Symmetry of electro- and acoustic myographic activity of the lumbar paraspinal muscles in normal adults.

The symmetry of paraspinal muscle activity was examined in 15 healthy adults (aged 31-60 years). Acoustic myography (AMG), which provides a measure of force, was recorded with electromyography (EMG) to assess electromechanical relationships during contraction. Bilateral recordings of EMG and AMG were made over the paraspinal muscles at the level of the 4th lumbar vertebra during a fatiguing test manoeuvre. Subjects were strapped to a hinged couch in the prone position. When the upper part of the couch was lowered, subjects maintained the upper body (above the anterior, superior iliac spines) unsupported, in the horizontal position for 60 sec. The EMG and AMG signals were full-wave rectified, integrated (IEMG, IAMG) and recorded on an ink-jet oscillograph during the fatigue test. By 60 sec of activity, IEMG had increased (right 122% +/- 8.7; left 125% +/- 11.8; mean +/- 1 SD as a percentage of initial values) while IAMG (reflecting force) remained unaltered (right 99% +/- 8.2; left 100% +/- 5.3). The IAMG:IEMG ratio (reflecting efficiency of activation) thus declined (right 0.81 +/- 0.08; left 0.80 +/- 0.08). The similarly of changes on both sides of the spine have quantified normal symmetry of paraspinal muscle activity and could be used to assess asymmetry in patients with spinal pathology.