Studying scapular movement, kinesiology and exercises.
The closer we look at movement or the potential movement of the human body, the more dynamic, intricate and specific it becomes. When studying the appendicular skeleton, personal trainers will benefit from understanding ideal range of motion (ROM), joint articulation and the influence of the axial skeleton on limb action.
The scapula’s position in natural stance often reflects an individual’s daily lifestyle and stress level. For example, many people stand naturally with the shoulder girdle in an abducted position; this is so common it often is overlooked. However, constant contraction of the pectoralis major and minor muscles can cause this chronic pattern to over lengthen the mid-back musculature. Other people stand naturally with the shoulder girdle in an adducted position, wherein the shoulder girdle has moved significantly backwards from the plumb line, causing the person to slouch.
It may seem that “standing up straight” with more erect posture would help correct this, but if muscle movers are constantly contracted and used for stabilizing an upright posture, they lose their movement efficiency. Why does this happen? Primary muscle movers help control movement. When tension relationships and joint congruity are optimal, the mechanical sequence of muscle firing works efficiently. Some muscles stabilize joints as others contract and shorten to provide movement. If a muscle is chronically contracted or hypertonic it becomes a poor mover. Understanding ideal ROM and the muscles affecting this range will be useful as you customize exercise programs.
The arms have great ROM and freedom of action, as well as stability to give their movements precision and force. The integrity of this stabilization rests upon the powerful musculature which secures the shoulder girdle against the thorax. The tone, attitudes, postures and structures of these muscles vary significantly among people, thus making it difficult to explain “normal relationships” of the scapula to the thorax without conceding a rather wide range of physiological fluctuations.
Mobility and free range of the arm is achieved by three individual joints, all mutually interdependent. The primary joint of the shoulder is the glenohumeral joint located between the head of the humerus and glenoid fossa of the scapula. The capsule of the glenohumeral joint attaches to the scapula, on the outer rim of the glenoid fossa. Superiorly, it goes up to the coracoid process and encircles the long head of the biceps at its origin. On the humerus, it attaches to the anatomical neck. The capsule is most weak inferiorly where it has no ligament or rotator cuff muscle support.
The scapula, clavicle and shoulder joint form a mechanical unit between the medial clavicle and manubrium of the sternum called the sternoclavicular joint. The clavicle rotates around the sternum and articulates laterally with the acromion process of the scapula, medially with the sternum. The scapula rotates around the clavicle and the humerus around the scapula.
The acromioclavicular joint is involved in shoulder movements between the distal clavicle and acromion process of the scapula.
Although it lacks some traditional characteristics of a joint, the scapulothroacic joint assists in motion of the shoulder at the glenohumeral joint. Scapular motion is controlled by the scapulothoracic muscles, which hold the scapula in close contact with the trunk and restricts its movements to the contours of the thoracic wall.
This article mainly covers the sternoclavicular and acromioclavicular joints and the movements and movers of the scapula and clavicle. It describes their attachments and how the muscles provide the support and motion necessary for optimal range of the glenohumeral joint (a future article will cover arm motion and the musculature that stabilizes and moves this joint).
In order to comprehend arm movement, scapular and shoulder girdle movement must be understood first. The position of the scapula in natural stance varies among people. In ideal alignment, the scapula lies directly over the posterior ribcage with the upper back in good alignment. It does not articulate with the ribs, rather it floats over them (approximately ribs 2 to 7). This suspension is created by a network of muscles and ligaments that provide scapular glide. While the scapula is very mobile, it has limited ROM.
The scapula is a triangular bone with three angles: superior, inferior and lateral. The three borders of the scapula—superior, medial (vertebral) and lateral (axillary)—create its sides. The lateral border contains the glenoid fossa, where the head of the humerus articulates. This is a very shallow oval-shaped depression, making it very susceptible to injury. The coracoid process arises from the superior border, medial to the glenoid fossa.
The anterior surface of the scapula is separated from the posterior ribcage by a thin, smooth layer of muscle that lets it glide over the ribs. The spine of the scapula, easily palpated and seen externally, is a strong ridge running diagonally near the superior border. The posterior surface of the scapula is a major attachment site for many muscles that create scapular movement. Scapular support and motion are provided by muscles that attach it to the thorax and vertebral column. These muscles are oriented obliquely and produce rotatory and linear motions of the bone in their direction of pull.
The movements ascribed to the scapula do not occur individually. Some degree of rotation or tilt accompanies abduction and adduction and, to a lesser extent, elevation and depression due to the contour of the rounded thorax. While there are no pure linear movements of the scapula, six basic movements are described in the next section.
