Center of Gravity
Hamilton & Luttgens Chapter 14
Objectives
1. Define center of gravity, and explain the
basis for its location in the human body
2. Estimate the location of the center of gravity
of individuals in any position
3. State the principles of equilibrium, and
explain and demonstrate applications of each
4. Locate the center of gravity of an individual
using either the reaction board or the
segmental method
CENTER OF GRAVITY (CG)
The “balance point” of
the body
The point where the
weight of the body acts
The point where all
forces acting on the body Fig 14.1
equal zero
– Linear forces must be
balanced
– Rotary forces must be
balanced
CENTER OF GRAVITY
The location of the
CG remains fixed as
long as the body
does not change
shape Fig 14.3
If an object’s shape
or position changes,
the location of the
CG changes
CENTER OF GRAVITY
As one changes
the relationship of
the body
segments to each
other, the CG may
even be located
outside the body
Fig 14.4
Placement of the Center of
Gravity in Humans
The location of the CG of a human being
in the normal standing position varies
with body build, age, and sex
Placement of Center of Gravity
Male’s CG is ~57% of standing height
In quiet standing, the CG can be
considered to be almost directly over the
center of pressure
Center of pressure is the point at which
the force vector for ground reaction force
is applied
Female’s CG is ~ 55% of standing height
STABILITY AND EQUILIBRIUM
All objects at rest are in equilibrium
All forces acting on them are balanced
The sum of all linear forces equals zero
The sum of all torques equals zero
However, all objects at rest are not
equally stable
Stable Equilibrium
Occurs when an
object is placed in
such a fashion that
an effort to disturb
it would require its Fig 14.5a
CG to be raised
Unstable Equilibrium
A slight disturbance
will drop the objects
CG to a lower point
An unstable objects
seeks stability by Fig 14.5b
falling from a
smaller to a larger
base of support
Neutral Equilibrium
Occurs when an
object’s CG is
neither raised nor
lowered when it is
disturbed Fig 14.5c
Remains the same
Inertial property
Adjusting the Center of Gravity
Humans spend most of their time adjusting
their positions to the type of equilibrium
best suited to the task and environment
Factors Affecting Stability
The ability to maintain one’s balance
under unfavorable circumstance is
recognized as one of the basic motor skills
The following factors affecting the stability
of a performer’s equilibrium state
– should make analysis easier
– may suggest means for improvement
Horizontal Center of Gravity
There are different types of support
systems in order to maintain equilibrium
and stability
Size of the base of Support
CG must remain
within the base of
support in order to Standing
maintain stable and Walking
in equilibrium
Easier with larger
base of support
Headstand
Fig 14.6
Shape of the base of Support
Fig 14.6c
Fig 14.6b
Resistance to Resistance to
AP forces lateral forces
Base of Support
Both feet and hands a quadruped
Babies crawl, roll and sprawl in their form
of locomotion
Children and adults biped
Seniors quadruped with the help of a
walker
Age and fitness specific
Vertical Center of Gravity
Height and location of Center of Gravity
along a vertical line (y axis)
Height of the Center of
Gravity
a>b>c
with x
respect to x
lateral
stability Fig 14.8
x
Height of Center of Gravity (CG)
Height of CG changes with body position
along the vertical line
As CG moves closer to base of support
more angular displacement can occur
before it goes beyond the base of support
Relationship of the Line of
Gravity to the Base of Support
To maintain equilibrium, line of gravity
must remain within its base of support
Fig 14.9 &
14.10
Mass of the Body
Only a factor when motion or an external
force is involved
Amount of force needed to effect a
change in motion is proportional to the
mass being moved
The greater the mass, the greater the
stability
Friction
Friction is related to the size of the base of
support
It has greater influence when body is in
motion or being acted on by an external
force
Inadequate friction makes it more difficult
to maintain equilibrium
Segmental Alignment
The human body consists of a series of
segments placed one above the other, the
problem of retaining equilibrium is a
multiple one
When segments are aligned in a single
vertical line, there is less likely hood of
strain to joints and muscles
When one segment gets out of line,
another segment must compensate for it
Visual and Psychological Factors
The effect of crossing a swirling river on a foot
bridge is a detriment to one’s equilibrium
Even if the supporting surface is adequate
The sense of balance may be disturbed,
extraordinary stimuli
Compensation: fix eyes on a stationary spot
above or beyond the “danger area”
Seems to facilitate neuromuscular control by
reducing the disturbing stimuli
Physiological Factors
Semicircular canals can affect equilibrium
– Colds, viruses, and other problems can affect
the inner ear may also interfere with balance
Any disturbance of the general physical
condition is likely to affect the sense of
balance
Principles of Stability:
I. Other things being equal, the lower the
CG, the greater will be the body’s
stability
II. Greater stability is obtained if the base of
support is widened in the direction of the
line of force
Principles of Stability
III. For maximum stability the line of gravity
should intersect the base of support at a
point that will allow the greatest range of
movement within the area of the base in
the direction of the forces causing motion
Principles of Stability:
IV. Other things being equal, the greater the
mass of a body, the greater will be its
stability
V. Other things being equal, the most stable
position of a vertical segmented body is
one in which the CG of each weight-
bearing segments lies in a vertical line
centered over the base of support
Principles of Stability:
VI. Other things being equal, the greater the
friction between the supporting surface
and the parts of the body in contact with
it, the more stable the body will be
VII. Other things being equal, a person has
better balance in locomotion under
difficult circumstances when the vision is
focused on stationary objects rather than
on disturbing stimuli
Principles of Stability:
VIII.There is a positive relationship between
one’s physical and emotional state and
the ability to maintain balance under
difficult circumstances
IX. Regaining equilibrium is based on the
same principles as maintaining it.
Mobility
Mobility & stability have an inverse
relationship
A critical point is the the change form a
position of stability to a state of mobility &
vice versa
Mobility
To initiate a step, line of gravity must be
shifted forward of the base of support,
swing then moves forward to reestablish a
base of support
Often is sport, it is necessary to alter
stability intentionally to become mobile
Mobility
Fig 14.14a
Mobility
Often is sport, it is necessary to alter
stability intentionally to become mobile
Putting the body into motion
Ability to start, stop, or change direction
quickly depends on manipulating the
stability of the body
Mobility
Both speed and direction of desired
mobility are used to determine the nature
of the change in stability required to
initiate motion
Mobility
To enhance the speed of a start, the line
of gravity should be as close as possible to
the edge of the base of support
A quick stop requires a large base of
support, lower CG, and move the line of
gravity away form the leading edge of the
base of support
FINDING THE CENTER OF GRAVITY
IN THE HUMAN BODY
Reaction Board Method
Segmental Method
Reaction Board Technique
Relies on the fact that the sum of the
moments acting on a body in equilibrium is
zero
The location of the gravitational line is found
for each plane
The CG is the intersection of the values for
each of these three planes
Limited to the body in a stationary position
Segmental Method
A highly useful procedure find the CG for
someone in action
Technique uses;
a photograph of the subject
Involves finding the CG of each body segment
The position of these gravity points are placed
on x and y axes
Knowledge of the ratio between the individual
segment weight and total body weight
Summary and Discussion
Methods of finding the Center of Gravity
Conceptual Framework
– Stability and equilibrium
– Mobility
– Psychological Factors
– Segmental Alignment
– Balance
– Center of Pressure
Center of Gravity
Vertical Alignments