The Shoulder Press Article

The Shoulder Press Single or double arms

The shoulder press combines motions at the scapulothoracic, sternoclavicular, acromioclavicular, glenohumeral and humeroulnar joint. Consequently, a multitude of muscular interactions are required to abduct the shoulder and extend the elbow to press the carriage away from the resting position. As we have seen the scapulothoracic joint serves as an important mechanical platform for all active movements of the humerus. Muscles such as the deltoid and rotator cuff require coactivation of the serratus anterior and trapezius to effectively stabilize the scapula and clavicle. In turn, the proximal skeletal attachments of the scapulothoracic muscles on the cranium, ribs and spine must be stabilized so that they can stabilize the scapula and clavicle.

Scapulothoracic, acromioclavicular sternoclavicular joint motion The shoulder press provides a good opportunity to explore arthrokinematics of the shoulder girdle in this common motion. Pressing the carriage away involves full shoulder abduction including about 60 degrees of scapular upward rotation. The upwardly rotated scapula provides a stable yet mobile base for the abducting humeral head and projects the glenoid fossa upward and anterior-laterally. Upward rotation also maximizes the volume within the subacromial space, preventing impingement, and preserves the optimal length-tension relationship of the abductor muscles of the glenohumeral joint, such as the middle deltoid and supraspinatus.

Scapulohumeral rhythm In the healthy shoulder a natural kinematic rhythm or timing exists between glenohumeral abduction and scapulothoracic upward rotation. The term “scapulohumeral rhythm” is used to explain this kinematic relationship. (See footnote below) After about 30 degrees of abduction this rhythm seems to remain remarkably constant, occurring at a ratio of 2: 1: for every 3 degrees of shoulder abduction, 2 degrees occurs by GH joint abduction and 1 degree occurs by scapulothoracic joint upward rotation. Based on a generalized 2: 1 scapulohumeral rhythm, a full arc of nearly 180 degrees of abduction is the result of a simultaneous 120 degrees of GH joint abduction and 60 degrees of scapulothoracic upward rotation. (See image 1)

IMAGE 1

Movements at the scapulothoracic joint are mechanically linked to the movements at both the sternoclavicular and acromioclavicular joints. This motion contributes a significant component of the overall upward rotation at the scapulothoracic joint during the exercise. Although controversial, up to 30 degrees of upward rotation at the AC joint occurs as the arm is raised fully over the head.1,2,3 (See images 1 and 2) Upward rotation contributes to both the quality and quantity of movement at the scapulothoracic joint. The scapula often follows a path closer to its own “scapular” plane during abduction (about 35 degrees anterior to the frontal plane) as opposed to true abduction strictly in the frontal plane. However, the AC and SC joints have the mobility to adjust to the virtually infinite number of paths that the scapula may take during elevation of the arm.

Although is it beyond our scope, it should be noted that horizontal plane adjustments at the AC joint occur around a vertical axis, described as internal and external rotation. Sagittal plane adjustments also occur at the AC joint around a near medial-lateral axis, termed anterior and posterior tilting.

(See image 2)

In contrast to the AC joint which permits more subtle movements between the scapula and lateral end of the clavicle the SC joint permits extensive motion of the clavicle, which guides the general path of the scapula. Complete upward rotation of the scapula occurs by a summation of clavicular elevation at the SC joint and scapular upward rotation at the AC joint.4,5 These coupled rotations are essential to the full 60 degrees of upward rotation at the scapulothoracic joint.6 (See Image 1 and 3)

IMAGE 3

IMAGE 2

IMAGE 4

The primary upward rotator muscles during the shoulder press are the serratus anterior and the upper and lower fibers of the trapezius. These muscles form a force-couple to effectively upwardly rotate the scapula 7, driving the upward rotation of the scapula and providing stable attachments for the more distal mobilizers, such as the deltoid and rotator cuff muscles.

The serratus anterior also secures the medial border of the scapula firmly against the thorax by generating an external rotation torque.

Image 4 The force-couple of the serratus anterior and trapezius rotates the scapula in the same rotary direction as the abducting humerus.

Acting simultaneously, the pull of the lower fibers of the serratus anterior on the inferior angle of the scapula rotates the glenoid fossa upward and laterally. These fibers are the most effective upward rotators of the force-couple, primarily because of their larger moment arm for this action. The upper trapezius upwardly rotates the scapula indirectly by its superiorand-medial pull on the clavicle.8 The lower trapezius upwardly rotates the scapula by its inferior and-medial pull on the root of the spine of the scapula.

The middle trapezius contributes a needed retraction force on the scapula, which along with the rhomboid muscles, helps to neutralize the strong protraction effect of the serratus anterior 9. The middle trapezius and serratus anterior muscles are synergists in upward rotation but are agonists and antagonists as they oppose, and thus partially limit, each other’s strong protraction and retraction effect.

