The Pelvic Girdle and Hip Joint 161of very strong ligaments that tether the bones
together, expansions from surrounding muscles,
and the shape of the involved bones (Dujardin et
al., 2002; Levangie and Norkin, 2001). Regarding
shape, the sacrum sits like a wedge between the two
ilia. Because it is wider at the top than the bottom,
it will resist the tendency to slide downward pro-
duced by the weight of the body from above. In
addition, there are interlocking convolutions on
the articulating surfaces of the sacrum and ilium that
add stability and limit movement in certain direc-
tions. However, very small (0.5 to 1.6 millimeters)
movements of the sacroiliac joints can occur and
are important for normal pelvic mechanics (Chen,
Fredericson, and Smuck, 2002). These movements
involve a combination of rotation and translation
about complex and unclear axes. Unfortunately, as
discussed later in this chapter, the sacroiliac joints
are a very common site of injury and chronic pain
in dancers, particularly older dancers.
Movements of the Pelvic Girdle
The limited movement permitted at the pubic sym-
physis and sacroiliac joints allows the pelvic girdle to
essentially function as a single unit. This arrangement
is advantageous in terms of stability and of protective
and support functions of the pelvis. However, it is
limiting in terms of movement and hence the lower
spine, particularly the lumbosacral joint discussed
in chapter 3, becomes very important for facilitating
positional changes of the pelvis. The movements
of the pelvis are termed anterior tilt, posterior tilt,
lateral tilt, and rotation. They will be discussed later
in the chapter in connection with alignment and are
shown in figure 4.15 on page 178.
Joint Structure and Movements of the Hip
In upright posture and movements such as walking or
running, the weight of the upper body is transmitted
down through the spine and pelvis through one or
both rotary hip joints to be supported by the limb or
limbs. In addition to withstanding these downward
forces of gravity, the hip joint also transmits forces
from the ground to the pelvis in these same move-
ments. These important force transmission functions
of the hip joint make joint stability and strength a
priority. However, while stability is favored, sufficient
mobility must be present to facilitate economical
locomotor movement and allow for desired posi-
tioning of the foot and lower limb in space. The
joint architecture with its unique arrangement of
ligaments and multijoint muscles helps the hip meet
the bias toward stability, while still allowing adequate
mobility.Hip Joint Classification
and Associated MovementsThe hip joint, or acetabularfemoral joint, is a ball-
and-socket joint formed between the acetabulum
and the head of the femur. The acetabulum faces
anterolaterally and slightly inferiorly. The head of
the femur forms about two-thirds of a sphere and is
fully covered by articular cartilage except for a small
pit at the top of the head of the femur called the
fovea. The head of the femur faces upward and for-
ward relative to its neck, with its convex surface fitting
well with the concave surface of the acetabulum. Due
to the depth of the socket and the broad surface areas
of contact between the articulating bones, joint stabil-
ity is favored. Approximately 70% of the head of the
femur articulates with the acetabulum, in contrast to
only 25% contact of the head of the humerus in its
socket with the shoulder joint (Hamill and Knutzen,
1995). The hip joint is considered the best example
of a ball-and-socket joint in the body.
As with other classic ball-and-socket joints, the
hip joint has three degrees of freedom of motion:
flexion-extension in the sagittal plane, abduction-
adduction in the frontal plane, and external-internal
rotation in the transverse plane as seen in figure 4.3.
In many dance movements, combinations of these
three movement pairs occur. The true axis of motion
for the hip joint goes through the center of the femo-
ral head, which can be visually estimated from locat-
ing the greater trochanter. However, the neck of the
femur serves an important function of increasing the
lever arm for the muscles that attach onto the greater
trochanter (gluteus maximus, gluteus medius, glu-
teus minimus, deep outward rotators) so that these
muscles can produce markedly greater torque.Hip Joint Capsule and Key Ligaments
A strong, dense joint capsule encloses the entire hip
joint. It attaches from the margin of the acetabulum
and runs distally, encasing the neck of the femur
like a tube, to attach posteriorly to the distal neck
of the femur and anteriorly to the intertrochanteric
line. The capsule also has thickened ligamental
bands, as shown in figure 4.4, that spiral around the
neck of the femur and are named according to the
bone from which they originate—the iliofemoral,
pubofemoral, and ischiofemoral ligaments. Due to