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June 6, 2023

Kinesiology of the Shoulder and Scapula

An analysis of the functional roles of muscles for the joint actions of the shoulder and scapula.

Brent Brookbush

Brent Brookbush

DPT, PT, MS, CPT, HMS, IMT

Kinesiology of the joint actions of the shoulder and scapula: functional roles of muscles by joint action.

Kinesiology of the Shoulder and Scapula:

by Brent Brookbush DPT, PT, MS, CPT, HMS, IMT

From the Course:

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Definitions:

  • Prime Mover : The muscle that can produce the most force for a joint action. Generally, this is the muscle that will generate the most force during a joint action when exerting force against an external load.
  • Agonist : Muscles that perform a joint action, including the prime mover and synergists. That is, unlike the terms prime mover and synergist, agonist refers to all of the muscles that can perform a joint action.
  • Synergist : Synergists are muscles that assist the prime mover in performing a joint action. By definition, all agonists that are not the prime mover are synergists.
  • Antagonist : Muscles that oppose the agonists (including the prime mover and synergists) for a given joint action. That is, all of the muscles that can perform the opposing joint action.
  • Neutralizer : During a joint action, neutralizers are muscles that oppose the unwanted or ancillary joint motions that result from the force generated by the prime mover and/or synergists.
  • Fixator : Fixators are muscles that act to reduce or prevent movement of joints proximal to the joint in motion.
  • Stabilizer : The role of a stabilizer is to improve arthrokinematics by maintaining optimal alignment of joint surfaces during joint actions. Most often, these muscles are the most intrinsic muscles of a joint. For example, the muscles of the rotator cuff of the shoulder.

Kinesiology of the Shoulder and Scapula

Scapulohumeral Rhythm: The actions of the shoulder are paired with the actions of the scapula. This serves to increase the available range of motion of the upper extremity and allows the glenoid fossa to be positioned in a more stable position in relation to the humeral head. For example, shoulder abduction is accompanied by upward rotation - this increases the amount of gleno-humeral abduction available by moving the acromion process “out of the way,” and this positions the glenoid fossa partially under the humeral head providing mechanical stability to downward force. The analysis below pairs each shoulder joint action with the accompanying scapular movement.

Shoulder Horizontal Adduction and Scapular Protraction:

Shoulder Horizontal Adduction (Horizontal Flexion)

Scapular Protraction

Shoulder Horizontal Abduction and Scapular Retraction:

Shoulder Horizontal Abduction (Horizontal Extension)

Scapular Retraction

Shoulder Flexion and Scapular Upward Rotation:

Shoulder Flexion

Scapular Upward Rotation

Shoulder Extension and Scapular Downward Rotation:

Shoulder Extension

Scapular Downward Rotation

Shoulder Abduction and Scapular Upward Rotation:

Shoulder Abduction

Scapular Upward Rotation

Shoulder Adduction and Scapular Downward Rotation:

Shoulder Adduction

Scapular Downward Rotation

Shoulder Internal and External Rotation:

Note: Shoulder rotation is not strictly paired with a specific movement of the scapula

Internal Rotation:

External Rotation:

Scapular Elevation and Depression:

Note: Scapular elevation and depression are not strictly paired with a specific shoulder movement or another scapular action. They likely function to allow movement of the glenohumeral joint in various transverse planes, or to extend movement in the frontal plane.

Scapular Elevation

Scapular Depression

© 2011 Brent Brookbush

Questions, comments, and criticisms are welcomed and encouraged -

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