Study type: Randomized controlled trialConfidence: Low

To Flex or Rest: Does Adding No-Load Isometric Actions to the Inter-Set Rest Period in Resistance Training Enhance Muscular Adaptations? A Randomized-Controlled Trial

Schoenfeld BJ, Grgic J, Contreras B, Delcastillo K, Alto A, Haun C, De Souza EO, Vigotsky AD

Year2020

Sample sizen=27

JournalFrontiers in Physiology

AuthorsSchoenfeld BJ, Grgic J, Contreras B, Delcastillo K, Alto A, Haun C, De Souza EO, Vigotsky AD

Evidence is still limited and needs further study

Summary

In this RCT, resistance-trained men (18-35y, training consistently at least 3x/week for 1+ year; 27 of 35 enrolled completed the study) were randomized to either traditional passive inter-set rest (TRAD, n=13) or 30 seconds of maximal-effort, no-load isometric flexing after every set (ISO, n=14, flexing the biceps, triceps, and quadriceps), during an 8-week, 3x/week resistance training program. Muscle thickness of the elbow flexors, triceps, and vastus lateralis showed no meaningful between-group difference; only the mid-thigh region showed a slight upward trend, and even that was not statistically decisive (the 90% confidence interval crossed zero). Lower-body maximal strength (leg press 1RM) even trended slightly worse in the flexing group.

Source (read the original)

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DOI: 10.3389/fphys.2019.01571

Key Findings

Key findings

Key point

1
Muscle thickness of the elbow flexors, triceps, and vastus lateralis showed no meaningful difference between the flexing (ISO) and passive rest (TRAD) groups
2
Only the mid-thigh region showed a slight upward trend, but the 90% confidence interval crossed zero, so it is not a statistically decisive effect (the study used an estimation approach with bootstrapped CIs rather than traditional significance testing)
3
Lower-body maximal strength (leg press 1RM) trended slightly worse in the ISO group
4
Small sample (n=27), young trained men only, single study — findings should not be generalized broadly

Related Research

Related research

Review

Inter-set stretch: A potential time-efficient strategy for enhancing skeletal muscle adaptations

Frontiers in Sports and Active Living, 2022

A narrative review summarizing a distinct rest-period strategy from no-load flexing: adding active or passive stretch of the target muscle during inter-set rest. Animal studies suggest passive stretch can activate mTORC1-dependent muscle protein synthesis pathways, and a small number of ~8-week human trials, mostly in untrained young men, report slightly greater muscle thickness gains with inter-set stretching. However, the authors themselves characterize the current evidence as 'preliminary,' and questions remain about optimal stretch intensity, hold duration, and how findings generalize across age, sex, and training status.

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Articles featuring this study

Research vs Bro-science
Does Posing Between Sets Actually Build Muscle? The 'Flex Rest' Claim vs. the Research
Between sets, you'll often see lifters set the weight down and flex — squeezing the biceps or quads as hard as possible. Social-media training advice frequently claims this 'flex rest' keeps the pump going and speeds up hypertrophy. But does what you do during rest actually change your gains? We compare a randomized trial that tested no-load isometric flexing between sets against a different rest-period strategy — inter-set stretching — to see which, if either, has real support.
Hirotsugu Yoshimura

Last checked: 2026-07-11