Neuromuscular Electrical Stimulation Therapy to Restore Quadriceps Muscle Function in Patients After Orthopaedic Surgery: A Novel Structured Approach

Spector, Paul MD; Laufer, Yocheved PhD; Elboim Gabyzon, Michal PhD; Kittelson, Andrew PhD; Stevens Lapsley, Jennifer PhD; Maffiuletti, Nicola A. PhD

Journal of Bone & Joint Surgery - American Volume:
doi: 10.2106/JBJS.16.00192
Current Concepts Review
Abstract

Despite evidence supporting the use of neuromuscular electrical stimulation (NMES) as an adjunct exercise modality to improve voluntary activation, muscle strength, and functional recovery after knee surgery, NMES therapy remains a clinically underutilized modality.

We propose a criteria-based treatment algorithm aimed at optimizing and simplifying the clinical application of NMES therapy following knee surgery.

The suggested algorithm includes a short preoperative phase for patient education (1 visit) and familiarization with NMES, followed by 2 home-based treatment phases (each lasting 3 weeks): (1) a high-intensity, high-volume phase initiated within the first few days following surgery, and (2) a high-intensity, low-volume phase.

Two evaluation sessions are also incorporated, with the first following 1 week of treatment to assess the responsiveness to NMES and the second after 3 weeks of treatment to assess whether voluntary activation failure has resolved.

Practical guidelines for maximizing muscle response while minimizing discomfort and fatigue, including optimal pulse characteristics, electrode size and location, knee joint position, and patient instructions, are provided.

Author Information

1Human Performance Laboratory, Schulthess Clinic, Zurich, Switzerland

2Physical Therapy Department, Faculty of Social Welfare and Health Sciences, University of Haifa, Haifa, Israel

3Muscle Performance Lab, School of Medicine, University of Colorado, Aurora, Colorado

E-mail address for N.A. Maffiuletti: nicola.maffiuletti@kws.ch

Article Outline

Neuromuscular electrical stimulation (NMES) involves the application of preprogrammed trains of stimuli to superficial skeletal muscles—by means of surface electrodes placed over the muscle belly—with the ultimate goal to evoke visible tetanic contractions. Unlike other electrical stimulation modalities such as transcutaneous electrical stimulation (that is commonly used for pain relief), NMES-based treatment programs have long been used to either preserve1 or restore2 skeletal muscle mass and function during and after a period of disuse due to injury, surgery, or illness. For example, multiple sessions of daily NMES markedly reduced quadriceps muscle atrophy in patients with a long leg cast by preventing the decrease in muscle protein synthesis associated with immobilization1. In the same way, the early addition of NMES proved to be effective in attenuating quadriceps muscle weakness and improving physical performance following total knee arthroplasty and anterior cruciate ligament (ACL) reconstruction in recently conducted randomized controlled trials3,4. Interestingly, NMES therapy is believed to be one of the most effective remedies to treat voluntary activation failure (also referred to as arthrogenic muscle inhibition)5, a common neuromuscular complication following knee surgery and injury resulting in the inability to maximally activate a muscle and, therefore, in weakness6. Despite this evidence, many practitioners continue to be reluctant to prescribe NMES postoperatively to their patients and its clinical value is not universally agreed on.

One of the major barriers impeding the successful implementation of NMES in postoperative care is that its clinical application remains extremely arbitrary because of poor knowledge of its physiological specificities. Too many parameters—in particular those related to the characteristics of the electrical current (e.g., pulse duration and frequency; see Figure 1 for a general overview of these parameters)—are often debated, while in fact only the quality of the evoked contraction (specifically, the level of evoked force) has been shown to be a main determinant of NMES treatment effectiveness7. In other words, the higher the force of the contraction evoked by NMES, the better the treatment-induced improvements in terms of both strength and voluntary activation7-12. Another problem related to the use of NMES in patients is the possibility of discomfort induced by the transcutaneous application of electrical current. This may affect the clinical applicability of this treatment modality, especially in patients sensitive to NMES sensation. Therefore, NMES therapy should be adequately dosed throughout a treatment program, with special care regarding progressive adaptation to repeated NMES exposure and identification (and eventual elimination) of nonresponders to the treatment13.

