What factors determine how stable an athlete is, and how do they manipulate balance to suit different sporting needs?
Explain the factors affecting stability and balance and apply them to situations requiring stability or rapid loss of balance
A focused answer to the WACE Year 12 Physical Education Studies Unit 3 content on stability and balance. The centre of gravity, line of gravity and base of support, the four main factors that change stability, and how athletes lower stability deliberately to start fast or raise it to resist being moved.
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What this dot point is asking
WACE expects you to define centre of gravity, line of gravity and base of support, list the factors affecting stability, and apply them in two directions: when stability is wanted and when instability is wanted. Application to a named position earns the marks.
Key terms
The centre of gravity is the point at which the body's weight is evenly balanced in all directions. It shifts as the body changes shape, so raising the arms lifts it and crouching lowers it. The line of gravity is the imaginary vertical line running down through the centre of gravity to the ground. The base of support is the area beneath and between the points of contact with the ground, such as the area between a person's feet.
The factors affecting stability
Four main factors change how stable a body is.
Height of the centre of gravity: a lower centre of gravity increases stability. Crouching makes an athlete harder to topple.
Size of the base of support: a larger base increases stability. Spreading the feet wide gives more room for the line of gravity to move before balance is lost.
Position of the line of gravity: stability is greatest when the line of gravity falls in the centre of the base of support, and balance is lost when it passes outside the base.
Body mass: a greater mass increases stability because more force is needed to move it, which is why a heavier player is harder to knock off the ball.
When stability is wanted
In sports such as wrestling, rugby scrummaging or defending in basketball, the athlete wants to be hard to move. They lower the centre of gravity by bending the knees, widen the base by spreading the feet, keep the line of gravity central, and use their mass against the opponent. A defensive ready position is essentially a stability position.
When instability is wanted
In a sprint start the athlete wants to begin moving forward as fast as possible, which means being on the edge of balance. In the set position the hips are raised, lifting the centre of gravity, and the line of gravity is shifted forward to the front edge of the base of support. The athlete is deliberately unstable, so the moment the hands lift at the gun the body falls forward and the legs drive into the run. The same idea explains a swimmer leaning out over the blocks at the start.
How this maps to the exam
A position image is common, with a command to explain stability. Identify whether the athlete wants to be stable or unstable, then use the centre of gravity, base of support and line of gravity to justify the body position. Naming the line of gravity relative to the base is the highest scoring detail.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20226 marksA rugby player setting for a scrum and a sprinter in the set position adopt very different body positions. Using the factors affecting stability, explain why one position maximises stability while the other deliberately reduces it.Show worked answer →
A 6 mark answer needs both positions explained through the stability factors and the line of gravity relative to the base.
- Scrum (maximising stability)
- The player bends the knees to lower the centre of gravity, spreads the feet to widen the base of support, keeps the line of gravity central within the base, and uses body mass. These make it hard for the line of gravity to leave the base, so the player resists being pushed.
- Sprint set position (reducing stability)
- The hips are raised, lifting the centre of gravity, and the weight is shifted forward so the line of gravity is at the front edge of the base. The athlete is deliberately near the point of imbalance, so at the gun the body falls forward and the legs drive into the run.
- Conclusion
- High stability suits resisting a force; low stability suits starting movement quickly. The decisive factor is where the line of gravity sits relative to the base.
Markers reward the correct use of centre of gravity, base of support, body mass and line of gravity for each position, and the link to the task.
WACE 20243 marksExplain why a body topples, with reference to the line of gravity and the base of support, and state what an athlete can do to avoid toppling when pushed.Show worked answer →
A 3 mark explain answer needs the toppling condition and a corrective action.
- Toppling condition
- A body loses balance and topples when the line of gravity passes outside the base of support.
- Avoiding it
- The athlete can widen the base of support (more room for the line of gravity to move), lower the centre of gravity, and lean into the push so the line of gravity stays central within the base.
- Why it works
- These actions keep the line of gravity inside the base even when an external force shifts the body, so balance is maintained.
Markers reward the line-of-gravity-outside-the-base condition and at least one valid corrective action with reasoning.