Low-Frequency Pulsed Current Versus Kilohertz-Frequency Alternating Current: A Scoping Literature Review

Neuromuscular electrical stimulation for facial wrinkles and sagging: The 8-week prospective, split-face, controlled trial in Asians

OBJECTIVE

This study aims to fill the knowledge gap regarding the effects of high frequency facial neuromuscular electrical stimulation (fNMES) on facial aging, using a device equipped with CERTEC (Cell Energy Regeneration Technology) operating between 40 and 190 kHz.

METHODS

This prospective split-face study was conducted at Tokyo University Hospital between March and May 2023 with 24 healthy adult women aged 30-59. The intervention group used the fNMES device along with basic skin care on one side of the face, and basic skin care alone on the other side for 8 weeks. Evaluations included changes in skin wrinkles, sagging, and blood flow.

RESULTS

This study found significant improvements in skin elasticity and degree of wrinkles in the areas intervened with fNMES (p < 0.05, respectively). In addition, the intervention resulted in significant improvements in jawline angle (p < 0.01), submental volume (p < 0.05), cheek volume (p < 0.05), maximum nasolabial fold depth (p = 0.03), and total volume of the nasolabial folds (p = 0.03). The fNMES intervention also showed improvement in blood flow (p < 0.05). These improvements were also subjectively assessed by the participants in subject questionnaires at 8 weeks after the intervention (p < 0.05).

CONCLUSION

This study suggests that high frequency fNMES effectively improves facial skin elasticity, reduces wrinkles and sagging, promotes blood flow, and contributes to overall facial appearance rejuvenation. Although further studies are needed, high frequency fNMES appeared promising as a noninvasive anti-aging therapy.

FUNDAMENTAL LIMITATIONS OF KILOHERTZ-FREQUENCY CARRIERS IN AFFERENT FIBER RECRUITMENT WITH TRANSCUTANEOUS SPINAL CORD STIMULATION

The use of kilohertz-frequency (KHF) waveforms has rapidly gained momentum in transcutaneous spinal cord stimulation (tSCS) to restore motor function after paralysis. However, the mechanisms by which these fast-alternating currents depolarize efferent and afferent fibers remain unknown. Our study fills this research gap by providing a hypothesis-and evidence-based investigation using peripheral nerve stimulation, lumbar tSCS, and cervical tSCS in 25 unimpaired participants together with computational modeling. Peripheral nerve stimulation experiments and computational modeling showed that KHF waveforms negatively impact the processes required to elicit action potentials, thereby increasing response thresholds and biasing the recruitment towards efferent fibers. While these results translate to tSCS, we also demonstrate that lumbar tSCS results in the preferential recruitment of afferent fibers, while cervical tSCS favors recruitment of efferent fibers. Given the assumed importance of proprioceptive afferents in motor recovery, our work suggests that the use of KHF waveforms should be reconsidered to maximize neurorehabilitation outcomes, particularly for cervical tSCS. We posit that careful analysis of the mechanisms that mediate responses elicited by novel approaches in tSCS is crucial to understanding their potential to restore motor function after paralysis.

Effect of muscle length on maximum evoked torque, discomfort, contraction fatigue, and strength adaptations during electrical stimulation in adult populations: A systematic review

