Test-retest reliability of wide-pulse high-frequency neuromuscular electrical stimulation evoked force

Motoneuron persistent inward current contribution to increased torque responses to wide-pulse high-frequency neuromuscular electrical stimulation

PURPOSE

To assess the effect of a remote handgrip contraction during wide-pulse high-frequency (WPHF) neuromuscular electrical stimulation (NMES) on the magnitude of extra torque, progressive increase in torque during stimulation, and estimates of the persistent inward current (PIC) contribution to motoneuron firing in the plantar flexors.

METHODS

Ten participants performed triangular shaped contractions to 20% of maximal plantar flexion torque before and after WPHF NMES with and without a handgrip contraction, and control conditions. Extra torque, the relative difference between the initial and final torque during stimulation, and sustained electromyographic (EMG) activity were assessed. High-density EMG was recorded during triangular shaped contractions to calculate ∆F, an estimate of PIC contribution to motoneuron firing, and its variation before vs after the intervention referred to as ∆F change score.

RESULTS

While extra torque was not significantly increased with remote contraction (WPHF + remote) vs WPHF (+ 37 ± 63%, p = 0.112), sustained EMG activity was higher in this condition than WPHF (+ 3.9 ± 4.3% MVC EMG, p = 0.017). Moreover, ∆F was greater (+ 0.35 ± 0.30 Hz) with WPHF + remote than control (+ 0.03 ± 0.1 Hz, p = 0.028). A positive correlation was found between ∆F change score and extra torque in the WPHF + remote (r = 0.862, p = 0.006).

DISCUSSION

The findings suggest that the addition of remote muscle contraction to WPHF NMES enhances the central contribution to torque production, which may be related to an increased PIC contribution to motoneuron firing. Gaining a better understanding of these mechanisms should enable NMES intervention optimization in clinical and rehabilitation settings, improving neuromuscular function in clinical populations.

Centrally mediated responses to NMES are influenced by muscle group and stimulation parameters

Wide-pulse high-frequency neuromuscular electrical stimulation (WPHF NMES) can generate a progressive increase in tetanic force through reflexive recruitment of motor units, called extra force. This phenomenon has previously been observed on different muscle groups, but little is known on potential inter-muscle differences. We compared extra force and sustained electromyographic (EMG) activity induced by NMES between plantar flexors, knee extensors, elbow flexors and within muscle groups using pulse durations of 0.2, 1 and 2 ms and stimulation frequencies of 20, 50, 100 and 147 Hz. Extra force production and sustained EMG activity were higher for plantar flexors compared to elbow flexors at all tested parameters (except 0.2 ms for extra force). When compared to elbow flexors, extra force of the knee extensors was only higher at 100 Hz and with 1 ms while sustained EMG activity was higher at all frequencies with pulse durations of 0.2 and 2 ms. Peripheral nerve architecture as well as muscle typology and function could influence the occurrence and magnitude of centrally-mediated responses to NMES. The present findings suggest that the use of wide-pulse high-frequency NMES to promote reflexive recruitment seems to be more pertinent for lower limb muscles, plantar flexors in particular.

Protocols Targeting Afferent Pathways via Neuromuscular Electrical Stimulation for the Plantar Flexors: A Systematic Review

This systematic review documents the protocol characteristics of studies that used neuromuscular electrical stimulation protocols (NMES) on the plantar flexors [through triceps surae (TS) or tibial nerve (TN) stimulation] to stimulate afferent pathways. The review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) statement, was registered to PROSPERO (ID: CRD42022345194) and was funded by the Greek General Secretariat for Research and Technology (ERA-NET NEURON JTC 2020). Included were original research articles on healthy adults, with NMES interventions applied on TN or TS or both. Four databases (Cochrane Library, PubMed, Scopus, and Web of Science) were systematically searched, in addition to a manual search using the citations of included studies. Quality assessment was conducted on 32 eligible studies by estimating the risk of bias with the checklist of the Effective Public Health Practice Project Quality Assessment Tool. Eighty-seven protocols were analyzed, with descriptive statistics. Compared to TS, TN stimulation has been reported in a wider range of frequencies (5-100, vs. 20-200 Hz) and normalization methods for the contraction intensity. The pulse duration ranged from 0.2 to 1 ms for both TS and TN protocols. It is concluded that with increasing popularity of NMES protocols in intervention and rehabilitation, future studies may use a wider range of stimulation attributes, to stimulate motor neurons via afferent pathways, but, on the other hand, additional studies may explore new protocols, targeting for more optimal effectiveness. Furthermore, future studies should consider methodological issues, such as stimulation efficacy (e.g., positioning over the motor point) and reporting of level of discomfort during the application of NMES protocols to reduce the inherent variability of the results.

