Shoulder Muscles: Osteokinematic Motions & Muscle Function
Muscles Acting on the Scapulothoracic Joint
There are three categories of movement at the scapulothoracic joint:
- Elevation and depression
- Protraction and retraction
- Upward and downward rotation
Elevation of the Scapula
The muscles involved in elevation are:
- Upper trapezius
- Levator scapulae
- Rhomboids
Depression of the Scapula
The muscles involved in depression are:
- Lower trapezius
- Latissimus dorsi
- Pectoralis minor
- Subclavius
Protraction of the Scapula
The primary muscle for protraction is:
- Serratus anterior
Retraction of the Scapula
The muscles involved in retraction are:
- Middle trapezius
- Rhomboids
- Lower trapezius
Upward Rotation of the Scapula
The muscles involved in upward rotation are:
- Serratus anterior
- Upper trapezius
- Lower trapezius
Downward Rotation of the Scapula
The muscles involved in downward rotation are:
- Rhomboids
- Pectoralis minor
Muscles Acting on the Glenohumeral (GH) Joint
The glenohumeral joint rarely moves in isolation; it typically moves in conjunction with the scapulothoracic joint.
Abduction and Flexion at the GH Joint
The muscles involved in abduction and flexion are:
- Anterior deltoid
- Middle deltoid
- Coracobrachialis
- Biceps brachii (long head)
Scapulothoracic joint involvement:
- Serratus anterior (protraction)
- Trapezius
Rotator cuff muscles (for GH joint abduction and flexion):
- Supraspinatus
- Infraspinatus
- Teres minor
- Subscapularis
Adduction and Extension at the GH Joint
The muscles involved in adduction and extension are:
- Posterior deltoid
- Latissimus dorsi
- Teres major
- Triceps brachii (long head)
- Sternocostal head of pectoralis major
External Rotation at the GH Joint
The muscles involved in external rotation are:
- Infraspinatus
- Teres minor
- Posterior deltoid
Internal Rotation at the GH Joint
The muscles involved in internal rotation are:
- Subscapularis
- Anterior deltoid
- Pectoralis major
- Latissimus dorsi
- Teres major
Force Couples at the Shoulder
Force couples occur when two or more muscles on opposing sides of a joint act together to provide joint stability or create movement. The forces are generally equal in magnitude and parallel to each other, often contracting in opposing directions.
Examples of force couples are:
- Abduction: Supraspinatus and middle deltoid contract together.
- Scapulothoracic Upward Rotation: Upper trapezius and lower trapezius contract in opposing directions.
- Scapulothoracic Downward Rotation: Rhomboids, latissimus dorsi, and pectoralis minor contract together.
Rotator Cuff Muscles
One of the primary responsibilities of the rotator cuff muscles during flexion or abduction of the glenohumeral joint is to provide an inferior glide of the humeral head within the fossa to allow this movement to occur.
- Supraspinatus: Drives superior roll of the humeral head compresses humeral head firmly against the glenoid fossa, creates a semirigid spacer above the humeral head restricting excessive superior translation of the humerus, a primary stabilizer within the GH joint.
- Infraspinatus and Teres Minor, Subscapularis: Exert a depression force on the humeral head to create an inferior glide. The infraspinatus and teres minor externally rotate the humerus.
The osteokinematic motion the infraspinatus and teres minor apply to the GH joint is purely external rotation.
Review
- Attachments, innervations, and actions of the muscles in the shoulder should be known.
- Understanding the peripheral nerve innervations of the shoulder and certainly down to the distal arm, ties into knowing the brachial plexus.
- Refer to the brachial plexus lecture for more information on how and where these specific peripheral nerves originate from.
- Review the tables and make sure you understand the details within them.