3.2 Biomechanical principles: stability and lever systems Flashcards
What is the definition of centre of mass?
The point at which a body is balanced in all direction; the point from which weight appears to act
How is the idea of centre of mass used in the Fosbury Flop technique to complete a high jump?
Uses a j-curve to allow velocity in the approach
Plants the outside foot to allow the inside leg to life, along with the arms, at take-off to raise the centre of mass as high as possible.
Fully extends the spine to rotate around the bar moving the centre of mass outside of the body and below the bar. Only one section of the body has to be above the bar at any one time ( in comparison to the scissor-kick technique where the whole body has to be above the bar at the same time).
Overall, because the centre of mass passes underneath the bar, the Fosbury Flop technique requires less force at take-off to clear the same heights as earlier techniques. When maximum force is generated at take-off, greater heights can be easily achieved.
What is the definition of stability?
The ability of a body to resist motion and remain at rest, for a body to withstand a force applied and return to its original position without damage.
What are the 4 factors that affect stability?
1 mass of the body
2 height of the centre of mass
3 base of support
4 line of gravity
How can mass of the body affect stability?
The greater the mass of a body, the greater its inertia, and therefore, the greater its stability.
What is an example of mass of the body affecting stability?
Sumo wrestlers typically have a high mass, as do prop forwards on a rugby squad, to withstand great applied forces.
How can the heights of the centre of mass affect stability?
The lower the centre of mass, the greater the stability.
What is an example of the height of the centre of mass affecting stability?
When a gymnast lands a jump, they flex at the hip and knee to lower their centre of mass and have a stable landing.
How does the base of support affect stability?
The greater the size of the base of support, the greater the stability. This can be moving two points of contact (e.g. feet) wider apart to create a larger surface area or it could be increasing the number of points of contact.
What is an example of the base of support affecting stability?
A table tennis player stands with feet wider than should width to increase stability, as do Olympic freestyle wrestlers when they spread their limbs out on the mat to pin down an opponent.
How does the line of gravity affect stability?
The line of gravity is an imaginary line which extends from the centre of mass downwards to the floor. The more central the line of gravity to the base of support, the greater the stability.
What is an example of line of gravity affecting stability?
A netball goal shooter’s line of gravity falls within her base of support where a goalkeeper who is marking the shot’s centre of mass falls outside and in front of her base of support, leading to over-rotation.
How and when would a sprinter maximise their stability?
A sprinter preparing in the blocks has maximum stability. The crouched position gives a low centre of mass. The base of support is large, with 5 points of contact (two feet, two hands and one knee). The line of gravity falls within the base of support and sprinters typically have a high mass due to their high proportion of muscle mass.
How would a sprinter reduce their stability just before starting their race?
When ‘set’ is called, the sprinter lifts their hips, raising their centre of mass, lifts one knee, reducing the points of contact, and leans forwards, shifting the line of gravity to the edge of the base of support. This reduces stability ready for movement.
How and when would a sprinter minimise their stability within a race?
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, and the 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 froward with great speed. This minimises movement time and gives the perfect start from which to drive forwards.