Downward rotation of the scapula occurs as the arm is returned to the side from a raised position. The motion is described similarly to upward rotation, except that the clavicle depresses at the SC joint and the scapula downwardly rotates at the AC joint. The motion of downward rotation usually ends when the scapula has returned to the anatomic position.

The GH joint Dynamic stability Earlier in this chapter, the dynamic stabilizing function of the rotator cuff muscles was discussed. As the carriage is pressed away during abduction of the GH joint the horizontally aligned supraspinatus muscle is ideal for directing the arthrokinematics of this motion. The supraspinatus rolls the humeral head superiorly toward abduction while also compressing the joint for added stability.

The remaining rotator cuff muscles (subscapularis, infraspinatus, and teres minor) exert a downward translational force on the humeral head to counteract excessive superior translation, especially that caused by deltoid contraction. 10 ,11 The long head of the biceps also contributes in this fashion.12 (See images 5 and 6)

IMAGE 5

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MUSCLES THAT ELEVATE THE ARM AT THE GLENOHUMERAL JOINT The prime muscles that abduct the GH joint are the anterior deltoid, middle deltoid, (image 5) and supraspinatus. Elevation of the arm through a combination of flexion/abduction and slight external rotation during the shoulder press changes the line of force so that the motion is performed primarily by the anterior deltoid, coracobrachialis, long head of the biceps Brachii and costal portion of Pectoralis major At the elbow, The primary elbow extensors are the triceps brachii and the anconeus. The triceps converge to a common tendon attaching to the olecranon process of the ulna. The triceps brachii has three heads: long, lateral, and medial. The long head has its proximal attachment on the infraglenoid tubercle of the scapula, thereby allowing the muscle to extend and adduct the shoulder. The anconeus is a small triangular muscle spanning the posterior side of the elbow providing important longitudinal and medial-lateral stability across the humero-ulnar joint.

(See Images 7 and 8)

IMAGE 7

Image 7 The 3 heads of the triceps from an anterior perspective. The medial head is almost totally obscured from the posterior view.

IMAGE 8

Image 8 The 3 heads of the triceps with the anconeus muscle highlighted.

A discussion about spring tension on the reformer allows us to explore muscle recruitment patterns at the elbow and how the Law of Parsimony operates at this joint also.

The law of parsimony13,14 states that the nervous system tends to activate the fewest muscles or muscle fibers possible for the control of a given joint action. Whilst Maximal-effort elbow extension generates near maximum levels of EMG activity from all components of the elbow extensor group, the law of Parsimony dictates that during submaximal efforts of elbow extension, however, different parts of muscles are recruited only at certain levels of effort.15 With little spring resistance the anconeus is the first muscle to initiate and maintain low levels of elbow extension force. As extensor effort gradually increases with increases in spring tension, the medial head of the triceps is next in line to join the anconeus.16 The medial head remains active for most elbow extension movements.17 Only after extensor demands at the elbow increase to moderate-to-high levels with further increases in spring resistance does the nervous system recruit the lateral head of the triceps, followed closely by the long head. The long head functions as a “reserve� elbow extensor, equipped with a large volume suited for tasks that require high work performance. With increased activation of the long head of triceps the anterior deltoid must oppose and exceed its shoulder extensor torque.

This hierarchic pattern of muscle recruitment makes sense from an energy perspective.

Consider, for example, the inefficiency of having only the long head of the triceps, instead of the anconeus or medial head of the triceps, performing very low-level maintenance types of stabilization functions at the elbow. Additional muscular forces would be required from shoulder flexors to neutralize the undesired shoulder extension potential of the long head of the triceps.

A simple task would require greater muscle activity than what is necessary. As electromyographic evidence and general intuition suggest, tasks with low-level force demands are often accomplished by one-joint muscles. As force demands increase, larger polyarticular muscles are recruited, along with the necessary neutralizer muscles.

In brief • The shoulder press combines motions at the scapulothoracic, sternoclavicular, acromioclavicular, glenohumeral and elbow joints • Pressing the carriage away is a concentric action that involves full shoulder abduction including about 60 degrees of scapular upward rotation • “Scapulohumeral rhythm” is a kinematic rhythm or timing between glenohumeral abduction and scapulothoracic upward rotation. For every 3 degrees of shoulder abduction, 2 degrees occurs by GH joint abduction and 1 degree occurs by scapulothoracic joint upward rotation • Movements at the scapulothoracic joint are mechanically linked to the movements at both the sternoclavicular and acromioclavicular joints.

• The primary upward rotator muscles during the shoulder press are the serratus anterior and the upper and lower fibers of the trapezius • As the carriage is pressed away during abduction the rotator cuff muscles play a dynamic stabilizing function • During this exercise shoulder abduction is performed primarily by the anterior deltoid, coracobrachialis, long head of the biceps Brachii and costal portion of Pectoralis major.