Acute Effects of Neuromuscular Electrical Stimulation on Contralateral Plantar Flexor Neuromuscular Function

Contralateral facilitation, i.e., the increase in contralateral maximal voluntary strength that is observed when neuromuscular electrical stimulation (NMES) is applied to the ipsilateral homonymous muscle, has previously been reported for the knee extensors but the neurophysiological mechanisms remain to be investigated. The aim of this study was to compare plantar flexor contralateral facilitation between a submaximal voluntary contraction (~10% MVC torque) and two evoked contractions (conventional and wide-pulse high-frequency NMES) of the ipsilateral plantar flexors, with respect to a resting condition. Contralateral MVC torque and voluntary activation level were measured in 22 healthy participants while the ipsilateral plantar flexors were at rest, voluntarily contracted or stimulated for 15 s. Additional neurophysiological parameters (soleus H-reflex and V-wave amplitude and tibialis anterior coactivation level) were quantified in a subgroup of 12 participants. Conventional and wide-pulse high-frequency NMES of the ipsilateral plantar flexors did not induce any contralateral facilitation of maximal voluntary strength and activation with respect to the resting condition. Similarly, no alteration of neurophysiological parameters was observed in the different conditions. This absence of contralateral facilitation contrasts with some results previously obtained on the knee extensors but is consistent with the absence of neurophysiological changes on the contralateral soleus.

Wide-Pulse High-Frequency Neuromuscular Electrical Stimulation Evokes Greater Relative Force in Women Than in Men: A Pilot Study

This study aimed to examine the potential sex differences in wide-pulse high-frequency neuromuscular electrical stimulation (WPHF NMES)-evoked force. Twenty-two subjects (10 women) completed this study. Prior to the stimulation, the visual analogue scale (VAS) for discomfort and the rating of perceived exertion (RPE) were measured, followed by the isometric strength of the dominant elbow flexor muscles. The subjects then completed ten, 10-s on 10-s off WPHF NMES (pulse width: 1 ms, frequency: 100 Hz) at maximum tolerable intensities. The subjects' RPE was recorded after each set, and the VAS was measured following the last stimulation. The stimulation induced significant increase in discomfort for both sexes, with women having greater discomfort than men (men: 22.4 ± 14.9 mm, women: 39.7 ± 12.7 mm). The stimulation amplitude was significantly greater in men than in women (men: 16.2 ± 6.3 mA, women: 12.0 ± 4.5 mA). For the evoked force, only the relative NMES-evoked force was found greater in women than in men (men: 8.96 ± 6.51%, women: 17.08 ± 12.61%). In conclusion, even at the maximum tolerable intensity, WPHF NMES evoked larger relative elbow flexion force in women than in men, with women experiencing greater discomfort.

Influence of wide-pulse neuromuscular electrical stimulation frequency and superimposed tendon vibration on occurrence and magnitude of extra torque

Low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation (NMES) can generate extra torque (ET) via afferent pathways. Superimposing tendon vibration (TV) to NMES can increase the activation of these afferent pathways and favor ET generation. Knowledge of the characteristics of ET is essential to implement these stimulation paradigms in clinical practice. Thus, we aimed to investigate the effects of frequency and TV superimposition on the occurrence and magnitude of ET in response to wide-pulse NMES. NMES-induced isometric plantar flexion torque was recorded in 30 healthy individuals who performed five NMES protocols: wide-pulse low-frequency (1 ms; 20 Hz; WPLF) and wide-pulse high-frequency (1 ms; 100 Hz; WPHF) without and with superimposed TV (1 mm; 100 Hz) and conventional NMES (50 µs; 20 Hz; reference protocol). Each NMES protocol consisted of three 20-s trains interspersed by 90 s of rest, with NMES intensity being adjusted to reach 10% of maximal voluntary contraction. The ET occurrence was similar for WPLF and WPHF (P = 0.822). In the responders, the ET magnitude was greater for WPHF than WPLF (P < 0.001). There was no effect of superimposed TV on ET characteristics. This study reported an effect of NMES frequency on ET magnitude, whereas TV superimposition did not affect this parameter. In the context of our experimental design decisions, the present findings question the clinical use of wide-pulse NMES and its combination with superimposed TV. Yet, further research is needed to maximize force production through the occurrence and magnitude of ET.NEW & NOTEWORTHY This study is the first to assess the effect of stimulation frequency and superimposed tendon vibration on extra torque characteristics generated by wide-pulse neuromuscular electrical stimulation. The percentage of subjects showing extra torque (i.e., considered as responders) was similar for low-frequency and high-frequency wide-pulse neuromuscular electrical stimulation. In the responders, the extra torque was greater for high-frequency than for low-frequency wide-pulse neuromuscular electrical stimulation. The superimposition of tendon vibration had no effect on extra torque occurrence or magnitude.

