1. ©PE STUDIES REVISION SEMINARS
STABILITY AND BALANCE
“Define and apply the principle of balance to a selected sport in relation to:
The centre of gravity i.e. Line of gravity, width of base of support, height of centre of gravity
Static balance
Dynamic balance
Curriculum Council of Western Australia. Physical Education Studies Support Document 2009.
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2. ©PE STUDIES REVISION SEMINARS
BALANCE
BALANCE: The ability to neutralise forces that disturb equilibrium.
Standing on one leg
Scooping a ball up on the run
“Bouncing” off a tackler without falling
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3. ©PE STUDIES REVISION SEMINARS
STABILITY
Object’s resistance to movement, from a balanced position. There are two types of stability:
STATIC STABILITY— when an object is at rest and not moving with linear or angular motion.
DYNAMIC STABILITY— when an object is in motion and moving with linear or angular motion.
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4. ©PE STUDIES REVISION SEMINARS
CENTRE OF GRAVITY
The point around which a body’s weight is equally balanced in all directions
Also referred to as the center of mass (need not be physically located inside of a body)
Standing still – centre of gravity is located in the abdominal cavity, about 6 inches above the pubis symphysis.
As your position changes – so does your centre of gravity.
The position of the centre of gravity will determine whether the body is in balance.
COG
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5. DETERMINING THE CENTRE OF GRAVITY
To determine ones COG, simply draw a box around the objects outer extremities
Then draw diagonal lines through the box, with the point of intersection determining the objects approximate COG.
Approximate COG
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6. CENTRE OF GRAVITY AND SPORT
COG passes through or beneath the bar whilst the athlete passes over the bar
In sports like High Jump and Pole Vault, where athletes are trying to attain maximum height, individuals will endeavour to position their COG outside of their body, having it pass beneath the bar whilst their body passes above it, to help achieve maximum height.
They achieve this by arching their back (hyperextension)
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7. ©PE STUDIES REVISION SEMINARS
BASE OF SUPPORT
What is the base of support?
Area bound by the outermost regions of contact between a body and support surface
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8. FACTORS AFFECTING BALANCE & STABILITY
MASS OF THE OBJECT
The greater the mass of an object, the greater its stability will be, given that all other factors are equal.
Sumo wrestlers have excellent stability as a result of their increased mass
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9. FACTORS AFFECTING BALANCE & STABILITY
SIZE OF THE OBJECTS BASE OF SUPPORT
The greater the area of support, the greater the degree of stability.
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10. FACTORS AFFECTING BALANCE & STABILITY
THE HEIGHT OF THE COG ABOVE THE BASE OF SUPPORT
The line of gravity or pull of gravity will always pass vertically through the centre of an object’s mass.
The higher the centre of gravity above the base of support, the less stable the object is. Athletes often lower their centre of gravity by bending the knees in order to increase their stability
Low COG =
↑ stability
High COG =
↓ stability
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11. FACTORS AFFECTING BALANCE & STABILITY
↑ Stability as line of gravity located well within base of support
THE POSITION OF THE LINE OF GRAVITY RELATIVE TO THE BASE OF SUPPORT
The line of gravity is an imaginary vertical line passing downwards through the centre of gravity. The closer the line of gravity is to the limits of the base of support, the less the degree of stability of the object.
Movement is easier when the line of gravity falls outside the object’s base of support
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12. FACTORS AFFECTING BALANCE & STABILITY
INCREASING THE SIZE OF THE BASE OF SUPPORT IN THE DIRECTION OF THE ONCOMING FORCE
Oncoming force
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13. FACTORS AFFECTING BALANCE & STABILITY
HORIZONTALLY POSITIONING THE COG NEAR THE EDGE OF THE BASE OF SUPPORT ON THE SIDE OF THE EXTERNAL FORCE
Oncoming force
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14. STABILITY VARIES WITH BODY POSITION
More stable Less stable
Low COG Higher COG
Wide base of support – 4 point contact Small base of support – 2 point contact
Line of gravity in middle of support Similar line of gravity
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15. Centre of Gravity, Stability & Balance
In many sports skills, players are required to unbalance quickly in order to have a fast movement time. This can be done by either;
Decreasing the size of the effective base and/or
Displacing the centre of weight quickly outside the effective base (move the line of gravity close to the base of support)
Line of gravity passes close to edge of base of support, reducing stability allowing the runner to get out of the blocks faster
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16. COG, STABILITY AND BALANCE
E.g. When receiving service a tennis player has to be prepared to move quickly in a sideways direction. In order to respond to a wide serve the player sways form side to side keeping the centre of weight in motion across the base of support. With the centre of weight in motion, less effort is required to unbalance the player when the decision to move in a particular direction is made. At the last moment players will also be seen to bring their feet closer together thereby decreasing the size of the effective base and facilitating an unbalancing effect.
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17. COG, STABILITY AND BALANCE
Sports Requiring Low Stability
Sports Requiring High Stability
Sprint starts – swimming and athletics
Tennis – service reception
Any aspect of sport requiring quick lateral movement or change of direction
Wrestling
Weight lifting
Collision sports
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