Fundamentals of muscle pain, referred pain, and deep tissue hyperalgesia.

A recent Danish national survey including approximately 16500 adults showed that 51% had experienced pain or unpleasantness of musculoskeletal origin within the 2 weeks preceding the interview (10, 11). The survey found that 12% took prescribed and 13% had used over-the-counter pain relievers due to musculoskeletal symptoms within the 2-week period. Approximately 10% of all visits to a clinical practice in the USA were found to be due to musculoskeletal pain complaints (12). In another Danish survey with 1504 subjects, the prevalence of muscle pain per se was reported to be 37% and 65% for males and females, respectively (13). In accord, an American survey of 1254 adults found a 53% prevalence of muscle pain (14), and about 9% of the respondents lost at least one working day within the previous year due to muscle pain. Moreover, 23% of pain patients visiting the general practitioner had muscle and soft-tissue pain, in contrast to only 0.1% with skin pain (15). Thus, musculoskeletal disorders with accompanying muscle pain have a considerable socioeconomic impact, in addition to the distress caused to the patients and relatives. Today the treatment of persistent musculoskeletal pain by currently available drugs is not optimal and an important factor in this is the lack of in-depth information about the neurophysiological mechanisms involved in deep tissue pain (16, 17). Deep tissue pain is a diagnostic and therapeutic problem, and further insights into the peripheral and central neurophysiological mechanisms are needed to improve diagnosis and therapy, and to pursue the implementation of a mechanism-based approach for treatment planning. Muscle pain is typically accompanied by a variety of characteristics such as referred pain to distant somatic structures, muscle hyperalgesia, disturbed sleep pattern, limited movements, autonomic reactions, and psychological effects (18–22). The present work focuses on the somatosensory manifestations of muscle pain; that is, localized muscle pain, referred pain, and deep tissue hyperalgesia. Experimental pain research is one way to attain new knowledge on the mechanism involved in muscle pain. By standardized induction and assessment of muscle pain in healthy subjects, information can be achieved regarding the normal nociceptive system. Most experimental pain research has, however, focused on cutaneous pain, although deep tissue pain is clinically more important.