Muscles of Shld/Shld Girdle



Name

Lab Section

Shoulder Complex Muscles

Purpose: To learn the actions, proximal attachments, and distal attachments of the muscles that act on the clavicle, scapula, and humerus at the glenohumeral, acromioclavicular, and sternoclavicular articulations.

Equipment: Textbook, Flash Anatomy Cards, or other resource for muscle actions & attachments

Handout – Learning Muscle Attachments (from web page)

Handout – Rules to Remember (from web page)

Articulated skeleton (to be provided by instructor)

Disarticulated skeleton (to be provided by instructor)

Background Information

As you begin your study of muscle attachments for muscles, know that I will use the terms proximal and distal attachments rather than origins and insertions. The reason for this is that literally defined, origin means the fixed bone and insertion means the moving bone. When a muscle contracts and attempts to shorten, it exerts force on all the bones to which it attaches. The bone that is more stable will not move, and the bone that is freer to move for whatever reason will move. Usually, the distal bone is lighter than the proximal bone and is, therefore, freer to move. Since the distal bone is usually the bone that moves, attachments on the distal bone were called the insertion points. However, you must understand that either bone (proximal or distal) can potentially move. If the distal bone becomes more fixed, or stable, then the proximal bone will move. An example of this would be in a pull-up, where contraction of the biceps brachii, an elbow flexor, cause elbow flexion by pulling the humerus up to the forearm. This is opposite what we usually associate with elbow flexion, where the forearm moves toward the stationary humerus as in a curl motion. In the case of the pull-up, the attachment of the bicep on the forearm would become the origin and the attachment on the scapula would become the insertion – in other words, they would be reversed from what we normally consider the origin and insertion. Therefore, to keep things simple, I will use the terms proximal and distal attachment rather than origin and insertion

For this lab, you are to learn the actions and attachments for the muscles of the shoulder complex. The shoulder complex consists of two distinct and anatomically independent units: the shoulder girdle and the shoulder (glenohumeral) joint. While they are functionally interdependent in many ways, these two units can also function independently of each other. Therefore, you should learn them and their muscles as individual units at this time. The functional interdependence of these two units will be presented to you later in this course if you are taking the course for 3 credits.

We will first discuss the shoulder girdle. The shoulder girdle consists of the scapula and clavicle, and the joints that connect these bones to the thorax [the sternoclavicular (SC) joint and the scapulothoracic (ST) joint] and to each other [the acromioclavicular (AC) joint]. Therefore, movements of the shoulder girdle (the scapula and clavicle) are not a result of motion about a single joint. Instead, movement of the shoulder girdle is the summation of movements that occur at all three joints – the AC, SC, and ST. Individually, these joints are classified primarily as nonaxial joints, which means that they do not allow rotation. At this time, we will not learn about these joints individually, but you should understand that the shoulder girdle movements that you learn are a result of motion about 3 joints simultaneously. When we consider the movements of all of these joints together, linear and angular movements of the shoulder girdle can be defined in all 3 planes. In the frontal plane, the linear movements of elevation/depression occur, and the angular movements of upward and downward rotation occur. In the sagittal plane, the angular movements of upward & downward tilt occur. In the transverse plane, the angular movements of protraction and retraction occur.

There are 6 muscles that are devoted to causing and controlling movements of the scapula and/or the clavicle: the subclavius, the serratus anterior, the rhomboids (rhomboid major and rhomboid minor), the trapezius, the pectoralis minor, and the subclavius. While the rhomboid major and rhomboid minor are considered two separate muscles anatomically, you can learn them as one muscle since their functions are identical. On the other hand, while the trapezius muscle is considered one muscle anatomically, you should learn it as three muscles (upper, middle, and lower trapezius) since each portion has a different function from the other portions. As you study the attachment sites for the shoulder girdle muscles, remember that a muscle that causes shoulder girdle movements must attach distally to the scapula or clavicle in order to be able to cause that bone to move. Therefore, proximal attachments (usually origins) must be on the thorax (ribs or vertebrae), and distal attachments (usually insertions) must be on the scapula or clavicle.

The shoulder (or glenohumeral) joint is the articulation formed between the head of the humerus and the glenoid fossa of the scapula. It is classified as a triaxial joint, allowing movement in all three planes: flexion, extension, hyperextension in the sagittal plane; abduction, adduction in the frontal plane; and medial rotation, lateral rotation, horizontal adduction, and horizontal abduction in the transverse plane. Because it does permit frontal and sagittal plane motions, it also permits circumduction. Muscles on the front will typically cause flexion, while muscles on the back will typically cause extension and hyperextension. Muscles on the lateral aspect of the shoulder will typically cause abduction while muscles on the medial aspect of the shoulder will typically cause adduction. Transverse plane motions must be understood by examining the line of pull of the muscle relative to the superior-inferior axis of the humerus. As you study the attachment sites for the hip muscles, remember that a muscle that causes shoulder joint movements must cross the shoulder joint. Therefore, proximal attachments (origins) must be on the scapula, clavicle, or thorax, and distal attachments (insertions) must be on the humerus, radius, or ulna. You should not confuse shoulder girdle muscles with shoulder joint muscles if you will remember that shoulder girdle muscles do not cross the shoulder joint and, therefore, cannot have attachments on the humerus or below.

