[DJ Hicks is a recent graduate of Houston Baptist University and the Athletic Lab Coaches Mentorship at Athletic Lab. DJ is currently an assistant coach at Valparaiso University]
Generally, it is agree that sprinting is comprised of three key phases: acceleration, max velocity, and speed maintenance. It is also commonly agreed upon that in sprinting, the ability to apply and manage force put into the ground under a time constraint is crucial. Throughout the phases of sprinting, this time constrained force application manifests in a few different ways.
Specifically speaking about the 100m dash, during acceleration ranging from about 0-30m, you will notice longer ground contact times, in which the body is most effective in overcoming inertia out of the blocks by projecting itself forward at roughly a 45° angle. You will also notice relatively big ranges of motion and a predominately horizontal application of force (hence the 45° angle). As the sprint progresses into the 30-60m range and so on, the above listed acceleration mechanics will be abandoned for more favorable ones that contribute to efficient running at top speed- max velocity mechanics. Here, some of the goals are elongated posture, striking directly under your center of mass, and minimal backside action. Top speed sprinting can be characterized predominately by vertical application of force, all the while moving horizontally down the track.
At no point during a sprint should force application be purely horizontal or vertical. At any point in a sprint, force application can be characterized as horizontal, vertical, or a slight blend for the sake of cueing an athlete. The action of propelling one’s self at a 45° horizontal angle is important during early acceleration because it puts you in the best position to push yourself to top speed. This is an easy point to convey to athletes because it is perfectly rational to accept that in order to progress down the track (to the finish line), a horizontal component must be present. Conversely, it is not as easy to convince athletes that while running at top speed, the fastest way they will sprint horizontally down the track is by focusing on contacting the ground in a vertical manner.
At maximum velocity (top speed), cueing a more vertical action is beneficial because it allows the body to express and counter force efficiently. As an example, when lifting a heavy object, keeping it closer to your center of mass (base of support) is much easier than holding it away. In sprinting at max velocity, it is much easier to both apply large amounts of force directly at and closest to your base of support. Additionally, accepting the reaction forces that follow each contact are best managed here as well, as opposed to behind or in front. Vertical application also aides in reducing breaking forces experienced at each ground contact.
Because humans cannot fly and gravity is alive and well, ground contact is inevitable, and if ineffective, can severely hinder performance. In normal walking, our hips are directly under us, and with heel strikes, our steps land in front of us, not under or behind. Here, with each heel strike we are producing braking forces that send reaction forces in the opposite way from which we are traveling. We don’t care though, because the goal is not speed, it’s just to get where we are going comfortably. Now consider creating a posteriorly tilted pelvis in which your hips are behind you and see if you can walk forward with ease…not so much. When sprinting, however, it is easier to run with this incorrect posteriorly tilted pelvis and trailing legs at top speed because the action is so much faster with less time on the ground. This is because the double support phase of walking is replaced with a double float action of sprinting. You will frequently see this in athletes who lack the necessary strength to hold the correct positions, or in those who believe what they are incorrectly doing is right.
With this explanation in mind, it’s also important to maintain a dorsi-flexed foot that is rigid and prepared for contact on its ball. At contact, a foot fixed in this manner hitting the ground in a vertical fashion will result in a diagonal resolution force that is the combination of both horizontal and vertical elements.
As stated, both horizontal and vertical forces are present at all times to some degree. Both are helpful. What’s important is cueing them in a helpful manner that creates correct body positions and prevents athlete confusion.