Fig. 1.

Positioning of stimulation and detection electrodes and M-waves. A, schematically shows the position of stimulation and detection electrodes in relation to the medial gastrocnemius (MG) muscle. After delivering current pulses over a relatively large region (dashed line) with a pen electrode, the pre-gelled cathode electrode was positioned at a specific location where the least injected current led to twitches observed primarily on skin regions closest to the MG muscle. The location of the stimulation electrode was presumably closest to the MG nerve branch. Arrows in the right panel indicate the direction along which electric charges flowed from the small cathode to the large anode electrode (monopolar stimulation modality; Merletti, Knaflitz, & DeLuca, 1992). Note that for a fixed current intensity, the amount of charges per unit of area (i.e., grey ellipses) reduces toward the anode electrode. Nerve fibers more distant from the cathode were therefore less sensible to variations in stimulation amplitude. B, shows raw M-waves (bottom panel) and their root mean square (RMS) amplitude obtained for the stimulation levels 2 (14 mA), 6 (42 mA) and 10 (70 mA) for the subject 2. Note the local representation of M-waves across the matrix. At stimulation level 2, small M-waves were predominantly represented at the more proximal rows. With the increase in stimulation amplitude large M-waves emerged laterally and distally.