Soleus H-reflex phase-dependent modulation is preserved during stepping within a robotic exoskeleton

Abstract

Objective: To investigate to what extent the phase-dependent modulation of the soleus H-reflex is preserved when bilateral leg movements are electromechanically driven by a robotic exoskeleton at different levels of body weight support (BWS) in healthy subjects.

Methods: The soleus H-reflex was elicited by posterior tibial nerve stimulation with a 1-ms single pulse at an intensity that the M-waves ranged from 4% to 9% of the maximal M-wave across subjects. Stimuli were randomly dispersed across the step cycle which was divided into 16 equal bins. At each bin, a maximal M-wave was elicited 100 ms after the test H-reflex and was used to normalize the associated M-wave and H-reflex. Electromyographic (EMG) activity from major hip, knee, and ankle muscles was recorded with surface bipolar electrodes. For each subject and muscle, the integrated EMG profile was established and plotted as a function of the step cycle phases. The H-reflex gain was determined as the slope of the relationship between the H-reflex and soleus EMG amplitudes at 100 ms before the H-reflex for each bin.

Results: During robotic assisted stepping, the phase-dependent soleus H-reflex modulation pattern was preserved and was similar at 25% and 50% BWS, a linear relationship between soleus H-reflex amplitude and background activity was found, and the reflex gain did not change with alterations of the BWS level. EMG amplitudes were smaller at 50% compared to 25% BWS.

Conclusions: Body unloading, decreased EMG amplitude of ankle extensors, and reduced ankle movement are not key factors for the soleus H-reflex phasic excitability to be manifested.

Significance: Robotic devices are utilized for rehabilitation of gait in neurological disorders. Based on our findings, spinal interneuronal circuits involved in the phase-dependent modulation of the soleus H-reflex will be engaged in a physiological manner during robotic assisted stepping in neurological disorders.