Changes in lumbar lordosis modify the role of the extensor muscles

Study Design. Fiber angles of longissimus thoracis and iliocostalis lumborum at L3 were documented in vivo, using high resolution ultrasound, with the lumbar spine in neutral curve and when fully flexed. Objectives. To evaluate the effect of changes in lumbar curvature on the mechanics of these muscles. Background. Full flexion modifies the failure tolerance of the lumbar spine, determines the load distribution among muscle and passive tissues, and modulates the types of tissue damage that occur. Related to this issue are the possible changes in muscle line of action with full flexion which changes the ability of the spine to support shear loads. Methods. Nine normal men and 5 normal women were scanned in three positions: (1) an upright standing posture; (2) with the hips flexed to approximately 30 degrees and the spine fully flexed; (3) hips flexed but the spine returned to a neutral curvature. Results. Mean longissimus/iliocostalis fiber angles for upright standing, hips flexed-spine flexed, and hips flexed-spine neutral lordosis were 25.7 degrees, 10.7 degrees and 28.3 degrees, respectively. Conclusions. Anterior shear load on the lumbar spine has been recently shown to be highly related to the risk of reporting a back injury. Bending forward allowing the spine to fully flex changes the line of action of the largest lumbar extensor muscles compromising their role to support anterior shear forces.