Stability of the Body

The body’s centre of mass (CoM) is the point at which the body’s mass is equally distributed or balanced in all directions. It is the point at which the weight of the body appears to act. In the analysis of objects when gravitational force is an influence then, the centre of mass is referred to as the centre of gravity (CoG) and this is the point at which weight appears to act. In the anatomical position, the height of your CoG is 55 to 57% of your standing height. The location of the CoG is important because mechanically a body behaves as if its mass were concentrated at the CoG and its position depends on the distribution of mass which can change position when body shape changes. For example it can move outside the body in movements such as the Fosbury Flop due to arching and hyperextension of back or move to different locations within the body.

In most sports skills where a performer is in contact with the ground, their line of gravity must fall within their base of support. The more stable an athlete, the more resistance an athlete generates against disruptive external forces, for example an opposing player trying to push a football player off the ball. The more stability a performer has, the more force required to upset their balance.

Stability: the capacity of an object to return to its original position after being displaced.

Base of Support: The area beneath and between the points of contact an object or person has with the ground. A sprinter at the start of the race will have four points of contact with the ground (both hands and both feet) and have a relatively large base of support compared to a netball player pivoting on only one foot.

Line of gravity: This is an imaginary line which passes vertically from the centre of gravity to the ground below an object

The conditions that foster maximal stability are:

1. The centre of mass being maintained over the base of support increases the performers of stability. If centre of mass moves close to the edge of the base of support the performer’s balance becomes less stable.

2. The nearer the line of gravity falls to the centre of the base of support, the greater the likelihood of maintaining balance.

3. The broader the base of support, generally the better the probability of maintaining balance; for example the base of support of a gymnast on the beam and gymnast on the high bar is small and less stable than the stable base of support of wrestler on the floor.

4. The probability of maintaining balance is increased when the CoG is lowered in relation to the base. Consider when athletes crouch during sports events e.g. slalom skiers when turning, ski jumpers on landing.

5. Athletes who increase their body mass increase their stability. This partly explains why there are weight divisions in most combat sports. An exception is when muscle mass is only added to the upper body as this shifts the CoM upwards so reduces stability.

6. Athletes increase their stability when they extend their base in the direction of an oncoming force. Consider a rugby player trying to maintain stability when hit by an opponent, they lean towards the opponent meaning the player making the hit must apply more force to shift the CoG.

7. Athletes increase their stability when they move their line of gravity towards an oncoming force for example moving CoG towards oncoming force enables you absorb the force over a longer period of time.

8. Some rotating movements increase stability. The spin or turn of a body creates gyroscopic stability, and this means the CoG can sometimes pass outside of the body but the performer can remain balanced. For example when a motorcycle rider takes a corner they will lean into the curve and the ‘centrifugal force’ that is acting on them will prevent them from falling over.

Instability

There can be instances where it is necessary to have instability in terms of balance. For example when a 100 m sprinter is in the blocks they will remain balanced but the line of gravity will only just fall within the base of support, just behind the hands. Once the race begins the centre of gravity is quickly shifted in front of the hands, which are also lifted from the floor, meaning they overbalance. This creates a very brief moment where the runner ‘falls’ forward which helps them to overcome inertia and assists in a fast start. To assist in situations where overbalancing is needed, performers will decrease their base of support, for example narrowing their stance or raising their base of support will help. In skills where rotational movements are required then having the line of gravity fall outside the base of support means that spins and turning can be created. To generate rotation a trampolinist needs to have an unstable position when they execute the take-off so they can produce an off-centre or eccentric force.