January 2002

Using Surface Electromyography

By Glenn Kasman, PT

Surface electromyography assessments of quadriceps muscle activity may be combined with manual and functional procedures during evaluation of patients with knee dysfunction.

Using Surface Electromyography

A multidisciplinary tool, sEMG can be a valuable asset to the rehab professional's muscle assessment arsenal.

Physical medicine practitioners routinely assess muscle activity while working with patients who have musculoskeletal and neuromuscular impairments. Surface electromyography (sEMG) is the recording of muscle action potentials with skin surface electrodes, used as an indicator of muscle recruitment. Both the magnitude and timing pattern of muscle recruitment can be displayed, along with the activity of certain muscles in relation to others.

Recording is noninvasive and painless. Clinicians incorporate sEMG to take much of the guesswork out of assessing muscle function. Muscle activity can be objectified, quantified, and documented during standard examination procedures as well as the performance of functional tasks. The efficacy of particular exercises and instructions in corrective movement patterns can also be assessed quickly and objectively.

In addition, the sEMG display is a rich source of motor learning information for patients. Using sEMG, patients gain access to muscle feedback that is far more sensitive than that obtained with the intrinsic senses acting alone. This feedback helps patients learn to relax overly tense muscles, better activate weak muscles, or change the coordination pattern among agonist, antagonist, and synergist muscles. Applications cover a broad scope of situations including athletic injury, repetitive strain and worker injury, injury due to motor vehicle accident, chronic pain management, neurological rehabilitation, and incontinence.

Vast Array of Uses
A multidisciplinary modality, sEMG is used by physical therapists to address movement dysfunction. Psychologists employ sEMG to intervene with problems attributed to excessive psychophysiological arousal. Occupational therapists incorporate sEMG into functional job analyses. Rehabilitation nurses and therapists treat incontinence related to dysfunction of the pelvic floor muscles with sEMG feedback. Physiatrists, orthopedists, and neurologists may be interested in sEMG procedures to enhance diagnosis of movement disorders as well as monitor the effects of medicines and surgical interventions designed to impact muscle activity.

The utilities of adding sEMG to practices in physical medicine and rehabilitation are numerous. Clinicians may increase the sophistication of patient evaluations through quantification of muscle activity. Third-party payors may be more inclined to authorize treatments when their efficacy can be objectively validated and documented. Patients are engaged by the sEMG feedback display and may be able to learn patterns of movement control that reduce pain and increase function. A myriad of opportunities for clinically applied research are present, including investigation of movement control and psychophysiological phenomena, as well as study of the effects of orthotics, manual therapies, and therapeutic exercises. Lastly, the addition of sEMG may differentiate a practice from competitors, and support the needs of referrers and payors in documenting muscle dysfunction.

SIGNAL DETECTION AND PROCESSING
Motor activity is subserved by commands that are generated in the central nervous system and transmitted along alpha motor neurons to the periphery. Following chemical transmission across the neuromuscular junction, action potentials are produced along the sarcolemma, and electrical excitation becomes coupled to sarcomere shortening via complex chemical and micromechanical processes. Fundamentally, electrodes placed in the vicinity of excitable membranes will detect action potential events. sEMG electrodes detect the algebraic sum of voltages associated with muscle action potentials within their pickup zone.¹ The sEMG signal represents the relative level of recruitment of an ensemble of motor units that underlie the electrodes.

The basics of the sEMG system have been described for clinicians in numerous sources.^1-5 Electrodes are usually in the shape of 0.5-1.0 cm discs coated with silver-silver chloride. Each recording channel detects activity from one muscle site and is composed of two active electrodes and a reference electrode. Active electrodes tend to be spaced with their centers about 2.0 cm apart. The difference in electrical charge between each active electrode and the reference makes for inputs to a differential amplifier with high input impedance. Filters are then used to pass frequencies related to muscle activity and to reject frequencies that are associated with electromagnetic noise. The signal may be viewed in its raw plus/minus form or full wave rectified, in which the plus-minus variations of the waveform are converted into a unidirectional signal. Several methods exist to smooth the peaks and valleys of the rectified waveform to ease inspection as well as to quantify the amplitude of the processed muscle signal.

Patient Assessment
The amplitude of the sEMG signal is usually expressed as some number of microvolts, noted as a series of relatively instantaneous measurements or averaged or integrated over a clinically meaningful period of time. Amplitude analyses are conducted to evaluate the magnitude and timing pattern of muscle activity. Inferences are drawn regarding a muscle's role in affecting a particular posture or movement and how pathologic processes alter that role. The sEMG activity of a homologous muscle pair or that of an agonist, compared with its antagonists or synergists, is examined to assess muscle balance.

