The Center of Gravity and Stability: Study Sheet.

1.  The CG of a body refers to its balance point or that point in which the body would balance without any tendency to rotate.  For the latter reason, the CG is that point where all of the weight of the body is concentrated.  The CG in an adult is normally at the upper third of the sacrum during the normal standing position.

2.  The human body is capable of assuming a variety of positions, therefore the CG tends to shift as we move.  As you change positions of the body segments, the CG may even be located outside the body.

3.   Body build, age and gender affects the location of the center of gravity of a human being.  Hellebrandt (1942) found the CG in women to be 55% of their standing height in a normal standing position.  Croskey et al. (1922) found the CG in men to be 56.18 percent of their height in a normal standing position. A women’s  CG tends to be lower because more weight is concentrated in the pelvis area and thighs compared to a man.

4.  Stable equilibrium results when CG is lowered.  Crouching, kneeling, or a sitting position will lower the center of gravity and increase stability.  A wrestler and defensive lineman will increase stability by lowering their CG.

5.  Unstable equilibrium exist when only a slight push or pull will destroy it.  Examples of unstable equilibriums include the swimmer in the starting block ready for a race, or a sprint runner is in an unstable equilibrium at the start of a race.

6.  Factors affecting CG include the size of the base of support, the relation of the line of gravity to the base of support, and the height of the CG.

  Points to remember:
a. The wider the base of support, the easier it is to maintain balance.

b. If you raise your arms the CG is shifted upward and it becomes more difficult to maintain balance over yourbase of support. Lowering the CG allows for greater angular displacement of the CG within the bounds of of the base of support, therefore you have less tendency to be off balance

c. The human body has less tendency to be off balance whethe line of gravity falls at the center of the base ofsupport. If you are not certain from which direction anexternal force is going to act on your body, it is bestto make yourself stable by having the line of gravity fallover the center of the base of support.

d. The greater the mass, the greater the stability.  Why?Force=mass x acceleration,  stability of an object isincreased if you increase the mass of the object.  It will   take a greater external force to set the object off balance.

e. The greater the friction, the greater the stability.  Improper friction makes it difficult to maintain one's   balance.

 7.  More principles of stability.

a.  The lower the CG, the greater the body's stability.

    Canoeing, should assume kneeling position for greatest
    stability and best position for paddling.

    An individual on the balance beam will squat when she
    feels she is losing balance.

b.  You obtain greater stability if the base of support is widened in the direction of the line of force.

When catching a fast moving baseball or a medicine ballit is best to widen the base of support in line withthe direction of the force.  Therefore, the catcher can give with the catch and therefore provide a greatestopping time (increase stopping distance) in which to reduce      or stop the motion of the ball.

c.  To maximize stability, the line of gravity should intersectthe base of support in such a way that it provides for thegreatest range of movement within the base.

A tennis player will keep the line of gravity centered sothat the CG can be shifted quickly in any direction without loss of balance.

The football player who knows he will be pushed from the frontshould shift his line of gravity forward (lean forward) so that he can give in a backward direction without losing his balance.

An individual in tug-of-war will lean backward (move the line  of gravity back) in order to absorb a strong forward pull from  the opponent.

d.  The greater the mass of a body, the greater will be its stability.
Heavy, solid individuals are more likely to maintain balance  than shorter individuals.

e.  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.

    In basketball, fencing, football, field hockey, and lacrosse,
    for example, the wearing of cleats and rubber-soled shoes
    aids in locomotion and provides for stability for positions
    held momentarily between quick and forceful movements.

f.  An individual maintains better balance in locomotion when
    focusing on stationary objects versus disturbing stimuli.