Humans are persistence hunting running animals, designed to run at a steady pace over long distances in pursuit of food. From an engineering stand point we are designed to run with a forefoot strike and heel striking or "jogging" is a learned behaviour encouraged by over cushioned shoes that mask the impact forces in running causing us to loose our proprioceptive feedback over time, yet around 75% of runners are heel strikers. Think of it this way; when you're wearing gloves it's really hard to get coins out of your wallet, or find something in a bag. this is because we cannot feel the surface that our fingers are reacting against. Our feet are the 'ears' to our hips and core muscles, what we feel through our feet lets our body know how to react to that surface. Thick soled shoes mask this communication and therefore we no longer run on the ground as we perceive it, but instead by memory. Still not sure? try running 20 metres barefoot at a moderate pace, most people adopt a forefoot strike and find that it is uncomfortable to heel strike.
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| Fig:1. Heel striking vs. forefoot strike |
A heel strike causes the leg to impact with a much straighter knee position. Impact causes loading of tibialis anterior while arch remains unloaded. Impact forces are transferred up through the knee.
A forefoot strike places the impact through the multiple bones and ligaments in the foot designed to act like springs. Impact loads the arch to absorb impact. Note also the double-impact graph from heel striking.
In heel striking the heel hits the floor with the full force of impact, however during forefoot striking the heel is lowered to the floor under the control of the posterior calf muscles and Achilles tendon.
Forces on impact
Lieberman et al. found that heel striking meant that the vertical momentum of the lower leg is absorbed on impact, whereas in a forefoot strike much of the vertical momentum is converted into rotational momentum. Imagine dropping a metal rod on a hard surface upright and the noise it would make because of the force of impact. If you dropped that same rod at an angle it would make less noise because the force of impact is displaced as the rest of the rod rotates to the floor.
Below is the impact transient and total impact forces for a heel striker and forefoot striker (Fig: 1)
Shod heel strike: 0.108 sec impact transient, 2.5 x BW
Shod forefoot strike: 0.124 sec impact transient, 2.49 x BW
The peak impact force for heel striking and forefoot striking are the same however this is the point at which body's centre of mass is lowest the ground, usually when the foot is already underneath the body, but at this point the forces are rising slowly, so it's thought that this is less likely to contribute to injury. However because a larger proportion of the foot and lower leg come to almost a dead stop during a heel strike, compared to a forefoot strike the there is a larger mass under the influence of that ground reaction force.
The speed at which the force is applied to the body during the impact transient is where we start to see differences. During a heel strike the impact transient is higher than during a forefoot strike, so that force is being applied to the body faster. Because of this there are those that believe that heel striking is a contributor to running injuries. However there are yet to be any direct studies in this area and most evidence is anecdotal. While heel-running shoes do reduce the force by about 10% and spread the force over a greater area, they do not eliminate the impact transient. And as a side note; heel striking causes forced ankle planter-flexion which can lead to overactive tibialis and is linked to shin splints.
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