The age-old question of why our dominant hand is so much better at tasks like writing, throwing, or wielding tools has finally been answered. A recent study by UCLA Health researchers, published in PNAS, challenges the long-held belief that this advantage stems from a hardwired brain superiority. Instead, the study reveals that the dominant hand's edge is built upon a lifetime of practice, not innate talent.
The research, led by neurologist Dr. Ahmet Arac, utilized motion-capture cameras to track the 3D arm movements of healthy adults. Participants engaged in various reaching tasks under different conditions, including normal reaching, reaching with added weight, and reaching while mimicking tool use. The study's most intriguing finding was that for simple reaching tasks, both arms performed similarly, even when burdened with extra weight.
However, when the participants were asked to use tools, which required curved, controlled movements, the dominant arm emerged as the clear winner. This advantage was attributed to the participants' extensive practice with these specific, complex movements throughout their lives. Interestingly, when the participants were asked to write with their elbows, a part of the body they had never used for writing before, the usual dominance effect disappeared. Both hands initially struggled, but with practice, they improved at the same rate, eventually surpassing the performance of the nondominant hand.
Dr. Arac explains, "The dominant arm isn't more capable because one hemisphere of the brain is simply better at controlling movement. It is because we’ve spent a lifetime practicing the specific, complicated movements that tools and handwriting demand. Take away that practice by switching to a body part like the elbow that’s never done the task before, and the advantage disappears."
This study redefines limb dominance as an experience-based phenomenon, shaped by the specific tasks we engage in and our initial preferences. It challenges the notion of a basic imbalance in motor control, instead emphasizing the role of practice and familiarity. The findings have significant implications for rehabilitation strategies, particularly for stroke survivors and individuals relearning motor skills. They also offer a fresh perspective on how skilled movements are learned and represented in the brain.
The research, conducted at UCLA with coauthor Nicolas Y.H. Jeong Lee and senior author Dr. John W. Krakauer, was supported by various grants and institutions, including the NIH and the US-Israel Binational Science Foundation. The study's journal reference is: Ahmet Arac, Nicolas Y. H. Jeong Lee, and John Krakauer. Arm dominance is an emergent effect of practice executing complex trajectory shapes required by tools and objects. PNAS. DOI: 10.1073/pnas.2601569123 (https://doi.org/10.1073/pnas.2601569123).
