FATIGUE OF THE ERECTOR SPINAE MUSCLES - A QUANTITATIVE ASSESSMENT USING FREQUENCY BANDING OF THE SURFACE ELECTROMYOGRAPHY SIGNAL

Study Design. The authors investigated fatigue-induced changes in the frequency content of the surface electromyographic (EMG) signal from t he erector spinae muscles. Objectives. The objective of the study was to understand the EMG changes in fatiguing muscle and to obtain a reli able index of fatigue. Summary of Background Data. Power spectral anal ysis has been used increasingly in recent years to monitor muscle fati gue, but parameters other than the mean or median frequency have recei ved little attention. Methods. Thirty-five healthy volunteers particip ated. They pulled upward with constant force on a handlebar attached t o a floor-mounted load cell while the EMG signal from the erector spin ae was recorded at the levers of T10 and L3 at 1024 Hz; 1.0-sec ''wind ows'' of the signal were analyzed using fast Fourier transforms, and t he resulting power spectra were divided into 10-frequency bands betwee n 5 Hz and 300 Hz. The median frequency, total power, and peak amplitu de-of the spectra were also calculated. Changes in the frequency conte nt of the EMG signal were examined during submaximal contractions of d ifferent intensity and duration. Results. Median frequency decreased s teadily during the contractions, whereas total power and peak amplitud e increased. The most-repeatable and linear index of change was the in crease in the EMG signal in the 5-30 Hz frequency band. The middle-to- high frequency component of the EMG signal increased during the early stages of the contractions, but decreased as the endurance limit was a pproached. Conclusions. Changes in the 5-30 Hz band of the EMG power s pectrum provide a more reliable and linear index of fatigue in the ere ctor spinae muscles than do changes in median frequency. In the erecto r spinae, the early effects of fatigue appear to be delayed by the rec ruitment of additional motor units.