- 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.