Motor dysfunction after stroke is generally associated with a reduction of central nervous system (CNS) circuit excitability and a decreased cortical representation area corresponding to the affected limbs (Yu et al., 2024). Successful rehabilitation with partial/entire functional recovery may enhance brain activities that control spontaneous movements (Joy and Carmichael, 2021). Recently, neuromuscular electrical stimulation (NMES) has been considered as one sufficient intervention to induce high-level cortical excitability or rapid reorganization in sensorimotor cortices for patients with physical impairments (Carson and Buick, 2021; Zhang et al., 2023). Powell et al. (1999) have reported that NMES could increase recovery of wrist extension over standard care in hemiplegic patients, and it has been also demonstrated to induce neuroplasticity by increasing the baseline level of spinal excitability such that low levels of input result in voluntary motor function in the patients with spinal cord injury (Karamian et al., 2022). Brain imaging studies showed that the NMES-evoked movements induced reliable cortical excitability enhancement and plastic changes in specific neural regions (Tenberg et al., 2023; Insausti-Delgado et al., 2021). Further research on chronic stroke patients confirmed that changes in sensorimotor area are accompanied with effective hand recovery (Kimberley et al., 2004). It is clear that the NMES-induced activity change and adaptive reorganization in brain play an important role in human behavioral restorations following rehabilitation treatments. Researchers have attempted to improve the rehabilitation-mediated functional brain changes through optimizing the NMES patterns (Zhao et al., 2018; Obayashi and Saito, 2022; Ryoki et al., 2017). Parameters of the NMES patterns, such as stimulation intensity, frequency or pulse width were optimized to enhance the sensorimotor cortical excitability and subsequently impact the efficiency of rehabilitation (Makii et al., 2015; Wegrzyk et al., 2017; Jiang et al., 2019).

Usually, neuromuscular electrical stimulations directly activate motor axons beneath the electrodes to induce isometric contractions (i.e., efferent/peripheral pathway) (Bergquist et al., 2011). Also, the electric pulse can generate contractions through reflexive recruitments of motoneurons by depolarizing the sensory fibers and ascending afferent volleys that contribute to cortical activity regulation (i.e., afferent/central pathway) (Qu et al., 2020). Therefore, the neuroplasticity changes in the central nervous system driven by NMES may be substantially related to increasing sensory axons recruitments. The sensory fibers and motor axons were reported to have different electrical excitabilities depending upon their intrinsic properties including strength-duration time constant and rheobase (Veale et al., 1973; Kiernan et al., 2004). Accordingly, the pulse duration was considered as a major factor affecting the priority of sensory or motor axon recruitments in NMES (Wegrzyk et al., 2017). Some recent studies have reported the effect of stimulation intensity and stimulation frequency on brain activation patterns (Insausti-Delgado et al., 2021; Backes et al., 2000; Kampe et al., 2000), but few studies have investigated whether stimulation pulse width affects cortical activities. The brain imaging study has just reported that the modulation of wide pulses resulted in differently activated brain regions, that is, NMES with wide pulses (100 Hz-1 ms) resulted in lower deactivation in the secondary somatosensory cortex and precuneus, while the conventional pattern (25 Hz–0.05 ms) induced larger hyperactivated in the bilateral thalami and caudate nuclei during stimulating the triceps surae of healthy subjects (Wegrzyk et al., 2017). Herein, the present study wanted to determine whether the optimized NMES pattern with wide pulses could better enhance cortical activities.

