Each specimen was mounted and tested in a biaxial Instron machine, undergoing nondestructive left and right axial rotation to peak moments of 4 Nm at a constant rotation rate of 8 deg. 
Instron
A motion tracking system (Optotrak) measured 3D displacements of individual vertebrae.  The range of motion, neutral zone size and stiffness were calculated from the moment-rotation curves and intervertebral range of motion was calculated from Optotrak data.



Optotrak