## Article  
EMG Signs of Motor Units' Enlargement in Stroke Survivors  
[PERSON] 1,2,3\({}^{\copyright}\), [PERSON] 4,5\({}^{\copyright}\), [PERSON] 2,3 [PERSON] 6 and [PERSON] 2,3,\({}^{\copyright}\)\({}^{\copyright}\)  
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in the paretic limb of stroke survivors with respect to healthy muscles (either of the non-paretic side of stroke patients or of a control group). This increased fiber density presumably indicates that the paretic muscle may be characterized by a greater innervation ratio [12]. Besides the invasive analysis of single-fiber electromyography [7; 8; 11], such motor units' enlargement following stroke was also suggested from surface electromyography. For example, [PERSON] and [PERSON] [9] and [PERSON] et al. [10] observed larger EMG amplitudes for the biceps brachii muscle on paretic limbs compared to contralateral limbs during voluntary isometric contractions performed at relative force levels. Similarly, [PERSON] et al. [13] observed an increase in the size of the excited muscle region within the gastrocnemius muscle in paretic limbs. These authors suggest that the increased EMG response may be associated with muscle fiber reinnervation. Assessing muscle reinnervation progress remains fundamental to comprehending factors that promote and hinder illness recovery. The muscle reinnervation process has been associated with the preservation of muscle strength [14]. On the other hand, the motor units' enlargement may impair the ability of a reinnervated muscle to produce force increments as finely as healthy muscles since a greater amount of muscle fibers innervated by motoneurons would be activated for similar synaptic inputs [15]. Therefore, the identification of techniques to track changes in the size of motor units (i.e., in the muscle's innervation ratio) would assist health professionals in assessing disease progression in stroke survivors. However, the methodologies applied in the above-mentioned studies investigating muscle reinnervation in stroke survivors present some limitations. Besides being an invasive technique, single-fiber electromyography assesses a relatively small portion of the entire pool of motor units in the muscle [16]. The other limitation is that the protocols considering voluntary contractions cannot be applied in hemiplegic patients (i.e., with complete paralysis of one side of the body), a common sequela of stroke [17]. Different methods involving incremental electrical stimulation, such as the \"motor unit number estimation--MUNE\" [18], the \"electrophysiological muscle scan\" [19], and the \"motor unit size index--MUSIX\" [14], have been proposed to indirectly assess structural adaptations on the neuromuscular system. The incremental stimulation technique gradually increases the stimulus intensity applied to a peripheral nerve to recruit successive motor units [18]. Since the motoneurons that innervate a muscle have different sizes and, therefore, different activation thresholds [20; 21], it is possible to recruit them by gradually increasing the intensity of current pulses applied to the nerve [18]. As the stimulation intensity increases, more motoneurons are activated and, consequently, more muscle fibers are recruited. Such progressive recruitment can be observed through increases in the amplitude of massed action potentials (M wave) [18; 19]. If no variations in the M-wave amplitude are observed as stimulation intensity increases, this indicates that a maximum M-wave response was elicited in the muscle, i.e., motor units corresponding to the pool of motoneurons stimulated were totally recruited [18]. Based on that, a potential method to non-invasively assess signs of muscle fiber reinnervation could be the combination of incremental neuromuscular electrical stimulation with high-density surface EMG.