Imbalance occurs when the relative stiffness of muscles that participate in concert to execute a specific movement is inappropriately coordinated.6 Muscle imbalance is presumably a function of both faulty central nervous system motor control and peripheral factors such as inefficient length-tension relationships and passive myofascial compliance. sEMG studies may therefore provide insight into the active component of muscle imbalance and can be linked by clinicians to the results of physical examination. Untoward motor programming may be influenced by nociception, perception, effect, beliefs, nervous system lesions, segmental and suprasegmental motor reflexes, sympathetically mediated reflexes, metabolic and nutritional issues, and a host of factors related to articular function and periarticular connective tissues. Analysis with sEMG can help clinicians identify relationships between muscle impairments and other physical and psychological impairments. Classification of impairments with observed functional limitations and disabilities can then be used to drive treatment planning in a thoughtful way.⁷

Clinically less common than amplitude analyses, investigation in the frequency domain is performed to study muscular fatigue. The voltage waveform seen at the sEMG display can be mathematically decomposed into a plot of component frequencies. Interestingly, the frequency spectrum of the sEMG signal shifts in a reliable way with fatigue during high intensity isometric muscle contractions.¹ That is, the frequency spectrum becomes compressed toward slower values due to complex neuromuscular and metabolic changes associated with sustained muscle activity. The frequency shift begins as a contraction and is continued beyond a short time, preceding the actual loss of force, and continues as force declines. This means fatigue monitoring may have certain advantages over other measures8 and successfully discriminates spinal pain patients from control subjects with impressive accuracy.^9-11

TRAINING
In addition to clinical and kinesiological assessments, the sEMG display is often used as a means of feedback for motor learning by patients.6 Muscle cues produced by an sEMG device are much more sensitive than those derived from a subject's intrinsic sensory apparatus. Initially, a patient may have little idea how to change the activity of a muscle that is not under intuitive voluntary control. The patient may not possess a suitable motor programming scheme to achieve the goal, ie, increased activation of one muscle relative to another, and may have difficulty distinguishing correct performance from error. Cues on the sEMG display are obvious and serve as a reference of correctness. Thus, the patient becomes able to evaluate various motor strategies for those that meet the goal.

Successful strategies are repeated and ineffective strategies are discarded. The patient identifies a progressively smaller subset of effective motor behaviors over time. sEMG feedback is used cognitively to label subtle intrinsic sensations as indicative of changes in muscle activity. Through the repeated association of artificial, extrinsic cues from the sEMG machine with natural kinesthetic sensations, an intrinsic reference of correctness is formed. The learner forms mature sensory identification and motor programming schema, and can then achieve the goal independently.

Surface electromyographic feedback training is readily combined with exercise prescription and instruction in corrective movement patterns.

The clinical objectives of feedback training with sEMG are relatively straightforward. Patients with muscle hyperactivity use feedback cues to reduce muscle output. For example, a patient with neck pain and upper trapezius hyperactivity could attend to the sEMG display to help improve posture, self-regulate responses to emotional stressors, or identify ergonomic improvements and motor skills for the workplace. A different patient with headaches and temporomandibular pain might produce chronic masseter and temporalis hyperactivity associated with chronic jaw clenching. Specific sEMG feedback techniques could be used to promote kinesthetic awareness, muscle relaxation, and reduction of parafunctional behaviors involving the temporomandibular region.

Patients with muscle hypoactivity incorporate sEMG feedback while learning to increase muscle recruitment. For example, a patient might show quadriceps inhibition after knee surgery that delays progress along a standardized clinical pathway. That patient could watch an sEMG display as his or her postoperative exercises are performed. Exercise variants, cognitive strategies, and adjunctive therapeutic agents would be trialed for those that facilitate quadriceps activity. Successful techniques would then be repeated while the patient attempts to raise the sEMG amplitude to match a goal marker on the display, set to progressively higher microvolt values over time.

Besides training greater and lesser muscle responses as separate objectives, patients may learn to simultaneously increase the activity of a hypoactive muscle while decreasing that of a hyperactive muscle. This coordination training takes place between an agonist with its antagonists or synergists. For example, the patient with neck pain and upper trapezius hyperactivity might also show hypoactivity of the lower trapezius. This patient would try to raise the amplitude of the lower trapezius signal and decrease the amplitude of the upper trapezius signal, during arm elevation maneuvers and simulated functional tasks. Successful training would presumably result in better muscle balance for upward rotation and stabilization of the scapula, leading to improved biomechanical relationships throughout the neck and shoulder girdle.

MUSCLE MONITORING
sEMG techniques offer distinct conveniences compared with other means of muscle monitoring. Because the methods are noninvasive and painless, their use tends to be readily accepted by patients and is generally quite safe. Although lead wires are used to connect the electrodes to the main instrument body (telemetry systems can be substituted if necessary), patients routinely are free to assume any position they desire, including those for functional tasks. Recordings are feasible where dynamometers would be impractical, for example with investigation of facial muscles or selective examination of the vastus components of the quadriceps. sEMG recordings can even be made underwater.¹²

The sEMG display resolves changes in the magnitude and timing of muscle activity with far greater precision than a clinician's or patient's eyes and hands. An entire range of activity levels can be captured for inspection, from voltages associated with activation of one or a few motor units to maximal effort recruitment. Within certain limits, the activity of particular muscles or muscle groups can be isolated. Setup becomes simple once the practitioner is experienced.

