3-DIMENSIONAL SCAPULAR ORIENTATION AND MUSCLE-ACTIVITY AT SELECTED POSITIONS OF HUMERAL ELEVATION

Abnormal scapular kinematics and associated muscle function presumably contribute to shoulder pain and pathology. An understanding of scapul ar kinematic and electromyographic profiles in asymptomatic individual s can provide a basis for evaluation of pathology. The purpose of this study was to describe normal three-dimensional scapular orientation a nd associated muscle activity during humeral elevation. Twenty-five as ymptomatic subjects, 19-31 years old, were evaluated. Digitized coordi nate data and surface electromyographic signals from the trapezius (up per and lower), levator scapulae, and serratus anterior were collected al static positions of 0, 90, and 140 degrees of humeral elevation in the scapular plane. The scapula demonstrated a pattern of progressive upward rotation, decreased internal rotation, and movement from an an teriorly to a posteriorly lipped position as humeral elevation angle i ncreased. Electromyographic activity of all muscles studied increased with increased humeral elevation angles. Differences between mean valu es at all elevation angles for all variables were significant (p < .05 ), except for the lower trapezius between the 90 and 140 degrees humer al angles. The results of this study suggest assessment of scapular ti pping and internal rotation as well as upward rotation may be necessar y to understand pathologies oi the shoulder that are related to abnorm al scapular kinematics.