The Underlying Mechanism of Spinal Manipulative Therapy and the Effect of Pain on Physical Outcome Measures

Inclusion Criteria: * Self ambulatory participants with or without acute/chronic LBP. LBP is defined as pain or discomfort between costal margin and above the gluteal folds, with or without leg pain. * All the included LBP participants must have modified Oswestry Disability Index score \> 12% * Have the ability to lie prone for at least 20 minutes. * Asymptomatic participants should be free from LBP at the time of visit. Exclusion Criteria: * With medical 'red flag' conditions such as cancer, cauda equine syndrome, spinal infection, fracture or systemic disease * History of arm surgery, shoulder or arm pain that may hinder the arm lifting in prone * History of orthopedic or neurological surgery to the spine, pelvis or hips * Inflammatory or active infective processes involving spine or shoulder * Spondylolisthesis, ankylosing spondylitis, scoliosis of greater than 20 degrees (Cobb's angle) * Pregnancy * Osteoporosis * Neurologic deficit or signs of nerve root compression * Congenital spinal disorder (such as spina bifida), * Participation in competitive sports more than 3 times per week * History of spinal manipulation or lumbar multifidus stabilization exercise training within the past 4 weeks * Additional exclusion criteria for asymptomatic participants include: back pain in the preceding year, or exceeding 1 week; missed at least 1 work day because of back pain; and consultation for a back problem.