What does electrical stimulation do in physical therapy

Physiother Can. 2017; 69(5): 1–76.

Language: English | French

Ethne L. Nussbaum, MEd, PhD, PT,

Abstract

Purpose: In response to requests from physiotherapists for guidance on optimal stimulation of muscle using neuromuscular electrical stimulation (NMES), a review, synthesis, and extraction of key data from the literature was undertaken by six Canadian physical therapy (PT) educators, clinicians, and researchers in the field of electrophysical agents. The objective was to identify commonly treated conditions for which there was a substantial body of literature from which to draw conclusions regarding the effectiveness of NMES. Included studies had to apply NMES with visible and tetanic muscle contractions. Method: Four electronic databases (CINAHL, Embase, PUBMED, and SCOPUS) were searched for relevant literature published between database inceptions until May 2015. Additional articles were identified from bibliographies of the systematic reviews and from personal collections. Results: The extracted data were synthesized using a consensus process among the authors to provide recommendations for optimal stimulation parameters and application techniques to address muscle impairments associated with the following conditions: stroke (upper or lower extremity; both acute and chronic), anterior cruciate ligament reconstruction, patellofemoral pain syndrome, knee osteoarthritis, and total knee arthroplasty as well as critical illness and advanced disease states. Summaries of key details from each study incorporated into the review were also developed. The final sections of the article outline the recommended terminology for describing practice using electrical currents and provide tips for safe and effective clinical practice using NMES. Conclusion: This article provides physiotherapists with a resource to enable evidence-informed, effective use of NMES for PT practice.

Key Words: critical care, orthopaedics, physical therapy modalities, rehabilitation, stroke, therapeutic electrical stimulation

Résumé

Objectif : en réponse à des demandes de conseils de physiothérapeutes pour optimiser la stimulation musculaire à l'aide de la stimulation électrique neuromusculaire (SENM), une revue, une synthèse et une extraction de données de la littérature ont été entreprises par six formateurs, cliniciens et chercheurs en physiothérapie dans le domaine des agents électrophysiques. L'objectif était de cibler des affections couramment traitées ayant fait l'objet d'une quantité suffisante d'études pour tirer des conclusions concernant l'efficacité de la SENM. Les études devaient porter sur la SENM produisant des contractions musculaires visibles et toniques. Méthodes : quatre bases de données électroniques (CINAHL, Embase, PubMed et Scopus) ont été parcourues à la recherche d'études pertinentes publiées entre la création des bases de données et mai 2015. D'autres articles ont été tirés de bibliographies de revues systématiques et de collections personnelles. Résultats : les données extraites ont été synthétisées par consensus des auteurs en vue de dresser des recommandations sur l'optimisation des paramètres et des techniques d'application de la stimulation dans le traitement de déficits musculaires associés aux affections suivantes: accident vasculaire cérébral (extrémité inférieure ou supérieure; aigu ou chronique), reconstruction du ligament croisé antérieur, syndrome fémoro-rotulien douloureux, arthrose du genou et arthroplastie totale du genou, ainsi que des maladies graves et en stade avancé. Les auteurs fournissent également un résumé des éléments clés de chaque étude incluse dans la revue. Enfin, ils recommandent une nomenclature de l'électrothérapie et présentent des conseils pour l'utilisation sécuritaire et efficace de la SENM. Conclusion : ce document constitue pour les physiothérapeutes une ressource permettant d'appuyer leur utilisation de la SENM sur des données probantes.

Mots clés : accident vasculaire cérébral, modalités de physiothérapie, orthopédie, réadaptation, soins intensifs, stimulation électrique neuromusculaire

Introduction

This article was developed by six Canadian physical therapy (PT) educators, clinicians, and researchers dedicated to evidence-informed practice in the use of electrophysical agents (EPAs). Although a previous publication, “Electrophysical Agents—Contraindications and Precautions: An Evidence-Based Approach to Clinical Decision Making in Physical Therapy,”1 has become a widely used reference, nationally and internationally, for informing safe application of EPAs, there is still no resource to guide the effective application of EPAs.

Impetus

The project was initiated in response to requests from physical therapists across Canada for guidance on which EPA parameters to select to effectively facilitate and enhance their patients' recovery from injury, disease, or immobility. Specifically, they asked for a resource that would provide (1) summaries of the best evidence to support the use of EPAs and (2) recommendations for the effective parameters and application techniques required to achieve optimal results. Many of the therapists' questions concerned the plethora of parameter options associated with neuromuscular electrical stimulation (NMES) devices (low-frequency current, medium-frequency current, monopolar pulses, bipolar pulses, etc.); thus, we selected NMES as the first EPA to address.

Numerous systematic reviews, with or without meta-analysis, have been published regarding PT interventions that use EPAs. In some instances, the research has been synthesized into clinical practice guidelines. In a meta-analysis, results from several studies can be pooled, and if the overall effect favours the treatment, it is considered the highest level of evidence to support the use of that treatment in clinical practice. However, most systematic reviews give little appraisal of the appropriateness of protocols or parameters used in individual studies, and they provide very little direction regarding the optimal parameters and application techniques for specific treatment interventions. Furthermore, systematic reviews commonly incorporate a wide range of approaches that use the treatment of interest and then pool results, so the benefits of a particular treatment protocol or specific approach can be missed. In this article, we have used a critical synthesis of the evidence to recommend specific parameters and techniques that are most likely to optimize effectiveness.

Scope

In this article, the abbreviation NMES refers to forms of therapy that apply electrical currents over muscles and nerves in a manner that produces smooth tetanic muscle contractions that simulate an exercise therapy session. However, NMES is distinct from exercise in that although the muscle is contracting, it is not voluntarily contracting; NMES is also not a passive modality because the muscle is active. The possible mechanisms by which NMES strengthens muscle and retrains limb movements, as well as the differences between voluntarily recruited and NMES-activated muscle contractions, are much debated; views are often contradictory. This article does not focus on the physiological basis of NMES effects, for example metabolic changes, neural adaptations, and fatigue resistance; for discussion of these issues, the reader is referred to alternative sources.2–5 Some physical therapists hold the view that NMES is useful only when combined with simultaneous voluntary contraction of the target muscle; however, the extensive literature that we reviewed for this project does not support this viewpoint. Rather, electrically stimulated muscle contractions may be appropriate therapy with or without patient participation and whether or not limb movement is produced. This article also includes a brief description of what some refer to as functional electrical stimulation (FES). However, we should note that there are differences between NMES and FES; these are explained in Section 5, “Terms and Definitions in NMES.”

To provide in-depth analyses, we limited this review to the following clinical conditions: stroke rehabilitation, orthopaedic conditions (hip and knee arthroplasty, anterior cruciate ligament [ACL] repair, patellofemoral pain syndrome [PFPS], and osteoarthritis [OA]), advanced disease states (mainly chronic obstructive pulmonary disease [COPD] and congestive heart failure [CHF]), and critical illness weakness associated with a stay in an intensive care unit (ICU). We included these conditions because they span the main areas of PT practice (neurology, orthopaedics, cardiopulmonary), represent patient groups seen in a variety of health care settings, and were those specifically requested by clinicians. Specialized use of NMES in conditions such as spinal cord injury, incontinence, and pediatric neurology were excluded. In all the conditions included in this review, NMES has been used to activate, strengthen, or retrain muscles to improve outcomes or hasten the achievement of treatment goals.

Throughout this article, we provide details and analysis of clinical studies in which NMES was used in a manner that is relevant to PT practice. We focused on the details of the treatment interventions—in particular, the stimulus parameters, application techniques, and treatment schedules evaluated in each included study. We hope that by taking this approach, and by recommending treatment protocols that are most likely to produce improvements in their patients, this document will be useful to clinicians. The final sections of the article include a guide to safe practice and definitions of the terms that we recommend clinicians use when working in this field. Ultimately, we aim to promote effective and safe EPA practice among physical therapists that is based on best evidence.

