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A biomechanical study was performed to determine the consequences of a simulation of muscle forces on the loads imposed on the functional spinal units.No biomechanical study has investigated the effect of incorporation of agonist and antagonist muscle forces on the loading of functional spinal units.Spinal disorders and low back pain are increasingly becoming a worldwide problem. Traditional conservative therapies are intended to strengthen the muscles of the trunk using a judicious regimen of physical exercises.Eighteen whole, fresh-frozen human cadaveric lumbar spine specimens (L2-S2; average age, 53.4 years) were tested in a spine tester using pure flexion-extension, lateral bending, and axial moments. The effects of coactivation of psoas and multifidus muscles on L4-L5 mobility were simulated in vitro by applying two pairs of corresponding force vectors to L4. The segmental stability was defined by the correlation of an applied moment to the resultant deformation as shown in load-displacement curves, and the range of motion was defined as the angular deformation at maximum load.The coactivation of muscles was accompanied by a 20% decrease in the range of motion(i.e., a significant increase in stability) during lateral bending and axial moments. Application of flexion-extension moments and muscle coactivation resulted in a 13% increase in the sagittal range of motion.The action of the intersegmental agonist and antagonist muscles biomechanically increases the overall stiffness (stability) of the intervertebral joints in axial torque and lateral bending, whereas it may destabilize the segment in flexion.