Chapter 7: The Biomechanics of the Human Upper Extremity

INTRODUCTION

After completing this chapter, you will be able to:

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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 text on smartphones. 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.

STRUCTURE OF THE SHOULDER

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.

Sternoclavicular Joint

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. Although a fibrocartilaginous articular disc improves the fit of the articulating bone surfaces and serves as a shock absorber, joint stability is provided primarily by the posterior joint capsule (30, 19). 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.