Why are humans more energy-efficient in running than animals with four legs?

The really simple answer is that humans run more efficiently because we let gravity do a lot more of the work. When we go forward we're basically putting one foot out and falling forward, then pulling ourselves forward and repeating the process with the other foot.

When quadrapedal animals run, they need to propel themselves forward with their front and back legs; the advantages of this are that they can put more of their total muscle mass into running and you get more sources of speed, and run faster/quicker; pretty much any quadraped can out-sprint a human. But humans are the undisputed champions of distance-running on Earth, partly because their run is more energy efficient.

The other thing that helps us run, just as a sidenote, is the fact that our bodies are really good at not overheating. A cheetah for instance can only keep up their vaunted 60 mph run-speed for a very short distance without overheating and exhausting themselves. But our ability to have the airflow of our forward motion wick heat away by evaporating sweat off of us is one of nature's best heat regulation mechanisms, and allows for humans to run for hours on end without stopping, when properly trained.

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