A recent study published in Neurology compared 25 boys and girls with ADHD and 25 boys and girls without ADHD. All of the children were right handed. They were asked to perform left handed finger tapping. Using video and a recording device measuring finger movements, the children with ADHD exhibited more mirror overflow movements with the right hand than the children without ADHD. This was particularly noticeably in the boys who showed almost four times as much mirror overflow.
In a second study, the researchers used trans-cranial magnetic stimulation to the motor control area of the brain on 49 children with ADHD compared to 49 children without ADHD. The results showed that the short interval cortical inhibition (the brain's "braking mechanism") was decreased by 40% in children with ADHD. This reduction in inhibition was strongly associated with the severity of the ADHD. In addition, the children with ADHD scored 60% lower on tests of motor development.
Reference: American Academy of Neurology. Abnormal Control of Hand Movements May Hint at ADHD Severity in Children. Retrieved from the web on 2/15/2011 from http://www.aan.com/press/index.cfm?fuseaction=release.view&release=908