Healthy volunteers will be accepted for that specific comparison group. Those volunteers must demonstrate no evidence of any systemic pathology, inflammatory, psychiatric, neurological or pain condition.  Inclusion Criteria:  diagnosis of primary chronic low back pain (performed in the initial evaluation or previously received by a healthcare professional), with or without leg pain minimum of three months of duration  Exclusion Criteria:  diagnosis of neuropathic pain in the lower extremity evidence of specific pathology affecting the lumbar spine diagnosis of psychiatric disorder or pain disorder affecting the hand/thumb or in the vicinity of the lumbar area intake corticosteroids, opioids or anticytokine medications pregnancy having been treated with spinal manipulation in the previous 12 months