• The triceps extend the elbow. Recruitment of all three heads is determined by spring tension • The carriage return is the eccentric phase of the exercise • The exercise is a closed chain movement

References 1. Inman VT, Saunders M, Abbott LC: Observations on the function of the shoulder joint. J Bone Joint Surg Am 26:1-32, 1944.

2. Ludewig PM, Phadke V, Braman JP, et al: Motion of the shoulder complex during multiplanar humeral elevation. J Bone Joint Surg Am 91:378-389, 2009.

3. Teece RM, Lunden JB, Lloyd AS, et al: Three-dimensional acromioclavicular joint motions during elevation of the arm. J Orthop Sports Phys Ther 38:181-190, 2008.

4. Fung M, Kato S, Barrance PJ, et al: Scapular and clavicular kinematics during humeral elevation: A study with cadavers. J Shoulder Elbow Surg 10:278-285, 2001.

5. Ludewig PM, Behrens SA, Meyer SM, et al: Three-dimensional clavicular motion during arm elevation: Reliability and descriptive data. J Orthop Sports Phys Ther 34:140-149, 2004.

6. Ebaugh DD, McClure PW, Karduna AR: Three-dimensional scapulothoracic motion during active and passive arm elevation. Clin Biomech (Bristol, Avon) 20:700-709, 2005.

7. Ekstrom RA, Donatelli RA, Soderberg GL: Surface electromyographic analysis of exercises for the trapezius and serratus anterior muscles. J Orthop Sports Phys Ther 33:247-258, 2003.

8. Ebaugh DD, McClure PW, Karduna AR: Three-dimensional scapulothoracic motion during active and passive arm elevation. Clin Biomech (Bristol, Avon) 20:700-709, 2005.

9. Ekstrom RA, Donatelli RA, Soderberg GL: Surface electromyographic analysis of exercises for the trapezius and serratus anterior muscles. J Orthop Sports Phys Ther 33:247-258, 2003 10. Halder AM, Zhao KD, Odriscoll SW, et al: Dynamic contributions to superior shoulder stability. J Orthop Res 19:206-212, 2001.

11. McCully SP, Suprak DN, Kosek P, Karduna AR: Suprascapular nerve block results in a compensatory increase in deltoid muscle activity. J Biomech 40:1839-1846, 2007.

12. Pagnani MJ, Deng XH, Warren RF, et al: Role of the long head of the biceps brachii in glenohumeral stability: A biomechanical study in cadaver. J Shoulder Elbow Surg 5:255-262, 1996.

13. Decker MJ, Hintermeister RA, Faber KJ, Hawkins RJ: Serratus anterior muscle activity during selected rehabilitation exercises. Am J Sports Med 27:784-791, 1999.

14. Endo K, Yukata K, Yasui N: Influence of age on scapulo-thoracic orientation. Clin Biomech (Bristol, Avon) 19:1009-1013, 2004.

15. Hara H, Ito N, Iwasaki K: Strength of the glenoid labrum and adjacent shoulder capsule. J Shoulder Elbow Surg 5:263-268, 1996.

16. Hara H, Ito N, Iwasaki K: Strength of the glenoid labrum and adjacent shoulder capsule. J Shoulder Elbow Surg 5:263-268, 1996.

17. Burkhead WZ Jr, Rockwood CA Jr: Treatment of instability of the shoulder with an exercise program. J Bone Joint Surg Am 74:890-896, 1992.

SCAPULOHUMERAL RHYTHM The most widely cited study on the kinematics of shoulder abduction was published by Inman and colleagues in 1944. This classic work focused on shoulder abduction in the frontal plane. Data from this study were collected using two dimensional radiographs and, most interesting, recording the movement of pins inserted directly into the bones of the shoulder in a live subject. This early study set the background for most subsequent studies on the kinematics of the shoulder.

A significant amount of research on scapulohumeral rhythm has been published since Inman’s classic in vivo work. Most of these studies used less invasive methods, including radiography, goniometry, photography, cinematography, and, more recently, fluoroscopy, magnetic resonance imaging, and electromechanical or electromagnetic tracking devices.

Published scapulohumeral rhythms vary across studies, ranging from 1.25: 1 to 2.9: 1, which are relatively close to Inman’s reported 2 : 1 ratio. Variations in scapulohumeral rhythms reflect differences in measurement technique, population sampled, speed and arc of measured motion, number of dimensions recorded, and amount of external load.

Regardless of the differing ratios reported in the literature, Inman’s classic 2:1 ratio remains a valuable axiom for evaluating shoulder abduction. It is simple to remember and helps to conceptualize the overall relationship between humeral and scapular movement considering the full 180 degrees of shoulder abduction.

Inman VT, Saunders M, Abbott LC: Observations on the function of the shoulder joint.

J Bone Joint Surg Am 26:1-32, 1944.