UCLA — A study published on July 8, 2026, in the journal Proceedings of the National Academy of Sciences by UCLA Health researchers challenges the hypothesis that the brain's dominant hemisphere is inherently superior at controlling movement. The research indicates that repetitive practice, rather than innate brain capability, accounts for the functional advantage of a dominant arm.

Ahmet Arac, the study's corresponding author and an assistant professor of neurology at the David Geffen School of Medicine at UCLA, said, "The dominant arm isn't more capable because one hemisphere of the brain is simply better at controlling movement." He added, "It is because we've spent a lifetime practicing the specific, complicated movements that tools and handwriting demand." Arac noted, "Take away that practice by switching to a body part like the elbow that's never done the task before and the advantage disappears."

Researchers tracked the 3D arm movements of healthy adults using motion-capture cameras as they reached toward targets under three conditions: normal reaching, reaching while wearing a four-pound wrist weight, and reaching while holding a lightweight stick strapped to the forearm. During simple reaching and when a weight was added, the dominant and nondominant arms performed about the same. However, when a stick was added to the forearm, requiring curved and controlled trajectories, the dominant arm outperformed the nondominant arm.

In a second experiment, participants wrote letters and numbers using a pen first with their hand, then with a pen taped to their elbow. When participants wrote with their elbow instead of their hand, the dominant-side advantage vanished. After practice, both the dominant and nondominant elbows improved by the same amount. Following this practice, both elbows performed better than the nondominant hand had performed.

Nicolas Y.H. Jeong Lee served as a coauthor of the study. John W. Krakauer of Johns Hopkins University and the Santa Fe Institute was the senior author. Support for the research was provided by the National Institutes of Health, the US-Israel Binational Science Foundation, NVIDIA, and UCLA's Department of Neurology.