## 4 Discussion
### Variations in M-Wave Amplitude Differed between Biceps brachii of Paretic and Non-Paretic Limbs  
Structural adaptations in the motor units' size of muscles affected after stroke were indirectly investigated by comparing the size of increments in the amplitude of M waves elicited in the biceps brachii of paretic and non-paretic limbs of stroke survivors. The M wave is the summation of action potentials of the synchronously activated muscle fibers in a muscle which is recorded from the surface electromyography [36]. Variations in the M-wave amplitude are usually assumed to reflect a change in the number of motor  
Figure 3: Boxplots in panels (**A**,**B**) show the _increment amplitudes_ and current ranges calculated for non-paretic and paretic limbs of patients, respectively. The asterisk indicates a statistically significant difference (\(p<0.05\)) between limbs. units recruited [18] and, therefore, in the number of muscle fibers activated. If the motor units' size (i.e., the innervation ratio) increases due to a reinnervation process, greater variations in the M-wave amplitude would be expected when motor units of a reinnervated muscle are gradually recruited during an incremental stimulation protocol [18; 19]. Thus, assuming collateral reinnervation takes place in muscles affected after stroke [7; 8; 11], we expected that increases in M-wave amplitude, evaluated by means of median increments (i.e., _increment amplitude_), would be greater in paretic rather than non-paretic muscles for similar, relative increases in current intensity. Our findings indicated that patients presented significantly greater _increment amplitudes_ in muscles of paretic with respect to non-paretic limbs (\(p\) = 0.016) (cf. Figure 3A). The average difference between _increment amplitudes_ of limbs was about 0.7%. Thus, increases in M-wave amplitude seemed to have been progressively larger in paretic than non-paretic biceps brachii for similar, relative increases in the current intensity. Side differences in _increment amplitudes_ may have been even greater for five out of the six patients analyzed. Two factors probably bias patients' results, upper-limb dominance and the interpolation of stimulus-response curves. In our previous study [27], we performed the same experimental protocol with 16 healthy, young subjects and found significantly smaller _increment amplitude_ in the biceps brachii of dominant than non-dominant limbs. Interestingly, for four patients evaluated in the present study the dominant limb was the paretic one and, therefore, increment amplitudes were greater in the dominant than the non-dominant limb. These results were contrary to those found for healthy subjects in [PERSON] et al. [27], and, therefore, patients' dominance probably underestimated side differences in _increment amplitudes_. In addition, for two patients (one within the just mentioned group), the current range was 16 mA greater in non-paretic than paretic limbs. It means linear interpolation was applied in the stimulus-response curve of paretic muscles, probably underestimating their _increment amplitudes_, as mentioned above. These findings corroborated the notion that collateral reinnervation occurs after a stroke, increasing motor units' size and, therefore, the magnitude of the muscle responses in the paretic limb. Previous accounts [7; 8; 11] indeed showed higher fiber density in muscles affected after stroke with respect to healthy, non-paretic muscles. As above-mentioned in the statistics section, two stroke survivors were excluded from the analysis because _increment amplitudes_ were smaller in paretic with respect to non-paretic muscles. These results indicate a muscle reinnervation process may not have occurred in the paretic biceps brachii of these patients. Such inter-subject variability observed in our findings regarding side differences of neuromuscular responses in stroke survivors was also reported by previous studies. Briefly, [PERSON] and [PERSON] [9] and [PERSON] et al.