ABSTRACT
PURPOSE: This study examined whether spinal excitability, as measured by the soleus Hoffmann reflex (H-reflex), is scaled to the difficulty level of the dual-task being performed. METHODS: Twenty-two participants completed a combination of three balance task and three secondary cognitive (visuo-motor) task difficulty levels for a total of nine dual-task conditions. An additional eight participants were tested while performing the same three balance task difficulty levels on its own (i.e., single-tasking). The balance task required participants to maintain their balance on a fixed or rotating stabilometer while the visuo-motor task required participants to respond to moving targets presented on a monitor. Throughout each single- and dual-task trial, H-reflexes were elicited from the soleus. RESULTS: Although dual-task performance, as quantified by visuo-motor task accuracy as well as the root mean square of the stabilometer position and velocity, decreased by 10-34% with increasing dual-task difficulty (p < 0.05), no changes in the soleus H-reflex amplitude were observed between dual-task conditions (p = 0.483-0.758). This contrasts to when participants performed the balance task as a single-task, where the H-reflex amplitude decreased by ~25% from the easy to the hard balance task difficulty level (p = 0.037). CONCLUSIONS: In contrast to the commonly reported finding of a reduced soleus H-reflex amplitude when individuals perform a less posturally stable task by itself, the results indicate that spinal excitability is not modulated as a function of dual-task difficulty. It is possible that when an individual's attentional resource capacity is exceeded during dual-tasking, they become ineffective in regulating spinal excitability for balance control.
