Shoulder Muscles: A Case Study

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Chapter I The Problem and Its Scope
The shoulder contains complicated groups of muscles, tendons, ligaments, and bursae all involved in humeral and scapular kinematics (Terry & Chopp, 2000). Regular use of the arms can result in fatigue of shoulder musculature, which could lead to altered scapular kinematics during humeral elevation (Tsai, McClure, & Karduna, 2003). These changes in scapular kinematics can potentially lead to shoulder problems such as impingement syndrome, rotator cuff tears, and glenohumeral instability (Ebaugh, McClure, & Karduna, 2006). It is important to investigate the effects of fatigue on individual shoulder muscles to further understand their role in shoulder injuries. This could help prevent future shoulder injuries
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In particular, the infraspinatus is involved in external rotation and stabilization of the humerus as a humeral head depressor by pulling the humerus towards the glenoid fossa of the scapula (Ngomo, Mercier, & Roy, 2013). There was an increase in superior translation of the humeral head while using a nerve block to cause dysfunction of the infraspinatus and supraspinatus. This increase is also often seen in individuals experiencing shoulder impingement syndrome (San Juan, Kosek, & Karduna, 2013). Reddy, Mohr, Pink, & Jobe (2000) found that the infraspinatus showed a significant decrease in electromyographic activity during humeral elevation when comparing subjects with shoulder impingement syndrome to a control group. This suggests that the infraspinatus is an essential muscle to investigate regarding alterations in activity due to fatigue. A good amount of studies have examined infraspinatus functions and fatigue peripherally (D. Ebaugh et al., 2006; Reddy et al., 2000; Tsai et al., 2003), but none have examined the effects of fatigue on the infraspinatus

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