The use of neuromuscular electrical stimulation as an adjunctive therapy for muscle strengthening in knee rehabilitation

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1 The use of neuromuscular electrical stimulation as an adjunctive therapy for muscle strengthening in knee rehabilitation Rainsford G Biomedical Research Ltd. Parkmore Industrial Park West, Galway, Ireland ABSTRACT: The aim of this review is to present some of the emerging evidence concerning the application of Neuromuscular Electrical Stimulation (NMES) in knee rehabilitation and offer theoretical justification as to the efficacy of this novel treatment approach.over the past two decades, numerous researchers have investigated the concept of electrical stimulation as a modality to prevent or retard disuse muscle atrophy associated with knee injury and pathology. Indeed a growing body of evidence would suggest that NMES offers a highly effective adjunctive therapy that can increase muscle strength, improve function and ultimately enhance patient outcomes. The primary muscle group stimulated has been the quadriceps femoris. Key topics that will be discussed will be the role of electrical stimulation in addressing arthrogenic muscle inhibition, the training load provided by electrical stimulation, the limitations of this training technique, the evidence for application of electrical stimulation in specific knee conditions and the overall clinical implications of this treatment approach. This review will identify the research literature on these topics, presenting some critical treatment requirements and suggested clinical implications for optimum treatment application. Keywords: Electrical Stimulation, Knee Rehabilitation, Clinical Implications Correspondence to: Gary Rainsford. grainsford@bmr.ie INTRODUCTION: Traditionally, the application of electrical stimulation in patients with knee problems was administered mostly as a pain management modality whereby the clinician would deliver Transcutaneous Electrical Nerve Stimulation (TENS) in an attempt to influence the pain gate system described by Melzack and Wall, 1 or through the release of chemical endorphins within the brain. While TENS attempts to influence the sensory nerves for the purpose of pain relief, NMES applies a more aggressive electrical current delivery that aims to deliver a stronger motor response, namely, muscle contraction. NMES is defined as the application of an electrical current to the peripheral nervous system via electrode pads placed on the surface of the skin. The current is typically delivered in close proximity to the muscle motor point with the therapeutic goal of achieving a strong muscle contraction. When repeated contractions of a sufficient training intensity are delivered to the muscle motor point over a sustained intervention period, the likelihood of training adaptation is implied. Synder-Mackler et al 2 were among the first clinicians to investigate the notion of applying electrical stimulation for the purpose of muscle strengthening in a cohort of patients with knee pain. The observation here was that stimulation offered a training stimulus that could augment more conventional voluntary strengthening programs. This is desirable given the fact that inherent to several knee conditions is muscle weakness, particularly of the quadriceps muscles. This can arise following sustained periods of reduced activity secondary to pain or as a direct result of trauma associated with corrective surgical procedures [e.g. Anterior Cruciate Ligament (ACL) repair, Total Knee Arthroscopy (TKA)]. NMES is an emerging intervention for which there is a growing body of research evidence to support its use in clinical practice. As such this review aims to identify the evidence base for this treatment modality and suggest some key clinical considerations to ensure its optimal implementation in practice. The role of Electrical Stimulation in addressing Arthrogenic Muscle Inhibition (AMI) Rice and McNair 3 suggest that injury of the knee joint is associated with a long lasting inability to fully activate the quadriceps muscle, a process known as arthrogenic muscle inhibition (AMI). This observation supports Mizner et al 4 who reported that the primary etiology of early quadriceps strength loss after TKA is voluntary activation failure. Rice and McNair 3 further report that AMI poses a significant barrier to effective rehabilitation in patients with arthritis and following knee injury and surgery, and that quadriceps AMI has long been of concern as it contributes to muscle atrophy and can delay or even prevent effective strengthening, hindering rehabilitation considerably. Several authors have postulated that the neuromuscular training adaptations associated with NMES training is not 36

