EMG recording and tibial nerve stimulation
EMG signal was recorded from the soleus and its antagonist tibialis anterior (TA) of the leg stimulated. Pairs of self-adhesive surface Ag–AgCl electrodes (2.2 × 3.5 cm, Nissha Medical Technologies, Buffalo, NY) were placed with their centers ~ 3 cm apart. The pair of soleus electrodes were placed just below the gastrocnemii and the TA pair were placed over the muscle belly. EMG signal was amplified and bandpass filtered at 10–1000 Hz using an AMT-8 EMG amplifier system (Bortec Biomedical Ltd., Calgary, AB, Canada), digitized at 3200 Hz, and stored.To elicit the soleus H-reflex and M-wave, cathodal stimulation was applied to the tibial nerve in the popliteal fossa using disposable surface self-adhesive Ag–AgCl electrodes (2 × 2 cm for the cathode and 2.2 × 3.5 cm for the anode, Nissha Medical Technologies) and a Digitimer DS8R constant current stimulator (Digitimer Limited, Letchworth Garden City, UK). For each participant, the stimulus electrode locations were carefully selected such that the least amount of current was required to elicit the H-reflex in the soleus while minimizing stimulation of the common peroneal nerve and other unwanted nerves. A single 1-ms square pulse was delivered to the tibial nerve stimulating electrode pair when the standing participant had maintained a pre-defined level of soleus (natural standing level) and TA (resting level) EMG activity for at least 2 s, and at least 5 s had passed since the last trial (Hill et al. At the end of the preliminary session, a thin, soft cast was fitted for each participant’s lower leg to mark the optimal EMG recording and nerve stimulating electrode locations. This cast was used for all subsequent experimental sessions to ensure consistency in electrode placement across multiple sessions.