This study will use microdialysis to investigate myofascial pain. This pain is characterized by "trigger points" (exquisitely tender spots) in a group of tense muscle fibers that extend from the trigger point to the muscle attachments. Trigger points in the trapezius, a large muscle lying between the neck and shoulder, are typically caused by emotional stress, postures such as hunching shoulders, certain activities like using a telephone receiver without elbow support, or by wearing certain articles such as a heavy coat or heavy purse. Microdialysis uses a very thin needle probe (about the size and shape of an acupuncture needle) to collect and measure chemicals directly from soft tissue. Analysis of these chemicals will show whether changes in the tissue around a muscle with trigger points are confined to that muscle, or if these changes also occur in more distant muscles.

The study will examine two types of trigger points. An "active" trigger point causes pain or other abnormal symptoms and often causes problems with movement. A "latent" trigger point often causes movement problems without causing pain. Many healthy adults have latent trigger points.

People between 21 and 65 years of age with the following characteristics may be eligible for this study: 1) no neck pain or trigger points in either upper trapezius muscle; 2) no neck pain but a latent trigger point in at least one upper trapezius muscle; or 3) neck pain of less than 3 months' duration and an active trigger point in at least one upper trapezius muscle. Participants undergo the following procedures:

• Physical examination of the muscles of the neck and shoulder area, testing strength and range of motion, and response to palpation to find trigger points.
• Pain inventory. Subjects complete a questionnaire for measuring pain and its intensity, location, quality, causes, relievers, and associated symptoms. The questionnaire is filled in before and after each microdialysis procedure.
• Microdialysis in upper trapezius muscle. An electrode patch is placed on either side of the site for insertion of the microdialysis probe and another electrode is placed on the outer edge of the shoulder. The electrodes are used to measure any electrical activity that occurs with insertion or movement of the probe. The subject lies face down and the probe is inserted in the upper trapezius muscle. It remains in place for 5 minutes while chemical substances are collected from the muscle. It is then advanced about 1.5 cm deeper into the muscle until a twitch response is obtained and remains in place for 10 more minutes while substances are collected.
• Microdialysis in the gastrocnemius muscle (large muscle of the calf). The same procedure for the upper trapezius muscle is done in the calf muscle.