Evaluating Function/Impairment of Low Back Pain Using SEMG
Electrodiagnostic evaluations, with one type being electromyography (EMG), are commonly performed in diagnosing nerve and muscle pain disorders. Conventional EMG assessment is usually referred to as “needle EMG,” where a fine concentric or monopolar needle electrode is inserted into a particular muscle. Quantitative analyses are then conducted on needle insertion-generated activity, as well as motor unit action potential waveforms. Another less intrusive form of EMG evaluation is surface EMG (SEMG). Rather than inserting needles, which many patients complain of as being painful and too invasive, electrodes are placed on the skin using simple adhesive collars and overlying the muscle(s) being studied. One drawback of SEMG, as compared to needle EMG, is that only superficial muscle activity can be recorded. Nevertheless, as will be reviewed, SEMG has been found to be useful in evaluating movement, gait, postural, and functional disturbances.
In this present article, we will focus on the use of SEMG in evaluating the degree of physical impairment in low back pain (LBP). Indeed, whenever one evaluates painful spinal disorders such as LBP—especially in workers’ compensation or personal injury populations—as recently reviewed by Gatchel, Ricard et al,1 the degree of potential physical impairment needs to be considered for employment/ injury compensation issues. Impairment refers to the alteration of a person’s usual health status due to anatomic or pathophysiologic abnormalities. For back pain, it is frequently evaluated by measuring strength, lifting capacity, range-of-motion, aerobic capacity, as well as measures of human performance.2 A traditional problem, though, has been the lack of universal agreement about what measure(s) should be used in impairment evaluations. The American Medical Association identified only range-of-motion in earlier versions of its Guides to the Evaluation of Permanent Impairment, but it is no longer included in the most recent 6th Edition. There are growing annual costs associated with the diagnoses and care of musculoskeletal disorders such as LBP, amounting to tens of billions of dollars in the United States alone.3 In fact, in a most recent survey of expenditures among adults with back and neck problems, Martin et al reported a 65% increase (adjusted for inflation) of expenditures from 1997 to 2005, which was a more rapid increase than overall health expenditures.4 Thus, there is a great need to develop valid measures to objectively quantify physical function in patients with these disorders. Such objective measures would aid in assessing both physical impairment needed to address compensation issues, as well as use in determining a therapeutic endpoint following treatment.
At the outset, it should also be kept in mind that a traditional problem faced by evaluators attempting to objectively measure musculoskeletal disorders such as LBP—where there is often primarily soft tissue involvement—is that psychosocial factors frequently influence the experience/reporting of pain.1,3 Some examples of such psychosocial factors are neuromuscular inhibition due to fear-avoidance of movement, secondary gain, etc.5,6 Nevertheless, there is still an urgent need for the ability to quantify physical function with appropriate validity criteria in place, in order to help evaluate both impairment and a therapeutic endpoint following treatment.1 SEMG may fill this need.
The use of SEMG to evaluate and treat LBP has made significant advances over the past few decades. As noted earlier—unlike traditional electromyography, which uses fine-wire needles inserted into muscles to measure their electrical activity directly—SEMG involves the placement of gel-filled electrodes onto the skin to measure the sum of the action potentials generated by the muscles underlying the electrode.7 Skin is usually prepped with alcohol to remove surface oil and to ensure good contact between the skin and the electrode. The signal detected by the electrode is proportional to the force and contraction intensity of the muscle.8 There have been three main types of SEMG evaluations for LBP.9 The first is static measurement, where the isometric activity of the paraspinal muscles is measured in a fixed position. The second is dynamic measurement, where muscle activity is measured during motion. Dynamic movements typically utilize exercises or stretches with maximal or submaximal muscular contractions. The third approach combines static and dynamic measurement. An example of this is the measurement of the flexion-relaxation phenomenon. Measurements are taken at rest in the standing position, during the motion of forward flexion, at rest during full flexion, and during the extension phase of the movement as the patient returns to the standing position. These measurements are combined into ratios to determine the presence or absence of flexion-relaxation.10
“More recently, several EMG patterns have been hypothesized to be associated with chronic LBP. . . Lofland et al found increased resting SEMG activity in the paraspinal muscles and was able to correctly classify patients and controls when combined with a physical symptoms checklist.13”
The first use of EMG as a diagnostic tool was in the 1940s, although this was needle EMG rather than SEMG. Researchers found that trigger points (small areas of contracted tissue) displayed higher levels of electrical activity when compared to the surrounding tissue.11 Price et al subsequently found that back pain patients had more incidence of abnormally high and low SEMG activity and more right-left asymmetry during movement.12 This was thought to be the result of abnormal protective posture and ischemia to the unused muscles. Other researchers found that chronic LBP patients showed lower levels of paraspinal muscle activity during movement. This was thought to result from disuse or deconditioning of the spinal muscle groups due to fear of increased pain.11 Still others found increased levels of EMG activity when standing, particularly when standing for long periods. The results, though, were not consistently replicated by other researchers. These early studies con-tained several methodological flaws which may have affected the validity of the results. For example, most of the studies failed to report the length of time patients had been experiencing back pain, and some even failed to distinguish between acute and chronic back pain. The studies were also handicapped by technical difficulties involving the recording equipment and the electrodes. In addition, studies varied widely in the “gain” or amplification power settings used for recording signal amplitudes, making the different studies difficult to compare.11