Your knees play a Huge part in your running (big shock, right?)!
Your knees are the pivot point for your God given leaf springs!
Run with your knees bent! Your leg should never fully straighten out, not when landing, not when lifting, not when moving your foot out for your next stride - always, always keep bend in your knees and that will Save your knees! It allows your legs to become springs/shocks, absorbing all the impact of your body! Bend your knees to the point where you feel a little funny, and then bend them a little more (you can't ever bend your knees too much).
Very basically - keep them pointing in the direction you are going! Don't torque your body one way or the other. Remember, chest, hips, belly button, knees, toes, shoulders, everything points straight forward.
Center of Gravity
As I mentioned earlier, in order for your foot to land
flat, it must be underneath your center of gravity.
You cannot possibly have your foot stretched way
out in front of you (over-striding) and land as
-Stand and march in place, that's how (and where) your feet
should land. Look down, you can't see where your shoes are tied
(you can't see the knot), that's correct. So, if you ever look down
and can see your shoe string knot when your foot lands, it means
you are over-striding too far and need to shorten your stride-
The “form” that has you over-stride was created by
heel striking, which was created by the invention of
“Jogging” (coined by Bill Bowerman – founder of
Nike and father of the modern running shoe).
It is virtually impossible to heel strike barefoot. Try
it. Once. You’ll never do it again.
So your strides are actually quite short, and in order
to compensate for using a shorter stride (as
intended), you turn your feet over faster.
As soon as one foot lands, you are lifting the other
off, you simply speed up that process (think of your
feet like a wheel) by turning them over at an even
faster rate. Think of it as running quicker, not harder
– it’s light, nimble, and natural vs. heavy and forced.
Read this article from Harvard University: