Why can I cycle faster than the fastest runner?

29 April 2012

Part of the show Is there such a thing as a "girls' throw"?

Question

Hiya - congratulations on your podcasts.

Here's a question for you:
Wikipedia tells me that the men's 1 mile running record is held by Hicham El Guerrouj at 3 mins 43.13 secs. This, according to my maths, equates to 16.03 mph. Needless to say El Guerrouj is an elite athlete! I go cycling on my (not very expensive) bike most weekends. I usually cover about 20 miles on a ride and my average speed is never less than 16 mph. I am not an elite althlete - in fact I'm almost 50, weigh 100 kg and have a BMI which puts me well into the obese range. Why am I able to move at the same speed as an elite athlete, for 15 times longer - especially when you consider that the bike adds 10% to my overall weight plus lots of friction from the moving parts?

Keep up the good work Best wishes
Trevor Barton

Answer

Dave - There are various different things which could be limiting how fast you can move. There are various mechanical reasons, like you might not actually be able to move your muscles quickly enough or if you're pulling a very heavy weight, it's limited by the amount of force you can apply on the weight. I think in this case it's limited by the amount of power you can put out. Elite athletes are going to be able to put a lot more power out than you are, but you're using a method of transport which is much more efficient. Bikes have been optimised over the course of 150 years to be incredibly efficient. You use very, very little energy to keep going along. In fact you need hardly any energy to keep them going along, it's just to accelerate them and to overcome air resistance, as there's no friction there. So, with a very relatively small amount of power, you can be going very, very fast.

Whereas when running, you've got to move these great big legs around all the time and you actually kind of reach a limit to how fast you can move the legs backwards and forwards. You've got to accelerate and decelerate every time you take a step.

Chris - You're basically accelerating a bag of water weighing at least 60, if not 100 kilos which you're elevating and dropping and decelerating with every step, aren't you? So, you're basically having to keep lifting this very heavy bag up and down, and it's not an efficient way to move.

Dave - And also, you've got your legs, at 10 or 15 kilos each, which you're accelerating backwards and forwards. So all of that takes a huge amount of effort whereas on a bike, all you've got to do is just push this off along through a very efficient mechanical train.

Diana - Perhaps one of the reasons why humans are so good at doing long distances for a long time is because they've only got two legs whereas four-legged animals like horses, although they can go faster, they can't go as fast for as long as humans can because they've got four things to lift up.