The simultaneous performance of multiple tasks is often required in daily life. The dual-task paradigm has been used extensively to evaluate the ability to perform simultaneous behavioral tasks. However, relatively few studies have been carried out to determine the muscle and brain activity underlying dual-task performance. This study determined the influence of single-task and dual-task conditions on muscle and brain activity by measuring ankle dorsiflexion, surface electromyography, and magnetoencephalography. Increased muscle activation and variability in the dorsiflexion interval was observed during dual-task performance compared with single-task performance, whereas brain activity in the contralateral motor and frontal areas was reduced. Incidental movement of the non-task-related ankle (contralateral ankle) showed a different pattern of brain activity during the dual-task performance in another experiment. These findings suggest that the activity decreased in several information-processing areas under dual-task conditions as a result of decentralization of attention. Activity in the right and left motor fields may play a role in the dual-task performance, as indicated by changes in brain activity during dual-task performance in two groups classified according to the activity level of the contralateral ankle.