CHAPTER 14:CHAPTER 14:THE CENTER OF THE CENTER OF

GRAVITY GRAVITY AND STABILITYAND STABILITY

CHAPTER 14:CHAPTER 14:THE CENTER OF THE CENTER OF

GRAVITY GRAVITY AND STABILITYAND STABILITY

KINESIOLOGYScientific Basis of Human Motion, 12th edition

Hamilton, Weimar & LuttgensPresentation 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

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ObjectivesObjectivesObjectivesObjectives1. 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. 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.

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Center of Gravity (C of G)Center of Gravity (C of G)Center of Gravity (C of G)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 14.1Fig 14.1

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CENTER OF GRAVITYCENTER OF GRAVITYCENTER OF GRAVITYCENTER 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

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CENTER OF GRAVITYCENTER OF GRAVITYCENTER OF GRAVITYCENTER 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

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Placement of the Center of Placement of the Center of Gravity in HumansGravity in HumansPlacement of the Center of Placement of the Center of Gravity in HumansGravity in HumansThe 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 heightMale’s CG is ~ 57% of standing heightIn 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.

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Stability and EquilibriumStability 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.

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Stable EquilibriumStable 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

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Unstable EquilibriumUnstable Equilibrium

When a slight disturbance will drop the objects’ C of G to a lower point.

Fig 14.5b

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Neutral EquilibriumNeutral EquilibriumWhen 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

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Factors Affecting StabilityFactors 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.

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Factors Affecting StabilityFactors Affecting Stability

Three primary factors:1. Size of the base of support.2. Relation of the line of gravity to the

base of support.3. Height of the center of gravity

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Size of the Base of SupportSize of the Base of Support

C of G must remain within the base of support in order to maintain equilibrium.

Easier with larger base of support.

Fig 14.6

Headstand

WalkingStanding

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Shape of the Base of SupportShape of the Base of Support

Resistance to lateral forces

Resistance to AP forces

Fig 14.6c Fig 14.6b

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Height of the Center of GravityHeight of the Center of GravityHeight 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.

a > b > c with respect to stability

Fig 14.8x

x x

a b c

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Relationship of the Line of Gravity to the Base of Support

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 14.10 Fig 14.9 Fig 14.10

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Mass of the BodyMass 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.

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FrictionFriction

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.

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Segmental Alignment 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.

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Visual and Psychological FactorsVisual 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.

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Physiological FactorsPhysiological 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.

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Principles of Stability:Principles of Stability:I. Other things being equal, the lower the C of

G, 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.

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.

IV. Other things being equal, the greater the mass of a body, the greater will be its stability.

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Principles of Stability:Principles of Stability:

V. 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.

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.

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Principles of Stability: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.

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MobilityMobility

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.

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MobilityMobility

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.

Fig 14.14

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MobilityMobility

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.

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Center of Gravity and PostureCenter of Gravity and PosturePosture 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.

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Postural AdaptationPostural Adaptation

Two primary strategies on a continuum:Hip strategy-

Achieved through trunk flexion/extension and hip flexion.Focus on balance in the sagittal plane.

Ankle strategyAnkle/foot adopts a position that

allows stability to be maintained.Used often for abrupt changes of

support surface

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Postural AdaptationPostural Adaptation

Dynamic adaptation

●Must interpret threats to stability

•Acceleration

•Change in base (i.e. one foot)

•Change in direction

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Dynamic PostureDynamic 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).

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Principles Applied to PosturePrinciples Applied to Posture

I. 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.

II. 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.

III. Standing posture is extension of the weight-bearing joints, not be accompanied by strain or tension

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Principles Applied to PosturePrinciples Applied to Posture

IV. Efficient posture would be a position that requires a minimum expenditure of energy for maintenance of alignment.

V. 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.

VI. 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.

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Principles Applied to PosturePrinciples Applied to Posture

VII. Posture requires coordination. This implies neuromuscular control and well-developed postural reflexes.

VIII. Adjustments in posture can be made more readily with good kinesthetic awareness of posture and muscle tension.

IX. 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.

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Finding the Center of Gravity in the Human BodyFinding 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.

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Segmental MethodSegmental 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

segmentThe 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.