Fundamentals of Biomechanics

76 FUNDAMENTALS OF BIOMECHANICS
forces of the large calf muscle group (soleus
and gastrocnemius).
BIOMECHANICS OF BONE
Unlike muscle, the primary loads experienced
by most bones are compressive. The
mechanical response of bone to compression,
tension, and other complex loads depends
on the complex structure of bones.
Remember that bones are living tissues
with blood supplies, made of a high percentage
of water (25% of bone mass), and
having considerable deposits of calcium
Application: Osteoporosis
Considerable research is currently being directed
at developing exercise machines as countermeasures
for the significant bone density loss in
extended space flight. A microgravity environment
substantially decreases the loading of the
large muscles and bones of the lower extremity,
resulting in loss of bone and muscle mass.There
is also interest in exercise as a preventative and
remedial strategy for increasing the bone mass
of postmenopausal women. The strong link between
the positive stresses of exercise on bone
density, however, is often complicated by such
other things like diet and hormonal factors. In
the late 1980s researchers were surprised to
find that elite women athletes were at greater
risk for stress fractures because they had the
bone density of women two to three times their
age. Stress fractures are very small breaks in
the cortical (see below) bone that result from
physical activity without adequate rest. What
was discovered was that overtraining and the
very low body fat that resulted in amenorrhea
also affected estrogen levels that tended to decrease
bone mass.This effect was stronger than
the bone growth stimulus of the physical activity.
High-level women athletes in many sports
must be careful in monitoring training, diet, and
body fat to maintain bone mass. Kinesiology professionals
must be watchful for signs of a condition
called the female athlete triad. The female
athlete triad is the combination of disordered
eating, amenorrhea, and osteoporosis that sometimes
occurs in young female athletes.
salts and other minerals. The strength of
bone depends strongly on its density of
mineral deposits and collagen fibers, and is
also strongly related to dietary habits and
physical activity. The loading of bones in
physical activity results in greater osteoblast
activity, laying down bone.
Immobilization or inactivity will result in
dramatic decreases in bone density, stiffness,
and mechanical strength. A German
scientist is credited with the discovery that
bones remodel (lay down greater mineral
deposits) according to the mechanical stress
in that area of bone. This laying down of
bone where it is stressed and reabsorption
of bone in the absence of stress is called
Wolff's Law. Bone remodeling is well illustrated
by the formation of bone around the
threads of screws in the hip prosthetic in
the x-ray in Figure 4.6.
The macroscopic structure of bone
shows a dense, external layer called cortical
(compact) bone and the less-dense internal
cancellous (spongy) bone. The mechanical
Figure 4.6. X-ray of a fractured femur with a metal
plate repair. Note the remodeling of bone around the
screws that transfer load to the plate. Reprinted with
permission from Nordin & Frankel (2001).