Mechanics of Shoulder Strength.

Last updated Thursday, February 10, 2005

Figure 1 - The moment arm
Figure 1 - The moment arm

Figure 2 - Muscles are usually stronger near the middle of their excursion
Figure 2 - Muscles are usually stronger near the middle of their excursion

Figure 3 - The points of application of a muscle force are not necessarily the same as that muscle's
Figure 3 - The points of application of a muscle force are not necessarily the same as that muscle's

Figure 4 - The anterior deltoid is a more effective internal rotator when the arm is in external rotation
Figure 4 - The anterior deltoid is a more effective internal rotator when the arm is in external rotation

Figure 5 - Supine press
Figure 5 - Supine press

Figure 6 - Anterior deltoid
Figure 6 - Anterior deltoid

Figure 7 - The cross-body and internal rotation moments of the anterior deltoid must be resisted by other muscles (such as the posterior deltoid and the infraspinatus)
Figure 7 - The cross-body and internal rotation moments of the anterior deltoid must be resisted by other muscles (such as the posterior deltoid and the infraspinatus)

Figure 8 - The fibers of the cuff sustain concentric loads when moving the humerus actively
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 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 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 11 - The glenoid rim abuts against the deep surface of the tendon insertion

Figure 12 - Tendon insertion
Figure 12 - Tendon insertion

Figure 13 - Coracoacromial arch
Figure 13 - Coracoacromial arch

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

Introduction

Strength is essential to carry out the functions of the shoulder. Many different muscles are required to power the shoulder because of the need to control both humeroscapular and scapulothoracic positions and to allow the vast range of motions of these articulations. For normal function, each muscle must be healthy, conditioned, securely attached, and coordinated.

About shoulder strength

For normal individual function each muscle must be intrinsically healthy, conditioned, attached securely to bone at both ends, and connected to the central nervous system by a healthy nerve supply. To contribute properly to the function of the shoulder, all the muscles around the shoulder must be activated by coordinated input from the central nervous system.

The strength of a given shoulder action is determined by the net torque created by the muscles responsible for this action. The torque resulting from a muscle's action is determined by the magnitude and direction of the muscle force and by the distance between the point of application of this force and the center of movement, the moment arm.

The magnitude of force deliverable by a muscle is determined by its size, health, and condition. It is also affected by the length of the muscle in the specified position of the shoulder: muscles are usually stronger near the middle of their excursion.

The points of application of a muscle force are not necessarily the same as that muscle's anatomic origin and insertion. For example, the effective humeral point of application for cuff tendons wrapping around the head is on the articular surface of the humeral head.

Finally, the direction of a given muscle force is also determined by the position of the joint. The anterior deltoid is a more effective internal rotator when the arm is in external rotation.

Changes in the direction and the point of application of a muscle force in different joint positions have a profound effect on the moment arm, and therefore, the torque that results from a given magnitude of muscle force.

Thus we see that each of the major determinants of a given muscle's contribution to torque is affected by joint position. It is apparent that maximizing shoulder strength must include positioning to optimize the contributions of the component muscles. This positioning is facilitated by the mobility of the scapula, which, for example, enables the anterior deltoid to remain within an optimal length as the arm is moved forward in an activity such as the supine press. This action can also be seen on the supine press exercise video listed below.

One of the relatively unexplored facets of active shoulder motion is the requirement for strict muscular balance. In the knee, the muscles generate torques primarily about a single axis: that of flexion-extension. If the quadriceps pull is a bit off-center, the knee still extends. In the shoulder, no such fixed axis exists. In a specified position, each muscle creates a unique set of rotational moments. The anterior deltoid exerts moments in forward elevation, internal rotation, and cross-body movement. If elevation without rotation in the plus 90 degree (sagittal) plane is desired, the cross-body and internal rotation moments of this muscle must be resisted by other muscles (such as the posterior deltoid and infraspinatus) at an additional energy cost. As another example, the latissimus dorsi cannot internally rotate the elevated arm unless other muscles resist its adduction moment. Conversely, it cannot act as a pure adductor unless other muscles resist its internal rotation moment.

The timing and magnitude of these balancing muscle effects must be precisely coordinated to avoid unwanted directions of humeral motion. For a ballerina to hold her arm motionless above her head, all the forces and torques exerted by each of her shoulder muscles must add up to zero. Thus the simplified view of muscles as isolated motors must give way to an understanding that the shoulder muscles function together in a precisely coordinated way to yield the desired effect. Opposing muscles work to cancel out undesired effects. Even the concept of force couples may be an oversimplification. Perhaps the best way to present the concept of muscle balance is to state that the summation of all the muscle actions around the joint must provide (1) joint stability, and (2) the torque necessary to carry out only the action desired. This degree of coordination requires a preprogrammed strategy or pattern that must be established before the motion is carried out.

Movies

The four muscles of the rotator cuff are uniquely adapted to contribute to muscle balance.

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.

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