Fundamentals of Biomechanics

Many professionals interested in human
movement function need information on
how forces act on and within the tissues of
the body. The deformations of muscles, tendons,
and bones created by external forces,
as well as the internal forces created by
these same structures, are relevant to understanding
human movement or injury.
This chapter will provide an overview of
the mechanics of biomaterials, specifically
muscles, tendons, ligaments, and bone. The
neuromuscular control of muscle forces
and the mechanical characteristics of muscle
will also be summarized. The application
of these concepts is illustrated using
the Force–Time Principle of biomechanics.
An understanding of mechanics of musculoskeletal
tissues is important in understanding
the organization of movement, injury,
and designing conditioning programs.
TISSUE LOADS
When forces are applied to a material, like
human musculoskeletal tissues, they create
loads. Engineers use various names to describe
how loads tend to change the shape
of a material. These include the principal or
axial loadings of compression, tension, and
shear (Figure 4.1). Compression is when an
external force tends to squeeze the molecules
of a material together. Tension is
when the load acts to stretch or pull apart
the material. For example, the weight of a
body tends to compress the foot against the
ground in the stance phase of running,
69
which is resisted by tensile loading of the
plantar fascia and the longitudinal ligament
in the foot. Shear is a right-angle
loading acting in opposite directions. A
trainer creates a shearing load across athletic
tape with scissor blades or their fingers
when they tear the tape. Note that loads are
not vectors (individual forces) acting in one
direction, but are illustrated by two arrows
(Figure 4.1) to show that the load results
from forces from both directions.
When many forces are acting on a body
they can combine to create combined loads
called torsion and bending (Figure 4.2). In
bending one side of the material is loaded
in compression while the other side experiences
tensile loading. When a person is in
single support in walking (essentially a
one-legged chair), the femur experiences
bending loading. The medial aspect of the
femur is in compression while the lateral
aspect is in tension.
RESPONSE OF TISSUES
TO FORCES
CHAPTER 4
Mechanics of the
Musculoskeletal System
The immediate response of tissues to loading
depends on a variety of factors. The
size and direction of forces, as well as
the mechanical strength and shape of
the tissue, affect how the material structure
will change. We will see in this section
that mechanical strength and muscular
strength are different concepts. This text
will strive to use “muscular” or “mechanical”
modifiers with the term strength to