Persons with LBP often have reduced muscle strength and endurance, which may compromise the functional capacity of the spine and increase the likelihood of re-injury (Andersson, 1989). Historically, paraspinal muscle impairment has been quantitatively assessed by the use of dynamometers to supplement standard clinical assessment procedures (Roy SH, 1992). Assessments based on surface electromyographic (EMG) techniques have been proposed to overcome some of the problems inherent in the use of dynamometers for back muscle evaluation and classification (Merletti R, 1994).
Recent approach of EMG variable is derived from the frequency rather than from signal amplitude of the signal. This is partly due to the fact that during a sustained contraction, the EMG signal propagates at a slower velocity and undergoes an alteration in shape associated with changes to the depolarization zone of the muscle membrane (De Luca CJ, 1985). These phenomena are referred to as "myoelectric manifestations of fatigue," and are typically measured during a contraction as a decrease in the median or mean frequency (MF) of the EMG signal.
Scheme of conducting an EMG for back muscles:
The performance state of the paraspinals muscles can be described by variables obtained from an array of EMG electrodes (De Luca CJ, 1993). Here is a description of how to conduct it
1. The electrodes are strategically placed at specific anatomical locations corresponding to contralateral and ipsilateral paraspinal muscles. Differences in the variable mapped at the beginning and end of a fatigue-inducing contraction are analyzed to assess impairment.
2. The issue of muscle performance objectivity the test is limited to the performance of submaximal constant-force isometric contractions in which the duration of the contraction is predetermined.
3. Furthermore, the useful information from the EMG signals is not derived from a single muscle group or a single parameter, but rather is the result of the concurrent behavior or mapping of many co-active muscle groups. It is hypothesized that the subject is likely to be unaware of, and cannot volitionally control, parameters derived from such a measurement scheme.
Roy et al used this method of EMG-based approach to assess and classify paraspinal muscle impairment in persons with LBP. An EMG device used to implement this technique, referred to as a Back Analysis System (BAS).