Quantification of the Lumbar Flexion-Relaxation Phenomenon: Comparing Outcomes of Lumbar Erector Spinae and Superficial Lumbar Multifidus in Standing Full Trunk Flexion and Slumped Sitting Postures

Outcomes for FRP during standing full flexion in asymptomatic participants appeared to be more robust and were not affected by the choice of either lumbar erector spinae or superficial lumbar multifidus. Conversely, during slumped sitting, FRP occurrence varied substantially depending on choice of muscle, although onset angles were relatively consistent between muscles. Although the choice of one muscle over the other may be warranted, it may be prudent to examine both muscles during FRP investigations in sitting postures, in order to fully characterize the behavior and activation patterns of the lumbar musculature.