OBJECTIVES: To assess low back muscles fatigue and to determine the demographic variables associated to fatigue on these muscles. METHODS: The electromyographic (EMG) activity of the right iliocostal (R-IL), left iliocostal (L-IL), right multifidus (R-MU) and left multifidus (L-MU) of 18 volunteers was recorded during submaximal isometric contractions. Root mean square (RMS) and median frequency (MF) values were correlated with isometric endurance time (IET). Positive RMS and negative MF slopes indicated occurrence of muscle fatigue. Multiple regression procedures were performed in order to verify the demographic variables related with the muscle fatigue. RESULTS: Fatigue was identified in all muscles and contraction intensities (p<0.01), except for MU-E at 5% in RMS slope analysis. Significant differences were found between the endurance time of 5% and 15% (p=0.01), 5% and 20% (p=0.0002). Higher levels of fatigue were found bilaterally in the multifidus muscles in the MF slope analysis. The combination of endurance time, age and body mass of the volunteers was identified as the determinant factor for the occurrence of muscle fatigue in the assessed muscles. CONCLUSIONS: Interventions designed to treat low back conditions must consider the several factors causing fatigue of muscles in this region.
Biomechanics; Electromyography; Spine; Muscle fatigue; Low back pain; Clinical protocols
