Therefore, the present study, which included methods similar to those used in other studies, such as discriminant statistical analysis, and which focused on sedentary subjects, represents a contribution toward the application of the technique to clinical practice.

Furthermore, although studies have been carried out with the aim of analyzing the relationship between the EMG signal and low back pain, before considering the capacity of the method to achieve a straight diagnosis, the intrinsic limitations of the technique need to be analyzed. 3, 6, 12– 14 However, in studies in which the objective has been to use EMG to discriminate between people with and people without pain, there has been a tendency to focus on small samples of athletes 12 people who are not athletes have received little attention. 8– 11 Scientific evidence has shown that people with low back pain exhibit earlier manifestations of muscle fatigue in low back muscles than people who are healthy. In this sense, muscle fatigue is understood to be a continuous process that starts with the initiation of neuromuscular activity and that is capable of causing changes in electrical activity, electrical propagation, excitation-contraction coupling, and various elements of the contraction process. Thus, in recent years, it has been suggested that chronic low back pain is intimately related to localized muscle fatigue.

7 On the basis of this spectral variable, an index of muscle fatigue during isometric contractions has been proposed as a guide for evaluating low back pain. During the fatigue process, the MF tends to decrease, reflecting the decrease in the motor unit action potential firing rate during a muscle contraction. 6 This system is based on muscle fatigue induction monitored by surface electromyography (EMG) in the frequency domain, which allows mapping of the median frequency (MF) during a sustained muscle contraction. The Back Analysis System allows for more “objective” diagnosis and follow-up. In 1993, an alternative low back pain evaluation procedure, the Back Analysis System, was proposed. Therefore, sometimes double-blind tests are used. 5 All of these situations involve a great degree of subjectivity that is, they depend on the patient's report or the physical therapist's interpretation. 3, 4 The evolution of low back pain during physical therapy treatment also can be monitored with questionnaires and physical performance tasks. Traditionally, the clinical diagnosis of low back pain has been made by a physician or physical therapist using methods such as palpation, anamnesis, and the Borg scale. 1 It is one of the principal causes of absenteeism from work and impairment of the performance of daily activities, thereby affecting an individual's quality of life. Low back pain is a syndrome of multifactorial etiology that affects 6.8% of the world population at any given time.

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