Electromyographic studies of erectores spinae: symmetrical postures and sagittal trunk motion.

During standing, the electromyographic (EMG) activity of erectores spinae is minimal. However, this activity increases when an object is held in front of the body. The size of the EMG response to this loading is dependent on both the weight of the object and the position of its centre of mass relative to the lumbosacral junction. Interpretation of EMG results becomes complex when movements of large amplitude are involved. Problematically, erectores spinae become electrically silent during trunk flexion, at a position which requires high tension in post-vertebral structures. This emphasizes that EMG does not record the state of passive connective tissue tension, which is sufficient to support the vertebral column in some flexed postures.