In-vivo quantification of the viscoelastic properties of the lumbar spine

Current clinical assessment of the spine is rather limited, focusing on joint mobility. The purpose of this study was to develop an in-vivo method to measure the viscoelastic properties of the lumbar spine and to understand how trunk motion contributes to postural control in unstable sitting. Thirty-two healthy participants were recruited. The participants were sitting on a custom-made chair that was able to swing freely in the sagittal plane. Each participant was asked to find balance after the seat was tilted backward about 10 and 20 degrees and released. The movement of the lumbar spine and pelvis were recorded. The seat was placed over the force plate. Seat, spine and hip angles and moment acting on the lower trunk were derived. Stiffness and damping coefficient were derived using a second order linear model. Stiffness was found smaller in female participants for both the experiment. Damping coefficient was low across all experiments. Increasing the angle of perturbation (from 10 to 20 degrees) the stiffness was decreasing in all participants. The spine angle and the hip angle represent how the participants responded to the perturbation. Peak to peak value of the spine and hip showed that the motion about the hip was greater than the lumbar spine motion and this, indicates the greater role the hip plays in responding to the seat motion and therefore in postural control while sitting.