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While it is very hard and energy-consuming to run a marathon, covering the same distance by bike is doable by most people. Where exactly does the gain come from?

Assuming for simplicity that we are on a flat surface, all the expended energy eventually dissipates, i. e. is used to produce work against friction. I can see the following possible explanations of why walking/running could lead to less efficient setup:

  • when running, at each step one needs to lift their center of mass, turning the energy into potential energy, which is then dissipated;
  • when the front foot reaches the ground, it moves in the forward direction until friction stops it, dissipating its kinetic energy;
  • our joints are just less suited for walking than for spinning peddles and dissipate more energy doing that.

So, I would like to know which of these reasons really contribute/are dominant? Or maybe there are some other reasons I did not think of? Are there some fundamental obstructions for walking to be more efficient, or is it just an "engineering" issue?

UPD: turns out this question has already been asked, which I somehow didn't manage to find before asking. See also this answer to another duplicate question.

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marked as duplicate by tpg2114 Jun 12 at 15:38

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  • $\begingroup$ Another thing to think about -- when speeds are very different, it's better to think in terms of total time. If I ran a marathon in X hours, and I rode a bike for X hours, I'd probably feel pretty similar at the end (assuming I was equally skilled/adapted for both). $\endgroup$ – tpg2114 Jun 12 at 15:19
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I suspect the single most important factor that makes cycling so much more efficient that running has to do with inertia, particularly on a flat surface. When I apply torque to the wheels due to my pushing down on the pedals I can immediately stop pedaling and my bike and I will continue coasting without any additional effort per Newton's first law, until my bike and I eventually slows down (air resistance and rolling friction) requiring me to give it another "push". There is no "coasting" when your run. You need to expend energy for every meter you cover.

While it is very hard and energy-consuming to run a marathon, covering the same distance by bike is doable by most people. Where exactly does the gain come from?

The gain comes from the fact that the cyclist does not need to expend as many calories as the runner in covering the same distance in the same time. Physical effort does not equal the physics work done. For one thing, our bodies are not 100% efficient. To illustrate this in the extreme, imagine you are standing still and holding a very heavy box in your hands. You need to exert an upward force of $mg$ just to hold the box in place. You are not lifting the box so you are not doing any work from a physics standpoint (force x distance). But you are certainly expending more energy holding the box than you would if your weren't holding it, causing you to tire. Physical effort does not equal work done.

The cyclist expends less energy by taking advantage of the fact that the wheels enable the inertia of the cyclist and bicycle to coast while not expending any energy Unfortunately the runner has no wheels.

Hope this helps.

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  • $\begingroup$ Trying to coast while running will quickly result in physical discomfort... There's also a mechanical efficiency gained on a bike through gears -- I bet 25 miles of riding on a 1:1 gear ratio would be quite exhausting. $\endgroup$ – tpg2114 Jun 12 at 15:14
  • $\begingroup$ @tpg2114 Nice point. I suppose if there was periodically ice on the road you could slide for a while (just kidding). $\endgroup$ – Bob D Jun 12 at 15:16
  • $\begingroup$ @tpg2114 Regarding gearing, true. There are probably a number of factors that make cycling more efficient than running. May answer was for what I felt is the single most important factor. Particularly on a level surface. $\endgroup$ – Bob D Jun 12 at 15:23
  • $\begingroup$ Thanks. I found and linked a similar question, and the most popular answer also mentions inertia. But I am not very convinced. If I place my foot on the ground at zero relative speed, then there will be no friction backwards. This means that I'll be still moving forward on inertia (of course, I will need to offset air resistance, but the same applies to cycling). $\endgroup$ – Kostya_I Jun 12 at 15:33
  • $\begingroup$ @Kostya_I That's OK, I'm not trying to convince you. When I was a kid I did lot's of cycling on the south shore of Long Island (very flat). I could ride for miles with very little effort. It comes down to how much work you need to do to cover a certain distance in a certain time. On a bike I can cover a lot of distance in a certain time without exerting any force (doing any work). I don't see how I can say the same for running. Hope you get the answer you are looking for. $\endgroup$ – Bob D Jun 12 at 15:58

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