Traditional NMES pattern is constant-frequency trains (CFT) which generally composes of successive narrow stimulus pulses (< 400 μs) and is applied at repetitive frequency of 15-40 Hz (Collins, 2007). This conventional CFT stimulations can artificially elicit muscle contractions to recruit motor units (MUs) resulting from motor neurons depolarization primarily through efferent/peripheral mechanism (Wegrzyk et al., 2017) and less sensory fiber activation through afferent/central pathway. The low priority of sensory axons recruitments in CFT stimulation is attributed to the differential sensitivity and antidromic collisions between sensory and motor fibers (Veale et al., 1973; Kiernan et al., 2004). Recently, wide duration pulses (pulse width of 0.5–1 ms) have been reported a tendency of preferably stimulating more sensory rather than motor axons, causing the resultant afferent inputs ascending through spine pathway to enhance cortical activities in the central nervous system circuit (Bergquist et al., 2011). Panizza et al. (1992) demonstrated that wider stimulus duration (0.5–1 ms) facilitated eliciting Hoffmann reflexes (H-reflexes) in healthy volunteers which reflected information of the sensory fiber excitation (Rongsawad and Ratanapinunchai, 2018). Lagerquist and Collins (2010) also observed that NMES with wide pulses applied to 15 healthy subjects evoked more afferent volley.

Compared to conventional CFT, Variable Frequency Trains (VFT) is an alternative stimulus pattern that can evoke muscle contractions in a more physiological manner (Deley et al., 2014; Karu et al., 1995; Doll et al., 2018; Carole et al., 2016). Each of those electrical pulses in VFT could only induce subthreshold depolarizations of axons, and different numbers of subthreshold depolarizations could be temporally summed to activate different axons with different activation thresholds, leading to recruiting MUs in asynchronous pattern (Carole et al., 2016). MUs recruitment in asynchronous pattern mainly resulted from directly activating motor axons (efferent pathway) and indirectly recruiting motoneurons in the spinal cord by activating sensory axons (afferent pathway) (Bergquist et al., 2011). Carole employed a special stimulation protocol of VFT that consisted of N high-frequency stimuli (called N-lets) with long inter-period intervals (Carole et al., 2016), and doublet-frequency train (DFT, N = 2) was acknowledged as the most successful train in force improvement (Karu et al., 1995). While VFT has been reported more profit to evoking muscle contraction compared to CFT, studies have not investigated the effect of VFT, especially DFT with wide pulses, on cortical activities. Herein, we hypothesized that the pulse assignment with wide-pulse-based VFT neuromuscular stimulation would better enhance sensorimotor cortical activities due to the activation of more sensory fibers during evoking elbow flexion movements of right upper limb. The purposes of the present study were to evaluate the effect of pulse assignment of NMES patterns on the neural activations in sensorimotor areas and investigate whether dual wide-pulses in doublets have an advantage in the sensorimotor cortical excitability improvement during evoking functional elbow flexion movements. We evaluated sensorimotor cortical excitability through quantifying the Beta ERD from EEG data from Electrode C3 located in left sensorimotor cortex with the corresponding ERD properties (amplitude and descending slope).

The present study investigated the impact of dual wide-pulses in doublets on sensorimotor cortical excitability improvement by combining the wide pulse with Variable Frequency Trains. Beta ERD of EEG signals from Electrode C3 located in left sensorimotor cortex was utilized to evaluate the activation level in cortex region during elbow flexion tasks (Wang et al., 2023; Tacchino et al., 2016; Wang et al., 2024). VFT stimulation could induce stronger sensorimotor cortical excitability than CFT stimulation, especially, the DFT with the dual 500 μs wide pulse trains was found the most effective pattern for brain activity improvements, which supports our hypothesis.

As mentioned above, to evoke similar elbow flexion movements, wide-pulse stimulation for both CFT and VFT utilized lower current intensity than narrow-pulse stimulation suggesting that compared to NMES with narrow pulses, NMES with wide pulses had more advantages in activating muscle contraction. Wide-pulse pattern not only directly activated motor axons but also favored to depolarize the sensory fibers, resulting in recruiting motor units to evoke muscle contraction by the combination of the peripheral efferent with afferent pathways (Bergquist et al., 2011). The activated sensory fibers can generate the afferent volley ascending to the central nervous system (CNS) which contributes to the cortical excitability regulation, and the enhancement of cortical excitability is the key to the improvement of motor rehabilitation (Obayashi and Saito, 2022).

