How the fitness tracker became a million dollar idea

Fitness trackers - those things you put around your wrist to measure your physical activity and your health - have been on the mass market for fewer than 10 years.

Nowadays, hundreds of millions are sold every year. But a lot of the technology inside these hi-tech gadgets was developed as long as a century ago, as the BBC's Aaron Heslehurst explains.

Video journalist: Jeremy Howell