Correlation between electromyographic spectral changes and subjective assessment of lumbar muscle fatigue in subjects without pain from the lower back

Objective. The purpose of this study was to correlate objective measurements of muscle fatigue in the lower back to the subjectʹs own assessment of fatigue. Design. Muscle fatigue in the lower back was assessed in healthy subjects using electromyography (EMG), endurance time and the Borg scale. Background. Muscle fatigue, measured with EMG and endurance time, in the lower back, is significant for patients with pain in the lower back. Methods. Fifty healthy subjects participated. EMG was detected from the lumbar extensor muscles during a modified Sørensenʹs test, an isometric contraction for the back extensors until exhaustion. During the test, subjects rated their subjective fatigue on a Borg CR-10 scale. Results. Borg scale ratings correlated with endurance time (0.68) and EMG median- and mean power frequency slopes (0.41–0.50). At a Borg rating of 3, median- and mean power frequency and endurance time were reduced by 30%. At a Borg rating of 5, median- and mean power frequency and endurance time were reduced by 50%. At a Borg rating of 7, median- and mean power frequency and endurance time were reduced by 60–70%. Conclusions. Significant correlation between the Borg scale, EMG and endurance time suggests a close relationship between subjective and objective assessment of muscle fatigue.