Purpose

The purpose of this document is to provide physical therapists with an evidence-based resource that can guide clinical decision making, thereby enabling clinicians to make effective use of electrical stimulation to improve muscle function in patients with musculoskeletal, neuromuscular, and critical care illnesses.

The specific objectives of this review are to

• 1.

  Increase awareness of the range of applications for NMES;

• 2.

  Demonstrate how NMES protocols are specifically designed to meet different treatment goals (e.g., strengthening vs. endurance training) and are customized to match the unique circumstances of each clinical situation (e.g., stage of recovery, level of fatigue);

• 3.

  Appraise the research related to NMES in the included conditions;

• 4.

  Provide general recommendations that will promote best practices for applying NMES in a safe and effective manner; and

• 5.

  Suggest terminology that should be used to describe NMES parameters and device features to facilitate communication among physical therapists, equipment suppliers, and other members of the clinical community.

Methods

Literature searches

We used a deliberate, collaborative, and consensual selection process that included all authors. Four electronic databases were used (CINAHL, Embase, PUBMED, and SCOPUS) to search for relevant literature that had been published between database inceptions through May 4, 2015. We worked in pairs to identify relevant citations, for which the full articles were then retrieved. Additional articles were identified by hand searching the bibliographies in the systematic reviews and searching our personal libraries. We reviewed the full text of selected articles to confirm that all included studies met predetermined criteria and fit the objectives of the review.

Selection of studies

Types of NMES interventions

Articles included in this review involved the application of NMES in such a way that visible and tetanic contractions of muscles were reported or could be expected to occur, even if not seen (e.g., some patients in ICU). We did not include studies in which only sensory-level electrical stimulation was applied (often called transcutaneous electrical nerve stimulation, or TENS) or in which electrical current was applied to muscle in experimental laboratory settings to elucidate underlying physiological effects. To ensure that clinicians could reasonably replicate the NMES protocols, we also eliminated studies that did not include at least three of the following parameters: frequency (measured in Hertz), ON:OFF duration (or use of a foot-pressure switch), amplitude, duration of application per session, and total number of NMES sessions or weeks of application.

Included in this review are studies that used NMES protocols that could be delivered within a typical PT treatment session and included patients who were at an acute or chronic stage of recovery. Treatment could be delivered in a variety of health care settings in which PT services would normally be provided, including outpatient clinics, community and home care, rehabilitation centres, acute care hospitals, and long-term care facilities. However, none of the included studies involved the use of NMES for denervated muscles (for an explanation of denervated muscles, see Section 4, “Equipment and Application”). We also included studies that evaluated NMES protocols that were quite complex, such as those in which NMES was applied using multiple channels or in which current was activated by an external trigger (electromyography [EMG] or foot pressure). We did not include those using equipment that is either not available commercially or not feasible for use in PT practice; examples are computerized (robotic) devices that are preprogrammed to sequentially activate several different muscle groups, proprietary devices that have undisclosed NMES parameters, and equipment that requires very complicated set-up, usually found only in experimental laboratories. These types of studies are usually labelled as FES.

We excluded studies requiring procedures out of the scope of PT practice, such as placement of indwelling or implanted electrodes or requiring medication to be injected immediately before stimulation—for example, botulinum toxin (Botox) and lidocaine. Studies were included even if they were of dubious quality to provide a fair representation of the literature. We have highlighted some of the flaws pertaining to individual studies in the Comments column of the even-numbered tables (Tables 2–16). Readers who desire a complete quality appraisal can consult the systematic reviews, when available, which are also listed in these tables.

Table 2

Details of Individual Studies on Use of NMES in Hemiplegic Shoulder Subluxation

Table 16

Details of Individual Studies for Use of NMES in Critical Illness and Advanced Disease States

In most instances, NMES was not the sole therapy but was applied in combination with other interventions considered to be conventional care for that condition or setting. Conventional care included supervised strengthening programmes; home exercise programmes; slings; braces; gait aids and other supports used to prevent tissue damage; other commonly used, hands-on therapies—for example, neuro-developmental treatment (Bobath) and manual therapy; and therapies provided by other health care professions that are considered to be usual care.

Types of study design

We selected studies that included subjects with the clinical conditions of interest and that had been designed to determine the effect of NMES on muscle strength, limb function, or both (see Sections 1–3 on clinical conditions). The controlled studies, whether randomized or not, compared the effect of NMES administered either alone or in combination with conventional care (which could include PT) to an appropriate control group, who received the same conventional care. Seldom was NMES compared with sham or placebo NMES because the visible muscle contractions produced by NMES make the blinding of subjects and therapists impractical.

The included studies had to systematically evaluate the effect of NMES and control treatments on outcomes such as strength, range of motion (ROM), and spasticity as well as on other standardized outcome measures of limb or body function and global performance measures, such as activities of daily living and quality of life (QOL). Because the primary objective of this article was to evaluate the effects of NMES on muscle function, we excluded studies that evaluated only outcomes such as QOL.

Within the three main clinical areas of interest, the literature search yielded studies clustered around certain common clinical conditions, giving us a body of literature to analyze in terms of parameters and effectiveness.

• 1.

  Stroke rehabilitation: NMES to promote muscle strengthening and recovery of limb function in adults (aged > 18 y) with hemiplegia who had recently been affected by a stroke (acute) and in those several months and even years after sustaining a stroke (chronic). Section 1 focuses on the three most common physical impairments affecting patient mobility and function: muscle weakness, abnormal muscle tone, and impaired motor control. The conditions reviewed are

  • a.

    shoulder subluxation (sublux);

  • b.

    loss of hand and upper extremity (UE) function; and

  • c.

    gait impairments resulting from foot drop and impaired control of leg muscles.

• 2.

  Musculoskeletal conditions: NMES treatment of orthopaedic conditions affecting muscles of the lower extremity (LE), including both acute injuries and chronic conditions. The conditions addressed in Section 2 are

  • a.

    post-operative management of ACL reconstruction (could include meniscal injuries);

  • b.

    pre- and post-surgical care after joint (hip and knee) replacement; and

  • c.

    treatment of chronic diseases and conditions affecting the knee, including

    • i.

      OA and

    • ii.

      PFPS.

• 3.

  Critical illness and advanced disease states: NMES used to prevent muscle deconditioning, which occurs in severe illness or gradually over time in those with chronic progressive diseases. The conditions reviewed in Section 3 are

  • a.

    those affecting patients admitted to an ICU and

  • b.

    chronic progressive diseases that cause muscle weakness and reduced endurance, such as

    • i.

      moderate to severe COPD and

    • ii.

      CHF.

Consensus process and presentation of findings

Individual study data were summarized by two assigned reviewers and entered into tables (even-numbered Tables 2–16) and checked by at least one other reviewer. Working pairs conducted a critical review of each study and, using the data, developed protocol recommendations for each main clinical area (odd-numbered Tables 1–15). Key articles were shared among all authors, and multiple iterations of the table entries were discussed until 100% agreement was reached. More important, the rationale for selecting specific NMES stimulus parameters and treatment schedules has been provided to enable clinicians to customize the specific parameters to meet the needs of a particular patient or stage of recovery.

