After training one arm, the other arm gets stronger because its motor neurons become easier to activate and receive more synchronized signals from the brain.

Scientific Claim

Greater shared synaptic input (CSI) and lower motor unit recruitment thresholds (MURTs) are associated with increased maximal voluntary force in the untrained limb following unilateral resistance training.

Original Statement

“the contralateral muscle MVF increase was associated exclusively with CSI and MURT (R² > 0.65, P < 0.01).”

From study:Neural determinants of the increase in muscle strength and force steadiness of the untrained limb following a 4 week unilateral training

Evidence Quality Assessment

Claim Status

overstated

Study Design Support

Design cannot support claim

Appropriate Language Strength

association

Can only show association/correlation

Assessment Explanation

The abstract uses 'associated with' but the claim implies a direct explanatory role. The study design cannot prove causation; the verb 'are associated with' is correct, but the phrasing must avoid implying mechanism as cause.

Evidence from Studies

Supporting (1)

Neural determinants of the increase in muscle strength and force steadiness of the untrained limb following a 4 week unilateral training

Cohort Study

Human

2025 Jun

Training one arm made the other arm stronger too—even though it wasn’t exercised—because the brain sent more coordinated signals to the muscles and made them easier to activate.

Contradicting (0)

No contradicting evidence found

Source Study

Neural determinants of the increase in muscle strength and force steadiness of the untrained limb following a 4 week unilateral training

Score: 37

DOI: 10.1113/JP288954

Similar Assertions

When your brain sends more coordinated signals to your muscles and less random noise, your movements become smoother and more controlled—even in muscles you didn’t train.

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After training one arm, both arms become easier to turn on—your brain can activate muscles with less effort, even in the arm you didn’t train.

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