2 entirely attributable to the training load delivered. 5-7 Indeed the observation that previous studies have demonstrated significant training gains among healthy athletic populations in a short time period (<6weeks) would imply that neural adaptation is a key contributing factor to the overall training effect observed following NMES intervention. 5-7 Here, Maffiulletti et al 5 reported a crosseducation effect following NMES training where contra lateral maximal voluntary contraction (MVC) increased after training by 8%. 6 In addition, neural drive as determined using the interpolated twitch technique to the trained quadriceps was enhanced after electrical stimulation from 94.9% to 98.8%. This concurs with Jubeau et al 7 who reported training-induced increases in muscle strength that paralleled an increase in muscle activation and EMG activity following electrical stimulation training. When applied to a selected group of patients with knee problems Petterson and Snyder-Mackler 8 demonstrated a 25% improvement in left quadriceps femoris maximal volitional force output following 16 treatments of combined NMES and volitional strength training over a 6- week period. This coincided with an improvement of volitional muscle activation that was 83% before treatment and 97% after treatment. Given the observation by Mizner et al 4 that 85% of muscle weakness 1 month post TKA surgery is attributable to failure of voluntary muscle activation and muscle atrophy, it is clear that this area should be a key area for redress in terms of rehabilitation goals for the therapist. Clinical Implications: The neuromuscular training adaptations associated with NMES are attributable to both neural and muscular mechanisms of action. NMES offers a training modality that may strengthen the muscle by overloading the structure and improving neural drive to the muscle. This is a highly desirable therapeutic effect given the clear detrimental effect of reduced activation on patient function and muscle strength in cases of knee injury / trauma. Training Load provided by Electrical Stimulation. In accordance with the principle of training overload, it is imperative to induce a sufficient training intensity in order to produce both measurable and clinically relevant training adaptations, irrespective of the training intervention provided (Voluntary Resistance Training vs. NMES). The goal of any training program should be to provide a sufficient training load to deliver meaningful training adaptations. The strength gains associated with progressive voluntary strengthening programs is well established. However, the presence of on going pain, decreased range of motion and reduced capacity pre and post surgical interventions is very often a limiting factor that restricts patients in engaging in beneficial voluntary training regimes. The use of isometric electrically elicited contractions represents an alternative strengthening modality that can supplement or temporarily replace voluntary exercises at a time when patients are unlikely to fully commit to voluntary exercise (e.g. during the immediate post operative window). In order to deliver a therapeutic effect the strength of the electrically elicited contraction is very pertinent, when considering the concepts of training overload. Previously Synder-Mackler et al 9 had argued that the use of battery operated portable muscle stimulators offered a training load that was suboptimal and unlikely to produce a meaningful clinical effect. This inference that portable stimulators represented a low intensity stimulation output was refuted by Lyons et al 10 who demonstrated comparable torque outputs when establishing the electrically elicited percentage maximal voluntary isometric contraction force delivered from a clinical unit (Verastim 380) and that produced from a portable commercial stimulator (Empi 300 PV). Figure 1. Adapted from Lyons Average peak electrically stimulated quadriceps femoris muscle torque production between stimulators (p=0.09). Torque is expressed as a percentage of the torque produced during a maximum voluntary contraction. Percentage of Maximal Voluntary Isometric Contraction Comparing Training Intensities Between a Clinical and Portable Based Stimulator Verastim 380 Stimulator Empi 300 PV The overall consensus among researchers as outlined by Laufer et al 11 is that a therapeutic window of 25-50% of MVC of the knee extensors is desirable to induce strength training benefits in knee patients from a muscle stimulator. That said, training benefits (preservation of muscle mass, increased strength) have been observed at training intensities much lower than this Overall, Rice and McNair 3 report that the benefits of NMES appear to be dose-dependent, with high-intensity, maximally tolerated stimulations proving more effective than those performed at lower intensities. CLINICAL IMPLICATIONS A key determinant of successful NMES application in knee patients is that a suitable training load be delivered. A therapeutic window ranging from 25-50% of MVC of the knee extensors is recommended although strength gains have been observed at lower intensities than this. Irrespective, the greater the training load delivered the more likely a training response. Similar to progressive voluntary exercise patients should be given an adequate habituation to the treatment technique with intensities of stimulation progressed as appropriate in response to patients familiarising themselves with the stimulation. 37

