RESUMO
BACKGROUND: Muscle activity is studied during trunk stabilization exercises using electromyograms (EMG) in time domain. However, the frequency domain analysis provides information that would be important to understand fatigue process. OBJECTIVE: To assess EMG of lumbar multifidus (LM) and erector spinae (ES) muscles, in time and frequency domains, during back bridge exercise. METHODS: Nineteen healthy young men performed the exercise for one minute and EMG was monitored by surface electromyography. Normalized root mean square (RMS) value and spectral median frequency (MF) were compared between beginning and final epochs of test. The dynamics of the MF during whole test was also obtained by short-time Fourier transform. RESULTS: RMS values were about 30% of maximum voluntary contraction, and LM muscle showed greater MF than ES, which did not decrease at the final of exercise. However, the slope of MF was significant mainly for LM. CONCLUSIONS: Muscle activation of 30% is sufficient to keep lumbar stability and is suitable to improve muscular endurance. The significance of MF slope without decreasing at the final of exercise indicates challenging muscular endurance without imply on high fatigability. Due to lower muscular demand, this exercise might be recommended for trunk stabilizing for low back pain patients.