Limitations
Like any clinical technique, sEMG has limitations. It is important to recognize that sEMG does not measure force, pain, anxiety, muscle length, joint position, or anything else other than voltage. With proper recording technique, the voltage pattern displayed with sEMG is representative of muscle recruitment. Inferences regarding clinical syndromes, however, are complicated by an interplay of neuromuscular, biomechanical, and psychological factors that may be difficult to separate.

Interpretation of sEMG activity can be subject to error brought about by certain effects of electrode configuration, tissue impedance, and other circumstances inherent to each recording setup. Thus, clinicians who wish to perform sEMG procedures should become well versed in technical aspects of electrophysiological recording as well as models for clinical intervention. sEMG skills are usually acquired in postprofessional continuing education courses and at meetings of professional societies as well as through review of scientific journals, textbook resources, and electronic media.

Cost and Reimbursement
Technological advances have enabled commercial sEMG units to be miniaturized for ambulatory recordings of one to four channels of muscle activity. Patients can perform functional activities in the workplace for a protracted time and the resultant sEMG data are downloaded for analysis. Portable units are easily incorporated into therapeutic exercise programs in the clinic or gym, or prescribed for home programs. Commercial systems that incorporate a desktop computer are capable of simultaneous recordings from eight or more channels; sophisticated statistical processing of amplitude, timing, and frequency variables; and a plethora of options for patient feedback. Software engineers continue to develop more powerful products while exploiting graphical user interfaces so that operation becomes easier. Manufacturers and vendors are able to deliver sEMG products to consumers with a cost value that outstrips the pricing of earlier models.

Reimbursement for sEMG procedures by health care insurers varies with the practice setting, payor, type of provider, medical diagnosis, functional status of the patient, and quality of provider documentation. In inpatient settings, providers should code sEMG services in whatever way makes sense for their dominant prospective payment, cost, or fee-based reimbursement systems while complying with regulatory standards.

Services in outpatient settings are typically coded using the American Medical Association's current procedural terminology (CPT) system. Patient examinations with sEMG are billed generally with a standard evaluation code for the provider discipline and may qualify for use of a higher level or specialized evaluation code with a greater fee schedule payment. sEMG feedback may be used as an adjunct to other treatments and bundled into those procedures or, depending on the circumstances, dedicated biofeedback CPT codes may be submitted. sEMG may add value even in capitated environments by adding precision to the diagnostic process, accelerating neuromuscular reeducation, and facilitating patient participation, return to function, and overall reduction of health care consumption.

Discussion in the community of health care providers who use sEMG extends to many patient populations. Numerous schools of thought can be found that embrace principles from psychology and movement science. sEMG procedures should not be employed solely because a patient has chronic dysfunction or pain, but rather when aberrant muscle activity is suspected as being a primary contributory factor to dysfunction and evaluation with sEMG will impact treatment planning. Feedback training with sEMG may or may not then be appropriate to facilitate motor learning by the patient. The important point is that sEMG should be used to enhance functionally meaningful outcomes that reduce patient disability, in ways that support patient satisfaction, while controlling the financial and social costs of care.

Controversy

A search using "surface AND electromyography" on the National Library of Medicine's PubMed engine returns a clear upward trend in the number of annually based hits for the period of 1985-2001. The trend in research is paralleled by developments in sEMG equipment and clinical procedures. This is not to suggest that clinical applications of sEMG are without controversy. For example, meta-analyses regarding superiority of sEMG feedback training with conventional therapies in stroke populations are mixed and confounded to some degree by methodological differences and limitations in experimental design.^13-15

There is an insufficient number of randomized, controlled designs to conclusively include or exclude sEMG feedback training's use with some musculoskeletal problems.16 Nevertheless, the fundamental validity and reliability of sEMG in movement system and psychophysiological analyses are well accepted^17-19 and recognized in professional practice guidelines.²⁰ It seems probable that the future will bring new applications of sEMG in performance enhancement in noninjured populations, new developments in forensic medicine, as well as refined approaches to sEMG with musculoskeletal and neuromuscular injuries.

Care providers may choose to make sEMG an integral part of their practice or reserve its use for occasional investigations of muscle activity and patient training. In any event, sEMG provides a unique means of monitoring muscle activity. Each clinician's repertoire of skills may be broadened by inclusion of sEMG, while patients are provided with powerful opportunities for motor learning. N

Glenn Kasman, PT, is the director of Physical Therapy at Good Samaritan Hospital, Puyallup, Wash. He has also coauthored two books on the subject of sEMG. He can be reached via email: kasmagl@goodsamhealth.org.

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