The present study focused on investigating sensorimotor cortical excitability induced by different NMES patterns through quantifying the beta ERD amplitudes of EEG signals at Electrode C3. Beta ERD amplitude has been reported to be strongly related to the level of sensorimotor cortical activation. The higher beta ERD amplitude is, the greater the cortical excitability is Tacchino et al. (2016). The results showed that compared to CFT patterns, VFT patterns favor the activation of sensorimotor cortical activities (i.e., significantly higher beta ERD amplitudes), which may be attributed to the way that VFT evokes muscle contraction. VFT was more prone to recruiting motor units in asynchronous pattern (i.e., more natural recruitment method) (Carole et al., 2016), and asynchronous motor units were generally recruited from the direct activation of motor axons (peripheral pathway) and the depolarization of sensory axons (central pathway) that activates motoneurons in the spinal cord (Bergquist et al., 2011). Together with those results in the present study, although CFT and VFT evoked similar elbow flexion movements, the stimulation current applied to right arm biceps of subjects in VFT pattern not only activated the motor axons, but also depolarized sensory fibers. This was also confirmed in the study of the MUs recruitment method during NMES-evoked muscle contraction (Collins, 2007). The sensory information generated by the activation of afferent fibers by electrical stimulation is transmitted to the central nervous system via the ascending afferent pathway. Siobhan’s research also demonstrated that afferent inputs generated by electrical stimulation through activating sensory axons could enhance cortical activity, driving transient or even long-term plastic changes to promote motor rehabilitation (Schabrun et al., 2012). Therefore, it may be speculated that VFT can induce larger beta ERD amplitudes due to the activation of more sensory axons.

Moreover, another important finding in this study was that the descending slope of beta ERD induced by VFT-evoked elbow flexion movements was significantly higher than that induced by CFT-evoked movements. This may be due to the fact that VFT activated more sensory axons and sensory volley ascending to the CNS for regulating cortical activities, thus developing the beta desynchronization more quickly. Previous study showed that electrical stimulation-evoked sensory inputs could regulate cortical activity networks (Carson and Buick, 2021). This was also verified in the monkey experiment that sensory afferents could encode cortical oscillations (Alonso et al., 2023). Therefore, the stimulation pattern of VFT in the present study was more inclined to depolarize sensory fibers so as to efficiently modulate cortical activities. The key to the improvement of motor function in motor rehabilitation lies in the enhancement of cortical excitability (Joy and Carmichael, 2021), and thus it is worthwhile to explore the potential of VFT in motor rehabilitation.

The results in the present study also showed that the pulse width of VFT could affect the sensorimotor cortical activities. We observed that wDFT induced higher amplitudes and the descending slope of beta ERD than nVFT did. By further comparison of the phase charges of the two VFT stimulation patterns, we found that to evoke similar elbow flexion motions, wDFT required significantly less phase charges than nVFT did. However, the beta ERD amplitude induced by the unit phase charge of wDFT was higher than that of nVFT, indicating that wDFT had higher phase charge efficiency to induce sensorimotor cortical excitability. In other words, the pulse assignment of VFT might regulate the level of cortical activities. Thus, the dual wide pulse train (wDFT) was more advantageous. The activation of axons is not only related to the accumulated charges applied to the membrane (Schwarz and Volkmer, 1965), but also affected by the physiological capacitance and input impedance of the membrane cells (Li and Bak, 1976). Due to the properties, such as large diameter, smaller input impedance and greater strength-duration constant, sensory fibers required less charges to be activated by the stimulation pulses with long sustained phase. Therefore, with longer pulse width (500 μs), wDFT in the present study required less phase charges, suggesting that more sensory axons could be activated by unit phase charge of wide-pulse VFT to better regulate cortical activities. Those results supported our hypothesis that VFT could induce stronger sensorimotor cortical excitability than CFT did. Further potential benefits on the modulation of cortical activities may be obtained by combining the wide-pulse with doublet VFT patterns.

