1. CHAPTER 14: THE CENTER OF GRAVITY AND STABILITY
KINESIOLOGY
Scientific Basis of Human Motion, 12th edition
Hamilton, Weimar & Luttgens
Presentation Created by
TK Koesterer, Ph.D., ATC
Humboldt State University
Revised by Hamilton & Weimar
Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.
McGraw-Hill/Irwin

2. 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.
Discuss the factors that affect the stability and energy cost of the erect posture.
5. Explain the effects that the postural adaptations have on static and dynamic postures.
6. Explain the value of both anticipatory and compensatory postural adjustments.

3. Center of Gravity (C of G)
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.
Torques must be balanced.
Fig 14.1

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

5. CENTER OF GRAVITY
As one changes the relationship of the body segments to each other, the C of G may even be located outside the body.
Fig 14.4

6. Placement of the Center of Gravity in Humans
The location of the C of G of a human in standing position varies with body build, age, and sex.
Female’s CG is ~ 55% of standing height
Male’s CG is ~ 57% of standing height
In quiet standing, the C of G can be considered almost directly over center of pressure.
Center of pressure - point at which the force vector for ground reaction force is applied.

7. 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.

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

9. Unstable Equilibrium
When a slight disturbance will drop the objects’ C of G to a lower point.
Fig 14.5b

10. Neutral Equilibrium
When a object’s C of G is neither raised nor lowered when it is disturbed.
Humans spend most of their time adjusting body positions for the type of equilibrium best suited to the task.
Fig 14.5c

11. Factors Affecting Stability
The ability to maintain one’s balance under unfavorable circumstances is recognized as one of the basic motor skills.
Understanding the factors affecting the stability of a performer’s equilibrium state:
should make analysis of a balance problem easier.
may suggest means for skill improvement.
We are examining stability from the point of view of the analyst rather than the performer.

12. Factors Affecting Stability
Three primary factors:
Size of the base of support.
Relation of the line of gravity to the base of support.
Height of the center of gravity
To make it read a little easier.

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

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

15. Height of the Center of Gravity
Height of C of G changes with body position.
As C of G moves closer to base of support more angular displacement can occur before it goes beyond the base of support.
Fig 14.8 x a b c
a > b > c with respect to stability

16. Relationship of the Line of Gravity to the Base of Support
To maintain equilibrium, line of gravity must remain within the base of support
Notice the hyper-extended the trunk to maintain the line of gravity within the base of support in Fig
Fig 14.9
Fig 14.10
Just rearranged to cut down the number of words.

17. 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.

18. 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.

19. 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 strain to joints and muscles.
When one segment gets out of line, another segment must compensate for it.

20. Visual and Psychological Factors
External visual cues may affect stability:
Even if the supporting surface is adequate,
The sense of balance may be disturbed by 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.

21. Physiological Factors
Semicircular canals can affect equilibrium:
Colds, viruses, and other problems that 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.

22. Principles of Stability:
Other things being equal, the lower the C of G, the greater will be the body’s stability.
Greater stability is obtained if the base of support is widened in the direction of the line of force.
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.
Other things being equal, the greater the mass of a body, the greater will be its stability.

23. Principles of Stability:
Other things being equal, the most stable position of a vertical segmented body is one in which the C of G of each weight-bearing segments lies in a vertical line centered over the base of support.
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.
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.

24. Principles of Stability:
There is a positive relationship between one’s physical and emotional state and the ability to maintain balance under difficult circumstances.
Regaining equilibrium is based on the same principles as maintaining it.

25. Mobility Mobility & stability have an inverse relationship.
A critical point is the change from a position of stability to a state of mobility & vice versa.
To initiate a step, line of gravity must be shifted forward of the base of support. The swing leg then moves forward to re-establish a base of support.

26. Mobility
Fig 14.14
Often in sport, it is necessary to alter stability intentionally to become mobile.
Ability to start, stop, or change direction quickly depends on manipulating the stability of the body.

27. Mobility
Both needed speed and direction are used to determine the change in stability required to initiate motion.
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 C of G, and movement of the line of gravity away from the leading edge of the base of support.

28. Center of Gravity and Posture
Posture implies shape or configuration of the body
The multi-segmented human body does not have a single posture.
Dynamic posture is of greater concern than static posture.
All posture is somewhat dynamic, including postural sway during quiet standing.
Represents a person’s habitual carriage.

29. Postural Adaptation Hip strategy- Ankle strategy
Two primary strategies on a continuum:
Hip strategy-
Achieved through trunk flexion/extension and hip flexion.Focus on balance in the sagittal plane.
Ankle strategy
Ankle/foot adopts a position that allows stability to be maintained.
Used often for abrupt changes of support surface

30. Postural Adaptation Dynamic adaptation
Must interpret threats to stability
Acceleration
Change in base (i.e. one foot)
Change in direction

31. Dynamic Posture Movement will disrupt the static posture.
Action/reaction flows through the kinetic chain.
Reaction transmitted from support surface.
Key for dynamic posture is the relationship between C of G, center of pressure, and momentum.
Postural adjustments will take place:
Some are anticipatory (feed forward).
Others are proprioceptive (feedback).

32. Principles Applied to Posture
The weight-bearing segments of the body are so aligned in good standing posture that the trunk and in the pelvic girdle are neutral (lack of either posterior or anterior tilt) and maintain normal spinal curves.
In a static posture the line of gravity will be close to the geometric center of the base. Maximum stability usually occurs when the line of gravity is over the center of pressure.
Standing posture is extension of the weight-bearing joints, not be accompanied by strain or tension

33. Principles Applied to Posture
Efficient posture would be a position that requires a minimum expenditure of energy for maintenance of alignment.
Efficient posture requires a minimum of muscle force. This implies adequate antigravity musculature to resist the pull of gravity maintain alignment without excessive effort or tension.
Posture requires sufficient flexibility in the weight-bearing joints to permit good alignment without strain. Flexibility should not be so great that excessive muscular effort is needed to keep the weight-bearing joints in alignment.

34. Principles Applied to Posture
Posture requires coordination. This implies neuromuscular control and well-developed postural reflexes.
Adjustments in posture can be made more readily with good kinesthetic awareness of posture and muscle tension.
The characteristics of normal posture change with age. Young children typically have a protruding abdomen and hollow lower back. Older persons show a wider stance, forward head and rounded upper back, and limited flexibility in the trunk, hips, and knees.

35. Finding the Center of Gravity in the Human Body
Reaction Board Method
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 C of G is the intersection of the values for each of these three planes.
Limited to the body in a stationary position.

36. Segmental Method
A highly useful procedure for finding the C of G for someone in action.
Technique uses:
A photograph or video frame of the subject.
Involves finding the C of G of each body segment
The position of the C of G points are placed on x and y axes.
Knowledge of the ratio between the individual segment weight and total body weight.