Osteopathic physicians may use regional diagnostic rotation tests of the spine during physical examinations for patient evaluations. Clinical judgments of these responses relate to symmetry as a criterion for mobility. This first part of a two-part study reports the authors' investigation of regional ipsilateral myoelectric activity during responses to active and passive shoulder and trunk rotations on the left- and right-side muscles with subjects seated. Results indicate symmetry existed in both active and passively induced regional rotation tests. Further, the distribution (profile) of ipsilateral myoelectric activity in the thorax reflected a bell-shaped activity curve that peaked at thoracic levels 6 and 7. This profile element demonstrates remarkable similarity between volitional and physician-induced rotation motions. These myoelectric data justify symmetry and profile as tangible standards for making clinical judgments of regional responses to spinal rotation diagnostic tests.