3 Limitations of Electrical Stimulation Training Maffiuletti et al 14 report the two main limitations of NMES training are the strong discomfort associated with stimulation and the limited spatial recruitment of muscle fibers which is quite superficial. Considering the requirement for a sufficient training load to induce a therapeutic training effect the value of stimulation is only viable if a suitable training intensity is achieved while maintaining patient comfort levels. The use of larger electrodes that distribute the current and avoid the build up of current density has vastly improved the tolerance of patients to stimulation training. This was noted by Porcari et al 15 who attributed enhanced training outcome when comparing two similar muscle stimulators to the ability of users with the large electrode device to engage in more intensive training sessions that were not limited by comfort issues. Again, it is vital that similar to voluntary exercise, patients have a suitable progressive program in place where they are allowed to accommodate to the training stimulus before progressing onto more aggressive training intensities. An inherent characteristic of NMES training as outlined again by Maffiuletti et al 14 is the high metabolic demand and early onset of muscle fatigue associated with repeated contractile activity within the same motorneuron pool. This phenomenon of fixed, synchronous muscle recruitment varies considerably from voluntary exercise where the recruitment order is asynchronous and spatially dispersed. Previously, some clinicians have attempted to minimize this limitation by changing the muscle length and varying electrode positions to vary the target muscle pool during treatment. Indeed some device manufacturers have attempted to address this inherent limitation with novel applications whereby current is passed between multiple electrodes in varying pathways to achieve greater recruitment and patient comfort. Using this approach Feil et al 16 showed better strength and functional scores in a group of ACL patients who supplemented their standard of care program with traditional stimulation approaches and those who used this multi-directional stimulation delivery system (See Figure 2 below). Figure 2. Adapted from Feil Electrode arrangement and Current distribution patterns in A) Traditional Electrode Configurations (PolyStim Device by Neurotech) and B) & C) Multipath arrangement (Kneehab Device by Neurotech) Clinical Implications: In order to optimize the likely therapeutic effect from using NMES training it is imperative that the clinician is aware of treatment limitations associated with NMES use. Implementing suitable refinements within the therapy is likely to have a marked impact on the likely training effect observed. These include using large medical grade electrodes that will ensure patient comfort at high training intensities and modifying patient positioning and or indeed current pathways to vary the motor neuron pool trained during the session. This is also likely to reduce the onset of fatigue within the muscle. EFFICACY OF NMES IN SPECIFIC KNEE CONDITIONS Anterior Cruciate Ligament Repair Despite the obvious benefits derived from surgical intervention in cases of ACL injury the persistent presence of muscle weakness post operatively can often impede patient outcomes and lead to unwanted compensatory strategies. The evidence base to support NMES use in ACL patient rehabilitation is growing. Lieber et al 17 showed that NMES and voluntary muscle contraction, when performed at the same intensity, are equally effective in strengthening skeletal muscle that has been weakened by surgical repair of the ACL. Feil et al 16 demonstrated greater patients outcomes (strength, functional scores) in a group of ACL patients treated with a novel NMES approach compared to those treated with traditional NMES and standard of care exercises. Kim et al 18 conducted a systematic review on NMES use following ACL reconstruction and reported that NMES combined with exercise may be more effective in improving quadriceps strength than exercise alone. This concurs with the seminal work of Synder-Mackler et al 2 who was among the first researchers to demonstrate the potential of NMES in an ACL cohort. Here patients were randomized to a volitional exercise alone or exercise plus NMES rehabilitation program. Results following a 4 week intervention showed patients in the NMES group to have a more normal flexion excursion of the knee, gait pattern and stronger quadriceps muscles compared to the exercise only group. The finding of improved muscle strength was repeated in a further study by Synder-Mackler et al 9 who demonstrated greater strength scores in a cohort of ACL patients treated with high intensity NMES plus intensive closed chain exercise compared to patients who trained with high intensity voluntary exercise or low intensity NMES plus intensive closed kinetic chain exercises. Fitzgerald et al 19 compared patient outcome among a group of ACL patients who were assigned to standard rehabilitation or standard care plus NMES. Here, the NMES group demonstrated moderately greater quadriceps strength at 12 weeks and moderately higher levels of self-reported knee function at both 12 and 16 weeks of rehabilitation compared to the standard rehabilitation group. In addition Fitzgerald et al 19 reported 38

