REF-1739 · Kikuchi et al., 2026 · International Journal of Sports Medicine · Paper · PubMed

Adding HIIT to strength training showed no significant difference from strength training alone in squat 1RM (SMD −0.06, p = 0.63) or countermovement jump (SMD −0.12, p = 0.38) in a meta-analysis of 21 studies (569 participants); by HIIT type, short and long interval formats went with a lower jump (SMD −0.53, p = 0.006) while sprint and repeated-sprint formats did not (0.14, p = 0.40), a split resting on few studies

Main caveat: The authors' own: the HIIT-type subgroup findings rest on a limited number of studies and need caution.
moderate evidenceMeasured on non-athletes to trained athletesRead from the abstract only

Kikuchi and colleagues pooled studies that compared strength training alone with strength training combined with high-intensity interval training, to see whether the added intervals cost strength, power or muscular endurance.

From 8,634 records found in PubMed, SPORTDiscus and Web of Science they included 21 randomised and non-randomised studies with 569 participants and pooled standardised mean differences in random-effects models. The combined training did not differ significantly from strength training alone in squat 1RM (SMD −0.06, p = 0.63) or in countermovement jump (SMD −0.12, p = 0.38).

Split by HIIT type, sprint-interval and repeated-sprint training showed no significant effect on jump (SMD 0.14, p = 0.40), while short and long HIIT showed a significant negative effect (SMD −0.53, p = 0.006), and the two subgroups differed significantly (Qbetween = 5.82, df = 1, p = 0.016).

The authors conclude that maximal strength and power did not differ significantly with HIIT added, that HIIT type may matter, and that the subgroup findings rest on few studies and need caution.

Participants

21 studies, 569 participants in total (about 27 per study by our arithmetic, both arms included).
Age, sex, training status and sport: not in the abstract.

Protocol

Search: PubMed, SPORTDiscus, Web of Science; search dates not in the abstract; 8,634 records identified.
Inclusion: randomised and non-randomised studies comparing strength training with concurrent HIIT plus strength training.
Outcomes: muscular strength (squat 1RM), power (countermovement jump); the abstract's aim also names endurance performance, but no endurance result is reported in it.
Analysis: random-effects meta-analysis of SMDs, significance at p < 0.05; subgroups by HIIT type (sprint-interval and repeated-sprint training against short and long HIIT) with a between-subgroup Q test.
Not in the abstract: confidence intervals, heterogeneity, risk-of-bias or certainty assessment, number of studies per subgroup, programme length, session spacing or order, HIIT mode, publication bias.

Key Results

Comparison (concurrent HIIT + strength vs strength alone)SMDp
Squat 1RM−0.060.63 (not significant)
Countermovement jump, all HIIT−0.120.38 (not significant)
Countermovement jump, sprint-interval and repeated-sprint training0.140.40 (not significant)
Countermovement jump, short and long HIIT−0.530.006
Subgroup difference (HIIT type)Qbetween = 5.82, df = 10.016
No confidence intervals are given, so how large a cost the non-significant results exclude is unknown.

Practical Application

Adding HIIT to a strength programme showed no significant pooled cost to squat 1RM or jump, but the short and long HIIT formats went with a lower jump (SMD −0.53) while sprint-interval and repeated-sprint formats did not. Not significant is not no cost: with no intervals reported, a small loss is not ruled out. The HIIT-type split is a between-study subgroup on few studies, not a randomised comparison.
Same direction as [SYNTH-21]. The guide says maximal strength is at most slightly affected and explosive strength is what concurrent training costs (REF-330, SMD −0.28); this pooling adds that the jump cost appeared with the short and long HIIT formats, not the sprint formats.
Inference, not tested here: if jump or sprint power matters and intervals must be added, short all-out sprints are the formats that showed no jump cost here; put them in a separate slot from heavy lifting where possible (the ≥3 h of REF-330).
Not independent of the other poolings. Sprint-interval trials added to lifting are also pooled in REF-1681 (9 RCTs, no significant strength, jump or sprint cost) and HIIT-plus-strength trials are described in REF-1737; the included trials are not listed in the abstract, so overlap cannot be checked, but these reviews should not be counted as separate confirmations.

Limitations

The authors' own: the HIIT-type subgroup findings rest on a limited number of studies and need caution.
Randomised and non-randomised studies pooled together.
Written from the abstract: who was studied, programme length, how HIIT and strength sessions were spaced or ordered, HIIT mode, heterogeneity, confidence intervals, risk of bias, publication bias and the endurance results named in the aim and title are all unknown.
The abstract groups short and long HIIT together against sprint formats; how each was defined is not stated.
Full text not read.
How this entry was checked and changed
Why this level

a meta-analysis capped at B+ because its risk of bias was not read (abstract only); non-randomised studies were also pooled