so this analysis extends the question to the interval formats most people now actually use. Its broadly reassuring answer — hypertrophy and upper-body strength unaffected — narrows the concern to lower-body strength specifically. The modality finding remains open, since neither the cycling nor the running sub-analysis reached statistical significance, and the optimal amount of rest between HIIT and lifting sessions is not pinned down here. More trials directly comparing interval modalities within concurrent programmes would settle it.

If you lift and also do interval training, this is a reason to be relaxed about the combination: across the pooled trials, muscle growth and upper-body strength did not suffer. The small, borderline reduction in lower-body strength gains is the one finding worth knowing about, and the trend-level data hint it may be more of an issue with cycling intervals than running ones. The authors' suggestion that longer gaps between the two kinds of session could ameliorate the effect is plausible but not proven here.