Abstract
Although voluntary muscle contraction modulates spinal reflex excitability of contracted muscles and other muscles located at other segments within a limb (i.e., intra-limb modulation), to what extent corticospinal pathways are involved in intra-limb modulation of spinal reflex circuits remains unknown. The purpose of the present study was to identify differences in the involvement of corticospinal pathways in intra-limb modulation of spinal reflex circuits among lower-limb muscles during voluntary contractions. Ten young males performed isometric plantar-flexion, dorsi-flexion, knee extension, and knee flexion at 10% of each maximal torque. Electromyographic activity was recorded from soleus, tibialis anterior, vastus lateralis, and biceps femoris muscles. Motor evoked potentials and posterior root-muscle reflexes during rest and isometric contractions were elicited from the lower-limb muscles using transcranial magnetic stimulation and transcutaneous spinal cord stimulation, respectively. Motor evoked potential and posterior root-muscle reflex amplitudes of soleus during knee extension were significantly increased compared to rest. The motor evoked potential amplitude of biceps femoris during dorsi-flexion was significantly increased, whereas the posterior root-muscle reflex amplitude of biceps femoris during dorsi-flexion was significantly decreased compared to rest. These results suggest that corticospinal and spinal reflex excitabilities of soleus are facilitated during knee extension, whereas intra-limb modulation of biceps femoris during dorsi-flexion appeared to be inverse between corticospinal and spinal reflex circuits.
Highlights
The spinal motoneurons receive excitatory and inhibitory synaptic inputs from sensory afferents and inputs from supraspinal centers either directly or via interneurons
motor evoked potentials (MEPs) of SOL and tibialis anterior (TA) were potentiated during knee extension and flexion, and MEPs of biceps femoris (BF) were potentiated during plantar-flexion and dorsi-flexion (Figs. 2, 3B)
The findings of intra-limb modulation of posterior root-muscle (PRM) reflexes were that PRM reflexes of SOL were potentiated during knee extension, and PRM reflexes of BF were depressed during dorsi-flexion
Summary
The spinal motoneurons receive excitatory and inhibitory synaptic inputs from sensory afferents and inputs from supraspinal centers either directly or via interneurons. H-reflex depression is observed in SOL during knee flexion (i.e., hamstring muscles contraction) (Izumi et al 2001), whereas H-reflex facilitation is observed in vastus medialis during plantar-flexion (i.e., triceps surae contraction) (Hwang et al 2000) These studies suggest that intra-limb modulation of the H-reflex occurs inversely between contracted and tested muscles in the lower limb. By applying tSCS, the present study could clarify the differences in intra-limb modulation of PRM reflexes among the lower-limb muscles, including the muscles in which it is difficult to evoke the responses by the H-reflex (e.g., hamstrings). Vertical dotted lines indicate the timing of the test stimulus by TMS and tSCS
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