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

Equilibrium

Static equilibrium

• System is at rest

The state of a system that is not experiencing any change in its direction or speed

Dynamic equilibrium

System is moving

with constant velocity

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

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

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

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

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

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

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

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

I ω I ω I ω

A

A

B C

B C

The inverse relationship between moment of inertia (I) and angular velocity (ω)