The Science of Energy in the Gym

21 January 2007


  • An exercise bike with a fancy energy readout in Calories or Watts.
  • A person like Dave, who knows how much energy appliances use
  • A person like Ali, who will do anything in the name of science


Ali had a go at powering a couple of small lights using a small hand cranked generator. When both lights were off turning the handle was easy, when the 5W bulb was turned on it got a bit harder and when the 10W bulb was turned on it got significantly more difficult.

Hand Cranked Generator

Considering a conventional light bulb uses 60-100W of power Ali wasn't doing very well so we decided to try using the biggest muscles in Ali's body - her legs.

We found a gym with an exercise bike with a power rating built in. When Ali worked quite hard on the exercise bike she generated around 150 Watts, enough to power a decent sized television. When Ali works really hard, she generates 280 Watts, enough to power a computer without a monitor. Unfortunately she's not fit enough to keep that pace up for long.

Ali Peddling Hard


If you move a magnet near a coil of wire, you will push the electrons back and forth. If there is somewhere for them to you they will flow producing an electric current.

Here you can see Ali moving a strong magnet in and out of a coil of wire. This makes a current in the wire which you can see on the meter.

Moving magents and coils is is how virtually all the power in the world is generated (except a little using photovolatic solar cells) the only thing that changes is what you use to move the magnets. Unfortunately this power does not come for free, a circulating electric current produces a magnetic field that fights the movements you are making. This means that the more current you draw from the could the harder it is to move the magnet and so you have to do more work to generate more power.

How much power would all the gyms in the country generate?

If you had 50 million people on the planet generating on average 100 Watts each, that's about 5 gigawatts or as much as a large coal powered station. However few people spend even 1% of their time in the gym and even less of that actually doing exercise, so the figure is probably closer to a few megawatts - about the same as 3-4 windmills and it would probably be cheaper and more environmentally friendly to install the windmills.

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