The purpose of this study is to compare changes in spinal stiffness and back muscle activity between spinal manipulative therapy (SMT) responders, non-responders and asymptomatic participants. Additionally, the investigators wanted to compare the amount of lumbar degeneration between SMT responders, non-responders and asymptomatic participants. This study also determines if the presence of pain modifies post-SMT spinal stiffness and back muscle activity.

SMT is a common manual therapy for treating patients with mechanical low back pain (LBP). SMT is defined as a high velocity, low amplitude thrust technique. Specifically, the clinician stands beside a supine patient. The patient is passively side-bent towards the side to be manipulated. The clinician passively rotates the patient and then delivers a high velocity, low amplitude thrust to the anterior superior iliac spine in a posteroinferior direction. SMT is indicated for patients with LBP judged to have spinal hypomobility or malalignment. Recently, our collaborators have discovered that LBP subjects who benefit from SMT can be identified prior to treatment by a five-item clinical prediction rule. Compared to non-responders, those who respond to SMT have: 1) more than 30% decrease in the modified Oswestry Disability Index score, 2) a significant and immediate decrease in spinal stiffness, and 3) a concurrent change in lumbar multifidus muscle activity. Taken together, the physical changes experienced by SMT responders alone provide an unique opportunity to better understand the mechanisms underlying SMT.

Since spinal stiffness is an important physical indicator of SMT response, it is important to understand the causal relation between pain, paraspinal/trunk muscle activity and spinal stiffness. Although research has shown positive correlation between pain and muscle activity, and between paraspinal muscle activity and spinal stiffness, the causal relation between pain, paraspinal/trunk muscle activity and spinal stiffness remains unknown. Given this background, an induction of temporary benign experimental pain to asymptomatic individuals can help clarify such causal relation and improve our understanding of physical responses in responders following pain resolution by SMT.

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.