While we are standing on two feet, our base of support is determined by an area enclosed by the outline of the feet. Figure 6.7 shows that, as you move your feet further apart, the size of the base of support increases from left to right. The use of crutches further increases our base of support, as this base
is determined by the area enclosed by whatever is in contact with the ground, shown in fi gure 6.8.
Depending on where the crutches are positioned, the base of support can be increased in size either forwards and backwards (fi gure 6.8b), side-to-side (fi gure 6.8a), or both (fi gure 6.8c).
For a person to maintain a stable position, the line of gravity must pass through the base of support.
If the line of gravity falls outside the base of support, it is impossible to be balanced and we must either adopt a new posture or else fall over. If we are in a position where the line of gravity is very close to the edge of the base of support, then we feel at risk of falling over because there is very little room to move without the line of gravity passing outside the base of support. A position close to the edge is problematic because we are never exactly stationary.
We are always moving slightly forward and back, left to right, and this normal level of movement risks passing the line of gravity outside the base if we stray too close to the edge.
Understand and apply
1 Obtain a number of different sporting implements, such as hockey sticks, tennis racquets and so on, and balance them to locate their centre of gravity.
2 Obtain some photographs of sportspeople. Try to fi nd photos where the person is adopting an interesting posture, such as a swim start, a high jumper crossing the bar or a dancer performing an arabesque. For each photo, estimate the location of the centre of gravity.
3 Balance a Barbie doll or a similar toy fi gure on your hand and locate its centre of gravity.
Explain why the location of Barbie’s centre of gravity may not be in exactly the same place as yours. Consider such factors as body composition, leg length compared to body, and the relative size of different body parts.
Line of gravity an imaginary line drawn vertically downwards from the centre of gravity.
Figure 6.7
Line of gravity and base of support
Several approaches may be used to increase the stability of the body. First of all, a larger base of support will increase stability. This is because more movement is allowed before the line of gravity gets close to the edge of the base of support. Lowering your body will also increase stability. With a lower centre of gravity, the body can tilt more from side to side without the line of gravity passing outside the base of support. Finally, increased mass will increase stability. A larger mass tends to remain steadier and requires more force to be accelerated. This point is explained later in the section Applying force to an object.
Figure 6.8
Line of gravity and base of support
Figure 6.9
A stable posture requires the line of gravity to be located within the base of support
a
a balance retained
c
c balance lost b
b balance precarious
Adopting a stable posture often requires us to maintain this stability in the face of some external force that might knock us over. This could be a deliberate attempt to make us unstable, such as a rugby tackle or could result from some other source of force, such as catching a ball. Whenever a force is applied to our body, we tend to get pushed in the direction of that force. If this moves our line of gravity outside the base of support, then we will fall over. However, there are strategies to minimise this effect. If we move our line of gravity closer to the edge of the base of support, in the direction that we expect the force will be applied we will become more stable. Moving your line of gravity towards the force before it makes contact will allow more movement to occur after the force is applied without us being pushed outside the base of support.
The benefi ts of this can be even greater if you make your base of support larger and set your base of support in the direction from which the force is expected. Doing this allows even more room for the line of gravity to move after impact.
Figure 6.10 illustrates an example of an athlete maximising his stability in anticipation of an external force. The athlete has increased the size of his base of support in the direction of the applied force, and moved his line of gravity towards the edge of the base of support where he expects the force to occur.
Understand and apply
1 Discuss why your feelings of stability change when you stand on one leg instead of two, when you close your eyes or when you lift your heel off the ground.
2 Sketch a view from above of an elderly person using a walking frame and identify the base of support for the person. Explain why the walking frame provides more stability than the use of a walking stick or no aid at all.
3 Obtain three wooden blocks with
approximately 10 centimetres square base, but different heights (10 centimetres, 20 centimetres and 30 centimetres). Place all three blocks onto a desk and gradually lift one edge of the desk until the blocks fall over. In which order do the blocks fall over and why? Explain with respect to line of gravity and base of support.
4 Consider the diagram opposite and discuss whether you think this man could remain
balanced in this position. Figure 6.11 Gymnast in a balance position Figure 6.10
Athlete widening the base of support and moving the line of gravity towards the direction of an anticipated force
Available range of motion for
line of gravity
Total base of support