Experimental simulation of cyclic, six degree-of-freedom, gait and sit-to-stand loading waveforms using a six-axis joint motion simulator

In vitro spinal load simulation is a critical tool for understanding the biomechanics of the spine. However, cyclic loading for these in vitro experiments is commonly limited to one or two degrees-of-freedom (DOF). While easily adopted, these methods do not capture the 6DOF loading associated with spinal movements and activities. The goal of this study was to develop and evaluate a comprehensive in vitro testing method to apply cyclic 6DOF gait and sit-to-stand (S2S) loading profiles to lumbar spinal motion segments. Eight cadaveric lumbar segments were subjected to 1DOF pure moment testing in load control for simulated flexion-extension, lateral bending, and axial rotation, followed by 6DOF gait and S2S simulations for short-duration tests (5 cycles). Gait and S2S tests were compared at quasi-static (0.5Nm/s) and dynamic (5Nm/s) loading rates. 6DOF gait simulations were also simulated over a longer-duration test (10,000 steps), with comparison of the pre- and post-cycle movement response examined. Load control testing with iterative learning control (ILC) was employed to ensure load accuracy during longer-duration gait simulations. Following 10,000 steps, no significant changes in spinal range of motion were observed. Root mean square error remained below simulator load cell resolution, except during the short-duration dynamic tests of gait and sit-to-stand in compression and flexion-extension. During longitudinal gait testing, convergence was reached at 4-10 % of total test length in all actuators. This study highlights the feasibility of simulating real-world loading conditions, such as walking and S2S activities, to better evaluate lumbar spine biomechanics under physiologically-derived loading conditions.