‘Gravity falling’

By Keith Bateman

Some people have been told that they should constantly fall forwards when running and that gravity provides a proportion of the forwards motion. If you have tried this and felt the falling sensation propelling you forward, sadly the effect is just an illusion.

This advice probably comes from the fact that most people land off-balance, leaning backwards and tilting more forwards will help to reduce braking. However, this interpretation of the physics of running conflicts with the known physical laws of motion. 

Gravity acts downwards (or, more correctly, towards the centre of the Earth). This means, as you fall, your centre of mass will be drawn downwards towards the ground. Not forwards, not backwards, just down. An orbiting satellite does constantly fall towards the Earth but it’s momentum stops it getting here. Here on Earth you will only experience the feeling of being pulled forwards as a result of gravity when running downhill.

If you stood vertically on ice skates and fell forwards keeping your body stiff like a plank, the top of your body would fall forwards, but the lower half of your body would slide backwards and “out from under you”. This is because gravity only causes your centre of mass to move directly down and not forward or back. If the top half of your mass moves forward then the lower half of your mass must move backwards to compensate and stop the centre of mass moving horizontally.

For anyone who hasn’t ice skated you may have experienced the reverse situation. You may have slipped on a wet bathroom floor and your feet shot forwards while your upper body fell backwards and you landed on your behind. Despite the sensation of falling backwards you weren’t propelled backwards, because your feet moved forwards. In other words, your centre of gravity just moved downwards and your body as a whole neither moved forwards or backwards.

If you’re still not convinced consider this simple thought experiment. Imagine a human sized tower built on the back of a wheeled trolley. If you push the tower so that it topples forward will it keep being propelled forwards forever on the back of the cart? No. It will fall forwards and the trolley will shoot backwards, maintaining the horizontal position of the centre of mass.

So while people seem to feel as though they are being propelled forward by gravity when they are running it is just a misleading sensation. The laws of physics dictate that gravity does not move your centre of mass forward, and hence cannot give you any forward propulsion.

If you still feel yourself falling forwards and you are not accelerating, falling over, or running into a strong wind then it is your head and torso that are forwards with these being supported or balanced by the lower half of your body.

You must be either:

  • kicking your feet up behind you using them to balance your torso
  • advancing your leg under your torso to maintain balance.

Neither of these situations represents a good running action.