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The interplay between motor and cognitive functions during performance of concurrent tasks is not fully understood but is known to vary depending on task characteristics and across clinical populations. Our controlled study examined how a concurrent digit span task affected a motor stability and motor overflow task in patients with multiple sclerosis (MS).We asked 22 patients with MS and 22 matched controls to exert force on a transducer using 1 index finger at a time. We measured their motor stability (accuracy of voluntary force production) and motor overflow (involuntary force produced by the opposite, inactive finger). During half of the trials, the participants concurrently performed a digit span task.Overall, the patients with MS had more motor overflow and less motor stability than the controls; these measures correlated with the patients’ disease severity. Adding the concurrent task affected motor stability; this relationship varied with the required level of exerted force. Motor overflow was lower during trials with the concurrent task. The concurrent task affected patients and controls similarly for both motor stability and overflow.This study demonstrates preserved motor function in a concurrent-task paradigm in patients with MS, and sheds further light on the relationship between attention and motor function in both the patients and controls. This research may help to inform rehabilitation for everyday life situations in which patients routinely perform cognitive and motor tasks simultaneously.