Using one arm or leg at a time might make your muscles work harder because your brain sends stronger signals to them.
Evidence from Studies
Supporting (3)
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Neuromuscular Adaptations to Unilateral vs. Bilateral Strength Training in Women
Doing exercises with one arm or leg at a time helped women activate their muscles more strongly through their nerves, even though both one-sided and two-sided training made them stronger overall.
Cross-education: motor unit adaptations mediate the strength increase in non-trained muscles following 8 weeks of unilateral resistance training
When people train one arm at a time, their brain and nerves get better at telling the muscles to work harder—even the other arm that didn’t train gets stronger because the nervous system learns to activate muscles more efficiently.
Training one arm made the other arm stronger too, even though it wasn’t exercised—this happened because the brain got better at sending strong, clear signals to the muscles, with less internal 'noise' holding them back.
Contradicting (3)
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Neuromuscular adaptations during bilateral versus unilateral strength training in middle-aged and elderly men and women.
The study found that lifting with both legs at once made people’s muscles fire more strongly than lifting with just one leg, which is the opposite of what the claim says.
This study didn't test if doing exercises with one arm at a time makes your muscles work harder—it looked at people who play tennis and found their muscles actually worked less when using both arms together, suggesting one-arm training might make muscles weaker, not stronger.
The study found that using both arms together with visual feedback made muscles work harder than using just one arm, which is the opposite of what the claim suggests.