Training one arm with heavy eccentric curls makes the other arm stronger too—even though it didn’t lift any weights—by about 10%, while the trained arm gets even stronger, by nearly 20%.
Scientific Claim
Eight weeks of unilateral eccentric biceps training increases maximal voluntary force by 10% in the untrained contralateral biceps brachii and by 19% in the trained limb in recreationally active adults aged 18–35, demonstrating that cross-education produces substantial strength gains without muscle hypertrophy in the untrained limb.
Original Statement
“MVF increased by 7% in untrained muscles at T1 and 10% at T2 (p < 0.05)... Trained muscles presented greater MVF (+11%, T1; +19%, T2)”
Evidence Quality Assessment
Claim Status
appropriately stated
Study Design Support
Design supports claim
Appropriate Language Strength
definitive
Can make definitive causal claims
Assessment Explanation
The study is a randomized controlled trial with pre-post measurements and a control group, allowing causal inference within the studied population and intervention. The verb 'increases' is appropriate for this design.
Evidence from Studies
Supporting (1)
Cross-education: motor unit adaptations mediate the strength increase in non-trained muscles following 8 weeks of unilateral resistance training
Training one arm made the other arm stronger too—even though it wasn’t worked—because the brain got better at telling the muscles to fire harder, not because the muscles got bigger.