الرحيم الرحمن الله بسم

• The balancing, equilibrium or pivoting point of the body.

• It is the point where the sum of all the forces and force movements acting on the body is zero.

• It is the point at which all the weight of the body may be concentrated and about which all the parts exactly balance.

1. In the human body, when standing in anatomical position it is located anterior to the second sacral vertebra.

2. The location remains fixed as long as the body does not change shape.

3. The location of the center of gravity can be outside the human body during activities depending on the relationship of body segments.

1. It is an imaginary vertical line passing through the center of gravity down to a point of the base of support.

2. It passes from the vertex through the body second sacral vertebra to point between feet when standing in the anatomical position

3. The gravitational pull acting at the center of gravity of any segment is also expressed by a vertical line called line of gravity

Factors affecting location of the Total Body Center of Gravity

Age

SexAddition or subtraction

Of weight

Body build

Change in position

Height

Age and Sex

• Age• Body segments differ in proportion to total height from

birth to maturity, the transverse plane of the center of gravity will lie in a different section of the body as age increases, but the proportion of height will be constant.

• The level of the COG will gradually decrease till it reaches the level of the second sacral vertebra at adulthood.

• Sex• Since the distribution of body mass differ from males to

females the COG will be located higher in males than in females

Addition or subtractionOf weight

• Addition of weight , carrying weight will move the COG toward the location of load. E.g. carrying weight above the head will raise it upward. Carrying weight behind the trunk will move the COG backward.

• Subtraction of weight, e.g. amputatin of one limb will move the COG upward and towards the sound side.

• Segmental COG:

By multiplying the segmental length by 4/7 segmental length x 4/7

Measured from the distal end of the segment

• Total body COG:

a) By location as a percentage of standing height above the base of support.

i. According to Palmer Formula: 55.7% of body height measured from the feet for male (- 1.4 cm for female).ii. According to the Crosky Formula:• 55.44% of body height measured from the feet (for men).• 55.18% of body height measured from the feet (for women).

iii. According to the Hellebrant and associates formula:

55.17% of the body height from the feet (for women).

b) Segmental method:

By calculation from a projection film image and is usually used in dynamic situation.

c) By using balancing board.

Definition:

• It is the resistance to over throwing or sudden change of the body’s position.

• It is the ability to maintain balance in static and dynamic situation.

Factors affecting stability:

• Relationship of line of gravity to base of support.• The height of the center of gravity.• The size and shape of base of support.• The mass of the body.• Supporting surface (Friction, inclination,

irregularities).• Segmentation.• Visual and psychological factors.• Physiological factors.• Speed (decrease requirement of lateral stability).

Definition:

• It is any condition in which all acting forces

are cancelled by others resulting in a stable balanced system.

Types of equilibrium:

• Stable.

• Unstable.

• Neutral.

Stable equilibrium

When the object is displaced slightly, it tends to return to its position.

It can be over turned without first raising its COG.

The greater the distance the COG is raised the greater the stability.

Human body can not be in a state of stable equilibrium except when fully supported in lying.

Unstable equilibrium

When the object is displaced slightly it tends to increase its displacement, the COG is lowered and fall outside the base of support immediately and the object fall.

The narrower the base of support and the higher the COG the more unstable a body

(It occur during activity)

Neutral equilibrium

When the object is displaced slightly, it remain in that displaced position.

The COG neither raised nor lowered.

( It occur during activities).