Electromyographic Median Frequency Changes During Isometric Contraction of the Back Extensors to Fatigue

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Study Design.This was a cross-sectional study involving 229 healthy, back pain-free individuals.Objectives.To examine the relationship between electromyographic manifestations of fatigue and endurance time during 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 been well investigated.Methods.Using skin-surface electrodes, electromyographic signals were recorded from thoracic (T10) and lumbar (L3) regions of erector spinae during an isometric endurance test, and the rate of change in median frequency of the electromyographic power spectrum (MFGRAD) was calculated.Results.MFGRAD was significantly higher at L3 than at T10. The best predictor of endurance time was given by the greater MFGRAD observed at either region. MFGRAD calculated over a submaximal time period (50% total time or 60 sec) also correlated significantly with endurance time. Women displayed a significantly longer endurance time and lower MFGRAD than men.Conclusions.Endurance appears to be limited by the most fatigable region of the muscle group. MFGRAD is a suitable technique for monitoring back muscle fatigue, even when it is determined over a submaximal time period. The back extensors of women are less fatigable than those of men when the same task is performed. [Key words: fatigue, electromyography, endurance, erector spinae, median frequency] Spine 1994;19:1223–1229

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