4 that a greater proportion of subjects in the NMES group achieved clinical criteria for advancing to agility training at 16 weeks. These findings concur with Risberg et al 20 who conducted a systematic review of treatment modalities implemented during ACL rehabilitation. Risberg reports that there is evidence that high intensity neuromuscular electrical stimulation in addition to volitional exercises significantly improves isometric quadriceps muscle strength compared to volitional exercises alone. Interestingly Pasternostro-Sluga et al 21 failed to demonstrate an additive effect from supplementing existing rehabilitation exercise with NMES. Here, NMES combined with an early exercise therapy regimen was not significantly more effective in reducing weakening than an early exercise therapy regimen alone after ACL surgery. Despite this finding the overall research evidence would suggest a combined approach of implementing NMES in addition to conventional strengthening programs can produce greater strength gains, than voluntary exercise alone. CLINICAL IMPLICATIONS The addition of NMES to a standard strengthening program post ACL surgery may well augment more conventional exercise. An apparent dose response exists suggesting high intensity stimulation to deliver greater strength gains. Total Knee Arthroscopy (TKA) Stevens et al 22 demonstrated that NMES combined with voluntary exercise produced more favorable strength gains and restoration of voluntary activation levels than exercise alone following TKA. While TKA can drastically reduce the pain levels associated with an osteoarthritic knee and contribute to greater functional independence post surgically, Lewek et al 23 reports that persistent quadriceps femoris muscle weakness following surgery can prevent patients from returning quickly and fully to functional activities. Indeed Mizner et al 4 observed in a group of 20 unilateral TKA patients postoperatively, quadriceps strength reductions of 62% while voluntary activation and maximal cross-sectional area was decreased by 17% and 10% respectively in comparison with the preoperative values. It would appear that in order to maximize the potential of surgical interventions a suitable rehabilitation program geared towards restoring muscle strength and subsequent functional capacity is essential. Further to this Mizner et al 4 also suggests that preoperative status in terms of strength and functional capability will be predictive of likely outcome and rehabilitation success following surgical intervention. Walls et al 24 have also highlighted the value of prehabilitation strengthening programs in terms of patient outcome post surgically. Their study investigated the compliance of a home-based, NMES prehabilitation programme in patients undergoing TKA. Results following intervention showed preoperative quadriceps strength increases of 28% (p< 0.05) with associated gains in walk, stair-climb and chair-rise times (p < 0.05). In addition there was a 99% compliance with the NMES programme. Following surgery there was a postoperative strength loss of approximately 50% in both the NMES and standard treatment groups. Here, only the NMES group demonstrated significant strength (53.3%, p =0.011) and functional recovery (p < 0.05) from 6 to 12 weeks post- TKA. While the concept of a voluntary prehabilitation programme is clearly a notable option the likely requirements for resources would limit the widespread availability in a clinical setting. As an alternative however the implementation of a home based solution such as a portable NMES device may well serve as a suitable training stimulus in the absence of suitable motivation/compliance or indeed availability of voluntary training facilities. Interestingly the most suitable application of NMES may be in patient subgroups for whom compliance with voluntary programmes is limited by pain during physiological movement. This observation would seem to be supported by the findings of Petterson et al 25 who observed enhanced clinical improvement (strength, activation and functional scores in stair climb and 6- minute walk test) in a cohort of 200 TKA patients who underwent a progressive strengthening programme with or without NMES. Clinical Implications: NMES is a viable option for clinicians looking to address strength loss and activation deficits among patients scheduled for TKA. The pre-operative status of the patient is a good marker to detect likely prognosis post surgically and clinicians should consider pre-operative programmes to optimize outcome. In the absence and or likely presence of poor patient compliance with voluntary exercise NMES may be considered as a suitable treatment option. NMES should not be considered as a replacement for voluntary programmes but more as an adjunct to optimize patient progression and in specific situations where poor patient adherence with exercise is present. CONCLUSION With the ever increasing growth of evidenced based practice, clinicians more than ever are bound by professional codes of conduct to implement intervention modalities that have a strong clinical justification and are supported by the research evidence. The emergence of NMES as an adjunctive therapy for muscle strengthening in knee rehabilitation is a well supported treatment approach that may well compliment existing voluntary strengthening programmes to produce more optimal patient outcome. This concurs with Paillard et al 27 who reported greater training adaptations in a combined (NMES + Voluntary) training approach compared to either modality on its own in post surgical knee patients. Specifically, Paillard et al 27 concluded that NMES is complementary to voluntary exercise because in the early phase of rehabilitation it elicits a strength increase, which 39

