Four-week exercise program does not change rotator cuff muscle activation and scapular kinematics in healthy subjects

Abstract

Rotator cuff and scapular muscle strengthening exercises are an essential part of shoulder rehabilitation and sports training. Although the effect of exercise training on pain and function have been widely investigated, few studies have focused on the changes in shoulder kinematics and muscle activity after exercise training. Therefore, the purpose of the present study was to investigate the effect of rotator cuff and scapular strengthening exercises on shoulder kinematics and the activation of rotator cuff and scapular muscles in healthy subjects. Thirty-six healthy subjects were recruited and randomly assigned into either a training or control group. Subjects in the training group were trained with rotator cuff and scapular strengthening exercises for 4 weeks. Scapular kinematics and shoulder muscle activity during arm elevation were measured before and after exercise training. After the 4-week training protocol, there was an increase in strength and a decrease in upper trapezius activation in the training group, which is consistent with previous studies. However, no difference was found in scapular kinematics and activation of rotator cuff muscles between the control and training groups after the training protocol. Although the exercise protocol resulted in strength gains for the rotator cuff, these gains did not transfer to an increase in muscle activation during motion. These results demonstrate the difficulty in changing activation patterns of the rotator cuff muscles. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:2079-2088, 2016.

Keywords: electromyography; kinematics; rotator cuff; scapula; strengthening exercise.

Figure 1

Testing position and sensor placement

Figure 2

Exercises for the rotator cuff and scapulothoracic muscles: a) full can, b) prone full can, c) sidelying external rotation, d) diagonal exercise, e) push up with plus and f) balance exercise

Figure 3

Percentage changes of the forces from pre-training to post-training in the control and training groups. The maximum voluntary contraction (MVC) forces of the deltoid, supraspinatus and upper trapezius were measured with 90° of elevation in the scapular plane with neutral axial rotation. The infraspinatus MVC force was tested by resisted external rotation (ER) with the arm at the side. The lower trapezius MVC was tested in a prone position with horizontal abduction (ABD) in 120° of elevation. The serratus anterior MVC was measured with 135° of elevation in the scapular plane.

Figure 4

Scapular kinematics of pre-training and post-training conditions in the control and training groups: a) scapular anterior/posterior tilt, b) upward/downward rotation, c) internal/external rotation.

Figure 5

Muscle activation of the deltoid and rotator cuff muscles of pre-training and post-training conditions in the control and training groups: a) deltoid, b) supraspinatus, and c) infraspinatus. MVC: maximum voluntary contraction.

Figure 6

Muscle activation of the scapulothoracic muscles of pre-training and post-training conditions in the control and training groups: a) upper trapezius, b) lower trapezius, and c) serratus anterior. MVC: maximum voluntary contraction.