Recent Advances in EMG Monitoring

Electromyographic (EMG) instruments have been used for many years to detect muscle action potentials through the application of surface or needle electrodes. Monitored activity can range from less than 0.1 microvolts to as high as several thousand microvolts. For example, relaxed muscles – such as those in the forehead region – generally exhibit voltages in the range of .75 to 3 microvolts, whereas tensed large muscles – such as the quadriceps – can exhibit activity as high as 2000 microvolts.

Even though most people are aware of the effects of gross muscle activity, lower chronic levels of activity often produce no visible movement or perceptible sensation and as a result go unnoticed, often resulting in muscle fatigue and pain.

Up until recently, input characteristics of EMG amplifiers and electrodes required considerable preparation of the skin surface and electrodes to obtain an accurate signal, especially in electrically noisy environments. This effort may still be worthwhile if research is being performed, but for day to day use of EMG, the preparation is cumbersome, generally inconvenient and often painful. I will discuss the recent breakthrough in EMG instrumentation later, but first I would like to review several of the newer applications for which EMG monitoring is being applied.

One of these fields is biofeedback. Biofeedback is simply the process of monitoring any physiological signal, and amplifying, conditioning, and displaying it to the monitored subject so that he or she can observe small changes in the signal, and gradually through trial and error, learn to control it voluntarily.

The goal of EMG biofeedback is to train subjects to increase, decrease, or stabilize muscle tension. Feedback is provided through proportional changes in a moving meter, bargraph, polygraph display, or an auditory tone. Training patients to decrease EMG activity below a predetermined criterion, is used clinically for tension headaches and other muscle pain. A similiar process in reverse is employed during muscle rehabilitation, where patients are trained to increase muscle activity of weak or flaccid limbs, such as may result from a stroke or accident.

For over 40 years, the electromyograph has been used to diagnose and treat neuromuscular disorders and to demonstrate improvements in muscle functioning following conventional treatment or neuromuscular stimulation. However, it is only since the mid 1960’s, and much more so in the last twenty years, that biofeedback has become an active component in the rehabilitation process.

Voluntary control of paralyzed or damaged muscles with residual functioning – such as results from stroke and nerve damage – can often be achieved through EMG biofeedback using either subdermal needle electrodes or surface electrodes for monitoring individual muscle groups. The process of surface monitoring is straight-forward. The body of the disabled muscle is identified, electrodes are placed over the length of the muscle and the subject is provided with visual and auditory feedback of increases in muscle activity while instructed to try to raise the tone or meter reading. Occasionally, a neuromuscular stimulator is combined with the EMG, such that when muscle activity exceeds a pre-set threshold, the stimulator is triggered, modelling a contraction. The threshold level is then increased slightly, and the process is repeated.

The greatest use of neuromuscular biofeedback has been in patients with hemiplegia and paraplegia. Paraplegia is usually due to spinal cord injuries resulting in paralysis below the waist, whereas hemiplegia is generally caused by a stroke resulting in muscular flaccidity or paralysis on one side of the body. Yet, most therapists working with these patients can attest that frequently there is some residual muscle activity in the single motor units, or even in totally paralyzed residual muscle tissue. Sensitive EMG devices can detect this low activity, and provide awareness and often a greater degree of control over the afflicted muscles.

In many reports with hemiplegic patients, following a combination of manual range of motion activity initiated by the therapist, with visual and auditory feedback of muscle activity, patients were able to produce strong voluntary contractions within one hour of training. Even in patients who had failed to respond to long term conventional treatment, clinically significant improvements were achieved over 8 to 12 months of auditory EMG training.

Reports of successful applications of EMG biofeedback in patients with injury or impairment of peripheral nerves rather than some central nervous system dysfunction include Bell’s Palsy (a facial paralysis of unknown origin), and crushing injuries. In several patients with Bell’s palsy, residual muscle activity was detected from muscles surrounding the mouth. Patients were successfully trained to increase the electrical activity, as well as to try to match the activity from the contralateral muscles displayed on a second EMG display.

