Assessment of Muscle Pain in Humans–Clinical and Experimental Aspects

SUMMARY

In recent years new experimental research has been initiated on investigations of muscle pain. This is important as the socio-economic impact of musculoskeletal pain disorders is substantial and new insight into the pathophysiological mechanisms can help to prevent chronicity. Several experimental models have been used to induce and assess muscle pain in humans. Intramuscular [i.m.] injection of algogenic substances [brady kinin, serotonin, capsaicin, hypertonic saline], i.m. electrical stimulation, ischemia or eccentric exercise are some examples. Injection of hypertonic saline has been used extensively in the past because the quality of the induced pain is comparable to clinical muscle pain with localized and referred pain. Injections of chemical substances are, however, not suitable if the muscle pain should be turned on and off in a more rapid way. We have developed a model based on continuous 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 combination of, e.g., serotonin and bradykinin is specifically effective to cause muscular hyperalgesia to muscle pressure stimulation. For methods we have employed, it seems that the size of the referred pain is related to the intensity and duration of the ongoing muscle pain and most likely also to the degree of central hyperexcitability. In patients with chronic pain syndromes such as fibromyalgia or whiplash the pain responses to experimental muscle pain are substantially exaggerated compared to controls. Experimental models are valuable to assess basic aspects of muscle pain in volunteers and in patients with musculoskeletal disorders.

KEYWORDS:

• Experimental studies
• muscle pain
• assessment
• referred pain
• hyperalgesia