Muscles do not perform motor or sensory functions unless energized by nerves originating in the spine. Problems can arise from muscle strains and tears, tendinitis where the muscles attach to the bones, or if nerves emanating from the spine and feeding the body's muscles are compressed, impinged, or avulsed. Additionally, the body has bursas and cartilage which absorb shocks between bones and muscles and bones and bones. Overstressed bursas can become inflamed. Further, synovial tissues further allows muscles to glide over one another.
Synovial fluids can fill joints creating pain and restricted motion.
The body is good at creating pain which is referred from another part of the body. The layperson, responding intuitively, seldom sees the connections between the location of the pain and its probable source. General practitioners, acting as guard keepers in most HMO and PPO healthcare structures, aren't trained in sports medicine and can miss or be unaware of these counterintuitive relationships, particularly as they relate to muscles or connective tissues and elude X-ray detection. Orthopedist further may not be aware of certain myofascial disorders which, at first appraisal, may appear to have an orthopedic source and resolution. Economics, competition, insurance companies, highly specialized training, old habits and antipathies between medical disciplines inhibit a true interdisciplinary approach to human performance and injury.