EMG activity of low back extensor muscles during cyclic flexion/extension

Cyclic trunk flexion/extension is known to be a risk factor for low back pain or disorders. Changes in the in vivo low back musculature associated with cyclic flexion/extension have not been adequately examined. The purpose of this study was to investigate the effects of cyclic flexion/extension on muscle activity of the low back extensors by quantifying changes in activation timing and mean amplitude of electromyography (EMG). Trunk flexion angle and EMG were recorded during 30 cyclic flexion/extension movements over a 5 min period, and during isometric extension performed before and after the 5 min period. Mean EMG in isometric extension increased (30% increase in average) significantly after cyclic flexion/extension, confirming a transfer of extension moment from viscoelastic passive tissues to the extensor muscles in isometric extension. However, in the extension phase of cyclic flexion/extension, a significant delay in the re-activation and a decrease in the mean amplitude of EMG were observed. The results of this study, together with findings in previous research, suggest that the biomechanical effects and potential risk associated with cyclic flexion/extension may be sensitive to the task demands. Further research is needed to investigate how different moment generating components function together to compensate for the loss of tissue stiffness under varying task conditions.