Asymmetry of erector spinae muscle activity in twisted postures and consistency of muscle activation patterns across subjects

Study Design. An experimental study involving a repeated measures design was performed. Objectives. To describe the muscle activation pattern in extending from a twisted posture; to evaluate interindividual variation of this pattern; and to study the effects of fatigue development. Summary of Background Data. Extending the trunk from a twisted posture is a risk factor for low back pain. Methods. Twelve male subjects performed contractions at 20%, 40%, 60%, and 100% of maximum in a neutral position and 15°, 30°, and 45° twisted posture. Surface electromyography of six bilateral back muscle tracts was recorded during these 5-second contractions and during sustained contractions at 0° and 30° and 40% of maximum. Results. The data show an asymmetry in muscle activation, increasing with angle of twist. Activity was higher contralateral to the direction of twisting. Linear regression analysis with angle and torque level against electromyographic amplitude yielded a significant fit in 92% of the data sets. In 75% of the subjects, the regression lines were similar. The divergence of the activation pattern in the other subjects appears to be explained by a higher co-contraction level of abdominal muscles. Fatigue caused a shift in activity to the more laterally situated muscle tracts. Conclusions. Twisting causes an asymmetry in back muscle activation. Resulting stress concentrations in spinal motion segments, and a reduced spinal stability could underlie the observed relationship between the task investigated and low back pain. Fatigue may reduce further spinal stability. Some subjects display a divergent activation pattern, which probably involves higher spinal compression forces.