1. Anterior Tilt. Movement about a coronal axis in which the coracoid process moves in an anterior and caudal direction (can be considered depressed anteriorly) while the inferior angle moves in a posterior and cranial direction. This movement is associated with elevation.
2. Elevation (Superior Glide). The scapula moves up and away from the ribcage, e.g., shrugging.
3. Depression (Inferior Glide). The scapula moves downward and against the ribcage.
4. Abduction (Lateral Glide). The medial border of the scapula moves away from the vertebral column and the lateral angle moves anteriorly. Following the contour of the thorax, it assumes a posterolateral position in full abduction (also called protraction).
5. Adduction (Medial Glide). The medial border of the scapula moves closer to the vertebral column and the lateral angle moves posteriorly (this relates to retraction).
6. Rotation (Inferior Angle Moves Toward or Away From Midline). Rotation of the humerus is associated with similar rotation of the spine. The movement is around a sagittal axis and is related to the glenoid fossa’s movement.
- Medial or Downward Rotation (Glenoid Fossa Moves Downward): Inferior angle (bottom of the blade) moves superomedially (up and toward the spine) as the lateral angle (nearest the humerus) moves inferolaterally (down and away from the midline).
- Lateral (Upward) Rotation: Conversely, inferior angle (bottom of the blade) moves superolaterally while the superior angle moves inferomedially again, using the glenoid fossa as the reference point.
The scapula and clavicle are moved by numerous muscles, many of which also provide movement of bones such as the humerus, spine and occiput. Depending on whether the scapula is fixed determines the action of these muscles. There are also many muscles that stabilize the scapula and clavicle during arm and head movement.
Serratus anterior is a broad, thin muscle covering the lateral ribcage. It originates from the upper eight ribs and inserts along the medial border of the entire scapula. It functions in abduction and rotation and helps hold the scapula in place. Serratus works to upwardly rotate and depress (lower fibers) the shoulder blade.
When the scapula is fixed, (in actions such as push-ups, it is important to keep the scapula fixed in place against the ribcage) the serratus fibers contract when the arm is pushing against resistance. In such a situation, the middle fibers of trapezius (an adductor) and serratus (acting as an abductor) contract simultaneously to stabilize the scapula. The inferior fibers of serratus assist in inspiration by helping elevate the ribs.
Subclavius originates from the first rib and its cartilage to the underside of the clavicle. It depresses the clavicle.
Pectoralis minor originates from ribs 3 to 5 and inserts on the coracoid process. When the ribs are fixed, it pulls the scapula downward and forward, and acts to stabilize the scapula. When the scapula is fixed, pectoralis minor assists in inspiration by elevating the ribs.
Sternocleidomastoid (SCM) acts primarily on the head and cervical spine. If the head is fixed, SCM can assist in inspiration by elevating the region where the clavicle and sternum meet (sternoclavicular joint).
Levator scapula originates from the transverse processes of C1 to C4. Levator scapula inserts on the superior angle of the scapula, elevates it and rotates it downward. When the scapula is fixed, it acts on the cervical spine with splenius capitis to extend the head and cervical spine.
Rhomboid major originates from T3 to T5 and inserts on the lower two-thirds of the vertebral border of the scapula.
Rhomboid minor originates from C7 to T1 and inserts on the upper third of the superior angle of the scapula.
These muscles adduct the scapula and assist levator scapula with downward rotation. When the scapula is fixed, their contraction pulls the thoracic spine laterally.
Trapezius originates on the occiput, nuchal ligament and spinous processes of C7 and T1 to T12. This large diamond-shaped muscle inserts on the lateral third of the clavicle (upper fibers), acromion and scapular spine (middle fibers), and a tubercle at the medial end of the scapular spine (lower fibers). The orientation of these fibers ranges from inferolateral to superolateral. When all of the fibers contract simultaneously, they adduct the scapula. The upper fibers acting alone elevate and upwardly rotate the scapula and elevate the clavicle. The lower fibers acting alone depress and upwardly rotate the scapula.
The upper fibers are very often over recruited in normal daily movements such as typing or sitting at a computer. These types of movements require prolonged periods of suspension of the arms causing “stiff neck” or a sensation of spasm or headaches. When force needs to be exerted or absorbed by the arms, the mid fibers of trapezius (adductor) act synergistically with serratus anterior (abductor) to stabilize the scapula.