Adding HIIT did not cost squat strength — a 21-study meta-analysis found interval type mattered for jumps
In short
A meta-analysis of 21 studies (569 participants) found no significant difference between strength training plus high-intensity interval training (HIIT) and strength training alone for squat 1RM (SMD -0.06) or countermovement jump (SMD -0.12). In subgroups, sprint interval and repeated-sprint training did not affect jump performance, while short and long HIIT reduced it (SMD -0.53). If maximal strength is the goal, HIIT can stay; if explosiveness matters, choose the interval type.
The fear that cardio eats strength did not hold up for high-intensity intervals. Pooling 21 studies and 569 participants, a meta-analysis found the squat 1RM difference between concurrent HIIT-plus-strength and strength alone was SMD -0.06 (p = 0.63) — essentially none. Countermovement jump showed no significant difference either, at SMD -0.12 (p = 0.38).
Which kinds of intervals hurt jump performance?
Splitting by interval type changed the picture. Sprint interval training and repeated sprints — all-out efforts of a few seconds up to about 30 seconds — had no effect on jumping (SMD 0.14, p = 0.40). Short and long HIIT, run longer at a lower intensity, significantly reduced jump performance (SMD -0.53, p = 0.006). The difference between those subgroups was itself significant (p = 0.016).
How should a lifter choose intervals?
- Goal is maximal strength — squat 1RM ended up the same with or without HIIT, so there is no need to drop intervals you do for conditioning or fat loss.
- Explosiveness matters — favour short, all-out sprint intervals. Longer HIIT bouts may blunt jump performance.
- Logging — note the interval type and duration alongside your lifts, so a stalled squat, bench or deadlift is easier to diagnose.
The fairlift Muscle Index is calculated from your big three 1RMs, so on these results HIIT alone is unlikely to drag it down. For low-intensity cardio as a recovery tool, see Zone 2 cardio is a between-sets recovery tool.
The subgroup analyses rest on a small number of studies, and the authors advise caution. The analysis does not address hypertrophy or long-term fatigue.
Frequently asked questions
Does HIIT reduce strength gains?
In a meta-analysis of 21 studies (569 participants), adding HIIT to strength training produced no significant difference in squat 1RM compared with strength training alone (SMD -0.06, p = 0.63).
Does HIIT affect jump performance?
It depends on the type. Sprint interval and repeated-sprint training had no effect on countermovement jump (SMD 0.14), while short and long HIIT reduced it (SMD -0.53, p = 0.006).
What kind of intervals suit lifters?
To protect explosiveness, all-out sprint intervals of a few seconds up to about 30 seconds are the better choice. For maximal strength alone, squat 1RM was similar regardless of HIIT type.
How certain are these meta-analysis results?
The overall result draws on 21 studies, but the interval-type subgroup analyses include few studies and the authors say they should be interpreted with caution.
Source: PubMed