Table 1

Summary of the Literature and Recommendations for Use of NMES in Hemiplegic Shoulder Subluxation

Table 15

Summary of the Literature and Recommendations for Use of NMES in Critical Illness and Advanced Disease States

Organization

This review is divided into five sections. Some readers may benefit from first reading Section 5, “Terms and Definitions in NMES,” because it lays out the language and terms we used when writing this article. Unfortunately, the language used in the literature to describe EPAs generally, and NMES protocols in particular, can be confusing; this is evidenced in the NMES parameters provided in the summary tables that follow, which in each case have been taken directly from the source. We believe that an important first step in promoting good practice in this field is to gain a good understanding of what terms mean and ensure that terms are used consistently in and across professions.

The layout of Sections 1–3 is similar: Indications and the rationale for using NMES for the specific condition are discussed, followed by a table (odd-numbered Tables 1–15) summarizing NMES treatment recommendations, the rationale for the recommendations, and a critical review of related research. The outcome measures listed in these tables are those used by investigators that showed significant improvements compared with control conditions. Even-numbered tables (Tables 2–16) report on the NMES protocols, outcome measures, and results of each study. Where detail is missing, the omission was by the original authors—that is, it was not reported. These tables are provided for the benefit of readers who are interested in the specifics from which the recommendations were derived. In addition, the tables highlight some of the strengths and weaknesses of each study and provide clinicians with an opportunity to compare and contrast NMES protocols used in positive and negative studies and to interpret the research and establish its relevance to their patient populations.

Section 4 of this article, “Equipment and Application,” is intended to support clinical decision making and describes a generalized approach to the use of NMES for patients with muscle impairments or motor control deficits. The section describes device features and treatment parameters that a clinician must set when designing an NMES protocol and provides a background rationale to assist clinicians in making these important choices. Furthermore, this section includes a general approach for the safe and effective use of NMES, recommending or discouraging common practices in PT on the basis of the potential benefit or risk.

Section 5 of the article describes terms and definitions related to the application of electrical currents.

1. Stroke Rehabilitation

1a. HEMIPLEGIC SHOULDER SUBLUXATION

Indications and rationale for using NMES

Shoulder subluxation resulting from weak muscles of the shoulder girdle is one of the underlying causes of shoulder pain and arm dysfunction post-stroke.6 Weakness can cause the joint and tendons to become stretched or torn and the joint surfaces to become abraded and inflamed; in addition, traction on nerves can alter sensory perception and interfere with muscle innervation. Susceptibility to shoulder problems is greatest immediately after stroke, when shoulder muscles are flaccid and unable to hold the humeral head in proper alignment. However, shoulder injury can also occur in later stages of recovery, when some shoulder muscles become spastic and produce muscle imbalance. NMES is applied to prevent disuse atrophy and increase muscle strength, thereby preventing or reducing subluxation and in turn improving active, pain-free ROM and promoting the recovery of UE function.

1b. Upper extremity stroke: wrist and finger extension

Indications and rationale for using NMES

Hemiplegia after stroke often results in flexor synergy of the wrist, hand, and fingers, which limits functional use of the hand and arm. Activation of the wrist extensors with NMES alone or EMG-triggered NMES (EMG-NMES) during purposeful hand movements can improve strength and active ROM of the wrist extensors. Repetitive, task-specific movements of the wrist and hand using NMES stimulation can prevent disuse atrophy and contractures and encourage functional use of the paretic hand.

Table 3

Summary of the Literature and Recommendations for Use of NMES or EMG-NMES for Wrist and Finger Extension

Table 4

Details of Individual Studies on Use of NMES or EMG-NMES Treatment of Wrist and Hand Post-Stroke

1c. LOWER EXTREMITY STROKE: FOOT DROP, PLANTAR SPASTICITY, AND GAIT IMPROVEMENT

Indications and rationale for using NMES

After a stroke, many individuals have foot drop, characterized by an inability to dorsiflex the ankle and requiring hip hiking to obtain sufficient toe clearance during walking. The abnormal gait causes walking speed to be slow, the physiological cost to be high, and the risk of stumbling and falling to increase. NMES is applied to improve muscle strength of weak foot dorsiflexor muscles, reduce foot drop, and decrease plantar muscle spasticity. By addressing these impairments, gait symmetry, speed, and walking distance can improve.

Table 5

Summary of the Literature and Recommendations for Use of NMES for Foot Drop, Plantar Spasticity, and Gait Improvement

Table 6

Details of Individual Studies on Use of NMES in Lower Extremity Stroke for Foot Drop, Plantar Spasticity, and Gait Improvement

2. Musculoskeletal Conditions

2a. ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

Indications and rationale for using NMES

Pain and weakness secondary to both ACL injury and post-surgical trauma is a common issue for patients after ACL reconstruction.90 Presynaptic reflex inhibition (alteration of neural signalling) of quadriceps (quads) inhibits appropriate recruitment of motor neurons.91 Muscle atrophy, particularly in type 1 muscle fibres post-injury and post-surgery, results in reduced muscle strength (60%–80% decrease in isometric quads strength), which jeopardizes joint function and has been shown to be linked to gait abnormalities (velocity, stride length, and pace).92 Weakness of the quads post–ACL injury has been reported to be related to reduced functional performance,93 a greater potential for re-injury,93 and a higher risk of developing OA.94 NMES is indicated post–ACL reconstruction to elicit an electrically induced muscle action to augment volitional recruitment and strengthen the quads; secondary to improved strength and biomechanics, NMES might reduce pain.

Table 7

Summary of the Literature and Recommendations for Use of NMES in Anterior Cruciate Ligament Reconstruction

Table 8

Details of Individual Studies on Use of NMES in ACL Reconstruction

2b. PATELLOFEMORAL PAIN SYNDROME

Indications and rationale for using NMES

Quads muscle weakness, indicated by reduced peak torque, is believed to play a key role in PFPS.118 Weakness of the vastus medialis (VM) is thought to be particularly important119 because the VM normally counterbalances the vastus lateralis muscle; VM weakness may be a cause of patellar mal-alignment, with the resultant abnormal tracking of the patella in the trochlear groove.120 It is uncertain whether quads weakness is the cause or a consequence of pain in PFPS.121 NMES activation of the quads, particularly of the relatively weaker VM, may facilitate normal tracking of the patella in the trochlear groove.

Table 9

Summary of the Literature and Recommendations for Use of NMES in PFPS

Table 10

Details of Individual Studies on Use of NMES in PFPS

2c. DEGENERATIVE ARTHRITIS AND OSTEOARTHRITIS

Indications and rationale for using NMES

Weak quads, loss of functional capacity and endurance (e.g., stair climbing, distance walking, timed up-and-go), pain, and stiffness are common reports of people with symptomatic knee OA.132 NMES is indicated to strengthen weak quads muscles, train endurance, minimize atrophy, and increase ROM at the joint.4,133,134

Table 11

Summary of the Literature and Recommendations for Use of NMES in Knee OA

Table 12

Details of Individual Studies on Use of NMES in Knee OA

2d. TOTAL JOINT REPLACEMENT

Indications and rationale for using NMES

Quads weakness secondary to end-stage knee OA146,147 and post-surgical trauma is very common in patients after total knee arthroplasty (TKA).146–148 NMES is commonly used after TKA to strengthen the quads and to provide an adequate training dose for those lacking sufficient volitional quads activation; it engages neurophysiological mechanisms thought to facilitate strength gains and provides a general physical stress to the quads' neuromuscular system. The goal is to attenuate the dramatic strength loss immediately post-operation, which typically persists for 1 year. NMES is also used to address quads weakness after total hip arthroplasty.

