When you train one arm, the other arm’s muscles start firing more easily and more intensely during contractions—even without being exercised—making it stronger through better nerve signals, not bigger muscles.
Scientific Claim
Unilateral eccentric biceps training reduces motor unit recruitment thresholds by 12% in the untrained biceps brachii and increases net discharge rate by 3.8 pps, indicating that neural adaptations—specifically earlier motor unit activation and greater firing rate gain—mediate strength gains in the non-trained limb.
Original Statement
“These gains were accompanied by significant decreases in motor unit recruitment thresholds (p < 0.01) and higher net discharge rate (i.e., gain in discharge rate from recruitment to peak) following intervention (p < 0.05)... Untrained limbs also presented a significant increase in the Net-DR following the first 4 weeks [∆DR = +3.1 pps] and 8 weeks [∆DR = +3.8 pps]”
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 RCT design with longitudinal motor unit tracking allows causal inference between training and neural adaptations. The study directly measured these parameters and found statistically significant changes.
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, not because it got bigger, but because the brain and nerves learned to turn on the muscles more easily and fire them faster.