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The most important is friction, without it running wouldn't be possible at all. The force of friction between the runner's foot and the ground is what propels the runner forward. The less friction there is, the less speed the runner can obtain.
Another is conservation of momentum. Here we can draw on Newton's third law of equal and opposing forces. A runner pushes into the ground and the ground pushes back up with an equal amount of force. This conservation of momentum is what allows the runner to move forward. If the ground gave way instead of pushing back the runner wouldn't get very far at all.
These first two explain why it takes more work to run on surfaces such as ice and sand.
Lastly, the form of the runner affects the efficiency of their run. For instance, if you run with your arms at anything other than 90 degrees you are wasting energy holding them in that position. Also, if you run by crossing your arms over your body you are wasting energy by adding a non-forward component of your velocity that acts against the forward component.
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