The aim of this Current Concepts Review was to propose a structured approach to NMES therapy for restoring quadriceps muscle strength and voluntary activation in patients after orthopaedic surgery, with special emphasis on total knee arthroplasty and ACL reconstruction. Over 1.8 million total knee arthroplasty and 230,000 ACL reconstruction procedures are performed annually in the United States and Europe combined, and their prevalence is expected to increase considerably in the near future14-16. Recovery of quadriceps muscle function (as well as of physical performance) is slow and incomplete in these patients5,6, even years after the operation17. The high prevalence of these surgeries and the necessity of rehabilitation for postoperative recovery highlight the need for a criteria-based user-friendly treatment algorithm aimed at optimizing and simplifying the clinical application of NMES. An examination of the most important physiological aspects of NMES will provide a rationale for the steps and recommendations in the treatment algorithm. Finally, we present some practical guidelines and promising perspectives for an optimal use of structured NMES therapy in clinical practice.

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NMES Therapy: Physiological Considerations

Contractions evoked by NMES demonstrate a unique motor unit recruitment profile, which differs in several spatial and temporal characteristics from normal voluntary muscle actions. Compared with voluntary activation, NMES recruits motor units in a superficial, incomplete, asynchronous, and nonselective pattern18-21, which overall limits the force evoked by the stimulation and increases the rate and amount of muscle fatigue. These physiological specificities of NMES, which are delineated below, have important implications with respect to its effectiveness and treatment methodology in the context of postoperative knee care.

The electrical fields generated by NMES, which are spatially fixed between the surface electrodes, evoke action potentials in the intramuscular motor axon terminal branches situated within the electrical field, as well as in the more superficial sensory nerve endings. As current intensity is increased, deeper motor units are recruited22,23. Yet, while motor fiber stimulation induces muscle contractions that emulate voluntary contractions, stimulation of afferent nociceptors in the skin evokes discomfort and pain that limit the ability to increase stimulation intensity and therefore to reach high force levels23,24. As a consequence, the force generated by NMES contractions rarely reaches the level of force produced during a maximal voluntary contraction (MVC)25. In patients, NMES-evoked forces of 30% to 60% of the MVC force, which correspond to an activated cross-sectional area of only 29% to 43% for the quadriceps muscle, have been observed26.

During voluntary contractions, motor unit recruitment generally follows the size principle27, in which recruitment progresses from small motor units innervated by small-diameter axons, which are typically slower and less fatigable, to larger motor units, which are faster and more fatigable. While it has often been suggested that the order of motor unit recruitment during NMES is reversed28, more likely, the recruitment order is random or nonselective (i.e., with no obvious sequencing related to fiber types), with the type of motor units that respond first related more to factors such as motor unit distribution within the muscle and distance from the stimulating electrode18,29. For the quadriceps muscle, it has been demonstrated that larger (more fatigable) motor units are mainly located in superficial regions of the vastus lateralis30, which would favor their recruitment during NMES compared with deeper (slower) units.

The difference in muscle fatigue between voluntary and NMES contractions is attributed to 2 additional phenomena. During voluntary contractions, motor units may alternate their activity by increasing the number of activated units when those initially firing are fatigued or when greater force is required19,31,32. Furthermore, motor units fire asynchronously during voluntary efforts, enabling smooth tetanic contractions despite the relatively low firing rate (range, 8 to 25 Hz) of each individual motor unit33. In contrast, during the spatially and temporally fixed NMES, all motor fibers reached by the electrical field respond synchronously at the pulse frequency delivered by the stimulator. Thus, to achieve a similar tetanic contraction, it is necessary to use higher pulse frequencies (range, 35 to 75 Hz), which also lead to higher fatigue rates.

Despite the above-mentioned limitations of NMES in terms of evoked force, discomfort, and muscle fatigue, its physiological specificities offer important advantages for use in patients after knee surgery. The relatively high proportion of large, fast motor units, which can be recruited at low contraction intensities during NMES—contrary to the situation with voluntary contractions—is particularly important in conditions of disuse muscle atrophy, in which fast muscle fibers are generally more affected34-36. Thus, in conditions of quadriceps muscle weakness as observed following knee surgery, NMES may offer “selective” treatment of the more affected muscle fibers. Such a therapeutic exercise modality has been shown not only to increase muscle strength and endurance4,37,38 but also to improve oxidative capacity and induce a shift toward a slower muscle phenotype39. Additionally, conditions such as joint effusion40,41, pain42-44, and altered proprioceptive input41,45 are characterized by difficulties in volitionally activating all available motor units (activation failure)6,46,47. Severe voluntary activation deficits may limit improvements in muscle strength in response to rehabilitation that utilizes voluntary exercise, possibly because of the inability to generate contractions of sufficient intensity to promote strength gains42. By bypassing the need to volitionally activate muscle fibers, NMES can reduce activation failure, thus promoting strength gains beyond those achieved by voluntary contractions38,48. In fact, diverse evidence indicates that NMES treatment programs may modify the excitability of specific neural paths at the spinal cord and at cortical levels49, thereby resulting in neural adaptations rather than merely muscle hypertrophy.

