When astronuats tried to run on the moon, they couldn’t. Gravity was not strong enough to pull them forwards. For that reason, they could not get to the next stride early enough, and it became hopping from one leg to the other, and was very slow.
If you stand something vertically on your desk, it should stay baanced, but if you tilt it enough to one side, it will have more weight on one side of the pivot point, than the other. And as gravity takes hold, there is nothing vertically under that side of the object to stop it from falling. It will fall over. If the leaning tower of Piza was not partially buried in the ground, it would fall over.
The above is what happens when we run. We slightly dissplace our weight, (especially in the acceleration phase) so that gravity can pull us forwards. What happens is that our reflexes are fast enough to catch us, by placing our feet on the ground, to stop us from loosing our balance. So our legs move backwards and forwards, ‘catching’ ourselves. It is safety reflexes, but also served a purpose to quickly escape things. If we put our feet too far in front, we will slow down (desireable if you want to avoid bumping in to something) , but if we dont put our feet in front of c.o.m, we can build the acceleration ( by not interfeering with gravity’s assertion on our momentum ).
There is no way we can create all of that acceleration momentum ourselves. Gravity is pulling us forwards, because we are allowing it to. (As a side note; this is why track athletes can ignore all the pilates and ‘balance’ exercises, that were designed for sedentry types.) If you had perfect balance, you would not go anywhere, and you would stay perfectly still. Fast running is “falling forwards quickly”, and I am not the first to describe it as thus.
Try the following experiment:Start in a standing potiion, and then jog forwards quickly, slow down quickly and imediately jog backward and then forwards. As you begin to jog forwards, it is much easier if you begin with a forwards lean, and when you try to de-accelerate, you will naturrally return to the upright position. If you try to stop very quickly, you will actually lean backwards. And if you try to reverse backwards quickly, you will also lean backwards in the first few strides of acceleration.
When you run, it is like your foot slipping on a banana skin (liek a in a cartoon) but the ground is the banana skin. So what happens is that the free leg swings forwards very quickly so that you can ‘catch’ yourself for the next propulsive stride. This is possibly the single most important action in sprinting as compared to other athletic endeavors. And so therefor; the hip flexor muscles, especially the psoas are of crucial importance to sprinting speed.
The ‘speed’ at which your leg pushes backwards on the ground is miss-understood. It is not realy pushing, so much as keeping you up, while gravity pulls you forwards. The speed at which your swing leg drives forwards, really influances the angle of support hip and leg and also its speed. It is not easy to describe. Charlie once said that jogging is running ON the ground whilst sprinting is running OVER the ground. The glutimus maximus muscles are not necesarily pushing your legs back against the ground. The gluteals are holding your torso upright (but not bolt upright like a soldier), otherwise you would lean forwards to much and hunch over. The hamstrings are not driving the leg backwards once foot is on the ground, so much as preventing the knee from buckling, so that you can quickly go in to the next stride.
Power is a factor. The more powerfull you run, the greater hip hieght, the less likely your foot will land in front of center of mass, and so the less you will interefere with gravities forwards pull on your deleiberately dissplaced body. Also, the more MU recruitment, the more musculoskeletal stiffness, and so the less ‘collapse’ when your feet strike the ground. But more than anything, the faster your forwards knee swing, the more you are being pulled forwards by gravity, rather than just driving away from it via hip extensors (vertical jump.)
If you were fifty times stronger, you would be able to run faster on Jupiter (if its surface was hard enough) than on earth.
So why don’t we maintain a constant and great forwards lean? Well for a number of reasons.
Firstly: the more the forwards lean, the less free you hips are, and the harder it is to have an open and ‘free’ long stride. And therefor: less chance of develpoing the level of musculoskeletal stiffness in the hamstrings required.
Also,there is slighltly less ability to cpe with and generate the required forces with a very pronounced lean, ad your foot will eventually return to striking in front of c.o.m and slowing you back down.
Top end speed, amongst other things, is using motor skills, to provide as little interferance on gravities forwards pull, as possible.
To get there, you need high accel, when feet can land behind the center of mass. In the first few strides of the race, the feet are landing obviously behind the body, and this ofcourse is where G-force is highest in the race. (Mass times acceleration.)
If you were to fall off a mountain, your first second of falling would be slower than the next second of falling, even though you are doing nothing physically to increase the speed.
When you accelerate in sprinting, especially at the beginning, gravity is taking hold of you and you have to do all you can to allow its own ability to accelerate your body forwards. (with power, quickness, spinal posture, hip rotation and its timing, relaxation, motor skills, physiology - energy etc…) otherwise you’ll collapse in a heap and you’ll ed up doing pilates or breakdancing on your ass on the ground.
You could write a whole book on this one concept, because everything feeds of everything else.
Just visualize Ben Johnsons first 60 meters in Korea. He ran like a propellar. Now imagine that propellar is being pulled along by an invisible cord (gravity) and this might be close to the feeling he had and also what I am trying to describe.