Fundamentals of Biomechanics

154 FUNDAMENTALS OF BIOMECHANICS
Figure 6.17. Raising a bowling ball in the approach stores more potential energy in the ball than the kinetic energy
from the approach. The potential energy of the ball can be converted to kinetic energy in the downswing.
energy in some non-useful form and never
achieve 100% efficiency. This is similar to
the energy losses (hysteresis) in strain energy
stored in deformed biological tissues
studied in chapter 4.
Strain energy is the energy stored in
an object when an external force deforms
that object. Strain energy can be viewed
as a form of potential energy. A pole
vaulter stores strain energy in the pole
when loading the pole by planting it in the
box. Much of the kinetic energy stored in
the vaulter's body during the run up is converted
into strain energy and back into kinetic
energy in the vertical direction.
Unfortunately, again, not all the strain energy
stored in objects is recovered as useful
energy. Often large percentages of energy
are converted to other kinds of energy that
are not effective in terms of producing
movement. Some strain energy stored in
many objects is essentially lost because it is
converted into sound waves or heat. Some
machines employ heat production to do
work, but in human movement heat is a
byproduct of many energy transformations
that must be dissipated. Heat is often even
more costly than the mechanical energy in
human movement because the cardiovascular
system must expend more chemical
energy to dissipate the heat created by vigorous
movement.
The mechanical properties of an object
determine how much of any strain energy
is recovered in restitution as useful work.
Recall that many biomechanical tissues are
viscoelastic and that the variable hysteresis
(area between the loading and unloading
force-displacement curves) determines the
amount of energy lost to unproductive energies
like heat. The elasticity of a material
is defined as its stiffness. In many sports involving
elastic collisions, a simpler variable
can be used to get an estimate of the elastic-