Neuromuscular electrical stimulation (NMES) can improve physical function in different populations. NMES-related outcomes may be influenced by muscle length (i.e., joint angle), a modulator of the force generation capacity of muscle fibers. Nevertheless, to date, there is no comprehensive synthesis of the available scientific evidence regarding the optimal joint angle for maximizing the effectiveness of NMES. We performed a systematic review to investigate the effect of muscle length on NMES-induced torque, discomfort, contraction fatigue, and strength training adaptations in healthy and clinical adult populations (PROSPERO: CRD42022332965). We conducted searches across seven electronic databases: PUBMED, Web of Science, EMBASE, PEDro, BIREME, SCIELO, and Cochrane, over the period from June 2022 to October 2023, without restricting the publication year. We included cross-sectional and longitudinal studies that used NMES as an intervention or assessment tool for comparing muscle lengths in adult populations. We excluded studies on vocalization, respiratory, or pelvic floor muscles. Data extraction was performed via a standardized form to gather information on participants, interventions, and outcomes. Risk of bias was assessed using the Revised Cochrane risk-of-bias tool for cross-over trials and the Physiotherapy Evidence Database scale. Out of the 1185 articles retrieved through our search strategy, we included 36 studies in our analysis, that included 448 healthy young participants (age: 19-40 years) in order to investigate maximum evoked torque (n = 268), contraction fatigability (n = 87), discomfort (n = 82), and muscle strengthening (n = 22), as well as six participants with spinal cord injuries, and 15 healthy older participants. Meta-analyses were possible for comparing maximal evoked torque according to quadriceps muscle length through knee joint angle. At optimal muscle length 50° - 70° of knee flexion, where 0° is full extension), there was greater evoked torque during nerve stimulation compared to very short (0 - 30°) (p<0.001, CI 95%: -2.03, -1.15 for muscle belly stimulation, and -3.54, -1.16 for femoral nerve stimulation), short (31° - 49°) (p = 0.007, CI 95%: -1.58, -0.25), and long (71° - 90°) (p<0.001, CI 95%: 0.29, 1.02) muscle lengths. At long muscle lengths, NMES evoked greater torque than very short (p<0.001, CI 95%: -2.50, -0.67) and short (p = 0.04, CI 95%: -2.22, -0.06) lengths. The shortest quadriceps length generated the highest perceived discomfort for a given current amplitude. The amount of contraction fatigability was greater when muscle length allowed greater torque generation in the pre-fatigue condition. Strength gains were greater for a protocol at the optimal muscle length than for short muscle length. The quality of evidence was very high for most comparisons for evoked torque. However, further studies are necessary to achieve certainty for the other outcomes. Optimal muscle length should be considered the primary choice during NMES interventions, as it promotes higher levels of force production and may facilitate the preservation/gain in muscle force and mass, with reduced discomfort. However, a longer than optimal muscle length may also be used, due to possible muscle lengthening at high evoked tension. Thorough understanding of these physiological principles is imperative for the appropriate prescription of NMES for healthy and clinical populations.

Application of facial neuromuscular electrical stimulation (fNMES) in psychophysiological research: Practical recommendations based on a systematic review of the literature

Facial neuromuscular electrical stimulation (fNMES), which allows for the non-invasive and physiologically sound activation of facial muscles, has great potential for investigating fundamental questions in psychology and neuroscience, such as the role of proprioceptive facial feedback in emotion induction and emotion recognition, and may serve for clinical applications, such as alleviating symptoms of depression. However, despite illustrious origins in the 19th-century work of Duchenne de Boulogne, the practical application of fNMES remains largely unknown to today's researchers in psychology. In addition, published studies vary dramatically in the stimulation parameters used, such as stimulation frequency, amplitude, duration, and electrode size, and in the way they reported them. Because fNMES parameters impact the comfort and safety of volunteers, as well as its physiological (and psychological) effects, it is of paramount importance to establish recommendations of good practice and to ensure studies can be better compared and integrated. Here, we provide an introduction to fNMES, systematically review the existing literature focusing on the stimulation parameters used, and offer recommendations on how to safely and reliably deliver fNMES and on how to report the fNMES parameters to allow better cross-study comparison. In addition, we provide a free webpage, to easily visualise fNMES parameters and verify their safety based on current density. As an example of a potential application, we focus on the use of fNMES for the investigation of the facial feedback hypothesis.

Electrical stimulation for investigating and improving neuromuscular function in vivo: Historical perspective and major advances

This historical review summarizes the major advances - particularly from the last 50 years - in transcutaneous motor-level electrical stimulation, which can be used either as a tool to investigate neuromuscular function and its determinants (electrical stimulation for testing; EST) or as a therapeutic/training modality to improve neuromuscular and physical function (neuromuscular electrical stimulation; NMES). We focus on some of the most important applications of electrical stimulation in research and clinical settings, such as the investigation of acute changes, chronic adaptations and pathological alterations of neuromuscular function with EST, as well as the enhancement, preservation and restoration of muscle strength and mass with NMES treatment programs in various populations. For both EST and NMES, several major advances converge around understanding and optimizing motor unit recruitment during electrically-evoked contractions, also taking into account the influence of stimulation site (e.g., muscle belly vs nerve trunk) and type (e.g., pulse duration, frequency, and intensity). This information is equally important both in the context of mechanistic research of neuromuscular function as well as for clinicians who believe that improvements in neuromuscular function are required to provide health-related benefits to their patients.

