Basic Biomechanical
Principles
Review Course for Assistive
Technology Practitioners and
Suppliers
Learning Objectives
• Basic concepts of biomechanics
• Relate basic biomechanical principles to
AT assessment and intervention
Basic concepts
• Mechanics: basic science dealing with
the motion of substances
– solids, liquids and gases
• Biomechanics: applying the
principles of mechanics to the human
body
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Basic Concepts
• Kinematics: the study of
movement
– Translational movement
– Rotational movement
• Kinetics: the study of forces
Newton’s Laws of Motion
• First: Inertia
– a body at rest remains at rest until acted on
by an external force
• Second: Acceleration of a body will be
directly proportional to any unbalanced
force applied
– Force =Mass X Acceleration or F=MA
• Third: For every action, there is an
equal and opposite reaction
Forces
• Cause an object to move or to
change direction
• Four characteristics
–
–
–
–
Magnitude
Direction
Action line
Point of application
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Types of Forces
• Compressive – two forces towards
each other along the same line
• Tensile – two forces away from each
other along the same line
• Shear – two forces away from each
other parallel to each other
Pressure
• Pressure = total force/ contact area
• If high forces are needed – increased
contact area will decrease pressure
• Example: pressure considerations may
affect the choice of planar vs. contoured
seating surface or materials used
Friction
• Force that resists motion of one body
relative to another body – surface force
– Resists ‘sliding’
– Increasing friction forces increases stability
– Increasing friction forces increases shear
within the body
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Torque
• A force that causes rotation of a body
around an axis (AKA - Moment)
Torque and lever arms
• Torque = Force X perpendicular distance from
the axis of rotation
• Longer lever arm = lower force required
• Example: when choosing a switch for an
individual with severe weakness, may want to
use a switch on a long lever to decrease force
necessary (e.g. wobble switch)
Motion concepts
•
•
•
•
Producing vs. preventing movement
Static equilibrium
Dynamic equilibrium
Balance of forces – essential in
preventing movement
• Provide “least restrictive” possible
equipment – essential in enabling
movement
Cook and Hussey (2002), p. 175
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Biomechanics References
• Cook and Hussey (2002), pp. 173-175
• Nwaobi, O. M.: Biomechanics of
Seating. In Trefler, E., editor; Seating
for Children with Cerebral Palsy; a
resource manual, Memphis, 1984,
University of Tennessee
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