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 subject's 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 Sørensen's 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.