Effects of Kilohertz Frequency, Burst Duty Cycle, and Burst Duration on Evoked Torque, Perceived Discomfort and Muscle Fatigue: A Systematic Review

ABSTRACT

Kilohertz-frequency alternating current is used to minimize muscle atrophy and muscle weakness and improve muscle performance. However, no systematic reviews have evaluated the best Kilohertz-frequency alternating current parameters for this purpose. We investigated the effects of the carrier frequency, burst duty cycles, and burst durations on evoked torque, perceived discomfort, and muscle fatigue. A search of eight data sources by two independent reviewers resulted in 13 peer-reviewed studies being selected, following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines, and rated using the PEDro scale to evaluate the methodological quality of the studies. Most studies showed that carrier frequencies up to 1 kHz evoked higher torque, while carrier frequencies between 2.5 and 5 kHz resulted in lower perceived discomfort. In addition, most studies showed that shorter burst duty cycles (10%-50%) induced higher evoked torque and lower perceived discomfort. Methodological quality scores ranged from 5 to 8 on the PEDro scale. We conclude that Kilohertz-frequency alternating current develops greater evoked torque for carrier frequencies between 1 and 2.5 kHz and burst duty cycles less than 50%. Lower perceived discomfort was generated using Kilohertz-frequency alternating currents between 2.5 and 5 kHz and burst duty cycles less than 50%.

Kilohertz Frequency Alternating Current Induces Less Evoked Torque and Less Neuromuscular Efficiency Than Pulsed Current in Healthy People: A Randomized Crossover Trial

CONTEXT

Pulsed current and kilohertz frequency alternating current are 2 types of neuromuscular electrical stimulation (NMES) currents often used by clinicians during rehabilitation. However, the low methodological quality and the different NMES parameters and protocols used in several studies might explain their inconclusive results in terms of their effects in the evoked torque and the discomfort level. In addition, the neuromuscular efficiency (ie, the NMES current type that evokes the highest torque with the lowest current intensity) has not been established yet. Therefore, our objective was to compare the evoked torque, current intensity, neuromuscular efficiency (evoked torque/current intensity ratio), and discomfort between pulsed current and kilohertz frequency alternating current in healthy people.

DESIGN

A double-blind, randomized crossover trial.

METHODS

Thirty healthy men (23.2 [4.5] y) participated in the study. Each participant was randomized to 4 current settings: 2 kilohertz frequency alternating currents with 2.5 kHz of carrier frequency and similar pulse duration (0.4 ms) and burst frequency (100 Hz) but with different burst duty cycles (20% and 50%) and burst durations (2 and 5 ms); and 2 pulsed currents with similar pulse frequency (100 Hz) and different pulse duration (2 and 0.4 ms). The evoked torque, current intensity at the maximal tolerated intensity, neuromuscular efficiency, and discomfort level were evaluated.

RESULTS

Both pulsed currents generated higher evoked torque than the kilohertz frequency alternating currents, despite the similar between-currents discomfort levels. The 2 ms pulsed current showed lower current intensity and higher neuromuscular efficiency compared with both alternated currents and with the 0.4 ms pulsed current.

CONCLUSIONS

The higher evoked torque, higher neuromuscular efficiency, and similar discomfort of the 2 ms pulsed current compared with 2.5-kHz frequency alternating current suggests this current as the best choice for clinicians to use in NMES-based protocols.

Effects of different electrical stimulation currents and phase durations on submaximal and maximum torque, efficiency, and discomfort: a randomized crossover trial

BACKGROUND

Neuromuscular electrical stimulation (NMES) is an important therapeutic tool for rehabilitation. However, best stimulation parameters remain to be determined.

OBJECTIVE

To determine the influence of different electrical stimulation currents and phase durations on torque, efficiency, and discomfort.

METHODS

Using a cross-over design, kHz frequency alternating currents (KFAC) and pulsed currents (PC) with narrow (200 µs) or wide (500 µs) phase durations were randomly applied on knee extensor muscles of healthy participants with a minimum of seven days between sessions. The NMES-evoked torque, NMES-efficiency, and discomfort (visual 0-10 cm analogue scale) were measured for each stimulation intensity increments (10 mA). Statistics were conducted using a three-way analysis of variances (phase duration x current x intensity), followed by Tukey post-hoc.

RESULTS

Twenty-four males (age 22.3 ± 3.5years) were included. No effect of NMES current was observed for torque, efficiency, and discomfort. For wide phase durations (500 µs), torque significantly increased for all stimulation intensities. For narrow phase durations (200 µs) evoked torque significantly increased only after 40% of maximal stimulation intensity. Phase durations of 500 µs produced greater torque than 200 µs. Discomfort was greater with 500 µs when compared to 200 µs. Submaximal relative torque, for example 40% of maximum voluntary contraction (MVC), was obtained with ∼ 60% and ∼ 80% of the maximal current intensity for 500 µs and 200 µs, respectively.