Table 13

Summary of the Literature and Recommendations for Use of NMES in TKA and THA

Table 14

Details of Individual Studies on Use of NMES in Total Joint Replacement

3. Critical Illness and Advanced Disease States

Indications and rationale for using NMES

Skeletal muscle proteins break down in advanced disease states, and during prolonged periods of immobilization, to provide energy for vital metabolic functions—for example, gluconeogenesis in the liver. This leads to varying degrees of loss of skeletal muscle mass and, in some patients, polyneuropathy. Muscle weakness and fatigue impede patients' capacity to exercise, are known to delay extubation, extend length of stay in ICU, and delay patients achieving independent mobility and returning to their former independence.164,165 The goal of NMES in advanced disease states is to prevent or reverse skeletal muscle wasting for persons who are not able to exercise. Conditions include advanced COPD, CHF, sepsis, and reduced consciousness during critical illness, malignancy, and periods of mechanical ventilation.

4. Equipment and Application

Stimulator

A wide variety of devices deliver NMES, including battery-operated, portable devices that deliver only NMES and combination units that deliver NMES as well as other electrical currents such as TENS, interferential current therapy, and high-voltage pulsed current (HVPC). NMES can also be applied using alternating current (AC)–powered (plug-in) devices that, in addition to offering multiple waveforms, offer other types of modalities such as ultrasound. Typically, devices with high power output (>80 mA) are required when using large electrodes (e.g., 10×13 cm), activating multiple muscles, or stimulating large muscle groups. Smooth tetanic muscle contractions are difficult to achieve when using devices with insufficient power output. The technical specifications should be listed in device manuals.

Stimulator features

Preprogrammed NMES protocols

Most NMES stimulators display parameters in digital rather than analog form (dials). One of the features associated with digital devices is the availability of preprogrammed protocols, in which the stimulus parameters are set by the manufacturer. These protocols would not be updated after purchase and may not even initially reflect the latest research, as shown in the tables in this document. Such protocols may be helpful for people who are not knowledgeable about NMES, but physical therapists must understand and be able to rationalize their choice of NMES parameters so that they can customize and modify treatment over time on the basis of a patient's characteristics and responses and the desired clinical outcomes.

Saved protocols

Devices commonly allow therapists to customize and save a few protocols. This feature saves time setting up the device for a repeat treatment of a particular patient. However, some parameters (e.g., pulse amplitude) cannot be saved and need to be set at each treatment.

Locking

Once settings have been selected for a particular patient, they can be “locked in” so that the patient or uninformed provider cannot adjust them inadvertently. This feature is particularly helpful when patients take equipment home or use equipment in unsupervised settings.

Compliance meters

Many devices permit tracking of how patients use them at home. Some devices track the total time the stimulator has been activated, and others track the duration and number of treatment sessions over a particular time period. This feature can be invaluable in understanding why NMES treatments appear to be ineffective for some patients.

Constant stimulation mode (continuously ON)

It is essential that therapists be aware of ON and OFF current cycles. Amplitude should be adjusted only when delivering current to the patient. Most devices have a safety feature that ensures that amplitude can be adjusted only during an ON cycle. For some portable devices, activating a “constant stimulation” button prevents the current from cycling OFF at the preprogrammed time, allowing more time to adjust the current amplitude to the desired level.

Reciprocal–synchronous (also called alternating–simultaneous)

Most NMES devices provide two channels, which can be used to deliver NMES to different muscles or to different locations on the same muscle. Devices with two channels usually have a switch that dictates whether the current flows simultaneously through both channels (synchronous) or automatically alternates between the channels (reciprocal) so that one muscle, or muscle group, is activated while the other channel is in the rest phase of the cycle. Reciprocal stimulation is helpful when the objective is to move joints through more than one direction of range—for example, wrist flexion and extension—in which case, it is important that the muscles contract reciprocally rather than simultaneously.

Automatic shut-off

It may be possible to program when a device will shut off completely, typically measured in minutes (15, 30, or 60 min) or following a preset number of work–rest cycles (ON:OFF times). Using this feature, patients will always receive the prescribed treatment program without the patient or clinician having to track the number of repetitions.

Electrodes

Self-adhesive electrodes

Self-adhesive, pre-gelled electrodes come in a variety of sizes and shapes and are relatively convenient to use because they do not require a clinician to use tape or straps to secure them in place. However, repeated use of pre-gelled electrodes leads to rapid loss of conductivity and deteriorating adhesiveness because of the buildup of skin cells and oils on the adhesive surface. In addition, loss of adhesion and drying of the gel may cause the edges to begin lifting, which can dramatically increase current density, cause uneven distribution of current, increase the risk of burn, and potentially result in electrode movement. At the very least, it can cause the patient discomfort. Patients occasionally develop sensitivity to the gum in self-adhesive electrodes, which may result in skin irritation (see “Safety Concerns” section).204 Clinicians should monitor the skin under self-adhesive electrodes: If an itchy rash develops, discontinue using them. The same self-adhesive electrodes should never be used for more than one patient.

Carbon rubber electrodes

Carbon-impregnated, silicone rubber electrodes used with electrode-specific gel and held in place using tape or straps produce the best electrical conduction and most even distribution of current across the electrode surface. The position of these electrodes can easily be adjusted, facilitating an optimal set-up for patients. The electrodes can also be used many times before they need replacing. However, patients can develop sensitivity to carbon rubber electrodes. Some carbon rubber electrodes have a pre-gelled adhesive layer; in this case, follow the precautions and procedures that apply to self-adhesive electrodes (see preceding section).

Electrode gel

Electrode gel that is specifically designed to optimize conduction of electrical current is recommended. Electrode-specific gel will promote optimal and even conduction of current and could be more comfortable for the patient because better electrode conduction means that the desired muscle contraction can be produced at lower current amplitude.

Electrode sponges

Sponges moistened with tap water may be used to couple carbon rubber electrodes and the skin; this is a good option when using larger electrodes. Sponges should be appropriately moistened (not too wet or too dry) and should be replaced when they become dirty to maintain their conductivity. It is recommended that a sufficient number of sponges be available to enable complete drying before reuse; this will limit the growth of water-borne bacteria such as Pseudomonas aeruginosa.205

Securing electrodes

When the optimal electrode placements have been determined, electrodes should be secured firmly with tape or straps to keep the entire electrode area, including the edges, in contact with the skin. Skin moves when the muscle contracts; thus, unsecured electrodes can lead to uneven current distribution and hot spots on the skin, which could cause an electrical burn or, at the very least, discomfort.

Patient set-up

Electrodes

The number, size, polarity, and location of electrodes need to be selected on the basis of patients' goals and target muscle characteristics.

Electrode polarity

Cathode: The negatively charged electrode. The lead wire is typically coloured black at one end.

Anode: The positively charged electrode. The lead wire is typically coloured red at one end.

Electrode positioning

Monopolar electrode placement: Place the cathode on the motor point (MP) of the target muscle and the anode proximally on the target muscle, on a nearby muscle supplied by the same nerve, or over the supplying nerve. This placement should be considered when the waveform produces more current flow in either the positive or the negative direction, thereby creating a circuit with clearly defined cathode and anode—for example, biphasic asymmetrical unbalanced pulsed current (PC). Monopolar set-up is often indicated when targeting small muscles.

Bipolar electrode placement: Place both electrodes on the muscle belly or at the proximal and distal ends of the muscle or muscle group. This placement should be considered when the waveform produces equal current flow in positive and negative directions. Both electrodes are considered active, and each electrode has a positive and negative phase (cathodal and anodal) during each pulse.

When possible, orient the electrodes parallel to the longitudinal direction of the muscle fibres to reduce resistance to current flow.124,125 Ask the patient where the stimulus is felt, and observe the resulting muscle action. Be prepared to move the electrodes if the desired muscle action is not elicited.

