H-reflex and M-wave recordings: effect of pressure application to the stimulation electrode on the assessment of evoked potentials and subject's discomfort.

This study aimed to compare the effect of different types of pressure applied to the stimulation electrode on assessing the efficiency of Ia-α-motoneuron transmission of the soleus muscle and the associated discomfort using electrical nerve stimulation. Twelve healthy young adults participated in three experimental sessions (one for each knee angle). The amplitudes of the maximal Hoffmann reflex (Hmax ) and motor potential (Mmax ) were recorded from the soleus muscle at 0°, 30° and 90° knee angles (0° full extension) through three pressure applications to the stimulation electrode: no pressure, pressure with manual application and pressure using adhesive tape. The soleus Hmax /Mmax were calculated to assess the efficiency of Ia-α-motoneuron transmission during varied knee angles and pressure application to the stimulation electrode. At the stimulation intensity evoking soleus Hmax and Mmax , subjects were asked to orally provide a value between 'no discomfort' (0) and 'worst possible discomfort' (10). The application of pressure on the stimulation electrode, particularly using adhesive tape, decreased both the stimulation intensity needed to evoke an electrophysiological response and the associated self-reported discomfort (P<0·05), while the Hmax /Mmax remained constant. At the stimulation intensity evoking Mmax , the electrical stimulation appeared to be more painful at 0° knee angle compared with 30° and 90° angles (P<0·01). To conclude, this study showed that a knee flexion and a pressure application to the stimulation electrode, especially using tape pressure, are recommended in the objective to reduce the patient/subjects' discomfort when eliciting evoked potentials on soleus muscle.