Balance: centre of gravity and base of support

1Why this matters

Think of a woman in Musanze carrying a full jerrycan on her head up a steep, stony path, or a footballer holding off a defender who is leaning into him. Neither falls, because both are controlling the same two things: where their weight is, and what is holding it up. Every sport, every dance and every gymnastics skill depends on balance, the ability to keep your body in a controlled position, whether still or moving.

2Three key ideas

• Your centre of gravity is the point where your body's weight can be treated as acting. When you stand upright it is roughly behind your navel. It moves whenever you move your arms, legs or trunk.
• The line of gravity is the imaginary vertical line from the centre of gravity down to the ground.
• The base of support is the area on the ground enclosed by the parts of you that touch it. For two feet, it is the area under and between them.

The rule: you stay balanced as long as your line of gravity falls inside your base of support. When it moves outside, you fall unless you move a foot or hand to make a new base.

3What makes you more stable

1. A wider base, for example feet apart instead of together.
2. A lower centre of gravity, for example bending the knees.
3. A line of gravity near the middle of the base, so there is room to sway before it leaves the base.

4Static and dynamic balance

• Static balance is holding a still position, such as a one-leg stork stand or a headstand.
• Dynamic balance is staying in control while moving, such as walking along a bench, landing from a jump or turning in a dance.

Three senses feed your balance: your eyes (where you are compared with the room), your inner ear (the vestibular system, which senses head position and movement) and proprioception (sensors in your muscles and joints that report where each body part is). Take one sense away, for example by closing your eyes, and balancing becomes much harder.

5Balance drills for school or home

1. Stork stand: stand on one foot and place the other foot against the inside of the standing knee, with your hands on your hips. Time how long you hold it, up to 60 seconds. Repeat with your eyes closed.
2. Line walk: walk heel-to-toe along a painted line or the edge of a bench, first forwards, then backwards.
3. Single-leg reach: on one leg, reach the free foot to touch three stones placed in front, to the side and behind, without putting it down.
4. V-sit: sit, lift both straight legs and your arms so that your body makes a V, and hold for 10 seconds.
5. Freeze landings: jump up, land on both feet with your knees bent over your toes, and freeze for three seconds without wobbling.

Safety: practise near a wall or a partner you can touch if you wobble, on a flat surface, and never on a raised object you could fall from.

Practice

Solution. On tiptoe on one foot the base of support is tiny (the ball of one foot) and the centre of gravity is higher. The line of gravity leaves the base with only a small sway.

Solution. (a) static; (b) dynamic; (c) dynamic; (d) static.

Solution. Closing his eyes removes vision, one of the three senses that control balance, so he must rely only on his inner ear and proprioception, and he corrects wobbles later and less accurately.

Solution. Widen his base in the direction of the push (one foot forward, one back) and lower his centre of gravity by bending his knees. Leaning slightly into the push also keeps his line of gravity inside the base.