Unilateral resistance exercises increase neural drive and muscle fiber recruitment compared to bilateral resistance exercises as a result of reduced neuromuscular inhibition during single-limb contractions.

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

When you lift with just one arm or leg, your brain learns to send stronger signals to your muscles—even to the other side that didn’t do the work. This makes your muscles more powerful, even without training them directly.

Study: 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.

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

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.