Locating the motor point

The MP is the point on the skin over a muscle where a contraction can be electrically induced with the lowest current amplitude. Because skin and tissue resistance to current is lowest at that point, patient discomfort is minimized, and tolerance is maximized. Placing electrodes over MPs is said to be crucially important in improving the effectiveness of NMES:206 Higher training intensity is associated with greater gains in muscle strength, so it is important to use all possible techniques to maximize motor unit recruitment.206 There are charts depicting MPs; however, these are approximate because MPs vary significantly among individuals, and a more precise location should be confirmed by “scanning.”206 To scan, or “surf,” for an MP, fix the anode over the nerve trunk or muscle belly of the patient's target muscle. Then fix the gelled cathode in the palm of your own hand and apply gel to the fingertip of that hand. Move your gelled fingertip over the approximate area of the MP; the spot that produces the strongest tingling sensation at your fingertip defines the MP.

A pen electrode can also be used to surf for the MP. Fix the anode over the nerve trunk or muscle belly of the patient's target muscle. Move the pen electrode (cathode) over the approximate area of the MP (holding it for 3–5 s in each spot using low amplitude) until you observe a visible muscle contraction.206 If a pen electrode is not available, you may use a small, gelled electrode, but be aware of unwittingly creating a large field of effect by spreading the gel over a large area.

Electrode size

The size of the electrode should be selected on the basis of the size of the target muscle and the required depth and spread of current. Larger electrodes promote deeper current penetration. In addition, using larger electrodes tends to be more comfortable for the patient because of reduced current density. Smaller electrodes are useful for isolating specific muscles and for stimulating smaller muscles. Current density is greater using smaller electrodes, and stimulation therefore tends to be less comfortable and poses greater burn risk.

Standard electrode sizes (e.g., 5×5 cm square or 5 cm in diameter) are used for medium-sized muscles (e.g., forearm, calf, shoulder). For larger muscles (e.g., quadriceps, hamstrings, lumbar spine), larger electrodes should be used (e.g., 5×10 cm, 10×10 cm, or larger) to allow for better dispersion of the current. Using small electrodes on a large muscle produces inadequate motor unit recruitment, whereas using electrodes that are too large can cause the current to activate unwanted adjacent muscles (e.g., upper trapezius fibres when treating shoulder subluxation).

Electrode spacing

When electrodes are placed close together, the current will travel more superficially; wider spacing will promote deeper penetration and greater spread of the current. Electrodes are generally placed further apart when using a monopolar electrode placement because the anode need not be placed on the muscle.

Limb position

Limbs should be positioned in the mid-range of muscle length to produce the strongest muscle contraction. For example, when stimulating quads for musculoskeletal conditions, the knee should be positioned in approximately 65° flexion.108 Avoiding lengthened or shortened positions of muscles should be incorporated into all NMES strengthening programmes. Muscle groups also need to be considered: For example, to enable the external rotators of the shoulder to be stimulated in their mid-length position, the patient's upper arm should be positioned in the coronal plane.

When muscles are very weak, consider placing the limb relative to gravity to enable an appropriate challenge to the existing muscle strength. For example, muscles with grade 1 or 2 strength should preferentially be stimulated with the limb in a gravity-assisted or gravity-neutral position; grade 3 muscles should be in a gravity-resisted position.

When motor relearning is a goal, the patient should preferentially use a functional position. For example, when retraining lower extremity muscles, patients may benefit from using NMES while standing or walking, rather than sitting with their lower leg dangling over the edge of the plinth or bed.

Voluntary contraction

Whether patients should voluntarily contract their muscles during NMES treatment depends on whether the goals of treatment include motor relearning, functional recovery (e.g., in neuro-rehabilitation programmes), or both or isolated muscle strengthening (e.g., many orthopaedic conditions).

The combination of voluntary effort, motor imagery (thinking or imagining the muscle action), and NMES appears to have greater potential to induce plasticity of the motor cortex post-stroke than either electrical stimulation or exercise training alone.207 Furthermore, carry-over of benefit after the end of NMES in a stroke treatment program is more likely when the muscle stimulation is superimposed on a functional and meaningful muscle action.32,41,150 Concurrent activation with both electrical stimulation and voluntary muscle contraction may recruit different types of muscle fibres and result in a more complete muscle contraction.

When the main goal of NMES is muscle strengthening, concurrent voluntary contractions are not required: The benefit of NMES without voluntary assistance has been shown in many studies. However, NMES is not intended as a stand-alone treatment. Patients receiving NMES should, in addition, undertake a comprehensive therapeutic exercise programme (supervised or at home). When patients are unable to perform voluntary contractions—for example, sedated patients in the ICU—NMES is applied alone.

Denervated muscles

This article focuses on applying NMES to select innervated muscles; this occurs through depolarization of the motor nerves rather than the muscle fibres directly. If there is damage to the lower motor neurons or neuromuscular junctions (i.e., partial or complete denervation), electrically induced muscle contraction occurs through direct depolarization of the sarcolemma. This requires a much longer phase duration for the NMES pulse (100–300 ms), thus more electrical charge, to produce a contraction. In fact, most portable NMES stimulators will not provide the parameters required to elicit a contraction of a denervated muscle. As a result, the possible benefit of applying NMES to denervated muscles has not been clearly established.208

Safety concerns

Lack of sensation

Several conditions for which NMES is indicated result in impaired sensation as a result of nerve damage. Although intact sensation is not considered to be an absolute contraindication, a lack of patient feedback significantly increases the risk of adverse reactions.1 When sensation is altered either by neurological condition (e.g., post-stroke, spinal cord injury) or by damage to superficial sensory nerves (e.g., as a result of surgical incision), it is important to determine whether the altered nerve supply has affected the ability to discriminate between different sensations (pins and needles vs. intense buzz) or the ability to detect a painful and potentially tissue-damaging stimulus. The physical therapist must monitor the situation very carefully, such as by performing frequent skin checks and assessing patient discomfort or potential damage.

Concurrent use of NMES and cold packs

Concurrent application of a cold pack over the electrodes during electrical stimulation will numb the area and block nerve transmission along the sensory fibres. Reducing a patient's awareness of pain or developing tissue damage resulting from the electrical stimulation creates an unsafe practice situation. In addition, the thin film of surface water that forms on the skin with the application of cold will allow a superficial passage of electrical current across the skin, rather than enabling the current to travel through the underlying tissues.

Skin irritation and skin burn

It is common to observe a slight reddening of the skin under the electrodes after applying NMES because of the increased blood supply to the area; however, it resolves spontaneously once the stimulation is switched off. Mild skin irritation is sometimes seen due to allergic factors (electrode compounds, electrode gel, self-adhesive gum, tape) or mechanical factors (skin abrasion from tape removal). Chemical and electrical factors can also be the cause of burns. A chemical burn may be caused when using direct current or monophasic PC (not typical for NMES) by the buildup of new acids and bases formed by electrolysis where the electrode sits on a patient's skin. An electrical burn may be caused by current density being too high; this is a particular risk when delivering high-current amplitudes through relatively small electrodes.

Common approach to applying NMES

A general approach to promoting safe and effective use of therapeutic modalities has previously been presented.1 Briefly, this approach involves taking the following steps:

• Consult a resource that provides a comprehensive list of relevant contraindications and precautions for NMES treatments as well as references and a rationale for conditions that increase the likelihood or severity of an adverse reaction or reduce intended benefits.1

• Develop a strategy to mitigate risks before, during, and after treatment. The most common risks associated with NMES treatment are (1) electrical surge or shock should the equipment malfunction; (2) skin irritation or allergy at the electrode sites; (3) pain during treatment if the current amplitude is not adjusted slowly and on the basis of patient feedback; and (4) post-treatment muscle soreness.

Most risks can be mitigated by establishing clear lines of communication between the therapist and the patient and creating a therapeutic relationship that encourages frequent and honest patient feedback.