Procedures to be completed prior to lab:

1. Review the bony markings listed below for the scapula, vertebra, sternum, clavicle, radius, ulna, and humerus. (Resource: BIOL 120 Lab Manual, or Flash Anatomy Cards – Bones)

Scapula Humerus

Acromion process Anatomical neck

Articular facet of acromion process Body

Lateral (axillary) border Deltoid tuberosity

Coracoid process Head

Glenoid fossa Intertubercular (bicipital) groove

Inferior angle Lesser tubercle (or tuberosity)

Infraglenoid tubercle Greater tubercle (or tuberosity)

Infraspinous fossa Surgical neck

Root of the scapular spine

Suprascapular notch Clavicle

Spine of scapula Conoid tubercle

Supraspinous fossa Sternal end

Subscapular fossa Acromial end

Superior angle

Superior border Vertebra

Supraglenoid tubercle Atlas (C1)

Medial (vertebral) border Axis (C2)

Coccyx

Sternum Sacrum

Xiphoid process Body

Manubrium Lamina

Clavicular notch Pedicle

Body Spinous process

Transverse process

Ulna Vertebral arch

Olecranon process Vertebral foramen

Radius

Radial tuberosity

2. Review the joint actions of the shoulder girdle and glenohumeral joint. On a separate sheet of paper, define the following terms for shoulder girdle (scapular) movements): elevation, depression, abduction (protraction), adduction (retraction), upward tilt, downward tilt, upward rotation, and downward rotation.

3. Use the Handouts – Learning Muscle Attachments & Rules to Remember – and your Flash Anatomy Cards or other resource to review diagrams, actions, and attachments of the following muscles:

Shoulder Girdle (Acromioclavicular, Sternoclavicular, and Scapulothoracic Joints):

Upper trapezius (I and II) Levator scapula Serratus anterior

Middle trapezius (III) Rhomboids (major & minor) Pectoralis minor

Lower trapezius (IV) Subclavius

Shoulder (Glenohumeral Joint):

Pectoralis major (sternal) Supraspinatus Anterior deltoid

Pectoralis major (clavicular) Infraspinatus Middle deltoid

Latissimus dorsi Teres minor Posterior deltoid

Teres major Subscapularis Biceps brachii (short head)

Triceps brachii (long head) Coracobrachialis Biceps brachii (long head)

Procedures to be completed during the lab session:

1. Use the bones, muscle models, and muscle diagrams to help you study these muscles and their attachments and actions. Strive to understand why each muscle has the action(s) that it has by applying concepts previously learned (torque and lines of pull).

2. Attempt to locate and palpate the superficial muscles on your lab partner. This will aid you in learning the location and actions of each of these muscles.

Study Questions

1. What are the posterior muscles of the shoulder girdle?

2. Which muscle would be considered the latissimus dorsi's little helper?

3. Which four muscles make up the rotator cuff group? What is their primary function at the shoulder joint as a group?

4. On what aspect of the arm would you expect to find muscles that flex the humerus?

Summary of Muscle Actions:

Shoulder Girdle

Elevation Depression Abduction/Protraction

Upper trapezius Lower trapezius Serratus anterior

Rhomboids Pectoralis minor Pectoralis minor

Levator scapula Pectoralis major Pectoralis major

Latissimus dorsi Latissimus dorsi

Subclavius

Adduction/Retraction Upward rotation Downward rotation

Middle trapezius Serratus anterior Rhomboids

Rhomboids Upper trapezius Pectoralis minor

Lower trapezius Lower trapezius Levator scapulae

Levator scapula

Upper trapezius

Upward tilt

Pectoralis minor

Shoulder

Flexion Extension Hyperextension

Anterior deltoid Pectoralis major (S) Posterior deltoid

Pectoralis major (C) Latissimus dorsi Latissimus dorsi

Biceps brachii Teres major Triceps brachii (L)

Coracobrachialis Posterior deltoid

Triceps brachii (L)

Adduction Abduction Lateral rotation

Latissimus dorsi Deltoid (all parts) Infraspinatus

Pectoralis major Supraspinatus Teres minor

Teres major Posterior deltoid

Triceps brachii (L)

Coracobrachialis

Medial rotation Horizontal adduction Horzontal abduction

Subscapularis Pectoralis major Posterior deltoid

Latissimus dorsi Anterior deltoid Infraspinatus

Pectoralis major Teres minor

Anterior deltoid

Teres major

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