The effect of motor imagery on gain modulation of the spinal reflex

Motor imagery is well known to have a facilitatory effect on the corticospinal tract, but conflicting opinions have arisen concerning its effect on spinal reflex excitability. The purpose of this study was to clarify the effects of motor imagery on gain modulation of the spinal reflex by focusing on the physiological differences between the H-reflex and the stretch reflex. In experiment 1, there were three conditions: rest, motor imagery of ankle dorsiflexion (MI-DF), and motor imagery of ankle plantarflexion (MI-PF). The subjects were instructed to imagine 100% maximum voluntary isometric contraction (MI-100) in each direction of movement. To examine the effects of the imagined effort level on spinal reflex excitability, the subjects also imagined 50% maximum voluntary contraction (MI-50) in experiment 2. The soleus H-reflex and the stretch reflex amplitude and background EMG (bEMG) activity were measured. There were no significant differences in bEMG activity between the H-reflex and stretch reflex measurements. In experiment 1, although the H-reflex amplitude did not change significantly among the three conditions, the stretch reflex amplitude increased significantly under the MI-DF and MI-PF conditions compared to the rest condition. In addition, the stretch reflex amplitude under the MI-100 condition was significantly larger than that under the MI-50 condition in experiment 2. These results indicate that motor imagery has a selective facilitatory effect on stretch reflex pathways. Furthermore, this excitability change may occur in untargeted antagonist muscles as well as targeted agonist muscles and may depend on the imagined effort level.