CONCLUSION

KFAC and PC current applied with the same phase duration induced similar relative submaximal and maximum evoked-torque, efficiency, and perceived discomfort. However, currents with 500 µs induced higher evoked-torque, current efficiency, and perceived discomfort.

Alternating Current Is More Fatigable Than Pulsed Current in People Who Are Healthy: A Double-Blind, Randomized Crossover Trial

OBJECTIVE

Tolerance level and rapid fatigue onset are limitations in the use of neuromuscular electrical stimulation (NMES) as an electrotherapeutic resource in rehabilitation and training protocols; however, it is unclear if pulsed current (PC) and alternating current (AC) produce different fatigue levels when applied at submaximal contraction level. The purpose of this study was to compare fatigue and discomfort levels between PC and AC during a submaximal contraction protocol in people who are healthy.

METHODS

In this double-blind, randomized crossover trial conducted in a laboratory setting, 30 male volunteers [23.23 years of age (SD = 4.59)] performed 2 submaximal fatigue protocols (with a 7-day interval) in a randomized order: PC (pulse duration = 2 milliseconds, pulse frequency = 100 Hz) and AC (2.5 kHz, pulse duration = 0.4 milliseconds, burst frequency = 100 Hz). NMES currents were applied to the knee extensor motor point of the dominant limb. The NMES protocol consisted of 80 evoked contractions (time on:off = 5:10 seconds) and lasted 20 minutes. The current was maintained at a constant intensity throughout the NMES protocol. The primary outcome measures were maximal voluntary isometric contraction, fatigue index (evoked torque decline), fatigability (number of contractions for a 50% drop in evoked-torque from the protocol start), total evoked torque-time integral (TTI), decline in TTI, and discomfort level.

RESULTS

AC at 2.5 kHz demonstrated higher maximal voluntary isometric contraction decline post-fatigue, higher fatigue index, higher fatigability (ie, fewer contractions to reach the 50% evoked torque decline from the protocol start), smaller total TTI, and higher TTI decline compared with PC. No between-currents difference was observed in discomfort level.

CONCLUSION

PC is less fatigable than AC at 2.5 kHz.

IMPACT

Based on this study, PC is the preferred current choice when the NMES goal is to generate higher muscle work, higher mechanical load, and smaller fatigability during training both for athletes who are healthy and for rehabilitation programs for people with disease or injury.

Comparison between Russian and Aussie currents in the grip strength and thickness muscles of the non-dominant hand: A double-blind, prospective, randomized-controlled study

Objectives

This study aims to compare the Russian and Aussie currents in the force gain and hypertrophy of the forearm muscles responsible for the grip.

Patients and methods

This double-blind, prospective, randomized-controlled study included a total of 30 healthy women (mean age: 20.2±1.7 years; range, 18 to 25 years) between May 2018 and July 2018. The participants were randomly divided into three groups: control group (CG, n=10), Aussie current group (ACG, n=10), and Russian current group (RCG, n=10). All three groups underwent a force test with a gripping dynamometer and the collection of images of the superficial and deep flexor muscles of the fingers with diagnostic ultrasound. The CG received a fictious current stimulus, while the other two groups received the designated stimuli from their currents. Further evaluations were performed after 24 h of the 12th application of the current.

Results

For grip, there were no significant differences in the moment of evaluation and interaction, while the effect size yielded certain points to advantages of force gain for the group using the RCG. The thickness of the superficial muscles showed a significant difference for the first evaluation between CG and RCG (p=0.014) and between RCG and ACG (p=0.010), indicating a larger effect size for RCG.

Conclusion

Our study results show that the Russian current is proven to be the mode which yields the most optimal results.

Russian and Low-Frequency Currents Induced Similar Neuromuscular Adaptations in Soccer Players: A Randomized Controlled Trial

CONTEXT

Neuromuscular electrical stimulation is widely used to induce muscular strength increase; however, no study has compared Russian current (RC) with pulsed current (PC) effects after a training program.

OBJECTIVES

We studied the effects of different neuromuscular electrical stimulation currents, RC, and PC on the neuromuscular system after a 6-week training period.

DESIGN

Blinded randomized controlled trial.

SETTING

Laboratory.

