Emphasize free leg in running like using free leg in jumping
One connection to try to establish in sprinting is having the trailing knee align with the opposite foot at ground contact. At top speed the knees will align, but in acceleration the knee will align with the foot due to the forward position of the bent knee as the foot contacts the ground. In either case, the knee aligns with the opposite foot.
Try to have the force generated by the legs create an immediate connection to the ground with a large horizontal component of velocity at ground contact. The knee drive forward and opposite foot pulling on the ground should occur simultaneously when the foot contacts the ground. This can be compared to a single-leg vertical jump where the free leg drives upward as the other leg extends. The difference in sprinting are the greater horizontal components of velocity and force, and the immediate horizontal force on ground contact. There should not be a two-count on ground contact and knee drive. If the foot contacts the ground before the trailing knee is brought forward, it takes more time for the horizontal force to develop and for the stride to finish. The foot’s horizontal component of the velocity will not be as high if it lands early and ahead of the hips. This is both due to the foot being in its downward rotation before it is under the hips, and due to the foot having a shorter path through which to accelerate before contacting the ground. If the knee is aligned with the foot at ground contact, the duration of the ground contact time is shortened, and there is greater horizontal component of velocity immediately on ground contact.
Another way to analyze this movement is to consider that both legs are moving their fastest as they cross under the hips. Having the legs moving their fastest as they cross at ground contact helps in two ways. It allows the foot to be moving backwards at a high velocity either to accelerate the body or to maintain top speed. The other benefit is having the trail leg quickly rotate ahead of the body. This has its own two benefits: 1) the thigh’s forward momentum drives the body’s center of mass forward, and also provides a force to counter with the glutes producing the thigh’s sudden change of direction creating a forward force. 2) the thigh’s high forward rotational speed at ground contact allows it to reach its maximum forward rotation faster so that the glutes can also pull early to repeat the process with the opposite movements. If the thigh is not in a position to pull with the glutes early enough, the foot cannot be accelerated before ground contact. A slower backward foot velocity means a slower running speed.