The value of NMES in motor recovery has been widely reported. The activation of sensory fibers has become a new method to improve its clinical efficiency in rehabilitation due to the activation and regulation of central nervous activities by evoked sensory volley through afferent pathway (Obayashi and Saito, 2022). Wide-pulse stimulation has been shown to be more conducive to activating sensory fibers which have a larger intensity-time constant and lower base intensity than motor axons (Veale et al., 1973; Kiernan et al., 2004), but traditional CFT induced rapid muscle fatigue (Doucet et al., 2012). In contrast, VFT has been reported to induce fatigue-resistant muscle contraction and increase muscle contraction torque (Lagerquist and Collins, 2010; Karu et al., 1995). Some studies have suggested that this feature was due to the non-linear summation in induced muscle strength (Karu et al., 1995) and an intrinsic catch like property of muscle fibers itself (Stuart et al., 1991). Others have shown that motor units were recruited asynchronously during VFT-evoked muscle contraction (Carole et al., 2016). However, few studies have investigated whether VFT was more likely to activate sensory fibers when evoking muscle contraction. Therefore, the present study designed a new stimulation pattern combining wide pulse with VFT to investigate whether it may induce stronger cortical activities when evoking upper limb movements, thereby indirectly evaluate its activation of sensory axons. The results confirmed that wide-pulse VFT could not only induce stronger central nervous activities, but also establish the activation pattern more efficiently. In addition, although similar elbow flexion movements could be evoked by different stimulation patterns, both wide pulses and VFT enhanced sensorimotor cortical excitability by activating sensory fibers which is necessary for promoting motor rehabilitation. Therefore, those results in the present study further suggested that rehabilitation training should not be limited to joint movements, but also enhance sensory inputs ascending to sensorimotor areas.

Based on previous studies of VFT-evoked muscle contraction pattern and wide-pulse NMES for the activation of sensory fibers, the present study combined the two stimulation patterns to investigate whether VFT, especially the wide-pulse DFT, had more advantages of central cortical modulation over traditional CFT from the perspective of cortical activity regulation. The results showed that compared to simply increasing the pulse width in several previous studies, the comprehensive optimization of NMES patterns through the combination of wide pulse and VFT pattern was more conducive to enhancing cortical excitability. Therefore, the proposed NMES pattern may provide a new idea to optimize NMES parameters for the improvement of motor rehabilitation efficiency. However, the present study also has some limitations. The first one is the fixed order of four stimulation patterns applied in all subjects. This may cause a confounding impact from the stimulation order, such as muscle fatigue. Although every subject was provided a 10-min rest between any two NMES patterns to reduce muscle fatigue, it could be better to avoid this confounding factor using a randomized order of the four NMES patterns in different subjects. Early research in human brain electrophysiology has already investigated the cortical response to various kinematic parameters, and concluded that sensorimotor oscillation activities were different with different movements (Yuan et al., 2010; Cassim et al., 2000). Hence, further evaluations according to different kinematic characteristics should be designed in future for a more comprehensive understanding in the cortical modulation induced via dual wide pulse trains. Moreover, the participants involved in our study were all healthy subjects. Considering the reduction of movement-related beta ERD at the contralateral sensorimotor cortex in stroke patients (Wang et al., 2023), it is also necessary to extend the present findings more specific to pathological populations with motor function disorders in further studies.

This study investigated the effects of NMES with wide-pulsed DFT on sensorimotor cortical excitability. Our findings established that the induced cortical excitability could be improved by optimized pulse width and VFT pattern, whereby the dual 500 μs wide pulse train (wDFT) maximized the benefits. The profound reason may attribute to the facilitation of afferent fibers activation enhancement which consequently increased the ascending volleys. Therefore, our results implied the potential of wide-pulse DFT in assisting cortical excitability modulation through modifying the related afferent inputs, which suggested a promising usage in the improvement of rehabilitation efficiency for patients with motor dysfunction.