Dance Anatomy & Kinesiology

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