Performing additional leg exercise did not attenuate muscle growth in the anterior upper arm compared with performing only arm exercise to failure, providing no evidence for competition of adaptations in muscle size and strength under uncontrolled nutritional conditions.
Key Findings
Results
Six weeks of low-load training to failure increased muscle size in the anterior upper arm regardless of whether additional leg exercise was performed.
105 untrained individuals were randomly assigned to one of three groups: LL-Failure (n=36), LL-Failure + Legs (n=33), or nonexercise control (CON, n=36).
Training consisted of 18 supervised sessions over 6 weeks at 30% 1RM to failure.
Muscle thickness was measured at 60% and 70% sites of the anterior upper arm at pre- and post-testing.
Both training groups showed increases in muscle size after 6 weeks of training.
Results
Adding leg exercise to arm training did not attenuate muscle growth in the anterior upper arm.
The LL-Failure + Legs group gained 0.19 cm in anterior upper arm muscle thickness.
The LL-Failure (arm only) group gained 0.18 cm in anterior upper arm muscle thickness.
The difference between groups (0.01 cm) was negligible, with no evidence of competition for muscle growth resources.
Changes were analyzed using the ANCOVA function of Bayes Factors for Informative Hypotheses with prevalues as the covariate.
Results
Changes in elbow flexor 1RM strength did not differ meaningfully between the arm-only and arm-plus-leg training groups.
The LL-Failure group improved 1RM elbow flexor strength by 0.32 kg.
The LL-Failure + Legs group improved 1RM elbow flexor strength by 0.25 kg.
Specific hypotheses were evaluated by comparing Bayes factors and posterior probabilities between models.
No evidence was found that recruiting additional muscle mass (legs) compromised upper arm strength gains.
Results
The study found no evidence for competition of adaptations in muscle size and strength between muscle groups under uncontrolled nutritional conditions.
Participants trained under uncontrolled nutritional conditions (diet was not standardized or monitored).
The LL-Failure + Legs group performed 4 additional sets of knee extension exercise per leg (20–30 RM) in each session, substantially increasing total muscle mass activated.
Despite the additional training volume and muscle mass recruited, upper arm hypertrophy was not compromised.
The authors note that whether greater training volume or limited nutrient intake induces a competition for resources warrants further investigation.
Methods
The study design used low-load training to failure for elbow flexion and a separate knee extension protocol to systematically vary the amount of muscle mass recruited during training.
Elbow flexion was performed at 30% 1RM to failure for 2 sets in both training groups.
The LL-Failure + Legs group added 4 sets of knee extension exercise per leg at 20–30 RM per session.
Muscle thickness at the 60% and 70% anterior upper arm sites and elbow flexor 1RM strength were the primary outcomes.
Statistical analysis employed Bayesian ANCOVA with informative hypotheses to compare models and evaluate specific directional hypotheses.
What This Means
This research suggests that training multiple muscle groups simultaneously does not limit how much any individual muscle can grow. In this study, 105 untrained people were split into groups: one group trained only their arm muscles (elbow flexors) to failure, another group trained the same arm muscles to failure and also performed leg exercises, and a third group did no exercise. After six weeks of training, both exercise groups gained similar amounts of muscle size and strength in their arms — about 0.18–0.19 cm of muscle thickness and around 0.25–0.32 kg of strength gain — suggesting the legs and arms were not 'competing' for growth resources.
The idea being tested here is sometimes called 'competition for resources' — the concern that if you work out more muscles in a session, your body might not have enough resources (like protein or hormones) to grow all of them equally. This study found no evidence of that happening under normal, everyday eating conditions (participants were not put on a controlled diet). Adding four sets of leg exercises to each session did not reduce the arm muscle gains at all.
This research suggests that people training multiple muscle groups in the same workout do not need to worry that working their legs will 'steal' growth from their arms, at least at moderate training volumes and under typical dietary conditions. The authors caution that whether very high training volumes or severely restricted food intake might create such competition remains an open question for future research.
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Kataoka R, Yamada Y, Hammert W, Sallberg R, Kang A, Song J, et al.. (2026). Skeletal Muscles Do Not Compete for Growth: Activating Additional Muscle Mass Does Not Compromise Changes in Muscle Size.. Journal of strength and conditioning research. https://doi.org/10.1519/JSC.0000000000005439