BACKGROUND: Spinal joint mobilization is a mainstay of clinical assessment of individuals with back pain. The clinician manually assesses stiffness and joint motion relative to segments above and below. Although clinical theory suggests that manually performed techniques can predict or detect intervertebral motion, this hypothesis remains untested. METHODS: Using a precision opto-electronic camera system and a custom spine testing machine, we measured intervertebral range of motion, neutral zone motion and three-dimensional flexibility in eight T5-T8 cadaveric specimens (mean age=81 years). We then measured stiffness when a cyclic posteroanterior load was applied at the spinous process of T6 using a servohydraulic material testing machine (Instron 8874), simulating the posteroanterior spinal mobilization technique. FINDINGs. There was a strong significant inverse relationship between stiffness during cyclic posteroanterior loading of T6 and flexion or extension range of motion of T6 relative to T7 (r=-0.88, P<0.01, extension; r=-0.81, P=0.01, flexion), and T6-T7 flexibility in all six directions. INTERPRETATION: Stiffness during simulated central cyclic posteroanterior mobilization in the cadaveric midthoracic spine is inversely correlated with flexion and extension range of motion and three-dimensional flexibility at the level at which the technique is applied. These findings provide biomechanical support for the inclusion of specific joint mobilization in the assessment of older adults with back pain.