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NMES Therapy: A Structured Approach

Proposal of a Criteria-Based Treatment Algorithm

There is no consensus on the optimal rehabilitation protocol following total knee arthroplasty or ACL reconstruction50,51. Quadriceps activation failure is typically pronounced following knee surgery, thus reducing the muscle overload that is possible with voluntary exercise alone. Therefore, in the absence of contraindications such as a pacemaker, pregnancy, thrombophlebitis, or postoperative hemorrhage52, NMES should be prioritized in the minds of clinicians as a potential treatment modality to complement other rehabilitation interventions (that include a variety of strengthening, flexibility, and educational strategies)4,53-55. To aid clinicians in applying and monitoring NMES therapy, we present a simple treatment algorithm (Fig. 2) that is intended to improve clinical decisions regarding (1) appropriateness of NMES therapy, (2) monitoring of patient progress, and (3) timing and rationale for NMES therapy modification or cessation.

First and foremost, patients and clinicians should understand the purpose of postoperative use of NMES: to reeducate neural pathways (affected by surgery) and to supplement voluntary muscle activation to deliver adequate training doses to the quadriceps muscle. Patients should be educated and familiarized with NMES, ideally before surgery. The first 3-week treatment phase should be initiated within the first few days following surgery, with a formal assessment of treatment response planned after 1 week and 3 weeks of treatment. A second 3-week treatment phase should follow only if the patient shows an adequate response to NMES therapy (as judged from the 2 evaluation sessions). The specific elements of our treatment algorithm are described in the following paragraphs.

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Preoperative Education and Familiarization with NMES

Patient independence and comfort with the operation of the NMES device, as well as the quadriceps response to NMES, are key factors in determining the appropriateness of therapy. A short visit for NMES education and a home-based familiarization period (consisting of a few days) might ultimately improve patient tolerance and allow for more precise parameterization and dosing during therapy. Ideally, this should occur prior to surgery, to allow adequate time for familiarization with the NMES device and protocol. Patients should be instructed to properly position the electrodes, to operate the device, and to increase current intensity progressively. While no adverse effects are expected, patients should be informed that some redness is expected at the site of the electrodes following stimulation. However, this reaction should subside within 2 to 3 hours and should disappear thereafter.

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Treatment Phase 1: High-Intensity, High-Volume NMES Therapy

NMES treatment protocols and conclusions regarding treatment effectiveness differ across recent clinical trials4,53,55,56. However, for trials demonstrating the benefits of NMES (i.e., when NMES resulted in greater gains in quadriceps strength and physical function compared with control interventions), some common themes emerge57. Key among these themes is a high-volume approach, in which NMES is performed on a daily basis (even multiple times per day), for at least 3 weeks. Therefore, we recommend multiple daily sessions of approximately 10 minutes (15 contractions) to maximize exposure when activation failure is greatest.

Patients should be encouraged to use high-intensity NMES, with stimulation amplitudes set at the highest tolerable level. Because the level of evoked force increases linearly with current amplitude29, this parameter is likely to be particularly critical to achieving therapeutic effectiveness. Clinicians and patients should be aware that amplitude may need to be increased periodically to accommodate for neural adaptations (tolerance) or factors such as adiposity or swelling, which can result in increased impedance, limiting contractile force.

Electrodes should be relatively large to minimize current density and maximize patient comfort and muscle activation, and they should be placed at opposite muscle ends (Fig. 3), ideally over both the vastus medialis (distally) and the vastus lateralis (proximally)58. Relatively long pulse durations (400 to 600 μs) should be used, as wide pulses are more likely to target motor fibers (thus maximizing quadriceps force production), while shorter pulse durations (<200 μs) might preferentially target sensory fibers and contribute to uncomfortable burning sensations during NMES59,60. Clinicians should pay special attention to device specifications to ensure that pulse duration is appropriately indicated on the device settings, as confusion of “pulse duration” and “phase duration” is common and could result in application of pulse durations that are half the intended ones (Fig. 1). Frequencies between 50 and 100 Hz are generally recommended25. Clinicians should choose the lowest frequencies within this range, thereby maximizing force production while limiting early muscle fatigue.

