Validation of lumped-parameter models of the human lumbar spine using in vivo driving-point compliance data during...

Previously published 5-DOF and 21-DOF lumped-parameter models of the human lumbar spine predict displacement responses to posteroanterior (PA) forces, yet neither has been validated against in vivo frequency response function (FRF) data. This study compared model-predicted driving-point compliance spectra to in vivo experimental data and estimated patient-specific mechanical properties through inverse analysis. Triaxial bone-pin accelerometers were mounted to the L2 and L4 spinous processes of two patients undergoing lumbar decompression surgery. An Impulse iQ instrument delivered 178 controlled impulses at the L3 spinous process across six conditions. Driving-point compliance FRFs were computed using the H1 spectral estimator. Patient-specific PA stiffness and damping ratios were estimated via coherence-weighted logarithmic least-squares optimization. Both patients exhibited a dominant first compliance resonance near 49 Hz, consistent with the 5-DOF prediction of 46 Hz (<7% error). The second and third modes (89-101 Hz and 107-110 Hz) agreed with model predictions of 84 and 120 Hz. Optimized stiffness converged to $52.3\pm 3.5$  kN/m (published: 50 kN/m), with damping ratios of 0.049--0.109. Time-domain Newmark-beta simulations predicted peak L3 displacements of 1.10--5.60 mm for a 3.2 ms impulse, consistent with cadaveric and in vivo spinal manipulation studies. These results provide the first in vivo validation of lumped-parameter spine models and demonstrate patient-specific mechanical property estimation from compliance FRF data.