An injured arm causes the brain to enlarge in areas that compensate the injured side and decreases the areas that are not being used owing to the cast or sling.
"These results are especially interesting for rehabilitation therapy for people who've had strokes or other issues," said study author Nicolas Langer from the University of Zurich in Switzerland.
"One type of therapy restrains the unaffected, or "good", arm to strengthen the affected arm and help the brain learn new pathways. This study shows that there are both positive and negative effects of this type of treatment."
For the study, researchers examined a group of right-handed people with an injury of the upper right arm that required a sling for at least 14 days. The entire right arm and hand were restricted to little or no movement during the study period, the journal Neurology reports.
As a result, participants used their non-dominant left hand for daily activities such as washing, using a toothbrush, eating or writing. None of the people in the study had a brain injury, psychiatric disease or nerve injury, according to a Zurich statement.
The group underwent two MRI brain scans, the first within two days of the injury and the second within 16 days of wearing the cast or sling. The scans measured the amount of gray and white matter in the brain.
Participants' motor skills, including arm-hand movements and wrist-finger speed, were also tested. The study found that the amount of gray and white matter in the left side of the brain decreased up to 10 percent, while the amount of gray and white matter in the right side of the brain increased in size.
"We also saw improved motor skills in the left, non-injured hand, which directly related to an increase in thickness in the right side of the brain," said Langer. "These structural changes in the brain are associated with skill transfer from the right hand to the left hand."