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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 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.
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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.
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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 (30).
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.
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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.
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As with the shoulder joint, several bursae are present in the
surrounding tissues to assist with lubrication. The most prominent
are the iliopsoasbursa and the deep trochanteric bursa. The iliopsoas
bursa is positioned between the iliopsoas and the articular capsule,
serving to reduce the friction between these structures. The deep
trochanteric bursa provides a cushion between ...