Adding leg work during arm training does not reduce arm muscle growth
A 6-week randomized trial in 105 untrained adults found that performing additional leg exercises alongside arm training did not reduce growth in trained arm muscles, suggesting muscles do not compete for adaptive resources under normal nutritional conditions.
Researchers randomly assigned 105 untrained individuals to three groups: 18 supervised sessions over 6 weeks of low-load elbow flexion training to failure (LL-Failure, n=36), the same arm training plus additional low-load knee extension sets (LL-Failure + Legs, n=33), or a time-matched control (n=36). The arm-only group performed 2 sets of elbow flexion at 30% of their 1-repetition maximum to failure per session; the arm-plus-legs group added 4 sets of knee extension work per leg at 20–30 repetitions per session.
Muscle thickness in the upper arm (measured at 60% and 70% of the arm's length) and elbow flexor strength were measured before and after the 6-week period. Both trained groups showed increases in arm muscle size and strength. Critically, the arm-plus-legs group—which recruited substantially more total muscle mass—showed nearly identical growth in the trained arm (0.19 cm) compared to the arm-only group (0.18 cm). Changes in arm strength also did not differ meaningfully between groups (0.32 kg versus 0.25 kg). The study found no statistical evidence that recruiting additional muscle mass through leg exercise reduced adaptations in the trained arm, challenging the idea that muscles compete for finite growth resources. The authors note that this conclusion holds under uncontrolled (ad libitum) nutrition; whether competition emerges under caloric restriction or very high training volume remains unknown.
The effect sizes were small but directional, and the Bayesian analysis specifically tested whether performing more total work *reduced* arm growth—it did not. Notably, the additional leg training was substantial (4 sets per leg), yet arm hypertrophy was uncompromised. One mechanism proposed in the literature is that systemic factors (hormonal response, nutrient partitioning) scale with total training volume; this study suggests the body's anabolic capacity is not a zero-sum game for different muscles. However, the study enrolled only untrained individuals and lasted just 6 weeks, so findings may not generalize to advanced lifters, longer training blocks, or conditions of energy deficit. The authors explicitly flag that this study used ad libitum feeding—caloric or protein restriction might reveal a different picture. Additionally, the arm stimulus was relatively light (30% 1RM), so it remains unclear whether competition would emerge if both muscle groups received very heavy, demanding stimuli simultaneously.
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Open in Cadence →References
- Skeletal Muscles Do Not Compete for Growth: Activating Additional Muscle Mass Does Not Compromise Changes in Muscle Size. — Journal of strength and conditioning research (Read the original)