Modulation of Hoffmann reflex excitability during action observation of walking with and without motor imagery

Action observation (AO) and motor imagery (MI) are used for motor impairment rehabilitation after neurological disorders, such as spinal cord injuries or strokes. Clinical studies have reported that rehabilitation using AO and MI is effective in restoring motor function. Our previous study showed a difference in the modulation of corticospinal excitability only during action observation (AO) of walking and AO of walking combined with motor imagery (AO + MI). However, it is unclear whether AO and AO + MI can modulate spinal reflex excitability as well as corticospinal excitability. Thus, the purpose of this study was to investigate the modulation of the Hoffmann reflex (H-reflex) during AO and AO + MI. We measured H-reflex in the soleus muscle under the following conditions: (1) control, (2) AO, and (3) AO + MI. Posterior tibial nerve electrical stimulation, which induces H-reflex, was applied at the following four walking phases: 1) mid-stance, 2) terminal-stance, 3) early-swing, and 4) terminal-swing. Our results showed that AO + MI significantly increases H-reflex over AO alone, regardless of phase and that AO significantly modulates H-reflex depending on the observed phase. These findings suggested that spinal reflex excitability can be modulated during AO and AO + MI and that neural effects are different in AO and AO + MI.