• Explain the risks and benefits before obtaining consent from the patient or substitute decision maker. Explain clearly what the patient is likely to feel and what common adverse signs should be watched for during treatment. Many physical therapists use consent forms that patients must sign; however, these documents should be used in conjunction with a dialogue with the patient or substitute decision maker that confirms understanding and provides an opportunity to ask questions.

• Conduct a sensory test using sharp–dull discrimination over the area where NMES is to be applied.

• Swab the relevant skin sites using an alcohol wipe or wet cloth to remove any topical products that could increase skin resistance to current flow.

• Apply the treatment, and encourage the patient to participate in the treatment in the manner determined (see “Voluntary Contraction” section). To protect the joint against potential injury, caution is required in eliciting strong muscle contractions when volitional muscle control is lacking.

• Check the skin under the electrodes after the stimulation is complete and more frequently during treatments, if indicated (see “Skin Irritation and Skin Burn” section).

• Remove all gel and tape residue from the skin using an alcohol swab or wet cloth.

• Remind and instruct patients and caregivers to monitor patients' reactions after NMES treatment. Provide clear instructions about what signs and symptoms to monitor, including both desirable and undesirable reactions, and advise when action should be taken.

Document the treatment parameters, electrode set-up, and patient positioning in enough detail that the treatment can be easily reproduced by another qualified clinician. Use valid outcome measures, and evaluate the measured outcomes (using minimum detectable change or minimal clinically important difference) to confirm treatment effectiveness.

Equipment care and maintenance

Electrode care

Carbon rubber electrodes should be rinsed with warm, soapy water after use and left to air dry, face up, or gently patted dry. They should not be aggressively rubbed because that can damage or remove the embedded carbon, thereby decreasing electrode conductivity.

It is essential to wash the electrodes and follow decontamination protocols that are consistent with health and safety requirements; in addition, be sure to use products that do not compromise the conductivity of carbon rubber electrodes.

Equipment cleaning

Equipment, leads, and electrodes should always be cleaned between patients. Consider using antiseptic solutions that are known to kill a broad spectrum of microbes while preserving electrode conductivity and equipment integrity. High-alcohol-content solutions (>70%) can rapidly erode the conductive surface of carbon rubber electrodes. Discussion with infection control professionals is recommended when using NMES for patients who are colonized with resistant or virulent microorganisms or for patients who have compromised immune function and reduced capacity to deal with a microbial burden.

Equipment checks

It is strongly recommended, and in some provinces it is mandated by college regulations, to check all equipment and supplies intended for use on people (patients, volunteers, students) at least once a year. In some instances, more frequent equipment inspections are warranted. When equipment stands unused for long periods, electrical components can accumulate dust, which can affect conduction and insulation in the unit, resulting in current flow that does not adjust smoothly or is intermittent. Physical therapists should test equipment that has been unused for 3–6 months on themselves before using it on patients. Annual equipment checks should be conducted by qualified biomedical technicians who can evaluate the integrity and patency of electrical circuitry and calibrate the device (typically using an oscilloscope) to confirm the accuracy of electrical output. Safety checks of AC-powered stimulators should include a check of the insulation of electrical cords, the circuit grounding, and the measurement of leakage currents. Faulty equipment should always be taken out of service immediately.

Checking leads and electrodes

Leads and carbon electrodes need to be checked regularly to confirm that they are conducting electrical current consistently and evenly and with low resistance. The metal wire used in most leads is easily damaged, especially when leads are bent or stretched excessively or repeatedly. When a damaged lead wire moves during treatment, intermittent current flow can occur, and this can be uncomfortable and potentially harmful to the patient. Physical therapists should test that lead wires are patent by applying electrodes to themselves and gently moving the leads during the current ON cycle, noting any change in sensation.

Carbon rubber electrodes should be replaced when their impedance is more than 500 Ohms per centimetre. Impedance can be measured by an ohmmeter; for instructions on carrying out this measurement, visit //cptbc.org/wp-content/uploads/2015/07/592107903-Practice-Standard-.pdf.

5. Terms and Definitions in NMES

A discussion of the NMES literature is confusing because of the inconsistency in electrotherapy terminology. A common set of terms to facilitate easy communication about EPAs is needed; however, the 2001 document most commonly cited by other authors209,210 needs updating to bring it in line with changes in equipment and recent modifications to traditional waveforms (e.g., Russian current). In this section, we define and describe terms that are relevant to the discussion of NMES; clinicians working with electrical stimulators may find it helpful to use our standard set of terms to reconcile the variety of terms used in research and industry.

Neuromuscular electrical stimulation (NMES)

Repeated application of current to produce contraction of innervated muscle by depolarizing local motor nerves. Repeated application may produce effects—for example, muscle strengthening “that enhances function but that does not directly provide function.”42(p.412)

Functional electrical stimulation (FES)

The use of electrical current to directly enable a functional movement.42 FES systems are commonly designed for the limbs, such as UEs for activities of daily living (ADLs) or LEs for gait. FES might replace a completely lost movement, as in paralyzed muscles in individuals with spinal cord injury, or replace or augment orthotics. FES may require sophisticated microcircuitry, multiple channels, and creative triggering mechanisms (voice, intact muscles, switches) and might need to be applied long term and during all waking hours to achieve the objectives.

Transcutaneous electrical nerve stimulation (TENS)

Application of current using surface electrodes to activate peripheral nerves; TENS (sometimes abbreviated TNS) is typically used for the purpose of modulating pain. A variety of current waveforms and pulse frequencies are associated with TENS; customarily, the approach produces sensory stimulation with or without small muscle twitches that are non-functional. Tetany is not normally required. TENS is not applied using the ON:OFF periods (measured in seconds) typical of NMES; rather, the current is delivered continuously for periods as short as 30 min or for many hours continuously.

Charge (coulombs)

A measure of how many electrons have been lost or gained by an object. Matter is either negatively or positively charged, or it has no net charge (neutral). One coulomb is the quantity of charge created when a current of 1 ampere flows for 1 second.

Current (amperes or milliamperes)

Movement of electrons or ions through a conductive medium. In human tissues and bodily fluids, this involves the flow of ions such as sodium, potassium, and chloride. A current flows according to Ohm's law (current=voltage/resistance) and is proportional to the magnitude of the electromotive force (voltage) divided by the opposition to current flow (resistance).

Voltage (volts or millivolts)

Electromotive force drives the movement of current from one location to another along a pathway or circuit. Current flow increases with an increase in voltage. Also known as the potential difference, voltage is created by the separation of negative and positive charges associated with two oppositely charged electrodes.

Resistance (ohms)

The opposition of a conductive material to the passage of an electrical current. Current flow increases with a decrease in the resistance of the conducting material. In the human body, high-resistance tissues (insulators) include skin, fat, and connective tissues, and low-resistance tissues (conductors) include muscles, blood, and other bodily fluids that have a high concentration of electrolytes.

Impedance (Z)

The opposition of a material to AC flow. It is also measured in ohms; however, it has the symbol Z. The relevance for clinicians is that impedance is lower for medium-frequency (1000 Hz) and high-frequency currents; therefore, they pass more easily through the skin layer than low-frequency currents.28

Resistance to ACs is complex because of the changing electrical and magnetic fields as pulse charge changes from positive to negative. Impedance factors into the resistance of capacitors in AC circuits. Skin is an insulator and stores an electrical charge on its outer surface—that is, it acts as a capacitor and resists current flow across it. Capacitor resistance is inversely dependent on frequency: As AC frequency increases, pulse duration decreases, allowing less time for a charge to be stored on the skin and, therefore, less impedance.

