There are a lot of people who express doubt that gravity, which is a downward force, can be used by runners to move forward. I believe that it has been adequately demonstrated that gravity is a significant component in forward movement when running and walking. However, Pose theory states that gravity is the only force moving runners forward. I believe that this is probably true, but it hasn't been "proven" the point of general acceptance yet.
The idea behind Pose theory is that we move forward using rotational torque which coverts the downward force of gravity into forward movement. If you can fall down, then you can move forward, using nothing more than the force of gravity. In order to run, you just have to string together a series of falls.
Now for the Monkeys
I happened to be watching a program about Darwin last night, and they showed a video of monkey swinging from branch to branch, and he was moving fast. He was able to move forward by grabbing each branch and falling which allowed him to move forward utilizing gravity via rotational torque. The main difference between the monkey and a runner is that instead of pulling his leg up to recover, the monkey had to reach for the next branch with his other arm to recover. The monkey did not have to push or pull to move forward, he simply had to fall and utilize rotational torque.
So What's My Point?
I'm not saying that this proves anything about the biomechanics of running. However, the point I'm trying to make is that it is possible to move forward using no force other than gravity. Instead of falling underneath our hands like a monkey, perhaps it is possible that we are falling over our feet when running.
This is only meant to be a thought experiment. Again, I'm not trying to "prove" anything here. I'm sure that any possible flaw in my thinking will be rapidly pointed out, but it would seem that by utilizing the principles from Pose theory, that a monkey is capable of moving forward extremely fast.