Why is riding a bicycle easier (requiring less work) than walking, say at natural walking speed?
Part of it is the fact that when you're walking, you are using your legs to support your weight and to propell yourself, whereas sitting on a bicycle seat releases your legs of the burden of your mass so that all the force they expand can be directed towards propulsion. But this is only a small part of the mechanical advantage provided by a bicycle.
I think the major factor involved is that human legs are made powerful enough so that we can run, should the need arise. This extra potential provides us the opportunity to take advantage of mechanical "gearing", where a little extra effort yields far greater results.
When walking, you spend a good deal of time out of equlibrium, you're center of gravity is not directly over the contact patch of your feet, so as posted, you spend engery supporting your weight. When you're running, there are times when both feet are in the air, so you're actually jumping up and down a bit.
As posted before, on a bicycle, you rest some (usually most) of your weight on the seat and handlebars, so more of your energy is going into just propelling the bike forward.
Also as posted, the gearing is designed to let your leg muscles operate at a speed and force that is optimized.
A bicycle is designed for work efficiency on a flat level surface, but legs aren't.
Try riding a bicycle up a real steep hill, or better still some stairs. Then you'll get a handle on what legs are good at!
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