Rotator cuff fatigue and glenohumeral kinematics in participants without shoulder dysfunction.

CONTEXT Researchers have established that superior migration of the humeral head increases after fatigue of the rotator cuff muscles. In these studies, the investigators used imaging techniques to assess migration of the humeral head during statically held shoulder positions. Their results may not represent the amount of superior humeral head migration that occurs during dynamic arm elevation. OBJECTIVE To investigate the effect of rotator cuff fatigue on humeral head migration during dynamic concentric arm elevation (arm at the side [approximately 0 degrees ] to 135 degrees ) in healthy individuals and to determine the test-retest reliability of digital fluoroscopic video for assessing glenohumeral migration. DESIGN Test-retest cohort study. SETTING Research laboratory. PATIENTS OR OTHER PARTICIPANTS Twenty men (age = 27.7 +/- 3.6 years, mass = 81.5 +/- 11.8 kg) without shoulder disorders participated in this study. INTERVENTION(S) Three digital fluoroscopic videos (2 pre-fatigue and 1 post-fatigue) of arm elevation were collected at 30 Hz. The 2 pre-fatigue arm elevation trials were used to assess test-retest reliability with the arm at the side and at 45 degrees , 90 degrees , and 135 degrees of elevation. The pre-fatigue and post-fatigue digital fluoroscopic videos were used to assess the effects of rotator cuff fatigue on glenohumeral migration. All measurements were taken in the right shoulder. MAIN OUTCOME MEASURE(S) The dependent measure was glenohumeral migration (in millimeters). We calculated the intraclass correlation coefficient and standard error of the measurement to assess the test-retest reliability. A 2 x 4 repeated-measures analysis of variance was used to assess the effects of fatigue on arm elevation at the 4 shoulder positions. RESULTS The test-retest reliability ranged from good to excellent (.77 to .92). Superior migration of the humeral head increased post-fatigue (P < .001), regardless of angle. CONCLUSIONS Digital fluoroscopic video assessment of shoulder kinematics provides a reliable tool for studying kinematics during arm elevation. Furthermore, superior migration of the humeral head during arm elevation increases with rotator cuff fatigue in individuals without shoulder dysfunction.