Body Balance and Stability Control

1. Body Balance and Stability Control • Balance is a very important factor in athletic performance • Generally depends upon the location of the centre of mass and how stable that centre of mass is

2. Equilibrium The state of a system that is not experiencing any change in its direction or speed Static equilibrium • System is at rest Dynamic equilibrium • System is moving with constant velocity

3. Balance • Process whereby the body’s state of equilibrium is controlled for a given purpose • The base of support and location of the centre of mass is manipulated • If the line of gravity passes through some part of the body’s base of support, the body will be balanced

4. Stability • A measure of the difficulty with which equilibrium can be disturbed • A net external force is required to overcome the static equilibrium of a sprint start • There is a trade-off between maximizing stability and acquiring speed off a mark

5. To increase stability in static equilibrium • Increase the base of support • Increase the inertia of the body • Decrease the vertical distance between the centre of mass and the base of support

6. To increase stability in dynamic equilibrium • Enlarge the body’s base of support in the direction of the external horizontal forces • Adopt a starting position in which the centre of pressure is close to the edge of the base of support whenever a quick acceleration is important • When slowing down or reversing directions in running, lean backwards but keep normal frictional forces high to prevent slipping • When tripping or falling, reflex movements may help to regain the loss of balance or to create a new base of support

7. Somersaulting – Angular Kinetics • Angular kinetics is concerned with the generation of rotations and the control of these rotations • All objects posses inertia and do not wish to begin rotating • If an external moment of force is applied to the object, it will experience an angular acceleration

8. Off-centre External Forces • Any external force that acts away from the body’s centre of mass will create a moment of force acting on the total body • Usually, the off-centre force is a reaction force from the ground or equipment, resulting from the internal muscle forces generated by the athlete • If an athlete has forward momentum and a pivot point is created, then some or all of the linear motion can be transferred into angular motion

9. Transfer of Momentum Within the Body • Sometimes athletes accelerate specific body parts while in contact with the ground • Upon leaving the ground, athletes lock these segments to the rest of their bodies • The joint angles these segments make with the trunk become fixed • The momentum created by these segments becomes part of the momentum of the total body

10. Rotations While Airborne • Rotations cannot be generated when airborne • Once the body is airborne, only gravity acts on the body • The angular momentum (the amount of angular motion) is conserved • Changes in body shape will decrease/increase the moment of inertia about the somersaulting axis • If the moment of inertia increases, the athlete will have more reluctance to rotating and will have slower angular velocity

11. A C B A B C I ω Iω I ω The inverse relationship between moment of inertia (I) and angular velocity (ω)