A Trigger Point
The trigger point model states that unexplained pain frequently radiates from these points of local tenderness to broader areas, sometimes distant from the trigger point itself. Practitioners claim to have identified reliable referred pain patterns which associate pain in one location with trigger points elsewhere. There is variation in the methodology for diagnosis of trigger points and a dearth of theory to explain how they arise and why they produce specific patterns of referred pain.
Compression of a trigger point may elicit local tenderness, referred pain, or local twitch response. The local twitch response is not the same as a muscle spasm. This is because a muscle spasm refers to the entire muscle contracting whereas the local twitch response also refers to the entire muscle but only involves a small twitch, no contraction.
Among MDs, many specialists are well versed in trigger point diagnosis and therapy. These include physiatrists (physicians specializing in physical medicine and rehabilitation), family medicine, and orthopedics. Osteopathic as well as chiropractic schools, however, include trigger points in their training. From Wikipedia*