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
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.
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
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.