5 is necessary to perform voluntary training during the later rehabilitation sessions. This may be particularly pertinent when there is confounding variables such as pain and weakness in the early rehabilitation phase that may impact patient compliance with standard voluntary programmes. Overall it would appear that NMES is an evidenced based treatment that may well augment traditional rehabilitation strengthening programs in knee rehabilitation. In order to optimize the therapeutic effect clinicians should familiarize themselves with this modality in order to ensure its optimal therapeutic application. CONFLICT OF INTEREST: The author of this article is a member of research and development team at Biomedical Research (BMR), a company that design and develop electrical stimulation devices for varying treatment applications including knee rehabilitation. REFERENCES: 1. Melzack R, Wall P.D. Pain Mechanisms A New Theory. Science. 1965; 150: Synder-Mackler L, Landin Z, Schepsis AA, Young JC. Electrical stimulation of the thigh muscles after reconstruction of the anterior cruciate ligament. Effects of electrically elicited contraction of the quadriceps femoris and hamstring muscles on gait and on strength of the thigh muscles. J Bone Joint Surg Am 1991; 73: Rice DA, McNair PJ. Quadriceps arthrogenic muscle inhibition: neural mechanisms and treatment perspectives. Semin Arthritis Rheum 2010; 40: Mizner RL, Petterson SC, Stevens JE, Vandenborne K, Snyder-Mackler L. Early quadriceps strength loss after total knee arthroplasty: The contributions of muscle atrophy and failure of voluntary muscle activation. J Bone Joint Surg Am 2005; 87: Maffiuletti NA, Zory R, Miotti D, Pellegrino MA, Jubeau M, Bottinelli R et al. Neuromuscular adaptations to electrostimulation resistance training. Am J Phys Med Rehabil 2006; 85: Gondin J, Guette M, Ballat Y, Martin A. Electromyostimulation Training: effects on neural drive and muscle architecture. Med Sci Sports Exerc; 2005; 37: Jubeau M, Zory R, Gondin J, Martin A, Maffiuletti NA. Late neural adaptations to electrostimulation resistance training of the plantar flexor muscles. Eur J Appl Physiol 2006; 98: Petterson S, Snyder-Mackler L The use of neuromuscular electrical stimulation to improve activation deficits in a patient with chronic quadriceps strength impairments following total knee arthroplasty. J Orthop Sports Phys Ther 2006; 36; Snyder-Mackler L, Delitto A, Bailey SL, Stralka SW. Strength of the quadriceps femoris muscle and functional recovery after reconstruction of the anterior cruciate ligament. A prospective, randomized clinical trial of electrical stimulation. J Bone Joint Surg Am 1995; 77: Lyons CL, Robb JB, lrrgang JJ, Fitzgerald GK. Differences in quadriceps femoris muscle torque when using a clinical electrical stimulator versus a portable electrical stimulator. A technical report. Phys Ther 2005; 85: Laufer Y, Snyder-Mackler L: Response of male and female subjects after total knee arthroplasty to repeated neuromuscular electrical stimulation of the quadriceps femoris muscle. Am J Phys Med Rehabil 2010; 89: Gibson JN, Smith K, Rennie MJ. Prevention of disuse muscle atrophy by means of electrical stimulation: maintenance of protein synthesis. Lancet 1988; 2; Stefanovska SK, Binder-Macleod SA, Stackhouse CA, McCarthy JJ, Prosser LA, Lee SC et al. Change in muscle force following electrical stimulation. Dependence on stimulation waveform and frequency. Scand J Rehabil Med 1985; 17: Maffiuletti NA. Physiological and methodological considerations for the use of neuromuscular electrical stimulation. Eur J Appl Physiol 2010; 10: Porcari JP, Miller J, Cornwell K, Foster C, Gibson M, McLean K et al. The effects of neuromuscular electrical stimulation training on abdominal strength, endurance, and selected anthropometric measures. J Sports Sci Med 2005; 4: Feil S, Newell J, Minogue C, Paessler H. The effectiveness of supplementing a standard rehabilitation program with superimposed neuromuscular electrical stimulation after anterior cruciate ligament reconstruction. Am J Sports Med 2011; 39: Lieber RL, Silva PD, Daniel DM. Equal effectiveness of electrical and volitional strength training for quadriceps femoris muscles after anterior cruciate ligament surgery. J Orthop Res 1996,14: Kim KM, Croy T, Hertel J, Saliba S. Effects of neuromuscular electrical stimulation after anterior cruciate ligament reconstruction on quadriceps strength, function, and patient-oriented outcomes: a systematic review. J Orthop Sports Phys Ther 2010; 40 (7): Fitzgerald GK, Piva SR, Irrgang JJ. A modified neuromuscular electrical stimulation protocol for quadriceps strength training following anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther 2003; 33: Risberg MA, Lewek M, Snyder-MackleR L. A 40

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