We covered that Force A is present in only a rearfoot strike, while Force B is present in both forefoot and rearfoot strikes.
But there is even more to this. Eliminating Force A by using a forefoot strike is more significant that just removing one of two forces. Because Force A is happening in such a small amount of time (hence my referring to it as instantaneous), your body can’t spread the force out as it can like Force B.
An example of the difference in forces (please don’t try this):
Force A: Kick a brick wall with your heel, as hard as you can
Force B: Kick the same brick wall, but do it in a slow, controlled, gentle fashion.
In the first example you can really hurt yourself, but in the second you won’t even feel a thing. Thinking again to Newton’s Second Law (F = M*A), you’ll note that the acceleration of your leg has decreased in the second example and therefore the total force is reduced to a level that your body can handle. If you were measuring, you would discover that reduced acceleration results in your foot touching the brick wall for a longer period of time. This is what is significant about Force A and why it’s so damaging. The angle of your heel and your stride results in a higher speed at which your foot meets the ground, resulting in the instantaneous impact. If you’re following, this is the acceleration part of the equation.
There is one more thing to consider between forefoot and rearfoot striking. When forefoot striking, your heel continues to fall as your ankle flexes, even after your foot impacts the ground. This spreads your body weight out throughout your entire foot and over a longer time period. You bear less than 2% of your body weight at the point of impact and much of that force converts into momentum as your ankle flexes and your heel lowers. Your tendons then use this energy to naturally propel your forward (like a spring).
When rearfoot striking, you don’t spread the same impact out over your foot, or over the same time period. As a result, you land with about 7% of your body weight, consisting of most of your foot and lower leg. In addition, this force shoots straight up into your leg and knee, because your foot isn’t absorbing the impact and momentum rotationally like a forefoot strike.
I realize this is a lot to take in all at once, so here is a handy little chart that explains the differences between Force A and Force B.
|Force A||Force B|
And this table shows the differences between a forefoot strike and a rearfoot strike:
|Forefoot Strike||Rearfoot Strike|
This illustrates that each stride you take using forefoot strike generates a single force that has almost no impact on your body. Each stride you take using a rearfoot strike generates that same force – and also generates a second, much more damaging force that impacts not only your feet, but your entire lower body.
If you are interested in learning more, visit Daniel Lieberman’s site on barefoot running. He has excellent videos, images and tables which show exactly what scientifically happens when using each type of foot strike. If you’re confused, this may help you understand exactly what happens (although I should warn you, it’s written very scientifically). He is also one of the authors of many of the studies that show the benefits of forefoot striking.