system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How it possible that using the same energy as walking, we can travel much faster with a bicycle ? Is there a limit to building other machines that make us travel faster using human energy ?

Because when I'm on my bike, I sometimes think it's a miracle that I travel so fast whereas I'm not even pushing at all.
gpt
Bicycles are one of the most efficient forms of transportation humans have come up with. The only thing that is more efficient are recumbents, which are just a derivative of bicycles where you lay down and are more efficient due to less air resistance, and "velomobiles", which are recumbents with a drag-reducing body.

There's a few reasons why it's more efficient than walking, but primarily, it's the fact that we raise and lower our body with every step we take forward while supporting our entire weight. It's not much, but that inch or two your entire body mass moves up and down every 2 feet (one step) you move forward really adds up over thousands of steps. On a bike, your body is fixed, and your legs are used to just move you forward-no vertical movement, no supporting your whole body weight. Far less energy is needed since the bicycle is "doing the work" of holding you up while making each "step", or pedal, move you further.

The limit to building machines that make us travel faster using human energy is air resistance and friction (but mainly air resistance since our friction reducing technology is very good these days). To put it in perspective, the world record for paced (car in front to reduce air resistance) on a bike is 167mph, but only 83mph un-paced.