Task-dependent changes in the cortical control of postural dual tasks: effects of secondary cognitive and motor tasks

This study investigated whether the motor cortical control underlying dual task performance depends on the type and difficulty of the dual task. Forty-four adults stood on a balance platform with or without performing a secondary motor task (i.e., maintain a ball in the middle of a tray) or cognitive task (i.e., 2-back test). Balance task difficulty was manipulated by changing the resistance on the balance platform. Transcranial magnetic stimuli (TMS) were applied to elicit motor-evoked potentials to assess corticospinal excitability and short-interval intracortical inhibition (SICI) from the tibialis anterior. Results indicated that TMS-evoked measures differed based on the type of secondary task. When the motor task was involved, individuals responded with an increase in corticospinal excitability (32.2 ± 5.4%; <i>P</i> < 0.001) but no change in SICI compared with the balance-only task. In contrast, when the secondary cognitive task was added, both corticospinal excitability and SICI were unaltered compared with when balancing on its own. These task-specific changes occurred in tandem with differential changes in balance performance. Compared with the balance-only task, performance worsened by 34.2%-41.9% (<i>P</i> < 0.001) when dual tasking with the secondary motor task but was either unaffected (hard balance task) or improved by 9.1 ± 4.0% (easy balance task; <i>P</i> = 0.028) when dual tasking with the cognitive task. Increasing balance task difficulty resulted in 38.7 ± 6.9% larger motor-evoked potentials (MEPs; <i>P</i> < 0.001) and 21.9 ± 9.4% reduced SICI (<i>P</i> < 0.001) but were unaffected by dual-tasking. Together, these findings provide insight into how individuals adopt different motor control strategies, leading to differences in performance, depending on the secondary task.<b>NEW & NOTEWORTHY</b> We examined motor cortical excitability when maintaining balance while performing a secondary motor or cognitive task. When the secondary motor task was involved, balance performance worsened and corticospinal excitability increased. Dual tasks involving the secondary cognitive task resulted in contrasting effects, with either no change or an improvement in balance performance, along with no change in cortical control. These findings demonstrate how different motor cortical strategies are adopted depending on the dual task requirements.