Alterations in shoulder kinematics and associated muscle activity in people with symptoms of shoulder impingement

Background and Purpose. Treatment of patients with impingement symptoms com monly includes exercises intended to restorer "normal" movement patterns. E vidence that indicates the existence of abnormal patterns in people with sh oulder pain is limited. The purpose of this investigation was to analyze gl enohumeral and scapulothoracic kinematics and associated scapulothoracic mu scle activity in a group of subjects with symptoms of shoulder impingement relative to a group of subjects without symptoms of shoulder impingement ma tched for occupational exposure to overhead work. Subjects. Fifty-two subje cts were recruited from a population of construction workers with routine e xposure to overhead work. Methods. Surface electromyographic data were coll ected from the upper and lower parts of the trapezius muscle and from the s erratus anterior muscle. Electromagnetic sensors simultaneously tracked 3-d imensional motion of the trunk, scapula, and humerus during humeral elevati on in the scapular plane in 3 handheld load conditions: (1) no load. (2) 2. 3-kg load, and (3) 4.6-kg load. An analysis of variance model was used to t est for group and load effects for 3 phases of motion (31 degrees-60 degree s, 61 degrees-90 degrees, and 91 degrees-120 degrees). Results. Relative to the group without impingement, the group with impingement showed decreased scapular upward rotation at the end of the first of the 3 phases of intere st, increased anterior tipping at the end of the third phase of interest, a nd increased scapular medial rotation under the load conditions. At the sam e time, upper and lower trapezius muscle electromyographic activin increase d ill the group with impingement as compared with the group without impinge ment in the final 2 phases, although the upper trapezius muscle changes wer e apparent only during the 4.6-kg load condition. The serratus anterior mus cle demonstrated decreased activity in the group with impingement across al l loads and phases. Conclusion and Discussion. Scapular tipping (rotation a bout a medial to lateral axis) and serratus anterior muscle function are im portant to consider in the rehabilitation of patients with symptoms of shou lder impingement related to occupational exposure to overhead work.