Elite sprinters captivate audiences with their blazing speed and acceleration, pushing off the blocks and propelling themselves to world records and gold medals. With each step, the ground reaction force pushes the runner up and forwards until they reach the finish line. This is why high net horizontal ground reaction force, summed over each step, is the strongest predictor of sprinting performance. Of course, that is the net effect of the ground reaction force impulse: it must push forward if the sprinter is to go forward. The instantaneous ground reaction force at any given point in a runner's step, and at different times while accelerating during a race, is slightly more complicated. After the initial push off the blocks, as the foot makes contact with the ground, there is firstly a braking impulse in which the ground pushes the runner in the reverse direction. Over the course of the step, the ground reaction force changes orientation from backwards to forwards, outstripping that small rearwards component and driving the runner forward. In other words, there are two phases to each step: a braking component and a pushing component.
With this in mind, we return to the elite sprinters. How do these outstanding athletes accelerate quickly? This is the question asked by Jean-Benoît Morin and a team of international collaborators working at the French Institute of Sport in Paris. The team noted that there are two possible ways elite sprinters may be increasing their acceleration capacity: they can ‘push more’, generating higher ground reaction forces in the forwards direction, or they can ‘brake less’, minimizing the ground reaction forces in the backwards direction. (Or, they could employ both strategies.)
To determine how elite sprinters accelerate so quickly, the team measured the ground reaction force impulses over multiple steps across the length of a 40 m sprint for nine sprinters ranging from international level (elite) to French national level (sub-elite) competitors. By measuring the different components of ground reaction force, the researchers could see which aspects of that force best predicted sprint time.
They found that even with a low sample size, there was a high correlation between sprint performance (mean velocity) and both the net horizontal ground reaction impulse and the positive component of the horizontal ground reaction impulse. In other words, the fastest sprinters had a bigger push-off with each step. There was no correlation between the negative ground reaction force impulse and average velocity; so, although the better sprinters were pushing more, they were not necessarily breaking less.
