2. CENTER OF GRAVITY (COG) :
Center of gravity (COG) : is an imaginary point
that the body weight can be assumed to be
concentrated and equally distributed, around
which body can rotate freely in all direction.
• The point where a body may be hung without
rotating.
• The point at which summation of all moments
equal zero
• The point where the weight of the body is
concentrated.
• Its symbol is COG , COM or CG .
• Anatomically; This point can be represented by
the point of intersection of the three cardinal
body planes (Sagittal , frontal and transverse).
Anti- Clockwise Moment = Clockwise Moment
5. • The center of gravity can be
located within or outside the
body depending on the bodies
configuration and position; it is
inside an object when the object
is uniform and outside the
object when it is not uniform.
6. LOCATION OF CENTER OF GRAVITY :
B. IN IRREGULARLY-SHAPED BODIES:
• The COG will be nearer the larger and
heavier end
7. LOCATION OF CENTER OF GRAVITY :
IN HUMAN BEING
• It’s is located anterior to the second sacral vertebra in
the anatomical position (standing) in normal adult
person.
1) In new born: above umbilicus.
2) At six month: at the level of six thoracic vertebra.
3) At two years: at the level of umbilicus.
4) At five years: below the level of umbilicus.
5) Adult: anterior to the second sacral vertebra.
8. FACTORS AFFECTING COG LOCATION:
1. Age : relatively higher in infants and
children compared to adults
2. Sex – higher in males than in females
3. Height
4. Body Type – Mesomorph, Endomorph
& Ectomorph
5. Weight : Added (pregnancy) or
Subtracted (amputation)
6. Position of Any Segment in Relation
to Total Body Segments: flexion of
right arm leads to movement of COG
upward, forward and to the right.
10. DETERMINATION OF COG LOCATION
• Total Body COG The total body COG can
be determined using mathematical or
laboratory methods.
A) Total Body COG:
1. Mathematical Method
2. Laboratory Method
B) Determination of Segmental Body COG
11. A) TOTAL BODY COG:
1) MATHEMATICAL METHOD
A. CROSKEY FORMULA
• In male:
Total body height x 56.18
100
=
height of COG measured from heel
• In female:
Total body height x 55.44
100
=
height of COG measured from heel
• E.g.:
176x 56.18
100
=98.87cm
B. PALMAR FORMULA
• regardless to the subject sex
•
Total body height x 55.7
100
+1.4 = height
of COG measured from heel
• E.g.:
176x 55.7
100
+1.4 =99.43cm
12. A) TOTAL BODY COG:
2) LABORATORY METHOD
• Board and Scale Method
13. B) DETERMINATION OF SEGMENTAL BODY
COG
• Approximately 4/7 of the segment
length measured from the distal end.
• E.G.: 30cm *4/7 =17.14cm
14. LINE OF GRAVITY (LOG):
• is an imaginary vertical line passing
through the COG toward the center
of the earth
• When the human body is in the
standing position the line of gravity
passes from the vertex through the
2nd sacral vertebra to just in front of
the ankle joint and between the feet.
15. BASE OF SUPPORT (BOS)
• is that part of a body that is in contact
with the supporting surface.
18. YOU MUST KNOW
• Equilibrium: is a state of balance in
which all forces are equal.
• Balance :is the ability to control
equilibrium during changing body’s
position.
• Stability: is a measure of the body’s
ability to maintain its original position.
19. EQUILIBRIUM
Definition;
Equilibrium is a state of balance in
which all forces are equal. A body is
said to be in equilibrium when the
resultant of all forces acting upon it is
zero.
Types of Equilibrium
Stable:
Unstable: a person
standing on one
leg
Neutral: a ball.
G
T
F M
20. 1-STABLE EQUILIBRIUM
The lower the COG and the wider the base of support the more and more equilibrium
will be established, E.g. person lying flat on the floor.
• when it returns to its original position after given a small displacement
Example :
The Traffic Cone
• Low Centre of Gravity
• Wide base & lower COG
21. 2- UNSTABLE EQUILIBRIUM
• If the object is displaced slightly and it tends to increase its displacement
• The COG drops to a lower point when compared to the original starting position e.g. a
cone
• An object becomes unstable when its Centre of Gravity falls outside of the object’s
base.
22. 3- NEUTRAL EQUILIBRIUM;
• if it comes to rest in a new position without a change in the level of the COG either
upwards or downwards
• E.g. A ball
25. CONDITIONS OF EQUILIBRIUM
• 1- If the body is at rest, with the velocity equaling zero, it is said to be in static
equilibrium.
∑F = 0, ∑M = 0
• 2- If the constant velocity is not zero, the equilibrium is called dynamic equilibrium
26. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface (The greater the BOS ,
the greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Also, stability increased if the increase in BOS in
the direction of applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
27. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface(The greater the BOS , the
greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Especially if the increase in BOS in the direction of
applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
28. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface(The greater the BOS , the
greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Also, stability increased if the increase in BOS in
the direction of applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
29. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface(The greater the BOS , the
greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Also, stability increased if the increase in BOS in
the direction of applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
30. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface(The greater the BOS , the
greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Also, stability increased if the increase in BOS in
the direction of applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
31. FACTORS AFFECTING STABILITY
1. COG height ( the Lower the COG, the Greater will be the body
Stability. Base of support (BOS) surface(The greater the BOS , the
greater the stability.)
2. BASE OF SUPPORT (BOS): An greater the BOS will be increase in
the stability. Also, stability increased if the increase in BOS in
the direction of applied force.
3. Line of gravity :For balance to be maintained, it is essential that
the line from LOG should fall within the BOS
4. PROPERITIES OF SUPPORTING SURFACE:
a) Friction (increase stability via friction)
b) Softness of the supporting surface(soft mattress , the less the
stability.)
c) Inclination of the supporting surface(The greater the inclination,
the less the stability)
5. Segmentation Principle If there is deviation of a part of the body
to certain direction, there is another compensator deviation of another
part of the body to the opposite direction to maintain balance during
this position
6. HE PERSON HIMSELF :Mass , Vision, Physical & Emotional State ,
Pain and Age
Editor's Notes
Features of a simple machine They do not use electricity They have one or fewer moving parts They give us mechanical advantage Even though they make work easier for us, they still need input (force or effort) from a person. They make tough jobs easier by changing the force, direction or speed of a movement
