Centre of mass and stability

Centre of mass

the point at which a body is balanced in all directions

Position of centre of mass

relies on the distribution of the body mass and can be manipulated to improve performance

Stability

ability of a body to resist motion and remain at rest

also the ability to withstand external forces

Factors affecting stability

1. Mass of the object/ body- the greater the mass the greater the inertia

2. height of the centre of mass- the lower the centre of mass the greater the stability

3. base of support- the wider the base of support the greater the stability

4. line of gravity- an imaginary line which extends down from the centre of mass to the floor. keeping the line of gravity within the base of support helps to maintain stability

How to maximise stability- centre of mass

lowering centre of mass/ centre of gravity

e.g. position in a squat

How to maximise stability- base of support

Increasing size of base of support or increasing the number of points of contact

e.g. landing on two feet in trampolining and landing on two feet in netball

How to maximise stability- Line of gravity 1

making line of gravity/ centre if mass/ centre of gravity central to base of support

e.g. weightlifter holding weight over their head

How to maximise stability- line of gravity 2

making line of gravity/ centre of mass/ centre of gravity closer to a resistance force/ point of contact

How to maximise stability- increasing mass

Increasing body mass

e.g. wrestler, shot put

To minimise stability

1. reduce movement time- shifting weight from foot to foot. dodging or marking

2. increase speed

3. change direction- small base of support, long line of gravity and leaning

4. rotation- to rotate from take off, taking the centre of mass outside of body

5. unpredictable- decrease stability of the ball in flight by using no spin

6. range of motion/ stretch- move line of gravity outside base of support/ decreased points of contact/ raised centre of mass

Phases of take off in a sprint

1. Maximising

• a sprinter preparing in the blocks has maximum stability

• crouched position gives a low centre of mass

• the base of support is large with 5 points of contact

• the line of gravity falls within the base of support

• sprinters typically have a high mass due to their high proportion of muscle mass

2. Minimising 1

• when ‘set’ is called the sprinter lifts their hips raising their centre of mass

• lifts one knee reducing the points of contact

• leans forwards shifting the line of gravity to the edge of the base of support

• reduces stability ready for movement

3. Minimising 2

• when the gun is fired, instability is maximised to aid performance

• the chest lifts raising the centre of mass

• hands come off the track minimising the base of support and points of contact

• line of gravity falls in front of the base of support causing the body to fall forwards

• this must be prevented by driving one leg forward with great speed

• this minimises movement time and gives the perfect start from which to drive forward