Biology Reference
In-Depth Information
Definitions and Basic Principles of Biomechanics
Force
The impacting force or type of applied load plays a key role in fracture creation and prop-
agation. Force is defined as an “action or influence” that is “applied to a free body” ( Turner
and Burr, 1993 ). In other words, a force is anything that alters the state of motion of an object
( Low and Reed, 1996 ). A force simply pushes or pulls on an object. Newton's first law of
motion states that a force must be applied to change the velocity or direction of movement
of an object. Newton's second law of motion states that the resulting change in momentum
of the object is proportional to the force applied ( Low and Reed, 1996 ). As an example, the
more force that is applied in hitting a baseball with a bat, the faster the ball will travel. Force
(F) is calculated as mass (m) times acceleration (a):
F
ma
¼
Force is measured in newtons (N) or pounds (lb). Force is a “vector quantity,” meaning
that it has direction or magnitude. This is an important concept for understanding skeletal
trauma biomechanics, where the direction of the force applied to the bone becomes an impor-
tant variable, as will be explained later.
Load
A load is a force, or combination of forces that is sustained by an object ( Low and
Reed, 1996; Frost, 1967 ). For example, the weight of the human body creates a load on
the feet.
Stress
When examining load type, the most common term used is “stress.” Stress is defined as
“force per unit area” ( Turner and Burr, 1993 ), thus calculated:
Stress
force
area
¼
=
Stress is calculated by newtons per square meter. The unit of 1 newton per square meter
(N/m 2 ) is 1 pascal. Stress is reported in pascals.
Stress is further subdivided into three areas: compressive stress, tensile stress, and shear
stress ( Alms, 1961; Nordin and Frankel, 1989; Turner and Burr, 1993 ). Compressive stress is
developed when a load acts to make the material shorter. Likewise, tensile stress is formed
when a load works to stretch the material. Shear stress results when one area of material
slides across another area of material. These three types of stress do not exist in isolation.
No matter how simple the loading scheme, compressive, tensile, and shear stress are always
occurring in combination.
Strain
The magnitude of load is referred to in terms of strain. Strain is defined as “percentage
change in length, or relative deformation” ( Turner and Burr, 1993 ):
Strain
increased length
original length
¼
=
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