The mechanical behavior of fourteen fresh human lumbar motion segments taken at autopsy from males with an average age of 29 yr was studied. Forces up to 1029 N were applied in anterior, posterior and lateral shear;​ and moments up to 95 Nm were applied in flexion, extension, lateral bending and torsion. In response to these loads endplate displacements up to 9 mm and rotations up to 18 were measured. Stiffness values ranged from 53 to 140 N mm⁻¹ in response to the shear forces and 6–11 Nm degree⁻¹ in response to the moments. Lumbar motion segments can develop significant passive resistances to loads in situations where they are allowed to undergo substantial deformations.