We also recommend an on:off ratio of 10:30 seconds, again to maximize exposure while still providing reasonable rest periods between contractions61. Because NMES stimulates a fixed volume between electrodes, the same muscle fibers may be repeatedly recruited during a session of NMES, which can result in reduced force production and relatively rapid muscle fatigue (see the section on NMES Therapy: Physiological Considerations). Clinicians should attempt to limit this phenomenon via subtle modifications in electrode placement or instruction regarding the frequency of sessions. Some muscle fatigue is likely necessary to induce strength gains, but as with any strengthening intervention, excessive levels of fatigue can result in deleterious effects, including pronounced soreness and muscle damage62. Such events, however, are rare in postoperative applications of NMES, given the dramatic activation failure that is typically present and other factors that limit muscle recruitment and current delivery.

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Evaluation Session 1: Is NMES Producing the Desired Motor Response?

After the first week of treatment, clinicians should assess the response of the quadriceps musculature to NMES therapy. In research settings, the intensity of the NMES-induced muscle contraction is typically measured using dynamometry, as a percentage of the isometric MVC. When a healthy contralateral leg is available for comparison (e.g., in patients after ACL reconstruction), forces of >50% of the contralateral MVC are desired to optimize the strengthening effects63. In practice, however, such assessments are rarely feasible, so a more qualitative approach is preferred. The criteria proposed by Fitzgerald et al.63 serve as a helpful guide: NMES should evoke a full, sustained, tetanic contraction of the quadriceps with visual or palpable evidence of superior patellar glide. If these criteria are not met, NMES may not achieve therapeutic doses, and alternative rehabilitation strategies should be considered.

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Evaluation Session 2: Is Voluntary Activation Failure Resolved?

Quadriceps activation failure may largely resolve in the first month following surgery, especially if NMES therapy proves effective4,53. Patients should be reevaluated after 3 weeks of treatment to determine if a high-volume approach is still warranted. The main risk of continued high-volume therapy is pronounced and chronic muscle fatigue64, which may result in reduced muscle force and a corresponding decline in the training dose delivered to the muscle.

Activation failure may persist beyond the initial postoperative period, and high-volume NMES therapy may still be indicated. The challenge lies in detecting persistent activation deficits, because common research methods (e.g., twitch interpolation) are impractical for use in a clinical setting. Clinical observations may help to rule in the presence of persistent activation failure. These observations would include factors such as (1) an inability to consistently perform a quadriceps set (superior translation of the patella during quadriceps contraction in knee extension), (2) an inability to perform a straight leg raise without extensor lag, or (3) subjective reports of difficulty with muscle control or recruitment. The presence of any deficit with any one of these clinical assessments suggests activation failure that would benefit from continued high-volume NMES.

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Treatment Phase 2: High-Intensity, Low-Volume NMES

Once a patient has progressed through treatment phase 1 and activation failure is largely resolved, a low-volume approach is recommended. Here, the goal is still to supply the quadriceps muscle with high-intensity NMES therapy, but with longer rest intervals between treatment sessions to allow for adequate recovery. Thus, NMES current characteristics and general settings are unchanged between the 2 treatment phases (Table I), but the duration of each session is increased to approximately 15 minutes and the frequency of treatment sessions is reduced to 4 to 6 sessions per week for treatment phase 2 (e.g., 1 session per day or every other day).

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What Is Next?

Once a patient has successfully completed treatment phase 2 (3 weeks with at least 12 sessions in total), we recommend discontinuing NMES therapy and focusing exclusively on voluntary strengthening exercise, which is by far more functional than NMES.