PATIENTS

A total of 27 male soccer players (age 22.2 [2.2] y, body mass 74.2 [10.0] kg, height 177 [0] cm, and body mass index 23.7 [2.9] kg/cm2 for the control group; 22.1 [3.1] y, 69.7 [5.7] kg, 174 [0] cm, and 23.0 [2.5] kg/cm for the PC group; and 23.0 [3.4] y, 72.1 [10.7] kg, 175 [0] cm, and 23.5 [3.4] kg/cm for the RC group) were randomized into 3 groups: (1) control group; (2) RC (2500 Hz, burst 100 Hz, and phase duration 200 μs); and (3) PC (100 Hz and 200 μs).

INTERVENTION

The experimental groups trained for 6 weeks, with 3 sessions per week with neuromuscular electrical stimulation.

MAIN OUTCOME MEASURES

Maximal voluntary isometric contraction and evoked torque, muscle architecture, sensory discomfort (visual analog scale), and electromyographic activity were evaluated before and after the 6-week period.

RESULTS

Evoked torque increased in the RC (169.5% [78.2%], P < .01) and PC (248.7% [81.1%], P < .01) groups. Muscle thickness and pennation angle increased in the RC (8.7% [3.8%] and 16.7% [9.0%], P < .01) and PC (16.1% [8.0%] and 27.4% [11.0%], P < .01) groups. The PC demonstrated lower values for visual analog scale (38.8% [17.1%], P < .01). There was no significant time difference for maximal voluntary isometric contraction and root mean square values (P > .05). For all these variables, there was no difference between the RC and PC (P > .05).

CONCLUSION

Despite the widespread use of RC in clinical practice, RC and PC training programs produced similar neuromuscular adaptations in soccer players. Nonetheless, as PC generated less perceived discomfort, it could be preferred after several training sessions.

Changes in microvascular oxygenation and total hemoglobin concentration of the vastus lateralis during neuromuscular electrical stimulation (NMES)

Background and Introduction: Neuromuscular electrical stimulation (NMES) is predicated on eliciting muscle contractions and increasing muscle demand to promote increase in strength. Previous studies have shown differences in the magnitude of elicited force among various NMES waveforms but less is known about metabolic demand of muscle during NMES.Objective/Purpose: The purpose of this study was to compare elicited force and muscle metabolic demand during electrically elicited contractions using different NMES waveforms.Methods: A single-session repeated measures design was used. Electrically elicited force (EEF), microvascular oxygenation (SmO2), total hemoglobin concentration ([THC]) of the vastus lateralis, and subject tolerance (VAS score) were measured using three NMES waveforms; burst modulated alternating current (Russian), biphasic pulsed current (VMS®), and burst modulated biphasic pulsed current (VMS-burst®).Results: A significant main effect for waveform was noted for EEF (F = 12.693, p < .001), SmO2 (F = 8.340, p = .001), and VAS (F = 4.213, p = .025), but not [THC]. Compared to Russian current, VMS-burst and VMS resulted in significantly greater EEF (p = .001; p = .009) and local metabolic demand (i.e. decreased SmO2) (p = .005; p = .003), but not [THC]. VAS was significantly greater (p = .023) for VMS (4.2) compared to Russian (3.07) but not different between VMS-burst and Russian and VMS-burst and VMS.Conclusion: Greater muscle force and local metabolic demand were observed with VMS-burst and VMS compared to Russian current. These data provide novel evidence to guide clinical decision making when selecting an NMES waveform.

Neuromuscular fatigue after low- and medium-frequency electrical stimulation in healthy adults

INTRODUCTION

In this study we investigated fatigue origins induced by low-frequency pulsed current (PC) and medium-frequency current (MF) neuromuscular electrical stimulation (NMES) after a clinical-like session.

METHODS

Eleven healthy men randomly underwent 2 NMES sessions, PC and MF, on quadriceps muscle (15-minute duration, 6 seconds on and 18 seconds off). Maximal voluntary contraction (MVC), central activation ratio (CAR), vastus lateralis electromyographic activity (EMG), and evoked contractile properties were determined before and after the sessions. Evoked torque and discomfort during the sessions were also measured.

RESULTS

Both currents produced decreases in MVC, EMG, and evoked contractile properties after the sessions. No difference was found between currents for all variables (P > 0.05). Evoked torque during sessions decreased (P < 0.05). No difference was observed in mean evoked torque and discomfort (P > 0.05).

DISCUSSION

Both currents induced similar neuromuscular fatigue. Clinicians can choose either PC or MF and expect similar treatment effects when the goal is to generate gains in muscle strength. Muscle Nerve 58: 293-299, 2018.