As I say often, moving the body forward is the ultimate goal of running. When a runner doesn’t bend their legs, as the legs move under the body, and then behind the body (or in front of the body, and then under the body), the body necessarily moves up and down.
A straight-leg swinging forward requires that we launch the body even higher, so the foot can clear the ground as we swing the leg forward. ..you laugh, but I’ve seen many people run this way. It’s like they can’t bend their knees, and maybe they can’t, but most likely because when they could, the refused to. And so, the ability unused, becomes lost.
Varying Leg Length
By bending the knees, we can alter the effective length of the legs as they pass under our body, and stretch out behind the body, thus minimize the amount of up and down movement of the body itself, so we can use our energy to move forward (instead of bouncing up-and-down).
Find Your Springs
If you stand up straight, and then lower your torso by bending your knees, you’ll find your “sweet spot”, this is the point at which your natural springs kick in, and as you’re bending your knees it will feel like if you want to go lower, you’ll need to force yourself lower… But don’t, it is this point that your natural springs kick in, and it is those natural springs (made up of your bones, joints, muscles, tendons, ligaments, etc.) which help propel us forward with very little conscious effort (as long as we land with our body moving over and in front of our supporting foot … and of course with our knee already bending as we land.