Chapter 15 Neuromuscular and muscular electrical stimulation
INTRODUCTION
To apply electrical stimulation effectively it is important to revise some basic principles on how nerves are activated by electrical signals and how muscle contracts in response to these signals (see Chapter 5). It is also important to understand types of muscle fibre, patterns of normal recruitmentof muscle fibres and how these are reversed when electrical stimulation is used. This is covered in Chapter 13, which also identifies the differences between electrical stimulation and voluntary exercise and discusses the mechanisms underlying increases in strength with electrical stimulation. Chapter 13 also discusses the types of current that can be used to produce an electrical response in muscle and nerve and the parameters that can be varied to produce different responses.
This chapter examines the clinical areas in which electrical stimulation has been used and reviews the relevant literature to identify what is known about the clinical effects of treatment and why these may occur. The practical application of neuromuscular electrical stimulation (NMES) for innervated muscle and electrical muscular stimulation (EMS) for denervated muscle will be discussed.
TYPES OF UNIT
A multitude of commercially available electrical stimulation units (using a variety of current types) are marketed under a variety of names. Units can either be portable (battery operated) or line powered and there has been some debate as to which type of unit is better for muscle strengthening. Some investigators have argued that line-powered units can produce greater strength gains as these units can cause higher training contraction force levels, particularly when used for larger muscle groups such as quadriceps (Robinson 1995). However, more recent evidence suggests that both types of unit can produce similar increases in peak torque along with comparable levels of discomfort (Fitzgerald et al 2003). It is essential that the user checks that the machine to be used has available the parameters required for treatment, although this chapter will show that there is some lack of clarity about the most effective to use on all occasions. Figure 15.1 illustrates a range of portable, battery-powered muscle stimulation units and a typical mains powered multimodal device that includes muscle-stimulating currents.
NOMENCLATURE AND TYPES OF ELECTRICAL STIMULATION IN NERVE AND MUSCLE
The Clinical Electrophysiology Section of the American Physical Therapy Association established a unified terminology for clinical electrical currents: direct current, alternating current and pulsed current (American Physical Therapy Association (APTA) 1990). Although the use of this terminology would make the task of classifying commercial stimulators and interpreting the results of research studies more straightforward, it does not appear to have been widely adopted and inconsistencies remain in the literature with regard to nomenclature. Investigators use terms interchangeably and sometimes the precise form of electrical stimulation can be gleaned only by careful review of the particular paper. The following terms tend to be used interchangeably.
FUNCTIONAL ELECTRICAL OR NEUROMUSCULAR STIMULATION (FES/FNS)
This term is used when the aim of treatment is to enhance or produce functional movement. The level of complexity of FES can range from its use (with dual-channel stimulators) to enhance dorsiflexion during gait in children with cerebral palsy (Atwater et al 1991) to multichannel FES to activate many muscles to restore stance and gait in patients with paraplegia (Hömberg 1997). FES is covered in detail in Chapter 18.
THERAPEUTIC ELECTRICALSTIMULATION (TES)
This term has been used specifically to describe a form of electrical stimulation that produces sensory effects only (Beck 1997, Pape 1997, Steinbok et al 1997). Unfortunately, the term ‘therapeutic electrical stimulation’ has also been used by some investigators to differentiate between electrical stimulation applied to promote function (FES) and that applied for some other therapeutic function, for example NMES for children with cerebral palsy (Hazlewood et al 1994) and adults with spasticity and spinal cord injury (Chae et al 2000, Pease 1998).
ELECTRICAL STIMULATION (ES)
The meaning of the generic term ‘electrical stimulation’ is further complicated by the expanding use of electrical stimulation. Some investigators may not simply be applying it to strengthen weakened muscles but may also be investigating its role in promoting functional recovery (Pandyan et al 1997, Powell et al 1999, Steinbok et al 1997) and reducing spasticity in neurological conditions (Alfieri 1982, Hesse et al 1998, Vodovnik et al 1984).
EVIDENCE OF CLINICAL EFFICACY
Some early studies do not include a comparison group and therefore have not identified the benefit of electrical stimulation over other forms of intervention. For example, electrical stimulation has been shown to strengthen atrophied muscle significantly (Singer et al 1983, William & Street 1976) but in some studies with a matched voluntary exercise group had no additional benefit (Grove-Lainey et al 1983).
The subject numbers in some studies are too small. Small studies produce findings both for (Delitto et al 1988, Snyder-Mackler et al 1991) and against (Grove-Lainey et al 1983, Sisk et al 1987)a modality, neither of which provides reliableevidence.
