400882 Introduction to Biomechanics - Centre of Gravity & Stability

What is meant by the term 'dynamic equilibrium'?

Linear and angular acceleration occurring in motion of a body (acc are non 0 because there is movement)

The centre of gravity refers to what part of a body?

COG is the unique point about which a body's weight is equally balanced in all directions or the point about which ΣT produced by the weights of body segments = 0

The COG is also synonymously referred to as what?

Mass centroid & centre of mass

Where is the COG located in a perfectly symmetrical object?

The geometric centre of the object

Is it possible for the COG to be located outside an object and if so, provide an example?

Yes - for example a donut or when the body curls over the bar in a pole vault

The COG is used as a reference for analysis during which types of motion?

Whole body motions

List TWO methods for determining the COG.

• Reaction board method (only works in that horizontal plane)

• Segmentation method (based on cadaver measurements)

Define Stability.

• Resistance to disruption of equilibrium

Define Balance.

Ability to control equilibrium

Skill based

Explain the differences between stable, unstable and neutral equilibrium and provide examples for each.

• If displacement is due to work done by the force & returns to original position = stable equilibrium → child on a swing

• If displacement is ↑ = unstable equilibrium → person doing a headstand

• If displaced and moves to a new position where it stays = neutral equilibrium → ball on a flat surface with force applied

List the FOUR factors stability is dependent on?

1. Where the line of gravity falls with respect to the base of support → i.e. COM closer to base of support = less stability (in that direction)

2. The height of the COM relative to the base of support

3. The total mass of the object (greater inertia → harder to move)

4. The friction between the surfaces

Explain why a position of minimal stability is advantageous in swimming starts.

So you can accelerate your mass quickly forwards when the gun goes off.

When all other factors are held constant, what FIVE principles of stability could improve a body's ability to maintain equilibrium?

1. Increasing the bodies mass

2. Increasing the friction between the body and the surface or the surfaces contacted

3. Increasing the base of support in the direction of the line of action of an external force

4. Horizontally positioning the COG near the edge of the base of support on the oncoming external force

5. Vertically positioning the COG as low as possible