6 Dance Anatomy and Kinesiology
7% of our bone mass is recycled in a week, and as
much as a half a gram of calcium can enter or leave
the adult skeleton in a single day (Marieb, 1995). So,
although bone is very hard, it is very alive and is con-
tinually changing in response to many factors includ-
ing the mechanical stresses to which it is exposed.
This relationship of stress to bone development was
actually expressed a long time ago (in 1892) by Julius
Wolff. Wolff’s law holds that changes in the internal
architecture of bone and the external conformation
of bone will occur in accordance with mathematical
laws and in response to the forces acting on bones.
The primary forces acting on bones are believed to
relate to the contraction of muscles and the loading
of bones in weight-bearing activities.
It appears that the longitudinal stresses (com-
pression) related to weight bearing are particularly
potent for instigating bone deposition and may be
due to the piezoelectric effect (G. piez , to press +
electricity). In the 1950s it was shown that when
bone is placed under stress, an electrical gradient is
generated. The side of the bone under compression
becomes electronegative while the side under ten-
sion becomes electropositive, creating an electrical
potential that appears to stimulate bone deposition
(Enoka, 2002; Mercier, 1995). Although Wolff’s law
has gone through some modifications in more recent
years to include additional factors, the concept is
still germane. Healthy bones remodel in response to
mechanical demands; they lay down new bone where
needed and resorb bone where it is not needed.
With Wolff’s law in mind, it is not surprising that
bone density is very much influenced by activity. The
mechanical stresses associated with walking, run-
ning, and dancing provide an important stimulus
to encourage maintenance of healthy bone density,
and bone density has been shown to be differen-
tially increased in relation to the associated stresses
of those activities. For example, some runners may
show increased bone density in the lower leg bones;
tennis players may show increased bone density of
the arm bones on their dominant side; and ballet
dancers may show thickening of the shaft of the
second metatarsal bone of the foot.
Although the most potent effect on bone density
appears to relate to high-impact weight-bearing
physical activity, forceful muscle contractions without
weight bearing can also positively influence bone
density; and greater bone density has also been
shown to be associated with stronger muscles and
greater muscle mass (Andreoli et al., 2001; Frost,
2000; Stewart and Hannan, 2000). Conversely, even
young children who are hospitalized, individuals of
any age whose limbs are immobilized in casts, and
healthy young individuals involved in space flight
(Hall, 1999; Roy, Baldwin, and Edgerton, 1996;
Zernicke, Vailas, and Salem, 1990) experience loss
of bone density (osteopenia) that can lead to gross
structural weakening of bones (increased porosity
in bone termed osteoporosis), probably due to
inadequate forces borne by bone. For example,
bed rest of four to six weeks can result in signifi-
cant bone density losses that are not fully reversed
with six months of normal weight-bearing activity;
and astronauts may lose up to 19% of their weight-
bearing bone on extended missions.
Bone remodeling and density are also influenced
by race, age, calcium availability, hormones, and
gender. For example, in terms of ethnicity, African
Americans tend to have greater bone density than
Caucasians, which is conjectured to be linked to
greater muscle mass in African Americans (Burr,
1997; Hall, 1999). In terms of age, bone deposition
outweighs bone resorption in healthy children,
resulting in net growth in bones. In younger adults,
bone resorption and bone deposition proceed
at similar rates. In older adults, bone resorption
FIGURE 1.3 Bone growth in youth. (A) Growth in bone
length via endochondral ossification; (B) growth in bone
girth via appositional growth and maintenance of propor-
tions via remodeling.
A Growth in length
via endochondral
ossifi cation
B Growth
in girth and
remodeling