Even some well-designed randomized controlled trials (RCT) make interpretation of the findings difficult as there is no consistency between electrical stimulation, exercise protocols or both. An example is differences in the ‘intensity’ used for NMES (‘intensity’ here applies to several parameters, i.e. not only the intensity of the applied current but also the frequency and duty cycle), which may account for the conflicting findings about the effectiveness of NMES to strengthen muscle. NMES has been found by Robinson (1995) to be significantly more effective at strengthening quadriceps than voluntary exercise, whereas Lieber et al (1996) and Paternostro-Sluga et al (1999) demonstrated that NMES was no more effective than voluntary exercise. However, the parameters used in the latter two studies were considered ‘low intensity’ by Robinson (1995), and so possibly not suitable for strengthening.
Even in studies in which the aim was to compare different types of electrical stimulation there are a number of varying factors, which makes it very difficult to establish which factor may be the important variable that leads to strengthening in a trial. Robinson (1995) showed that ‘high-intensity’ NMES (as defined above) caused significantly more strengthening than both ‘low-intensity’ NMES and voluntary exercise. Robinson (1995) argue that the difference in results can be accounted for by the fact that the ‘high-intensity’ group trained harder than the ‘low-intensity’ group. There is evidence that the higher the training contraction force, the greater the improvement in quadriceps strength (Robinson 1995) and these authors concluded that these results supported the use of line-powered units. However, it is important to note that the protocols for battery-operated and line-powered units in this study were very different. Some of the differences may be explained by the placebo effect of a bigger line-powered unit or the therapist–patient interaction, which was absent when patients used a portable unit at home.
Nevertheless, there does appear to be evidence for the clinical effectiveness of electrical stimulation for strengthening muscle, improving function and reducing tone in patient populations. The shortcomings in the research base, however, mean it is not possible to assign particular effects to certain interactions of parameters and only broad guidelines can be given. The following section examines the evidence for clinical efficacy in a number of areas; possible treatment parameters to achieve these effects are presented in the section on practical application (p. 238).
STRENGTHENING IN NON-NEUROLOGICAL CONDITIONS
Two mechanisms for strengthening muscle with NMES have been proposed. First, strength gains may be achieved in the same manner as standard voluntary strengthening programmes, which use a low number of repetitions with high external loads and a high intensity of muscle contraction (at least 75% of maximum). The second mechanism by which strengthening can occur is the preferential recruitment of type II phasic muscle fibres, which have a lower threshold for NMES (Delitto & Snyder-Mackler 1990, Lake 1992).
Electrical stimulation of healthy muscle
In general, the research evidence does not support the use of electrical stimulation for increasing either strength or endurance in healthy muscle. Although there is evidence that electrical stimulation is more effective than no exercise, it has been clearly shown that the combination of electrical stimulation and exercise is no more effective than exercise alone (Bax et al 2005). This systematic review of 17 articles (published between 1983 and 2000) investigated the effect of NMES on healthy quadriceps muscle; 14 of these articles compared NMES to no exercise and 10 compared it to volitional exercise. The results of the meta-analysis of 12/14 studies (235 subjects) confirmed the view from previous single studies that NMES is more effective than no exercise whereas there is nodifference (or indeed volitional exercise may be better) than NMES (meta-analysis, 8/10, n = 155 subjects) in unimpaired quadriceps. Further systematic review work is planned in this area and will update these findings in due course (Mizusaki et al 2006).
There is, however, some controversy over whether NMES is more effective for strengthening abdominal muscles than voluntary exercise. Although multiple-muscle-group NMES (which includes stimulation of the abdominal muscles) as used in muscle-toning clinics has proved totally ineffective in muscle strengthening (Lake 1988, Lake & Gillespie 1988), there is some evidence that NMES combined with voluntary exercise can be more effective than exercise alone for abdominal training in healthy subjects (Alon et al 1987, Alon & Taylor 1997). This latter finding may be explained by the fact that in many healthy adults the abdominal muscles are atrophic, or that use of NMES makes it easier to learn the correct activation of the abdominal muscles. A similar argument could be put forward for the fact that one study has shown that NMES is more effective than exercise for strengthening of the back musculature (Kahanovitz et al 1987). Further trials are therefore required to clarify these results but it is worth highlighting here that future trials should identify whether NMES is superior to voluntary exercise rather than showing NMES to be superior to no exercise, as in Porcari’s recent trial (2005).
Electrical stimulation of atrophied muscle
Electrical stimulation for strengthening is useful clinically in cases involving immobilization or contraindications to dynamic exercise to prevent disuse atrophy (Selkowitz 1989), in early rehabilitation by facilitating muscle contraction, and in selective muscle strengthening or musclere-education (Lake 1992).
Quadriceps
Recent review evidence concludes 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 (Arna Risberg et al 2004) and that voluntary muscle training programmes were only able to produce equivalent results when the latter’s intensity was higher (Lieber et al 1996, Paternostro-Sluga et al 1999). This would suggest that NMES is a very useful modality when the patient is unable to exercise voluntarily at high levels. However, in order to produce therapeutic strength changes the patient will need to tolerate intensive NMES training regimens and the clinician needs to use high power outputs to produce more positive results (Bax et al 2005).
