2. Happy gadara( )

3. 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

4. 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 equal zero
Linear forces must be balanced
Rotary forces must be balanced
Fig 14.1

5. CENTER OF GRAVITY
The location of the CG remains fixed as long as the body does not change shape
If an object’s shape or position changes, the location of the CG changes
Fig 14.3

6. 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

7. 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

8. 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

9. 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

10. Stable Equilibrium
Occurs when an object is placed in such a fashion that an effort to disturb it would require its CG to be raised
Fig 14.5a

11. Unstable Equilibrium
A slight disturbance will drop the objects CG to a lower point
An unstable objects seeks stability by falling from a smaller to a larger base of support
Fig 14.5b

12. Neutral Equilibrium
Occurs when an object’s CG is neither raised nor lowered when it is disturbed
Remains the same
Inertial property
Fig 14.5c

13. Size of the base of Support
CG must remain within the base of support in order to maintain stable and in equilibrium
Easier with larger base of support
Standing
Walking
Headstand
Fig 14.6

14. Shape of the base of Support
Fig 14.6c
Fig 14.6b
Resistance to
AP forces
Resistance to
lateral forces

15. 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

16. Vertical Center of Gravity
Height and location of Center of Gravity along a vertical line (y axis)

17. Height of the Center of Gravity
a > b > c with respect to lateral stability x x
Fig 14.8 x

18. 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

19. 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

20. 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

21. 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

22. Mobility
Fig 14.14a

23. 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

24. 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

25. Center of Gravity
Vertical Alignments