Abstract
We investigated how corticospinal excitability of the resting digit muscles was modulated by the digit movement in the ipsilateral limb. Subjects performed cyclical extension-flexion movements of either the right toes or fingers. To determine whether corticospinal excitability of the resting digit muscles was modulated on the basis of movement direction or action coupling between ipsilateral digits, the right forearm was maintained in either the pronated or supinated position. During the movement, the motor evoked potential (MEP) elicited by transcranial magnetic stimulation (TMS) was measured from either the resting right finger extensor and flexor, or toe extensor and flexor. For both finger and toe muscles, independent of forearm position, MEP amplitude of the flexor was greater during ipsilateral digit flexion as compared to extension, and MEP amplitude of the extensor was greater during ipsilateral digit extension as compared to flexion. An exception was that MEP amplitude of the toe flexor with the supinated forearm did not differ between during finger extension and flexion. These findings suggest that digit movement modulates corticospinal excitability of the digits of the ipsilateral limb such that the same action is preferred. Our results provide evidence for a better understanding of neural interactions between ipsilateral limbs, and may thus contribute to neurorehabilitation after a stroke or incomplete spinal cord injury.
Highlights
Movement of one limb enhances corticospinal excitability of muscles employed in the movement and the excitability of resting muscles of other limbs (Carson et al, 1999; Baldissera et al, 2002; Borroni et al, 2004; Byblow et al, 2007; Mcintyre-Robinson and Byblow, 2013)
Statistical analysis revealed that there was a significant main effect of movement (F(1,16) = 4.94, p = 0.041) on motor evoked potential (MEP) amplitude of the finger extensor, whereas neither a main effect of forearm position nor an interaction between movement × forearm position was significant (F(1,16) = 0.30, p = 0.593 and F(1,16) = 0.62, p = 0.443, respectively). This indicates that MEP amplitude of EDC was greater during toe extension than during toe flexion irrespective of forearm position
Neither a main effect of forearm position nor an interaction between movement × forearm position was significant (F(1,16) = 2.57, p = 0.128 and F(1,16) = 0.67, p = 0.424, respectively). This indicates that MEP amplitude of FDS was greater during toe flexion than during toe extension irrespective of forearm position
Summary
Movement of one limb enhances corticospinal excitability of muscles employed in the movement and the excitability of resting muscles of other limbs (Carson et al, 1999; Baldissera et al, 2002; Borroni et al, 2004; Byblow et al, 2007; Mcintyre-Robinson and Byblow, 2013). When a wrist is cyclically extended and flexed, corticospinal excitability of the contralateral resting wrist extensors and flexors increased during the extension and flexion, respectively (Carson et al, 1999). Wrist movement of one limb enhances the corticospinal excitability of homologs muscles of the other limb. When cyclically moving the foot upward (dorsiflexion) and downward (plantarflexion), corticospinal excitability of the ipsilateral resting wrist extensors and flexors increased during dorsiflexion and plantarflexion, respectively, with the forearm in a pronated position on an armrest (Borroni et al, 2004). When the forearm is in a supinated position, wherein activation of wrist extensors produces downward movement of the hand, the corticospinal excitability of wrist extensors increases during plantarflexion, but not dorsiflexion (Borroni et al, 2004). Little is known how digit movement of the upper limb affects the corticospinal excitability of digits of the lower limb, and vice versa
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