Rheumatoid arthritis
A Cochrane Review by Pelland et al (2002) identified one RCT that showed that a patterned form of NMES (derived from the fatigued motor unit of the first dorsal interosseous in a normal hand) and NMES at 10 Hz had a significant effect on hand function compared to a control no-treatment group. However, these conclusions are limited by the low methodological quality of the single trial. More well-designed studies are therefore needed to provide further evidence of the benefits in the management of rheumatoid arthritis.
Pelvic floor muscles
Although studies examining the effect of NMES have focused largely on rehabilitation of knee injuries, it has also been shown to be useful in rehabilitation of patients with pelvic floor dysfunction, which can lead to faecal (Fynes et al 1999) and urinary incontinence (Sand et al 1995).
Urinary incontinence
A recent review in Clinical Evidence (Onwude 2005) included RCTs and three systematic reviews (Berghmans et al 1998, Herbison et al, 2002, Moehrer et al 2002) that investigated the effects of NMES on female urinary stress incontinence. Onwude (2005) concluded that there was no evidence for a difference between NMES + exercise, NMES + weighted cones or NMES + oestrogen supplements. However, NMES was significantly better than no treatment or sham treatment. It was noted that some of the included RCTs might have lacked the power to detect a clinically important difference (Onwude 2005) and therefore it would still be pertinent to use the parameters from apositive study to guide treatment. For example, significant improvements in urinary stress incontinence were found after 15 weeks of pelvic floor muscle stimulation (Sand et al 1995). A protocol pertinent to this topic can be found in the Cochrane library, which will provide an update on stress, urge and mixed urinary continence in due course (Berghmans et al 2004).
Faecal incontinence
There have been two Cochrane Systematic Reviews in this area (Hosker et al 2000, Norton et al 2006), which identifiedonly two RCTs (Fynes et al 1999, Mahony et al 2004) that evaluated the effectiveness of NMES on faecal incontinence. In the study by Fynes et al (1999), electrical stimulation was performed via an endoanal probe using low-frequency (20 Hz) and high-frequency (50 Hz) settings to target static (slow-twitch) and dynamic (fast-twitch) fibre activity with a 20% ramp modulation time. Over 12 weeks of treatment (one session per week),electrical stimulation combined with audiovisual biofeedback of muscle activity significantly improved continence scores (Fynes et al 1999). Mahony et al (2004) compared EMG biofeedback with biofeedback plus anal electrical stimulation at 35 Hz and a 20% modulation ramp over 12 weeks, and showed no differences between the two groups. Further work is therefore required to establish the effectiveness of NMES for this condition.
Electrical stimulation of denervated muscle
Despite more than a century of using EMS to stimulate denervated muscle, controversy over its use and efficacy continues (Davies 1983, Delitto et al 1995). This is primarily due to the variety of treatment protocols that have been used to assess care. Although there is no current consensus about the duty cycle that should be used, the frequency of stimulation or the number of contractions that should be employed, Snyder-Mackler and Robinson (1995) suggest that EMS can delay atrophy and its associated changes. However, they also note that there is no evidence to suggest that such a delay is significant in terms of final recovery.
USE OF ELECTRICAL STIMULATION IN ADULTS WITH NEUROLOGICAL CONDITIONS
The effects of electrical stimulation in neurological rehabilitation can be divided into improved motor function (Barecca et al 2003, de Kroon et al 2005), reduction in spasticity (Alfieri 1982, Hesse et al 1998, Vodovnik et al 1984, Weingarden et al 1998), increase in muscle strength (Glanz et al 1996, Powell et al 1999), increase in range of movement of the wrist (Pandyan et al 1997, Powell et al 1999) and reduction of shoulder subluxation in stroke patients (Ada & Foongchomcheay 2002). Some of these trials use FES, which is explored in more detail in Chapter 18.
Motor recovery
de Kroon et al (2005) reviewed 19 trials that evaluated the effect of electrical stimulation on motor control. Several types of electrical stimulation were explored: (1) the patient was not actively involved in the muscle contraction; (2) two forms of stimulation that relied on the patients either voluntarily contracting their muscle to a predefined level to trigger the electrical stimulation (EMG-triggered electrical stimulation) or moving the joint to a predefined position for the electrical stimulation to be triggered (positional feedback training). Twelve trials were RCTs, two non-RCTs, two had a multiple baseline design and three were case series (de Kroon et al 2005). Thirteen comparisons produced results in favour of electrical stimulation, whereas nine comparisons were in favour of the control group or showed no difference. So, on balance, the results would appear to be positive, although this needs to be tempered by the methodological shortcomings of some of these trials.
Strength
A meta-analysis of studies that used various forms of electrical stimulation in stroke patients showed that the strength of wrist, knee and ankle extensors was significantly increased after 3–4 weeks of treatment (Glanz et al 1996). More recent randomized controlled trials confirmed this finding of increased strength (de Kroon et al 2005).
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