Contraction of the scapular retractors in different shoulder-abduction angles while standing may alter the length–tension relationship of the muscles, resulting in variable activation of the parts of the trapezius muscle. However, how the shoulder-abduction angle affects the activity of the trapezius muscle parts during scapular retraction in a standing position is unknown. The shoulder-abduction angle is often modified during scapular-retraction exercise, especially to produce lower UT/MT and UT/LT ratios. Scapular retraction in an upright standing position is more functional as it provides core activation as well as lower extremity muscle activation compared with the prone and sitting positions. In previous studies, 8, 14, 17– 19 the scapular-retraction exercises were performed in predominantly prone positions, and some authors 20– 22 also compared trapezius muscle-activation levels among different retraction exercises performed in prone, sitting, and standing positions. Individuals with either excessive scapular internal rotation–anterior tilt or reduced scapular upward rotation may benefit from LT-enhanced exercises. 7 Individuals with prominence of the medial scapular border may benefit from incorporation of MT-enhanced exercises in their training programs. 2, 16 One of the most important points to consider when applying scapular-retraction exercises is to keep the UT/MT and UT/LT muscle-activation ratios at low levels.
10, 12, 15 The retracted scapula also serves as a stable base for the rotator cuff muscles and improves their force generation and stabilization function. 10, 13, 14 Conversely, scapular retraction is thought to minimize the anterior tilt and internal rotation of the scapula, 8 maintaining the subacromial space during shoulder elevation. 7– 9 Some researchers found that scapular protraction narrowed the subacromial space, which may be linked to subacromial impingement syndrome 10– 12 and poor scapulothoracic muscle-activation ratios. Scapular-retraction exercises are often prescribed in both the prevention and treatment of shoulder injuries. 1 Previous investigators 4– 6 have shown that postural, kinematic, and muscle-activity changes were all linked to symptoms of shoulder disorders. 1, 4– 6 These kinematic alterations may reduce the subacromial space and place excessive loads on the rotator cuff muscles as the arm elevates. Alterations in the activity of the parts of the trapezius muscle, such as excessive activation of the UT combined with decreased activation of the LT and MT, result in reduced scapular upward rotation, increased internal rotation, and anterior tilt.
3 These scapular movements occur via coordinated activation of the serratus anterior, upper trapezius (UT), middle trapezius (MT), and lower trapezius (LT) muscles. 2 During glenohumeral elevation, the scapula rotates upwardly and externally and tilts posteriorly. 1 Notably, scapular stabilization is vital for accurate and effective glenohumeral-joint movements during overhead activities. The trapezius muscle, with its 3 distinct parts, plays an important role in scapular stabilization.