Just one dose of the ADHD drug methylphenidate can temporarily improve affected children's muscular control and movement, a small study shows.

The study, reported in the online journal Behavioral and Brain Functions, focused on 24 boys newly diagnosed with hyperkinetic disorder (HKD), a diagnosis nearly identical to attention-deficit hyperactivity disorder-combined type (ADHD-C).

In this form of ADHD, children not only have attention problems, but also act impulsively and have difficulty sitting still and otherwise controlling their movement.

Norwegian researchers looked at the effects of a single dose of methylphenidate — best known by the brand-name Ritalin — on the boys' movement control.

Children with ADHD-C commonly have increased muscle tension, which can hinder normal movement.

This, in turn, may manifest as problems such as stiffness, restlessness as a child repeatedly shifts to get comfortable, and even poor handwriting, explained Liv Larsen Stray of the University of Stavanger, the lead researcher on the study.

"Our study shows that a single dose of methylphenidate typically led to improvement of the muscular tone and to a more fluent movement in children with ADHD-C/HKD," Stray told Reuters Health.

For the study, the researchers observed 24 8- to 12-year-old boys with ADHD-C/HKD on two separate days. On each day, the children underwent tests of their coordination, balance and movement control.On one day, the boys took the tests before and 90 minutes after a dose of methylphenidate; on the other day, they were given an inactive placebo instead of the drug.

In general, Stray's team found, the boys' test performance benefited from the dose of methylphenidate, with temporary improvements in activities like throwing a ball or holding a leg in the air.

There were also improvements in "thumb movement," the researchers note, which may — along with better elbow and shoulder movement — explain why children's handwriting often improves once they start methylphenidate.

The findings, the researchers write, suggest that movement-related difficulties "may be a more integrated part of the core problems" of ADHD than generally thought.

According to Stray, relatively little has been known about the specific effects of methylphenidate on movement problems in ADHD.

"Our study," the researcher said, "suggests that methylphenidate has a much broader and more immediate positive impact on muscular function and motor control in ADHD than hitherto believed."