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After completing this chapter, you
will be able to:
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- Explain how anatomical structure affects movement capabilities
of upper-extremity articulations.
- Identify factors influencing the relative mobility and stability
of upper-extremity articulations.
- Identify muscles that are active during specific upper-extremity
movements.
- Describe the biomechanical contributions to common injuries
of the upper extremity.
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The capabilities of the upper extremity are varied and impressive.
With the same basic anatomical structure of the arm, forearm, hand,
and fingers, major league baseball pitchers hurl fastballs at 40 m/s,
swimmers cross the English Channel, gymnasts perform the iron cross,
travelers carry briefcases, seamstresses thread needles, and students
type on computer keyboards. This chapter reviews the anatomical
structures enabling these different types of movement and examines
the ways in which the muscles cooperate to achieve the diversity
of movement of which the upper extremity is capable.
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The shoulder is the most complex joint in the human body, largely
because it includes five separate articulations: the glenohumeral
joint, the sternoclavicular joint, the acromioclavicular joint,
the coracoclavicular joint, and the scapulothoracic joint. The glenohumeral
joint is the articulation between the head of the humerus and the
glenoid fossa of the scapula, which is the ball-and-socket joint
typically considered to be the major
shoulder joint. The sternoclavicular and acromioclavicular joints
provide mobility for the clavicle and the scapula—the bones
of the shoulder girdle.
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Sternoclavicular
Joint
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The proximal end of the clavicle articulates with the clavicular
notch of the manubrium of the sternum and with the cartilage of
the first rib to form the sternoclavicular
joint. This joint provides the major axis of rotation for movements
of the clavicle and scapula (Figure 7-1). The sternoclavicular
(SC) joint is a modified ball and socket, with frontal and transverse
plane motion freely permitted and some forward and backward sagittal
plane rotation allowed. A fibrocartilaginous articular disc improves
the fit of the articulating bone surfaces and serves as a shock absorber.
Rotation occurs at the SC joint during motions such as shrugging
the shoulders, elevating the arms above the head, and swimming.
The close-packed position for the SC joint occurs with maximal shoulder
elevation.
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Acromioclavicular
Joint
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The articulation of the acromion process of the scapula with
the distal end of the clavicle is known as the acromioclavicular
joint. It is classified as an irregular diarthrodial joint,
although ...