REF-1525 · Conceição et al., 2026 · Journal of Applied Physiology · Paper · PubMed

Does HIIT block muscle building when I also lift?

In 19 untrained young men protein synthesis and type II fibre size rose with or without it, but strength rose less with it (P = 0.016). Markers, small trial; abstract only.

Main caveat: Abstract only. Group sizes, training protocols, order, effect sizes and intervals are unread.
limited, mixed evidenceMeasured on non-athletes

Conceição and colleagues randomised 19 previously untrained young men to concurrent or resistance training for 16 weeks.

Biopsies were taken before and 48 h after a standardised session at weeks 4 and 16, with myofibrillar protein synthesis measured by deuterium oxide incorporation, along with satellite cells, myonuclei and gene expression; strength, aerobic capacity and fibre cross-sectional area were measured at baseline and after training.

Protein synthesis rose 48 h after exercise at both time points with no group differences. Type II fibres grew and myonuclei increased; strength gains were greater with resistance training alone, and aerobic capacity improved only with concurrent training.

The authors conclude that concurrent training did not inhibit the molecular mechanisms associated with hypertrophy in untrained people. Our reading: that closing sentence is narrower than the title and is the fair one. Protein synthesis is a marker, not performance. A non-significant group difference in a trial of 19 across two groups is weak evidence of no difference.

And the outcome the athlete actually lifts with went the other way.

Protocol

Design: randomised, two groups, 16 weeks (indexed in PubMed as a randomised controlled trial). Registration is not in the abstract.
Who: n = 19 previously untrained young men; split between groups, age, and body size are not in the abstract.
Interventions: concurrent training vs resistance training. The title gives the endurance mode as long-interval HIIT; its protocol, the resistance protocol, session order and spacing are not in the abstract.
Biopsies: before and 48 h after a standardised session at weeks 4 and 16.
Outcomes: myofibrillar protein synthesis by deuterium oxide incorporation; satellite cell content; myonuclear number; myogenic regulatory factor expression; muscle fibre cross-sectional area, strength and aerobic capacity at baseline and after training. Which strength test is not in the abstract.
Funding and interests: São Paulo Research Foundation (FAPESP), grants 2014/00985-0 and 2015/19756-3 (PubMed). The conflict-of-interest statement was not read.

Key Results

OutcomeResult as reportedP
Muscle protein synthesis, 48 h after exercise, weeks 4 and 16Increased; no group differences0.0105 (over time)
Satellite cell contentIncreased over time in type II fibres only0.0021
Myonuclear number, type I / type II fibresIncreased in both0.0301 / 0.0009
Myonuclear number, type IHigher in the concurrent group0.0027
MYF5 / MYF6 expressionIncreased over time0.0141 / 0.034
MYOD1 expressionElevated after exercise only in concurrent0.0009
Type II fibre sizeIncreased0.016
Strength gainsGreater with resistance training alone0.016
Aerobic capacity (VO2peak)Improved only with concurrent< 0.001
No effect sizes: the abstract gives P values only, with no group means, percentage gains or intervals, so the size of the strength difference and how large a protein-synthesis or fibre difference could be excluded are unknown.
Does the title overstate? Yes, for a reader who takes "interference" to include strength. The title and the highlight text say there is little evidence of interference and that molecular remodelling stayed intact, while the same abstract reports lower strength gains with concurrent training. The abstract's conclusion is limited to molecular mechanisms in untrained people.

Mechanisms

Measured: protein synthesis by deuterium oxide over the window of a standardised session at two time points; satellite cells, myonuclei, gene expression; fibre size.
Authors' interpretation: long-interval HIIT preserved the adaptations related to hypertrophy, supporting concurrent training as a way to build muscle and aerobic fitness at once.
Inference: strength fell behind while protein synthesis showed no group difference (a weak null in 19 men, no interval read), so this trial gives no sign that suppressed protein synthesis caused the strength cost, and does not exclude it. Neural or fatigue explanations are possible and were not tested in what was read.
Inference: a 48-h window after one session is not the whole training week; no group difference there does not show equal accretion across 16 weeks, for which fibre size is the better measure, and the abstract gives its time effect only.

Practical Application

Does HIIT block muscle building when I also lift? In 19 untrained young men protein synthesis and type II fibre size rose with or without it, but strength rose less with it (P = 0.016). Markers, small trial; abstract only.
Do not quote this as "no interference": the group difference in strength is the interference result here. A signal staying intact is not the same as performance staying intact.
It fits the view that the AMPK-mTOR story is too simple ([PHYS-19]): molecular machinery looked intact and strength still lagged, in this population. It says nothing about trained athletes.
Not for session planning: session order and spacing are not in the abstract, so no scheduling rule comes from it. Not from this paper; the base's own practice for spacing is SYNTH-31.

Limitations

Abstract only. Group sizes, training protocols, order, effect sizes and intervals are unread. The publisher's page returned a bot check and the repository copies listed by OpenAlex returned an empty response, so we stopped there; those copies may hold the full text.
Small. 19 men across two arms; non-significant group differences say little, and the abstract reports no equivalence test.
Population. Previously untrained young men only; no women, no athletes.
Markers. Protein synthesis, satellite cells, myonuclei and gene expression are not performance; a fibre-level change comes from small biopsy samples.
Conflict of interest not read; funder is a public research foundation.
How this entry was checked and changed
Why this level

a parallel RCT under 20 is B; held at B- under the rubric because this is an abstract-only small study whose headline outcomes are markers (protein synthesis, satellite cells, gene expression), not what an athlete performs, and key methods, group sizes and between-group effect sizes are unread