Mechanics of Shoulder Strength.

Last updated Thursday, February 10, 2005

Figure 8 - The fibers of the cuff sustain concentric loads when moving the humerus actively

Figure 9 - The fibers of the cuff sustain eccentric loads when resisting humeral motion or displacement

Figure 10 - Tendon fibers are subjected to bending loads when the humeral head rotates with respect to the scapula

Figure 11 - The glenoid rim abuts against the deep surface of the tendon insertion

Figure 12 - Tendon insertion

Figure 13 - Coracoacromial arch

Figure 14 - Rotator cuff tendon fibers become weaker with disuse and age

The rotator cuff

Muscle balance

These muscles are relatively small in size and have small moment arms, in comparison with the deltoid and the pectoralis major. Through their role in muscle balance, these muscles make a major contribution to shoulder strength. Their insertion into a continuous cuff around the humeral head permits these muscles to provide an infinite variety of moments to oppose unwanted components of the stronger motors. To hold a glass of water straight out in front of the body, for example, one needs to use the infraspinatus to balance the internal rotation moment of the anterior deltoid. When the function of the cuff muscles is compromised, the shoulder loses both its direct contribution to strength and the effectiveness of the muscles it balances.

The fibers of the cuff are subjected to many different challenges. They sustain concentric loads when moving the humerus actively. They sustain eccentric loads when resisting humeral motion or displacement. The tendon fibers are subjected to bending loads when the humeral head rotates with respect to the scapula. The glenoid rim abuts against the deep surface of the tendon insertion when the humeral head is rotated beyond the limits of the glenohumeral articular surfaces. In certain circumstances, the superficial fibers of the cuff may be abraded by the coracoacromial arch. Like the rest of the body's connective tissues, rotator cuff tendon fibers become weaker with disuse and age. As they become weaker, less force is required to disrupt them.

Movie

• Anatomy of the rotator cuff (3.32 MB)

Disclaimer

This resource has been provided by the University of Washington Department of Orthopaedics and Sports Medicine as general information only. This information may not apply to a specific patient. Additional information may be found at http://www.orthop.washington.edu or by contacting the UW Department of Orthopaedics and Sports Medicine.

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