The untrained arm gets stronger without getting bigger—proving that the improvement comes from better nerve signals, not from muscles growing larger.

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Evidence from Studies

Supporting (1)

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Cross-education: motor unit adaptations mediate the strength increase in non-trained muscles following 8 weeks of unilateral resistance training

Randomized Controlled Trial

Human

2024

When one arm gets stronger from lifting weights, the other arm gets stronger too—even though it didn’t lift anything—because the brain and nerves get better at telling the muscles to work harder, not because the muscles get bigger.

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No contradicting evidence found

Source Study

Cross-education: motor unit adaptations mediate the strength increase in non-trained muscles following 8 weeks of unilateral resistance training

Score: 46

DOI: 10.3389/fphys.2024.1512309

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