ELECTROMYOGRAPHIC MEDIAN FREQUENCY CHANGES DURING ISOMETRIC CONTRACTION OF THE BACK EXTENSORS TO FATIGUE

Study Design. This was a cross-sectional study involving 229 healthy, back pain-free individuals. Objectives. To examine the relationship be tween electromyographic manifestations of fatigue and endurance time d uring isometric contraction of the back extensors to fatigue. Summary of Background Data. Despite the widespread use of electromyography to monitor muscle fatigue, its relationship with endurance time has not b een well investigated. Methods. Using skin-surface electrodes, electro myographic signals were recorded from thoracic (T10) and lumbar (L3) r egions of erector spinae during an isometric endurance test, and the r ate of change in median frequency of the electromyographic power spect rum (MF(GRAD)) was calculated. Results. MF(GRAD) was significantly hig her at L3 than at T10. The best predictor of endurance time was given by the greater MF(GRAD) observed at either region. MF(GRAD) calculated over a submaximal time period (50% total time or 60 sec) also correla ted significantly with endurance time. Women displayed a significantly longer endurance time and lower MF(GRAD) than men. Conclusions. Endur ance appears to be limited by the most fatigable region of the muscle group. MF(GRAD) is a suitable technique for monitoring back muscle fat igue, even when it is determined over a submaximal time period. The ba ck extensors of women are less fatigable than those of men when the sa me task is performed.