## 2 Materials and Methods
### Assessment of Muscle Responses in Paretic and Non-Paretic Limbs  
Firstly, monopolar EMGs were visually inspected to identify channels with contact problems or powerline interference. Low-quality signals, if present, were replaced with the linear interpolation of the neighbor channels [30]. Monopolar EMGs signals were then band-pass filtered with a fourth-order Butterworth filter (10-400 Hz cutoff; bidirectional filter). Single-differential EMGs were obtained by differentiating monopolar signals along consecutive rows. M waves were triggered from EMGs (30 ms epochs) and then averaged across the four stimulation pulses, separately for each channel and stimulation intensity. The presence of innervation zones, either in the first or between the first and second rows  
Figure 1: Frontal (**left image**) and lateral (**right image**) views of the experimental setup performed with the paretic limb of a patient. Electrodes for surface EMG detection and stimulation were positioned in the distal and proximal portions of the biceps brachii muscle, respectively. The tape attached to the patient’s fingers is for rehabilitation purposes and, therefore, is not related to the experimental protocol. of the grid, was observed for 9 out of 28 muscles assessed. Thus, given that the innervation zone leads to a spurious decrease in the amplitude of surface EMGs [23], channels over the innervation zone were excluded, and only M waves with the greatest peak-to-peak amplitude in each column of the grid were considered for analysis. Amplitude values were then averaged across columns, producing a single, representative biceps brachii response per stimulation intensity. Variations in the mean peak-to-peak amplitude with stimulation intensity were considered to assess whether biceps brachii responses are greater on the paretic than the non-paretic side. First, the range of current intensities leading to the smallest and greatest M waves was identified from the distribution of peak-to-peak amplitudes obtained for all stimulation intensities. The highest current intensity below which the peak-to-peak amplitude was smaller or equal to the first amplitude mode was then identified and defined as the _motor threshold_. Likewise, the smallest current intensity over which peak-to-peak amplitude equaled or exceeded the second or last amplitude mode was identified and defined as the _maximal muscle response_. These values define the current range (cf. shaded rectangles in Figure 2A,C) within which increases in stimulation intensity resulted in increased muscle response; i.e., increased M-wave amplitude. Because the current range differed between patient limbs, the increases of 2 mA steps in stimulation intensity represented a different percentage of the current range for paretic and non-paretic muscles. Therefore, to ensure like-with-like comparisons, the amplitude of M waves obtained from the limb with the smaller current range was linearly interpolated considering a fixed, stimulation step smaller than 2 mA (Figure 2C). This new stimulation step was computed as the value of the smaller current range divided by the number of stimulation levels within the greater current range between limbs. Then, the differences between peak-to-peak values (_increments_) obtained for consecutive stimulation levels were computed and normalized with respect to the difference between the amplitudes corresponding to the _maximal muscle response_ and the _motor threshold_. _Increments_ were then averaged across all current levels (hereafter referred to as _increment amplitude_) and considered to assess limb differences in biceps brachii responses. Figure 2: M-wave peak-to-peak amplitude for each current intensity is shown for the non-paretic (**A**) and paretic (**C**) limbs of a single, representative patient. Gray circles correspond to amplitude values averaged across the four stimulation pulses and channels. Black circles correspond to the interpolated amplitude values within the paretic limb’s current range (shaded rectangle).