Assessment of muscle pain in humans - Clinical and experimental aspects

In recent years new experimental research has been initiated on investigati ons of muscle pain. This is important as the socioeconomic impact of muscul oskeletal pain disorders is substantial and new insight into the pathophysi ological mechanisms can help to prevent chronicity. Several experimental mo dels have been used to induce and assess muscle pain in humans. Intramuscul ar [i.m.] injection of algogenic substances [bradykinin, serotonin, capsaic in, hypertonic saline], i.m. electrical stimulation, ischemia or eccentric exercise are some examples. Injection of hypertonic saline has been used ex tensively in the past because the quality of the induced pain is comparable to clinical muscle pain with localized and referred pain. Injections of ch emical substances are, however, not suitable if the muscle pain should be t urned on and off in a more rapid way. We have developed a model based on co ntinuous intramuscular electrical stimulation where the local and referred pain vanish immediately when the stimulation is terminated. Infusion of a variety of algogenic substances have been tested and combinat ion of, e.g., serotonin and bradykinin is specifically effective to cause m uscular hyperalgesia to muscle pressure stimulation. For methods we have em ployed, it seems that the size of the referred pain is related to the inten sity and duration of the ongoing muscle pain and most likely also to the de gree of central hyperexcitability. In patients with chronic pain syndromes such as fibromyalgia or whiplash the pain responses to experimental muscle pain art: substantially exaggerated compared to controls. Experimental mode ls an valuable to assess basic aspects of muscle pain in volunteers and in patients with musculoskeletal disorders.