Download Basic Biomechanics, (5th edition) by Susan J. Hall, Ph.D.

Chapter 13
Equilibrium and
Human Movement
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
McGraw-Hill/Irwin
© 2012 The McGraw-Hill Companies, Inc. All rights reserved.
Equilibrium
What is torque?
• the rotary effect of a force about an
axis of rotation
• measured as the product of force and
the force’s moment arm - the
shortest (perpendicular) distance
between a force’s line of action and
an axis of rotation
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-2
Equilibrium
Where do torques occur within the
human body?
The product of muscle tension and
muscle moment arm produces a
torque at the joint crossed by the
muscle.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-3
Equilibrium
Skilled pitchers often maximize the length of the moment arm
between the hand and total-body axis of rotation during the
delivery of a pitch to maximize the effect of the torque
produced by the muscles.
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-4
Biomechanical Levers
• Levers: simple machines
• Lever: rigid bar that turns about an axis.
• Levers increase amount of torque generated
by a muscle.
Parts of a lever
•
•
•
•
•
Fulcrum
Force point
Resistance point
Force arm (Farm)
Resistance arm (Rarm)
Classes of levers
• Levers are classified according to the
arrangement of the force point, resistance
point, and fulcrum.
Equilibrium
F
Relative locations
of the applied
force (F), the
resistance (R),
and the fulcrum
or axis of rotation
determine lever
classifications.
R
First class
R
F
Second class
F
R
Third class
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-8
Mechanical Advantage
• If the force arm > resistance arm, the lever
favors force.
• If the resistance arm > force arm, the lever
favors speed and range of motion.
• If they are equal = favors neither.
Third class lever
• Force point between fulcrum
and resistance.
• Rarm > Farm
• Favors speed and ROM
• Most common in human body
Second class lever
• Resistance point between
fulcrum and force point.
• Farm > Rarm
• Favors force
First class lever
• Fulcrum between Force point
and Resistance point
• Length of Farm and Rarm
depend on location of fulcrum.
• Can favor either force or speed,
depending on length of Farm
and Rarm.
Equilibrium
What is torque?
• the rotary effect of a force about an
axis of rotation
• measured as the product of force and
the force’s moment arm - the
shortest (perpendicular) distance
between a force’s line of action and
an axis of rotation
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-13
Center of Gravity
What is the center of gravity?
• the point around which a body’s weight
is equally balanced in all directions
• also referred to as the center of mass
or mass centroid
• (need not be physically located inside
of a body)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-14
Center of Gravity
• Point at which all body’s mass and weight is
evenly distributed in all directions.
• Center of gravity is dependent upon:
– Body shape (anthropometrics, gender)
– Body position
• Base of support
Stability and Balance
What is stability?
• resistance to disruption of equilibrium
What is balance?
• ability to control equilibrium
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-16
Stability and Balance
What is the base of support?
(area bound by the outermost regions
of contact between a body and
the support surface)
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-17
Stability and Balance
What can increase a body’s stability?
• increasing body mass
• increasing friction between the body
and the surfaces of contact
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-18
Stability and Balance
What can increase a body’s stability?
• increasing the size of the base of
support in the direction of an
external force
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-19
Stability and Balance
What can increase a body’s stability?
• horizontally positioning the center of
gravity near the edge of the base of
support on the side of the external
force
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-20
Stability and Balance
What can increase a body’s stability?
• vertically positioning the center of
gravity as low as possible
The higher the
CG, the greater
the amount of
torque its motion
creates about
the support
surface.
d
d
d
Basic Biomechanics, 6th edition
By Susan J. Hall, Ph.D.
13-21