Electromyography of lumbar erector spinae muscles--influence of posture, interelectrode distance, strength, and fatigue.

The aims of the study were to obtain information (1) on surface electromyograms (SEMG) from the lumbar erector spinae muscles at different interelectrode distances and postures during short isometric contractions with constant force, (2) on the relationships between SEMG and extension force at different postures, and (3) on changes in SEMG during fatiguing isometric contractions at different postures and strengths. Six male subjects developed target forces in prone postures without gravity confounding the measurement of the extension torque. The angles between the constantly horizontal upper trunk and thighs were 90 degrees (P1), 135 degrees (P2), 170 degrees (P3), and 190 degrees (P4). Standard deviations of the distribution of SEMG amplitudes (RMS values), autoregressive (AR) time series models of the 15th order and spectral densities, including mean power frequency (MPF), were computed. Smaller interelectrode distances accompanied smaller RMS values and higher MPF. At a constant extension torque of about 110 Nm, RMS values and MPF increased from P1 to P4. Changes of interelectrode distance were of relatively minor importance, compared with the variation in the posture. With increasing torque, the increase in RMS values was steeper at P3 than at P2. The AR structure and MPF did not exhibit distinct effects of force. During sustained contractions at P2 and P3, only the highest force (mean = 140 Nm) at P3 caused a significant decrease of the MPF at the very beginning of the contraction. Endurance at P2 was greater than at P3. Higher forces and/or shorter muscles (P3) induced more pronounced and earlier relative decreases of the MPF and residual variance of AR models. Up to the "failure point", RMS values increased slightly, but without significant differences.