Stability and balance: WACE Physical Education Studies Unit 3
“Explain the factors affecting stability and balance and apply them to situations requiring stability or rapid loss of balance”
Stability is the resistance to losing balance. It increases with a lower centre of gravity, a larger base of support, keeping the line of gravity within the base, and greater body mass. Athletes raise stability to resist being pushed (a low, wide wrestler) and deliberately lower it to start quickly (a sprinter in the set position with the line of gravity at the front of the base).
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.
The single decisive condition for toppling is the line of gravity passing outside the base of support. All the other factors (low centre of gravity, wide base, more mass) work by making it harder for the line of gravity to reach the edge of the base. Framing your answer around keeping the line of gravity inside the base, or pushing it outside, gives a clear mechanical reason for stability or instability.
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.
Students assume an athlete should always maximise stability, but sport often needs the opposite. A sprinter, a fielder ready to dive and a winger about to change direction all want low stability so they can move quickly. Match the stability to the task: high stability to hold a position or resist a force, low stability to start or change direction fast. State which the situation needs before applying the factors.
Step 1: decide whether stability is wanted
For a basketball defender holding ground against an attacker, the task is to resist being moved, so high stability is wanted. Always state the goal before applying the factors.
Step 2: lower the centre of gravity
Bending the knees and sinking the hips lowers the centre of gravity, which increases stability because the line of gravity has further to travel before it leaves the base.
Step 3: widen the base and centre the line of gravity
Spreading the feet widens the base of support, and keeping weight evenly balanced keeps the line of gravity central within that base, maximising the resistance to being toppled.
Step 4: bring in body mass and conclude
A greater body mass adds to stability because more force is needed to move it. Conclude that the low, wide, balanced stance keeps the line of gravity inside a large base, making the defender hard to push off position. Naming the line of gravity relative to the base is the highest-scoring detail.
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 questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 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 2024-style3 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.