Stability and Balance
Definition of Terms
Center of Gravity
The center of gravity is the theoretical point where all the body weight is concentrated or the theoretical point about which the body weight is evenly distributed. If a body is of uniform density and has a symmetrical shape the center of gravity is in the geometric center. If the object is not symmetrical and does not have uniform density, it is more difficult to describe the location of its center of gravity.
Because the human body is not symmetrical and is of non-uniform density, the center of gravity is difficult to locate. In fact, the body’s center of gravity shifts with each body movement. When the distribution of a person’s body weight changes, the center of gravity shifts toward the greater weight concentration.
In addition to the shifting of the center of gravity caused by body movements, external loads alter the position of the center of gravity. When an external load is added, the concern is the combined center of gravity of the load and the person. When the person lifts or carries a load, the combined center of balance must be kept over the base to be in balance. The greater the external load, the greater the distance the load is held from the person’s body, the farther the combined center of gravity will be from the person’s center of gravity.
The person’s weight may be distributed in such a way as that the center of gravity falls outside the body. A person who assumes a pike position in tumbling or diving provides an excellent example of the center of gravity falling outside the body.
LINE OF GRAVITY
The line of gravity is an imaginary vertical line that passes through the body’s center of gravity. The line of gravity represents the direction gravity acts on a body – it is a straight line perpendicular to the earth’s surface through the body at the center of gravity.
Balance is a physical ability that may be improved through purposeful practice. There are two types of balance:
1. Static balance, when a person remains over a relatively fixed base and
2. Dynamic balance, when a performer is in motion.
Stability is a quality relating to the degree to which a body resists being upset or moved. The major factors that affect a person’s stability are:
a. the area of the base of support
b. the relation of the line of gravity to the edge of the base
c. the height of the center of gravity and
d. the mass of the person.
AREA OF THE BASE OF SUPPORT
The larger the base of support, the greater stability a person has. A person is more stable when standing on two feet than does a person standing on one.
Several structural characteristics of the human body relate to the base of support. Since the size of a person’s feet is a factor in determining the area of the base, there is considerable individual variety. In a more complex manner, the size of the pelvis is a limiting factor. Whenever a stance is larger than the dimensions of the hips, the legs form an oblique angle to the surface. This position could be detrimental to stability if there is inadequate friction to keep the feet from sliding out. (Try a straddle position while standing on the ice!!)
RELATION OF THE LINE OF GRAVITY TO THE EDGE OF THE BASE
Increasing the horizontal distance between the line of gravity and the edge of the base enhances stability. If a higher degree of stability in all directions is desired, the performer should assume a stance with the line of gravity falling in the middle of the base.
HEIGHT OF THE CENTER OF GRAVITY
Lowering the center of gravity increases one’s stability because it enlarges the distance the center of gravity must be raised before it falls outside the base. The height of the center of gravity varies with the age, gender and body build of the individual.
Generally speaking, the center of gravity of an adult standing erect lies within the pelvis. Because of the proportionally larger size of young children’s heads compared to their bodies, their center of gravity tends to be higher. Also men tend to have higher center of gravity’s than women due to relatively larger shoulder regions.
Mass is defined as the quantity of matter in a body. Stability is related to mass in that the greater the mass, the better the degree of stability. For a body to be moves, there must be sufficient force to overcome inertia and friction, both of which increase proportionally with an increase in mass.
PRINCIPLES OF STARTING, STOPPING AND CHANGING DIRECTIONS
1. When a quick start is essential, and the intended direction of movement is known, the performer should decrease the horizontal distance between the line of gravity and the edge of the base in the intended direction and position the center of gravity relatively high.
2. When a quick start is essential, but the direction of movement cannot be anticipated, the performer should raise the center of gravity, keep the line of gravity over the middle of the base, and redace the size of the base.
3. When a quick stop is to be made, lower the center of gravity and increase the horizontal distance between the line of gravity and the initial placement of the lead foot on the stopping surface.
4. When performers wish to fake an opponent and then change directions quickly, their center of gravity should not be moved in the direction of the fake.
As skill proficiency increases, the number of segmental body parts used in its execution decreases. Any body shape that forms a symmetrical line can be called a segment. The first example in the first figure illustrates a body that is essentially a one-segment shape. As seen in the second example, when an obvious part of the body changes position to the point that the line appears broken or lacking in symmetry, a two-segment shape is then formed. The remaining examples demonstrate that the number of segments is simply a manner of symmetrical lines or shapes observed.
The symmetry formed by ‘various body parts need not be straight lines to qualify as segments. Many of the movement patterns observed in gymnastics is characterized by dynamically curved lines. Any abrupt irregularity in a curved line becomes, by virtue of the original definition, a two-segment shape.
The three sets of body shapes shown in the second figure illustrates the use of both curved and straight lines and compares the number of segments needed to execute each skill. The more aesthetically pleasing position is the one with the fewest number of segments.
When designing skill progressions, this principle may be used to gradually increase the difficulty level of basic movements. This principle is applied in the tumbling variations covered in class as well as in the pre-vaulting progressions.