Surface electromyography (sEMG) is a method of measuring the electrical activity of muscles. Muscle tissue conducts electrical potentials similar to the way nerves do and the name given to these electrical signals is the muscle action potential. Surface EMG, therefore, is a method of recording the information present in these muscle action potentials. The scientific literature has documented the relationship between muscle force production and sEMG amplitudes as well as the relationship between muscle fatigue and the frequency information present in the sEMG signal. The knowledge of these documented relationships along with the basic timing information available from sEMG allows the experienced clinician to determine levels of muscle asymmetry, postural disturbances, dysfunctional muscle patterns, recruitment of muscles, guarding, and perhaps most importantly muscle fatigue. This type of information can prove to be most valuable in the assessment and treatment of myofascial pain, as found in work-related and/or traumatic injuries. Advances in computer technology have allowed sEMG information to be quickly processed and presented in visually meaningful ways thereby making it a viable and useful tool in the assessment and treatment of WMSD (RSI) injuires. Now clinicians and patients can obtain immediate feedback about the state of their muscles.

One can measure the electrical signal of the muscles by two methodologies: indwelling (fine wire or needle) electrodes or surface electrodes. Indwelling electrodes are inserted directly into the muscle fibres while surface electrode are placed on the skin overlying the muscle. There are pros and cons to each of these methods, suffice it to say here that for a number of reasons surface electrodes provide a more quick and comfortable method without significantly sacrificing accuracy or completeness. In order to collect sEMG data electrodes are placed on the surface of the skin over the muscle of interest. The placement of these electrodes is determined via a number of factors, including the orientation of the muscle fibres, depth of the muscle tissue, anatomical landmarks, and avoidance of certain muscle components. The skin is generally prepared with an alcohol swab and highly conductive electrodes are put in place with the use of conductive gel and tape. This type of preparation reduces skin impedance and ensures a clean signal for processing. Once the signal is picked up from the muscle it is amplified by a differential amplifier which helps to further reduce "noise" (unwanted common signal) from the sEMG signal. Further processing can now take place to present the sEMG in meaningful formats. The most common processing strategies for sEMG are: rectification, integration, and fast Fourier transformation (FFT). Rectification of the EMG signal leaves only the positive values without decreasing the information available in the signal. Rectified EMG has often been shown, in scientific studies, to be linearly related to the force produce by the muscle. Integration is the process of determining the area under the rectified EMG curves. Integrated EMG signals often does not give more information than rectified EMG but can be easier to compare in a visual format. Fast Fourier transformation allows the examination of the frequency components of the EMG signal. The frequency spectrum of the EMG signals has been shown, in controlled studies to be sensitive to muscle fatigue. The raw (unprocessed) EMG signal can also provide meaningful timing information in muscle use patterns.

The timing, force and fatigue data provided by sEMG can be extremely valuable to the experienced clinician. This information can be used as one part of a full assessment protocol to determine if a patient has significant muscle asymmetries, problems with guarding, possible postural disturbances, and significant muscle fatigue. When this data is taken into consideration along with that of complete physical and psychological assessments from qualified clinicians it becomes possible to develop and implement an effective rehabilitation or preventative program. Not only is sEMG important as an assessment tool it can also be used as an adjunct to available treatment modalities. The immediate presentation of EMG data (processed and/or raw) can be used for relaxation therapy and biofeedback sessions. These additional treatment techniques can be extremely effective at decreasing myofascial pain levels by helping to correct the underlying faulty muscle patterns which can be a factor in causing pain.

As mentioned above, the scientific findings to date have shown that sEMG can provide useful information for the clinician interested in myofascial pain. In fact, recent research has shown that fatigue measures from sEMG are extremely sensitive to injury states. These studies are beginning to show how important sEMG can be in the diagnosis of myofascial pain problems. However further research must still be completed to determine the differences between patient and uninjured populations. There are a number of interesting research questions concerning the clinical uses of sEMG that still must be addressed. The RSI Clinic is currently involved in ongoing research of this nature.