The time-varying response of the in vivo lumbar spine to dynamic repetitive flexion

Objective. To quantify the time-varying stiffness and kinematic responses of the in vivo lumbar spine exposed to dynamic repetitive flexion movements. Design. Changes in in vivo passive lumbar moment-angle relationships were monitored in response to dynamic repetitive flexion. Background. While previous in vitro studies have provided conflicting evidence on the effects of repetitive flexion movements on the stiffness of the lumbar spine, no previous studies have quantified the time-varying changes of the in vivo lumbar spine to dynamic repetitive flexion. Methods. Subjects lifted and lowered a 4.5 kg load over two barriers at a rate of 7 lifts per minute for 1.5 h inducing at least 80% of the lumbar flexion range of motion. Prior to lifting and at 30 min intervals passive moment angle relationships were obtained by pulling the subject into flexion on a customized frictionless table. Results. Repetitive lifting induced a decreasing stiffness trend after 30 min, followed by a recovery towards initial stiffness levels with further loading. The trends were non-significant for all measures studied. Conclusions. The results indicate that after 30 min of lifting, creep within the passive tissues may allow workers to exceed their initial range of motion, altering joint mechanics and loading patterns potentially leading to an increased risk of developing low back pain.