Constant voltage (CV) stimulator (current measured in milliamperes)

Maintains the voltage set by a clinician at the start of treatment. Current flow varies inversely with skin–electrode resistance, meaning that if the contact area between electrode and skin changes during treatment, the resistance, and therefore current flow, also change. A problem can arise when the initial skin–electrode contact is poor and full contact suddenly occurs: Resistance drops dramatically, current flow increases dramatically, and a patient feels a sudden surge of current, which could be uncomfortable or painful. In the reverse situation, when full skin–electrode contact changes to partial contact, voltage is maintained by the stimulator, but, because of higher resistance, the current flow drops. The drop could mean that current flow is below beneficial level.

Constant current (CC) stimulator (current measured in volts)

Delivers current to the electrodes at a constant amplitude by varying the voltage output whenever resistance changes. This means that if the contact area between skin and electrode is suddenly reduced during treatment, the same amount of current will flow through a smaller skin area, and the increased current density could be uncomfortable, even painful, for the patient. This type of stimulator has a built-in safety factor in that the maximum voltage adjustment is limited to a safe range. The advantage of CC stimulators is that they ensure that the current level is maintained at that set initially by the clinician. They may be more draining on battery-operated units.

Some stimulators permit selection of CV or CC. If a choice is not available, CC versus CV can be determined by slowly lifting an electrode corner off the skin while the stimulator is on. If the patient perceives that the current intensifies, the stimulator is delivering constant current; if the patient perceives that the current weakens, the stimulator is delivering constant voltage.

Cathode

The negatively charged electrode; it attracts positively charged cations. The cathode is considered more active because it can more readily depolarize a nerve; therefore, it is often placed over the muscle MP. The negative lead wire is typically coloured black at one end.

Anode

The positively charged electrode; it attracts negatively charged anions. The anode is often placed over the nerve innervating the target muscles or proximal or distal to the cathode. The positive lead wire is typically coloured red at one end.

Waveform

Diagrammatically represents a change in stimulus amplitude over time. This “picture” of an electrical event begins when the current flows and stops when the current returns to zero. The amplitude and direction of the current flow is reflected in the shape of the waveform and depends on the polarity of the electrodes.

Many AC (plug-in) stimulators offer one or more different waveforms, which can be selected by a therapist (see monopolar and bipolar set-up, under “Electrode Positioning”). Portable stimulators usually provide limited waveform choices.

Waveforms can be described as monophasic or biphasic, then further described by their shape (e.g., monophasic rectangular, symmetrical biphasic rectangular, asymmetrical biphasic rectangular, sinusoidal). More description follows under “Pulsed Current.”

Please note that the terms rectangular and square waves are commonly used interchangeably. Both correctly describe a pulse with a rapid rise of amplitude and variable pulse duration. This article uses rectangular.

Types of current

Direct current (DC)

Current that flows in one direction continuously for a period of at least 1 second. It is also called galvanic current. Electrode polarity (positive or negative) remains constant until it is changed manually by the operator. This form of current results in an accumulation of charged particles (ions) under the electrodes, which, if excessive, will cause an electrochemical burn. DC has limited clinical application (e.g., iontophoresis and wound healing).

Alternating current (AC)

Continuous bidirectional current that changes in direction at least once every second. The most common type of AC is a sinusoidal wave, in which both phases are equal and opposite and no net charge accumulates. Unlike pulsed current, there is no OFF time between cycles or phases. AC is almost exclusively available on plug-in, multi-modal units and is not present on most portable devices.

Russian current as an example of burst-modulated AC (BMAC)

This classic waveform is a medium-frequency sinusoidal current that is balanced and switches polarity 2,500 times per second (2500 Hz). A type of BMAC, Russian current is interrupted (modulated) into 20-millisecond bursts, consisting of 10 milliseconds of AC current followed by 10 milliseconds of no AC current (50% duty cycle). This is repeated 50 times per second (burst rate of 50). The background 2500-Hertz AC is called the carrier frequency.

More recently,211,212 devices have been designed to deliver different configurations of the traditional Russian current with adjustable levels of carrier frequency (1000–5000 Hz), burst frequency (50–75 bursts per second), or burst duration (2–10 ms). Modulated AC therapeutic currents are normally available only on wall-powered stimulators.

Some portable stimulators indicate that they offer Russian current, but the current characteristics show that it is a burst-modulated PC, which is distinct from true Russian current and BMAC. This current consists of three or more biphasic, balanced, rectangular wave pulses of 120- to 400-microsecond phase duration separated by a 100-microsecond interpulse interval. These bursts of three or more pulses are delivered 50 times a second.

Although these three forms of current would be indistinguishable to a patient, some research has suggested that the ability to elicit near-maximal force without considerable discomfort can be influenced by the waveform.211,212

Pulsed current (PC)

Pulsed current is a brief, intermittent current flow interrupted by periods of no current flow. Current can flow in one direction (monophasic) or both directions (biphasic). Each pulse is an isolated event described by waveform shape (e.g., rectangular, twin peaked), amplitude, and duration. With PC, the duration of the pulse is very short, typically only a few hundred microseconds (one-millionth of a second), and the total charge delivered using PC is extremely low. For example, during a 10-second muscle contraction with a pulse duration of 300 microseconds and pulse frequency of 50 Hertz, current would be delivered for a total of 0.15 seconds. PC is the type of current most commonly used for therapeutic purposes because the risk of tissue injury is minimal, and it can be delivered using small, battery-powered devices.

Monophasic pulsed current

Current flows in only one direction, and the polarity of the electrodes does not change. Most often, it appears as a rectangular waveform, with a range of pulse amplitude, pulse duration (commonly 100–400 μs), and pulse frequency (commonly 50–100Hz) that can be selected by the clinician. It is erroneous to describe monophasic PC as pulsed DC because current does not flow in one direction for periods of 1 second or longer. Because the pulse has a very short duration, very little charge accumulates under the electrode. Common types of monophasic PC include monophasic rectangular waveforms and high-voltage PC (twin-peaked monophasic PC).

High-voltage pulsed current (HVPC)

Pulses are characterized by high initial voltage, up to 500 volts, followed by a rapid, exponential fall in voltage. Pulses are delivered in pairs (so-called twin peaks), with minimal time between peaks. The duration of each phase is very short (20–60 μs), taking only 5–10 microseconds to reach 50% of peak amplitude. Because of the rapid decay in voltage, the total charge of individual pulses (about 15 μC) is lower than that of most other waveforms. During each pulse, current flows in only one direction, resulting in a small accumulation of charge under each electrode. However, because of the very short pulse duration, this charge is negligible and does not change skin pH. This combination of high amplitude and brief pulse duration produces a relatively comfortable electrical stimulation. However, it is ineffective for activating muscles, other than small muscles (e.g., in the hand). HVPC is most commonly used to stimulate tissue repair and promote the closure of many types of chronic, open wounds.

Biphasic pulsed current

Bidirectional flow of current with two distinct phases. A current flows in one direction for a defined period, and then the polarity of the electrodes switches, causing the current to reverse and flow in the opposite direction.

Biphasic symmetrical pulsed current

A current with a waveform that has two identical phases; each has an equal and opposite current flow so that no net charge accumulates on the skin.

Biphasic asymmetrical pulsed current

A current in which the polarity of electrodes changes during each phase of the pulse, but the shape of each phase of the waveform is not the same. The two phases of the pulse may be balanced or unbalanced. If balanced, the charge in each phase is equal and opposite, resulting in no net charge accumulating on the skin. If unbalanced, the two phases of the pulse have different amounts of charge, leaving a net balance of charge on the skin. The waveform most often used in NMES stimulators has a leading phase that is rectangular, followed by a second phase with the current flowing in the opposite direction at a lower amplitude for a longer duration. In this way, the phase charge is balanced so that the pulse is electrochemically neutral—that is, there is no net charge.

