Active trunk extensor contributions to dynamic posteroanterior lumbar spinal stiffness

Background: Assessments of posteroanterior (PA) spinal stiffness using mobilization apparatuses have demonstrated an increase in PA spine stiffness during voluntary contraction of the lumbar extensor muscles; yet, little work has been done to this degree in symptomatic subjects. Objective: To use a previously validated dynamic mechanical impedance procedure to quantify changes in PA dynamic spinal stiffness at rest and during lumbar isotonic extension tasks in patients with low back pain (LBP). Methods: Thirteen patients with LBP underwent a dynamic spinal stiffness assessment in the prone-resting position and again during lumbar extensor efforts. Stiffness assessments were obtained using a handheld impulsive mechanical device equipped with an impedance head (load cell and accelerometer). PA manipulative thrusts (approximate to 150 N, <5 milliseconds) were delivered to skin overlying the L3 left and right transverse processes (TPs) and to the L3 spinous process (SP) in a predefined order (left TP, SP, right TP) while patients were at rest and again during prone-lying lumbar isotonic extension tasks. Dynamic spinal stiffness characteristics were determined from force and acceleration measurements using the apparent mass (peak force/peak acceleration, kg). Apparent mass measurements for the resting and active lumbar isotonic task trials of each patient were compared using a 2-tailed, paired t test. Results: A significant increase in the PA dynamic spinal stiffness was noted for thrusts over the SP (apparent mass [17.0%], P = .0004) during isotonic trunk extension tasks compared with prone resting, but no statistically significant changes in apparent mass were noted for the same measures over the TPs. Conclusions: These findings add Support to the significance of the trunk musculature and spinal posture in providing increased spinal stability.