Sprinting: What's more important-horizontal or vertical force application?
A very large portion of the coaching community would suggest that horizontal force application is what drives the runner forward. It does seem reasonable that movement down the track requires the sprinter to apply a concentric contraction to create maximum force in the horizontal direction. Not only does it seem reasonable, it certainly looks like the runner gives a powerful push against the ground at toe off.
But what you see is not always what you get...
In the previous post on this topic, I referred to the Young-Hui Chang, et al. study, "The Independent Effects of Gravity and Inertia on Running Mechanics" to show that athletes are able to alter their running technique, when necessary, without conscious effort.
One of the two general hypothesis tested by the researchers was that altered gravitational forces affect the vertical (but not horizontal) forces generated by the runner. One of their specific predictions was that no change in horizontal braking and propulsive impulses generated during running would be effected by reduced gravitational forces. Simply stated, they believed that gravity did not affect the horizontal direction of running.
The results of the research did not match their expectation. In the discussion section of the paper, they stated, "contrary to our original hypothesis and intuition, these data indicate that gravity affects not only the generation of active vertical forces but also indirectly affects the generation of horizontal forces. Gravity appears to exert the major influence over both vertical and horizontal force generation during running." In addition to the preceding, they stated their surprise that "... horizontal impulses generated against the ground to brake and accelerate the runner with each step were also affected by gravity."
To fully appreciate the importance of understanding the debate over horizontal/vertical force application and its affect on speed, let's take a closer look at what occurs during the stance phase of high speed running:
An examination of what happens during the stance phase reveals that at its onset, horizontal ground reaction force is negative and decelerates the forward movement of the center of mass. While the center of mass is decelerating, it's also rotating over the grounded foot while continuing its horizontal direction. Just prior to mid-stance, vertical ground reaction force is at its maximum and horizontal forces are nearly zero. During the second half of the stance phase, ground reaction force is positive, accelerating the forward movement of the center of mass as the runner's leg is extended. As toe-off approaches, the horizontal ground reaction force again returns to zero (note: for a person running at a steady speed, braking force and propulsive force must be equal).
Here's a high speed force tracing that should give a clearer picture :
"During the drive phase, the power comes from, a pushing action off the ball of the foot. Recall that stride length, and therefore sprinting speed, is the result of a pushing action. The goal of the drive phase is to create the maximum push off the ground. The ball of the foot is the only part of the foot capable of creating an efficient and powerful push."-Sport Speed. George Dintiman, Bob Ward, Tom Tellez. Human kinetics, 1988.
The quote above represents the predominant understanding of what occurs at ground contact. If there is an efficient and powerful push, then there should be a concurrent spike in the high speed force tracing where the push occurred. Here's the conundrum: the image of the high-speed force tracing above shows maximum force prior to mid-stance. That means that there is more than half the stance time remaining for a second spike representing the powerful push off described by the authors above. Instead, the high speed force tracing shows a continuous dissipation of force from its peak to the end of the stance phase. If the authors meant that the pushoff occurred where the high-speed tracing peaks, then it would mean that the pushoff would have occurred halfway through the stance phase. This of course would mean that the end of the stance phase was halfway through the stance phase-obviously a contradiction!
Recall what I said above: What you see is not always what you get!
The peak of the high-speed force tracing occurs when the runner lands. What's being measured is a multiple of bodyweight, created by the runner as a free falling body making contact with the ground. The high-speed force tracing clearly shows the magnitude of the difference between the horizontal and vertical forces. The researchers concluded, "... In human running, gravity (not inertia) exerts the major influence on both vertical and horizontal forces generated against the ground. Horizontal forces are modulated so that they change in proportion to vertical force."
Most importantly, look at the vertical force measured...almost 2000 newtons. The horizontal? About 1/10 of the vertical.
Should the focus of training be on the horizontal?
I think not.