The Babinski sign (extensor plantar reflex) and clonus are signs of trouble in the brain. Clonus may also be seen in spinal cord damage. The motor pathway of nerve supply to muscles involves two neurons (nerve cells), an upper one in the brain that connects to a lower one in the spinal cord. Damage to the upper motor neuron releases abnormal reflex responses such as these.
It’s good to check for these reflexes when evaluating back pain. When found, they indicate a problem that is not coming from something in the lower back. One thing to rule out, with a thoracic MRI, would be a tumor in that area compressing the spinal cord causing lower back pain.