Biphasic asymmetrical PC is the most common type used in portable TENS and NMES machines. The initial active phase behaves similarly to monophasic PC, in which there is one clearly defined, negatively charged electrode (cathode) and another positively charged electrode (anode).

NMES parameters

Frequency (pulse rate; Hertz or pulses per second [pps])

The number of pulses in 1 second (a biphasic pulse has two phases but still counts as a single pulse when considering pulses per second).

Phase and pulse duration (microseconds)

Pulse duration is the time elapsed from when the current (or voltage) leaves the isoelectric (zero) line until it returns to baseline. It includes both positive and negative phases when the pulse is biphasic as well as any interphase interval. Because pulse duration is measured in units of time, it is incorrectly, although commonly, referred to as pulse width.

Pulse amplitude (millivolts or milliamperes)

The magnitude of the current or voltage deviation from zero or isoelectric line (current or voltage, depending on whether the stimulator is a CV or CC device). Often described as peak or peak-to-peak amplitude, which is the maximum or largest deviation from zero.

ON time

The time over which a series of pulses is delivered. With NMES protocols, this time reflects the duration that the muscle will be activated (work cycle).

OFF time

The time over which the stimulator automatically cycles OFF and no current is delivered. With NMES protocols, this is the period between muscle contractions (rest cycle).

ON:OFF ratio

A ratio of the ON time of each cycle to the OFF time (e.g., ON:OFF 10:30 s=1:3 ratio). Higher ratios (1:5) have more rest time between muscle contractions and cause less muscle fatigue.

Ramp-up time

The amount of time it takes for the stimulating current to reach the set amplitude of an ON cycle, commonly 1–2 seconds. Devices usually count the ramp-up time as part of the total ON time.

Ramp-down time

The amount of time it takes for the stimulating current to return to zero intensity at the end of an ON cycle, commonly 1–2 seconds. Devices commonly count the ramp-down time as part of the total OFF time.

Conclusion

The tables in this document provide data that have been extracted from a large body of evidence and critically analyzed to inform clinical practice. There is moderate to strong evidence that NMES is effective as a treatment for some UE and LE problems post-stroke, for weakness post-ACL repair and total knee replacement, for muscle weakness in knee OA, and for debilitation and weakness after critical illnesses. The benefit of NMES for PFPS is uncertain.

These data informed our recommendations for the key NMES parameters for effective treatment. For quads muscle strengthening, after knee surgery and in OA and PFPS, the optimal approach includes tolerance-level current amplitude and isometric contraction without voluntary assist, but with an additional voluntary strengthening programme performed at another time; also important are adequate pulse duration and a limited number of repetitions within a session, approximately 10–15 contractions three times a week. In contrast, for motor relearning and strengthening for patients post-stroke, the key factors are high levels of repetition within sessions applied on a daily basis at relatively lower current amplitudes and lower pulse frequency; shorter pulse durations are acceptable.

Optimal outcomes using EMG-NMES or NMES alone might be achieved when muscle stimulation is applied during functional activities post-stroke. For managing severe muscle weakness and atrophy and the deconditioning associated with critical illness and advanced cardiopulmonary disease, the optimal parameters are generally similar to those used post-stroke, although different outcomes are measured—namely, muscle strength and cardiopulmonary function, each of which has been reported to benefit from NMES treatment; relatively lower pulse frequency and amplitude and a high number of daily repetitions are indicated. For patients in the ICU, voluntary exercise is usually not an option, and patient positioning is determined by feasibility. For all these clinical conditions, an adequate total number of sessions is important to improve outcomes.

The authors of this article have clearly identified the positive effects of the use of NMES in a variety of clinical situations, and they have provided clinicians with appropriate information and parameters to promote the effective use of NMES on patients in these or similar clinical conditions.

Index

Advanced COPD 49

Anterior cruciate ligament reconstruction 24

Consciousness disturbance 49

Degenerative arthritis and osteoarthritis 37

Electrodes

• Anode and cathode 61

• Bipolar electrode placement 61

• Carbon rubber electrodes 61

• Cathode 61

• Checking leads and electrodes 64

• Common approach to applying NMES 63

• Concurrent use of NMES and cold packs 63

• Denervated muscles 62

• Electrode care 63

• Electrode gel 61

• Electrode size 62

• Electrode spacing 62

• Electrode sponges 61

• Equipment care and maintenance 63

• Equipment checks 64

• Equipment cleaning 64

• Lack of sensation 62–63

• Limb position 62

• Locating the motor point 61

• Monopolar electrode placement 61

• Safety concerns 62–63

• Securing electrodes 61

• Self-adhesive electrodes 60–61

• Skin irritation and skin burn 63

Heart failure 49

Hemiplegic shoulder subluxation 5

Lower extremity stroke: foot drop, plantar spasticity, and gait improvement 18

Malignant disease 49

Mechanical ventilation 49

NMES parameters

• Frequency (pulse rate; Hertz or pulses per second [pps]) 67

• ON:OFF ratio 67

• Phase and pulse duration (microseconds) 67

• Pulse amplitude (millivolts or milliamperes) 67

• Ramp-down time 67

• Ramp-up time 67

Patellofemoral pain syndrome 33

Sepsis 49

Stimulator 60

Stimulator features

• Alternating–simultaneous 60

• Automatic shut-off 60

• Compliance meters 60

• Constant stimulation mode 60

• Locking 60

• Preprogrammed NMES protocols 60

• Reciprocal–synchronous 60

• Saved protocols 60

Terms and Definitions in NMES

• Anode 65

• Cathode 65

• Charge (coulombs) 65

• Constant current (CC) stimulator (current measured in volts) 65

• Constant voltage (CV) stimulator (current measured in milliamperes) 65

• Current (amperes or milliamperes) 65

• Functional electrical stimulation (FES) 64

• Impedance (Z) 65

• Neuromuscular electrical stimulation (NMES) 64

• Resistance (ohms) 65

• Transcutaneous electrical nerve stimulation (TENS) 64–65

• Voltage (volts or millivolts) 65

• Waveform 65–66

Total joint replacement 43

Types of Current

• Alternating current (AC) 66

• Biphasic asymmetrical pulsed current 67

• Biphasic symmetrical pulsed current 67

• Direct current (DC) 66

• High-voltage pulsed current (HVPC) 66–67

• Monophasic pulsed current 66

• Pulsed current (PC) 66

• Russian current as an example of burst-modulated AC (BMAC) 66

Upper extremity stroke: wrist and finger extension 12

Voluntary contraction 2, 62

Abbreviations

Units

cm – centimetre(s)

mm – millimetre(s)

mA – milliampere(s)

Hz – Hertz

μV – microvolts

AC – alternating current

HVPC – high-voltage pulsed current

Muscles

gastrocs – gastrocnemius muscle

hams – hamstring muscles (biceps femoris, semitendinosis, semimembranosis)

MP – motor point

quads – quadriceps muscle

VL – vastus lateralis muscle

VM – vastus medialis muscle

General

ACL – anterior cruciate ligament

CCT – controlled clinical trial

CHF – congestive heart failure

COPD – chronic obstructive pulmonary disease

EMG – electromyography

EPAs – electrophysical agents

Ex – exercise

FES – functional electrical stimulation

ICU – intensive care unit

LE – lower extremity

NMES – neuromuscular electrical nerve stimulation

OA – osteoarthritis

PFPS – patellofemoral pain syndrome

PT – physical therapy

QOL – quality of life

RCT – randomized controlled trial

SR – systematic review

sublux – subluxation

THA – total hip arthroplasty

TKA – total knee arthroplasty

UE – upper extremity

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