Q: How much force is really going through my legs when I run?
A: Forces on the body from ground impact can be considered in two parts: those associated with initial foot strike on the ground, and those related to pushing off. These 'ground reaction forces' have been calculated using sophisticated force measuring called "force plates". The ground reaction force is the force associated with the equal and opposite force of reaction between your foot and the ground.
The ground impact associated with initial foot strike during running can be up to 3-4 times your body weight. A quick calculation will show that even for a 100lb runner, each limb experiences up to 400lbs of force during each step. Another important feature of ground impact is the rate at which the force is applied because ground reaction forces that peak very quickly can be more damaging than those that peak slowly. Imagine for example, hitting your thumb with a hammer while driving a nail. Even with a 1 lb hammer the force on the thumb can be very large and we all know the damage that this can cause. But if a 1 lb weight (like a hammer) is placed very slowly onto your thumb, just rested there, you experience very little force and there is probably no damage. What this shows is that the rate at which your foot and leg experiences force is very important, and if the rate can be reduced, your body experiences less shock. With the correct type of shock absorbing material under your heel, both the magnitude and rate of ground impact can be reduced.
In the later case, the forces of push-off also exceed your body weight. This force is not from impact, but from muscular effort trying to move the body forward. To move the body, to accelerate it up and forward, you push against the ground, and the ground pushes back. Because these forces are large, it is important that your leg is aligned well and has a good platform to push from.
Question: The wear on my shoes is to the outside border of the heel. Does this mean I under-pronate?
Answer: Let's start with some definitions: - Under-pronation is somewhat of a misnomer particularly because what most people are referring to when they say under-pronate is actually supination. Supination is the opposite motion to pronation, (just as flexion is to extension). Since under-pronation implies a lack of or less pronation, supination and under-pronation are not equivalent.
Your question - "Does shoe wear on the outside of the heel mean you supinate?" Very few runners actually supinate, that is, land on the outside of the foot and then continue to roll to the outside. Landing with the foot in a supinated position - outside of the heel striking first - is common to most runners, but the majority of runners then roll to the inside, that is, they pronate. If your shoes wears on the outside border of the heel, than you have a better chance of being a pronator than a supinator. Really what this statement means is that if you land with your foot rolled to the outside, there is a strong likelihood that you roll in - pronate - to compensate for the strike position.
Dr. Nick Brown
Dr. Brown is a postdoctoral researcher in the Department of Biomedical Engineering at The University of Texas.
