The effects of scapular muscle strength, muscle stiffness, and coordination on scapular kinematics and subacromial space in overhead athletes with rotator cuff tendinopathy

Abstract

Scapular muscle deficits have been proposed as one of the causes of rotator cuff (RC) tendinopathy. Despite deficits in muscle strength and coordination of scapular muscles having been reported in people with RC tendinopathy, the relationship between these changes and the control of scapular kinematics for the maintenance of the subacromial space have not been investigated. It is crucial to identify the underlying pathomechanics and pathophysiology of scapular muscles in controlling scapular motion and the subacromial space in people with RC tendinopathy. Such information would provide a better understanding of the aetiology of RC tendinopathy for the prevention and management for RC tendinopathy. A comprehensive literature review was conducted to assess current research on the importance of scapular muscles in the control of scapular motion in overhead athletes during dynamic shoulder abduction, and to preserve the subacromial space to prevent rotator cuff pathologies (Chapter 1). Chapter 2 investigates the change in the subacromial space in overhead athletes with RC tendinopathy and its association with the strength of scapular muscles. Athletes with RC tendinopathy demonstrate strength deficits of the scapular muscles and more reduction in the subacromial space in the early phase of shoulder abduction from 0° to 30° when compared to asymptomatic athletes. Also, a decreased strength of middle and lower trapezius is associated with more reduction in the subacromial space. Chapter 3 establishes a reliable method to assess muscle stiffness of the upper trapezius using Supersonic Shear Imaging, and Chapter 4 shows that athletes with RC tendinopathy exhibit a stiffer upper trapezius during arm at rest and during static arm holding when compared to asymptomatic athletes; and an increase in the stiffness of the upper trapezius is associated with lesser reduction of the subacromial space in athletes with RC tendinopathy. Chapter 5 determines the effects of scapular muscle strength and changes in the activity onset of scapular muscles in relationship with scapular motion during dynamic shoulder abduction in athletes with RC tendinopathy. Decreased scapular upward rotation and external rotation were observed in the early phase of shoulder abduction (from 0° to 30°) in athletes with RC tendinopathy when compared to asymptomatic athletes. Delayed activity onset of middle trapezius relative to upper trapezius is associated with less scapular upward rotation during early arm abduction from 0° to 30°. Also, decrease strength in middle trapezius is associated with less upward rotation during dynamic shoulder abduction from 30° to 60°. Chapter 6 and Chapter 7 investigate whether modulation of muscle stiffness and muscle coordination can change the subacromial space and scapular motions in athletes with RC tendinopathy using taping as an example. With the application of tape applied on the scapula in athletes with RC tendinopathy, maintenance of the SAS is associated with a decrease in the stiffness of the upper trapezius during static arm positioning. Also, scapular taping facilitates early activation of the middle trapezius, lower trapezius, and serratus anterior during dynamic shoulder abduction, and can effectively increase scapular upward rotation in the early phase (0° to 30°) and late phase (60° to maximum abduction) of shoulder abduction in athletes with RC tendinopathy. To conclude, scapular muscle strength, muscle stiffness, and muscle coordination are related to the control of the subacromial space and scapular kinematics during static arm positioning and dynamic shoulder abduction. Deficits in these muscles are evidenced in overhead athletes with RC tendinopathy. Modulation of scapular coordination, scapular kinematics and the subacromial space are evidenced after scapular taping in athletes with RC tendinopathy.