Modulation of torque evoked by wide-pulse, high-frequency neuromuscular electrical stimulation and the potential implications for rehabilitation and training

The effectiveness of neuromuscular electrical stimulation (NMES) for rehabilitation is proportional to the evoked torque. The progressive increase in torque (extra torque) that may develop in response to low intensity wide-pulse high-frequency (WPHF) NMES holds great promise for rehabilitation as it overcomes the main limitation of NMES, namely discomfort. WPHF NMES extra torque is thought to result from reflexively recruited motor units at the spinal level. However, whether WPHF NMES evoked force can be modulated is unknown. Therefore, we examined the effect of two interventions known to change the state of spinal circuitry in opposite ways on evoked torque and motor unit recruitment by WPHF NMES. The interventions were high-frequency transcutaneous electrical nerve stimulation (TENS) and anodal transcutaneous spinal direct current stimulation (tsDCS). We show that TENS performed before a bout of WPHF NMES results in lower evoked torque (median change in torque time-integral: - 56%) indicating that WPHF NMES-evoked torque might be modulated. In contrast, the anodal tsDCS protocol used had no effect on any measured parameter. Our results demonstrate that WPHF NMES extra torque can be modulated and although the TENS intervention blunted extra torque production, the finding that central contribution to WPHF NMES-evoked torques can be modulated opens new avenues for designing interventions to enhance WPHF NMES.

Effect of reflexive activation of motor units on torque development during electrically-evoked contractions of the triceps surae muscle

The aim of the study was to identify stimulation conditions permitting the occurrence of extra torque (ET) and to examine their impact on spinal and corticospinal excitabilities. Twelve subjects received stimulation trains over the tibial nerve (20 s duration, 1 ms pulse duration) that were delivered at 3 stimulation frequencies (20, 50, and 100 Hz) and at 5 intensities (110%, 120%, 130%, 140%, and 150% of the motor threshold). Torque-time integral (TTI) of each stimulation train was calculated. Spinal [maximum H-reflex (H_max)/maximal M-wave (M_max)] and corticospinal [maximal motor evoked potential amplitude (MEP_max)/M_max] excitabilities were assessed at rest before and after each stimulation train by tibial nerve stimulation and by transcranial magnetic stimulation, respectively. Moreover, a twitch at each stimulation intensity was delivered before and after each stimulation train. The EMG activity associated with this twitch was analyzed to identify the initial motor unit (MU) recruitment pathway before each stimulation train and discriminate trials to H-trials (indirect recruitment) and M-trials (direct recruitment). TTI was higher for H-trials compared with M-trials for all tested frequencies. There was a decrease in H_max/M_max for the 20 Hz-H trials and an increase for the 100 Hz-H trials, whereas MEP_max/M_max remained unchanged at post measurements. Present results demonstrate that the initial MU recruitment pattern plays a main role in the ET occurrence, with the indirect recruitment via the afferent volley being substantial for its development. The modulations of H_max/M_max without changes in MEP_max/M_max suggest that the ET development affects spinal excitability and that these changes are frequency dependent. NEW & NOTEWORTHY This study brings new insights into the stimulation conditions permitting the development of extra torque. An initial indirect recruitment of motor units, inducing reflex activation of spinal neurons through Ia afferent solicitation, appears a prerequisite for extra torque development. Under these conditions, spinal excitability modulations were frequency dependent.