Effects of repetitive movement on range of motion and stiffness around the neutral orientation of the human lumbar spine

In loading experiments on the lumbar spine, typically three consecutive loading cycles are applied of which the third cycle is used for analysis. The aim of this study was to investigate whether the use of ten instead of three loading cycles reduces effects of viscoelastic behavior in the assessment of range of motion (ROM) and stiffness around the neutral orientation of the human lumbar spine. To this end, twelve cadaveric human lumbar spines (L1-L5) were obtained (mean age: 76.9 years). Before testing, the spines were subjected to a compressive load of 250 N for 1 h. To each spine, ten consecutive loading cycles were applied (−4 Nm to+4 Nm) in flexion and extension (FE), lateral bending (LB) and axial rotation (AR). The ROM and stiffness within the neutral zone were calculated per motion segment (L2-L3, L3-L4 or L4-L5) from load-displacement data. It was found that the ROM increased significantly (all p<0.001) in all directions after three (FE: 0.07 degree/1.0%, LB: 0.08 degree/1.5%, and AR: 0.04 degree/1.5%) and after ten loading cycles (FE: 0.20 degree/2.9%, LB: 0.16 degree/3.3%, and AR: 0.09 degree/3.3%). Stiffness was not significantly affected, but varied considerably over cycles. Although effects were small, assessment of the tenth cycle instead of the third cycle reduces viscoelastic effects in repeated measurements of ROM, because the spine is closer to a steady state condition, while averaging over loading cycles would improve the assessment of stiffness estimates.