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# Clinical Biomechanics

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```									Chapter 12- Clinical
Biomechanics
Brendan McElligott
Kimmi Dotseth
Joe Kotansky
Clinical Biomechanics?

• Biomechanics is the study of forces, and
their effect on living organisms
• Clinical Biomechanics is defined as the
application of biomechanics to the
treatment of patients, e.g., by orthopedic
specialists or physical therapists
Why does this pertain to us?
• Orthopedists, physical therapist’s,
occupational therapists, and athletic
trainers are health professionals who use
biomechanical concepts to evaluate and
treat patients
• Bioengineers, ergonominists, and human
factors specialists use biomechanical
concepts to understand how individuals
physically interact with their environment
What this chapter entail?

• In this chapter, the book looks at both
statics and dynamics, statics is when an
object is primarily stationary, whereas
dynamics is when there is movement
especially when applied to sports and
exercise.
The Scope of Clinical Biomechanics

• Based on content areas of anatomy,
mathematics, physics, and clinical sciences
• Additional content areas include specific
rehab techniques, wheelchair design,
anthropology, specific tissue repair,
surgical techniques, and architecture
Kinesiology in Biomechanics
• Kinesiology, the study of human movement, is
an important content area within biomechanics
•   Kinesiology involves the study of the skeletal
system, including the major joint articulations
and the major muscles and muscle groups that
are prime movers during exercise
•   This is essential to Exercise Science students
because they need to know which produce
movements and why
Focusing on science in Clinical
Biomechanics
• The main property in biomechanics is
force, which can be defined as a push or
pull
• A force that is applied externally to an
object is a load, when motion occurs,
force is the factor that causes a mass to
accelerate
Force continued….

• This is shown through the equation F= ma
• The exact definition of force however
must consist of four things: point of
application, line of application, direction
of push or pull, and magnitude
• All applications of forces and motions on
objects in biomechanics are subject to
Newton’s laws of motion
Gravity

• Gravity is the mutual attraction between
two objects
• The earths gravity on an object is called
the object’s weight
• The earth’s pull on an object is what we
consider “down”
Contact
• Whenever two object are in contact, a force acts
between them
•   This goes along with Newton’s 3rd law
•   Forces acting in the body can cause a few
different adverse things such as compression-
the process in which 2 forces act along the same
line in opposite directions toward each other and
tension, the process in which 2 forces act along
the same line in opposite directions away from
each other. The forces tend to pull the object
apart
Inertia
• “An object at rest
tends to remain at
rest, and an object in
motion tends to
remain in motion at a
constant velocity
unless acted on by an
external force.” – This
is inertia
Muscle
• It is important in biomechanics because it generates the
bodies forces
•   Different times of lifts/contractions mean different things
in biomechanics
•   Isometric contractions are when the muscle force is
equal to the resistance offered and there is no change in
length in the muscle
•   Concentric contraction occurs if the muscle force
exceeds the resistance offered and the distance between
the attachments decreases
•   A eccentric contraction occurs when the muscle force
exceeds the resistance offered and the muscle increases
in length
Elasticity

• Is defined as the capacity of an object to
reform to its original size and shape once
it has been deformed
• This can be seen through F=-kl, where k
is the material and l is the amount of
deformation
Composition and resolution of
forces
• Combining forces is called the composition of
forces
•   The process of dividing forces is called resolution
of forces
•   When 2 or more forces are subjected on an
object, the single force is called a resultant of
the forces
•   Because forces are vectors, most all forces are
associated with arrows to which the forces are
pushing or pulling
Resolution

• The process of resolution separates the
force into two perpendicular components
• This can be done either graphically or
mathematically, often found in geometry
Equilibrium

• When in equilibrium, the sum of the forces
and torques equal zero
• Called “static equilibrium” when an object
is at rest (Newton’s 1st law)
• First Condition:
Ʃ F = 0 (sum of forces equal zero)
• Second Condition:
ƳM = 0 (sum of torques equal zero)
Second Condition
• ƳM = 0
• M= moment
• OR: application of a force at a distance from axis
• Since the force does not act through the pivot
point, the object rotates
• In order to remain in static equilibrium (rest),
what must happen?
• The ability to determine force components is
essential to evaluate effects of moment on an
object
First-Class Levers (EOR)
• Point of axis (O) between two forces, Effort
(E) and Resistance (R)
•   One force will tend to rotate the object
clockwise, the other will tend to rotate the
object counterclockwise
•   The distance from the axis can determine the
magnitude of force needed to keep
equilibrium
•   Axis force will equal the sum of effort and
resistance
Second-Class Levers (ORE)
• Resistance is between the axis and effort
• Magnitude of effort is always less than
resistance
• Magnitude of force at axis point will
always be less than then force at
resistance

• Example: wheelbarrow
Third-Class Levers (OER)

• Effort between resistance and axis
• Magnitude of effort is always greater than
resistance
• Resistance will always move faster and
farther than effort
• Force at axis will be less than at effort

• Works well for throwing or kicking a ball
Strength of Materials
• Strength of a material is an object’s ability to
resist deformation when a load is placed on it
•   Strain- the measure of the change in dimensions
of an object
•   Mechanical stress- the property of a material to
resist deformation (units are force per unit area)
•   Three principal stresses and strains: tension,
compression, and shearing
Stresses and Strains
• Tension: two or more collinear forces act away
from each other
– Material ____________
• Compression: two or more collinear forces act
towards each other
– Material ____________
• Shearing: two or more non collinear, parallel
forces pointed in opposite directions act on the
material
– Material ____________

• Cause stresses and strains to arise
• Axial, bending, and torsion
• May occur alone or in combination

• Compression, torsion, and shearing
usually all occur to some degree

– EX: Intervertebral disc

• Will mainly have compression stress
• The widening of the disk suggests torsion
stress as well
• Shearing occurs at 45 degree angle to the
• Forces act in coplanar manner, but not
collinear
– EX: a beam supported at both ends, or foot
– EX: Cantilever
• Compression stress occurs in top part of
beam
• Tension occurs in bottom part of beam
• Shearing occurs parallel and
perpendicular to forces
Cantilever
– A horizontal beam is anchored at one end and loaded
at the other
– EX: diving board, proximal end of femur
•   Beam tends to bend
•   Compression occurs on lower side of beam
•   Tension occurs on upper part of beam
•   Shearing occurs perpendicular and parallel to
forces

• Rod or shaft is loaded so that it twists
around the long axis
– EX: removing lid from jar, spiral fracture of
tibia
• Compression and Tension occur along
spiraling lines
• Shearing occurs perpendicular and parallel
to the rod
Tissue
• Wolff’s law: the ability of the bone to
adapt (by changing size, shape, internal
structure) depends on mechanical stresses

• Important in early development
• Too much or too little can be dangerous

• EMG (Electromyography)- established technique that
records electrical signal from muscle motor units
– Used to determine muscle function
– Can be used to determine appropriate exercise and
rehabilitation programs
• Ergonomics
– Study of interaction between humans, the objects they use
and the environments in which they function
– Analyze tasks, define risk factors, redesign object or
environment for increased safety

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