EMG biofeedback has also been used successfully to retrain muscular control following accidental lacerations and subsequent surgical repair of hand tendons. Combined with conventional physiotherapy, the patients provided with biofeedback made the greatest improvements in voluntary range of motion activity.

Spasmodic torticollis, also known as wryneck, is indicated by the pulling of the neck to one side due to neck muscle contractions of unknown etiology. Treatment included EMG biofeedback affected sterno-cleido-mastoid muscle activity alone, or combined with contralateral flexion. Approximately 40% of the patients receiving this feedback were markedly improved at a 6 months follow-up.

Several other neuromuscular disorders have responded well to EMG biofeedback. Some of these include: training facial expressions to blind persons by providing auditory feedback of appropriate increases and decreases in muscle patterning; decreasing blepharospasm (involuntary eye blinking) activity; and training to enhance retention in fecal and urinary incontinence, both with vaginal and rectal electrode sensors, as well as with peri-anal electrodes.

In summary, the use of EMG biofeedback in neuromuscular rehabilitation, has demonstrated considerable effectiveness in a wide variety of neuromuscular disorders, and should certainly be considered as an inexpensive and positive adjunct to conventional therapy.

EMG biofeedback has also been used very successfully to produce voluntary reductions in muscles exhibiting excessive muscle activity for general relaxation, and specifically for conditions including tension and migraine headaches, insomnia, hypertension, and pain.

In disorders involving musculature in the region above the shoulders, generally the goal is to keep muscle activity at a ‘normal’ resting level, and to help patients learn to both perceive increases in muscle activity during the day, and to develop a strategy to reduce tension before it leads to ischemic muscles and pain. Similarly, chronic pain sites often respond well to decreased EMG activity, providing – in many cases – some relief from pain.

Dental applications include TMJ (temporomandibular joint pain and dysfunctions) and bruxism, which are dysfunctional clenching and grinding of the teeth which often leads to TMJ pain as well as tooth damage. EMG treatment for TMJ pain and bruxism uses EMG monitoring of the TMJ region, both to provide feedback for diurnal use to keep activity below a threshold level as well as to correlate the occurrence of excess activity with stress, and for nocturnal use, to set a threshold, above which an alarm will sound, awakening the patient and aborting the bruxing episode.

Recently, another application of EMG monitoring has become popular with chiropractors, orthopaedic specialists, and physiotherapists – muscle imbalances. The most frequent use of this new technology is to document these imbalances in a dynamic fashion, to register changes due to therapeutic interventions, and to exhibit them to patients and insurance carriers. This is done by using two EMG sensors placed on either side of the spine, while the instrumentation compares readings as the two sensors are moved from site to site down the spine.

Until the mid 80’s, the process of muscle scanning was difficult and slow, due to the requirements for skin preparation, attachment of the electrodes, and interpretation of the instruments. There are now several excellent instruments available which eliminate these shortcomings, in several ways. For example, MyoTrac and MyoTrac 22 use a miniature scanning sensor – MyoScan – with internal electronics which are so sensitive and advanced that generally no skin preparation is required, thereby allowing a site to be scanned within seconds. For those of you interested in specifications, the system has a d.c input impedance of greater than 1 million megohm (that’s a million, million ohms), the ability to detect signals of less than .08uV, and a CMRR of greater than 180 decibels, thereby allowing signal detection even in extremely electrically noisy environments.

The MyoTrac 2 can also be connected to interfaces which simply plugs into the serial port on IBM-PC compatibles providing immediate access to powerful ProComp software.

Other more sophisticated computerized biofeedback systems are available such as ProComp+TM and FlexCompTM which monitors up to 16 channels of EMG or other signals. Powerful and flexible DOS and Windows® software is available to provide clinicians and patients with meaningful and compelling displays, animations and statistics.

1Paper presented at June 1989 Conference of Physiotherapists of Quebec. (updated 5/2000)

2Thought Technology Ltd., Montreal Quebec, Canada.

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