Palmer hypothesized that vertebral joint misalignments, which he termed vertebral subluxations, interfered with the body's function and its inborn ability to heal itself. D. D. Palmer repudiated his earlier theory that vertebral subluxations caused pinched nerves in the intervertebral spaces in favor of subluxations causing altered nerve vibration, either too tense or too slack, affecting the tone (health) of the end organ. D. D. Palmer, using a vitalistic approach, imbued the term subluxation with a metaphysical and philosophical meaning. He qualified this by noting that knowledge of innate intelligence was not essential to the competent practice of chiropractic. This concept was later expanded upon by his son, B. J. Palmer, and was instrumental in providing the legal basis of differentiating chiropractic from conventional medicine. In 1910, D. D. Palmer theorized that the nervous system controlled health:
Among people seeking back pain relief alternatives, most choose chiropractic treatment. About 22 million Americans visit chiropractors annually. Of these, 7.7 million, or 35%, are seeking relief from back pain from various causes, including accidents, sports injuries, and muscle strains. Other complaints include pain in the neck, arms, and legs, and headaches.