Chapter 8

### INTRODUCTION

After completing this chapter, you will be able to:

• Explain how anatomical structure affects movement capabilities of lower-extremity articulations.

• Identify factors influencing the relative mobility and stability of lower-extremity articulations.

• Explain the ways in which the lower extremity is adapted to its weight-bearing function.

• Identify muscles that are active during specific lower-extremity movements.

• Describe the biomechanical contributions to common injuries of the lower extremity.

The lower extremity is well structured for its functions of weight bearing and locomotion. JUPITERIMAGES/Brand X/Alamy.

Although there are some similarities between the joints of the upper and the lower extremities, the upper extremity is more specialized for activities requiring large ranges of motion. In contrast, the lower extremity is well equipped for its functions of weight bearing and locomotion. Beyond these basic functions, activities such as kicking a field goal in football, performing a long jump or a high jump, and maintaining balance en pointe in ballet reveal some of the more specialized capabilities of the lower extremity. This chapter examines the joint and muscle functions that enable lower-extremity movements.

### STRUCTURE OF THE HIP

The hip is a ball-and-socket joint (Figure 8-1). The ball is the head of the femur, which forms approximately two-thirds of a sphere. The socket is the concave acetabulum, which is angled obliquely in an anterior, lateral, and inferior direction. Joint cartilage covers both articulating surfaces. The cartilage on the acetabulum is thicker around its periphery, where it merges with a rim, or labrum, of fibrocartilage that contributes to the stability of the joint. Hydrostatic pressure is greater within the labrum than outside of it, contributing to lubrication of the joint. The acetabulum provides a much deeper socket than the glenoid fossa of the shoulder joint, and the bony structure of the hip is therefore much more stable or less likely to dislocate than that of the shoulder.

###### FIGURE 8-1

The bony structure of the hip.

Several large, strong ligaments also contribute to the stability of the hip (Figure 8-2). The extremely strong iliofemoral or Y ligament and the pubofemoral ligament strengthen the joint capsule anteriorly, with posterior reinforcement from the ischiofemoral ligament. Tension in these major ligaments acts to twist the head of the femur into the acetabulum during hip extension, as when a person moves from a sitting to a standing position. Inside the joint capsule, the ligamentum teres supplies a direct attachment from the rim of the acetabulum to the head of the femur.

###### FIGURE 8-2

The ligaments of the hip.

The hip is inherently more stable than the shoulder because ...

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