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Additional Clinical Pearls for Effective NMES Application

NMES is most effective in patients who have voluntary activation failure. These patients most commonly include those who have had knee injury or surgery (e.g., ACL reconstruction or total knee arthroplasty), but they could also include patients with anterior knee pain, knee osteoarthritis, or hip arthroplasty5,6. Regardless of the specific population, there are a few additional considerations that are important for effective NMES applications. First, electrode size is critical because current density (and patient discomfort) is inversely proportional to the electrode size; smaller electrodes result in greater discomfort and, therefore, smaller NMES doses. Electrodes with approximately 200 cm2 of total surface area are recommended (e.g., two 8 × 12-cm rectangular electrodes) (Fig. 3). Second, high intensity is meant to be tolerable, resulting in a tetanic muscle contraction; however, in most patients, it should be uncomfortable to achieve the most effective dose. It is important to push patients toward some discomfort to get the maximum benefit as there is a strong relationship between the level of force evoked by NMES (i.e., the NMES dose) and the resulting strength gains7,8,12. Stimulators that allow for ≥100 mA of intensity are often necessary, especially when postoperative swelling is present. Third, it is helpful to stimulate the muscle near its optimal length-tension relationship to maximize the level of evoked force and thus muscle tension65. For example, for the quadriceps muscle, it may be more effective (and comfortable) to use NMES with the knee between 60° and 75° of flexion rather than at full extension66. As mentioned earlier, modifying electrode placement and the knee (but also hip) flexion angle slightly from session to session may optimize recruitment of the various parts of the muscle. Finally, it is unknown whether using NMES with or without a concomitant voluntary muscle contraction is more effective. However, clinicians may want to consider encouraging patients to simultaneously (and submaximally) contract the stimulated muscle, particularly during the first treatment sessions, as a strategy for minimizing discomfort.

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Perspectives and Overview

Growing research evidence supports the use of NMES as an adjunct exercise modality enhancing voluntary activation, muscle strength, and functional recovery in patients after orthopaedic surgery4,37,57,67. Furthermore, the advancements in technology leading to readily available and user-friendly stimulators render home self-treatment a viable option. Yet, NMES remains a clinically underutilized modality. This may be attributed to the multiple and often confusing and redundant parameters that must be considered during NMES treatment, as well as to the discomfort and muscle fatigue associated with the stimulation. The present study provides a concise summary of the physiological and methodological considerations guiding the selection of optimal stimulation parameters, as well as evidence-based recommendations for postoperative use of NMES of the quadriceps. The suggested treatment algorithm should help to streamline the clinician’s decision-making process, thus increasing the clinical utility of NMES following knee surgery.

The proposed approach ensures early identification of patients who are more likely to respond to the treatment. Furthermore, the recommendation to initiate familiarization with NMES prior to surgery is expected to improve patients’ tolerance of higher current intensities that are necessary to reach therapeutic levels of contraction68. While the presented protocol limited the preoperative treatment to a short habituation period, research has indicated that NMES-based treatment prior to total knee arthroplasty not only improved strength preoperatively but also contributed to muscle and functional recovery following surgery69. Thus, in the future, clinicians should also consider incorporating NMES as a preoperative treatment modality.

One should keep in mind that, at the current intensities necessary to induce muscle contractions that are strong enough to overcome activation failure, patient discomfort is almost unavoidable. The recommendations in the present article with regard to pulse characteristics, electrode size and location, joint position, and patient instructions are aimed at minimizing the discomfort as well as the muscle fatigue associated with NMES. Current research in the field of NMES is directed at further optimizing parameter settings to reduce these unwanted effects. For example, promising results have been recently reported with the use of 4 large electrodes placed over the quadriceps muscle3,70,71, which are designed to deliver current via multiple paths (distributed or multipath NMES) as opposed to the more traditional unidirectional current flow. Early studies with this application demonstrated stronger quadriceps muscle contractions combined with reduced discomfort and fatigue compared with conventional NMES70,71. More importantly, this relatively novel NMES modality proved to be more effective than both traditional NMES and standard rehabilitation for improving functional recovery after ACL reconstruction in a recently conducted randomized controlled trial3. Thus, distributed or multipath NMES may represent a valid alternative (or complement) to the NMES procedures described in the present article. Finally, although our treatment algorithm only refers to quadriceps NMES for the treatment of patients after knee surgery, the proposed principles can potentially be applied to other muscle groups and orthopaedic patients having muscle weakness and activation failure.

Investigation performed at the Human Performance Laboratory, Schulthess Clinic, Zurich, Switzerland

Disclosure: The authors indicated that no external funding was received for any aspect of this work. On the Disclosure of Potential Conflicts of Interest forms, which are provided with the online version of the article, one or more of the authors checked “yes” to indicate that the author had a relevant financial relationship in the biomedical arena outside the submitted work.

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