The Skeletal System and Its Movements 9greater risk of stress fractures than male athletes
(Browning, 2001), and a study of ballet students
found that young females had about twice the risk
of developing stress fractures as young males and
that this risk is further heightened during adoles-
cence; 70% of the stress fractures in female dancers
occurred during the late adolescent period of 15 to
19 years of age (Lundon, Melcher, and Bray, 1999).
Furthermore, a study of female dancers found an
older age of onset of menstruation (menarche), and
the incidence of menstruation stopping (secondary
amenorrhea) was twice as high among dancers with
stress fractures as compared to dancers without stress
fractures (Warren et al., 1986). Even more dramatic,
another study of professional ballet dancers found a
female dancer who had amenorrhea longer than 6
months had an estimated risk for stress fracture 93
times that of a dancer who did not have amenorrhea
(Kadel, Teitz, and Kronmal, 1992).
Training errors, such as an increase in exercise
intensity or duration that is too great, can also be
important (Brukner, Bradshaw, and Bennell, 1998),
and one study of runners with stress fractures found
27% of cases developed after rapid commencement
of training (Taunton, Clement, and Webber, 1981).
Another study of runners found training errors in
22.4% of 320 stress fractures (Matheson et al., 1987).
Although not substantiated, in dance, a sudden
increase in workload (especially pointe work or
jumps); rapid changes in dance style, technique, or
floor surfaces; and excessive fatigue may contribute
to stress fracture risk. One study of professional ballet
dancers showed a 16 times greater risk for those
dancing more than 5 hours per day when compared
with those dancing less than 5 hours (Kadel, Teitz,
and Kronmal, 1992).
Stress fracture prevalence has been reported to
be as high as 61% in professional ballet dancers
(Warren et al., 1986), and further research will be
necessary to understand the relative significance
of the various causative factors in dancers. In the
interim, current study results suggest that application
of sound training principles, swift medical referral
when amenorrhea is present, healthy nutritional
practices including adequate calcium intake, and
smoking cessation can aid in the prevention and
treatment of stress fractures.
The Human Skeleton.
There are 206 bones in the adult human skeleton,
177 that can engage in voluntary movement (Ham-
ilton and Luttgens, 2002). The major bones of the
skeleton are shown in figure 1.4.
The Axial and Appendicular Skeleton
The skeleton has two major divisions—the axial
skeleton (L. relating to an axis) and the appendicu-
lar skeleton. As its name implies, the axial portion
forms the central upright “axis” of the skeleton,
and includes the skull, vertebral column, sternum,
and ribs (figure 1.4A). The skull contains 28 bones,
which form the face (facial bones) and remainder
of the skull (cranial bones). This book will simplify
this area and simply refer to the bones of the skull
as a unit. The skull provides an essential protective
function for the vulnerable brain and plays an impor-
tant part in housing the senses of sight, smell, taste,
and hearing. The sternum (commonly referred to
as the breastbone; G. sternon, the chest) and the
12 ribs with their adjoining cartilages help form
the thorax, which provides important protection
for the lungs and heart. Thirty-three vertebrae
form the vertebral column (commonly referred to
as the spine). The segmented property of the spine
allows it to be flexible and capable of a wide variety
of movements. Consecutive vertebrae form a canal
that houses and protects the very important and
fragile spinal cord.
The appendicular skeleton is composed of the
bones of the limbs (appendages), which are hung
upon or attached to the axial skeleton as seen in
figure 1.4B. The appendicular skeleton contains two
additional subdivisions, the paired upper extremity
and the paired lower extremity. The upper extrem-
ity is composed of the bones of the shoulder girdle,
upper arm, lower arm, wrist, and hand. The shoulder
girdle consists of the paired clavicles (commonly
called collarbones) and scapulae (L. the shoulder
blades). The upper arm bone is called the humerus
(L. shoulder), while bones of the lower arm are the
radius (on the thumb side; L. rod, ray) and ulna
(L. arm). The upper part of the hand contains two
rows of small bones called the carpals (eight bones);
followed by five rays of bone found in the “palm” of
the hand, called the metacarpals; and the 14 digits
of the fingers called the phalanges.
The lower extremity is composed of the bones
of the pelvic girdle, thigh, lower leg, and the ankle-
foot. The pelvic girdle is composed of two paired
hip bones called os innominatum or os coxae that
connect to each other in the front and to the sacrum
behind. In the young child, each os coxae is made
up of three separate bones: the ilium (upper wing-
like portion of the pelvis), ischium (lower portion),
and pubis (front portion). These bones later fuse
together. The thigh bone is called the femur, and the
lower leg bones are the tibia and fibula. The tibia is