Physics : In Biology and Medicine : Intro

Mechanics is the branch of physics concerned with the effect of forces on the
motion of bodies. It was the first branch of physics that was applied successfully
to living systems, primarily to understanding the principles governing the
movement of animals. Our present concepts of mechanics were formulated
by Isaac Newton, whose major work on mechanics, Principia Mathematica,
was published in 1687. The study of mechanics, however, began much earlier.
It can be traced to the Greek philosophers of the fourth century B.C. The
early Greeks, who were interested in both science and athletics, were also
the first to apply physical principles to animal movements. Aristotle wrote,
“The animal that moves makes its change of position by pressing against
that which is beneath it. . . . Runners run faster if they swing their arms for
in extension of the arms there is a kind of leaning upon the hands and the
wrist.” Although some of the concepts proposed by the Greek philosophers
were wrong, their search for general principles in nature marked the beginning
of scientific thought.
After the decline of ancient Greece, the pursuit of all scientific work
entered a period of lull that lasted until the Renaissance brought about
a resurgence in many activities including science. During this period of
revival, Leonardo da Vinci (1452–1519) made detailed observations of animal
motions and muscle functions. Since da Vinci, hundreds of people have
contributed to our understanding of animal motion in terms of mechanical
principles. Their studies have been aided by improved analytic techniques
and the development of instruments such as the photographic camera and
electronic timers. Today the study of human motion is part of the disciplines

of kinesiology, which studies human motion primarily as applied to athletic
activities, and biomechanics, a broader area that is concerned not only with
muscle movement but also with the physical behavior of bones and organs
such as the lungs and the heart. The development of prosthetic devices such
as artificial limbs and mechanical hearts is an active area of biomechanical
research.
Mechanics, like every other subject in science, starts with a certain number
of basic concepts and then supplies the rules by which they are interrelated.
AppendixA summarizes the basic concepts in mechanics, providing a review
rather than a thorough treatment of the subject. We will now begin our discussion
of mechanics by examining static forces that act on the human body.
We will first discuss stability and equilibrium of the human body, and then we
will calculate the forces exerted by the skeletal muscles on various parts of
the body.