Running Physics: How to Choose Shoes Based on Weight

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Do you really know how to choose the perfect pair of running shoes for you? You know how mass, impact force And racing style do they affect the type of shoes to choose? The impact force running shoes have to withstand can be calculated, and do you know how? Thanks physics! But don’t be afraid, ours comes to our aid Sandro Mastrolembophysicist, runner and friend of LBM Sport.

In this article, we’ll tell you why it’s essential to change running shoes every few miles, as impact degrades their cushioning and support properties and why it’s so important to choose a shoe that’s right for you. your body and your running style, given that among these
two factors depend on the force of impact to which the shoe will be subjected.

What impact force do our shoes have to withstand?

To explain the impact force that running shoes have to withstand, there are two models that we see in the image and that we will explain.

Technical diagram of running shoes explained with physics

Multi mass-spring model

It tells mathematically how the ground reaction forces produced by ours Arts with those produced by the model (which aims to represent our body with simpler mechanical elements, that is, blocks of well-defined weight and springs with well-known forces).

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2 mass model

The two-mass model is a computational model (i.e. used in automated calculation using computer algorithms) that uses readily known mass data (weight in kg).

to summarize

We can say that the multi-mass-spring model is a mechanical representation of our body, while 2 mass model is a translation in terms of a computer algorithm.

How is the runner’s weight calculated?

It is obtained by multiplying its mass by the acceleration of gravity equal to 9.8 m/s 2. Both in the case of a man and a woman, the weight is divided according to these percentages (m2 92% and m1 8%).

Practical example

If we have a runner or runner weighing 70 kg, then m1 will be 92% of 70, or 64.4 kg, while m2 will be 8% of 70 kg, or 5.6 kg.

The runner in the image is a man, but the gender is irrelevant to the model. The only thing that matters is the total mass of the runneri.e. its weight.

How does running style affect impact resistance?

The force of impact depends on the runner’s vertical speed with which they hit the ground, as well as the runner’s running style. Generally, a distinction is made between:

  • RSF (rearfoot forwards) runner supported mainly with the heel;
  • SBB (forefoot forwards) runner who is mainly supported on the forefoot.

For runners who support themselves primarily with the heel, in general the impact force during the
landing is always more important than runners who rely primarily on the forefoot because strength is developed in a much shorter time.

So how do you choose running shoes based on weight and running style?

Runners support heel they are, in general, more prone to injury and should consider buying more padded shoes.

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The runners we mainly accompany forefoot (usually the most experienced riders) can afford lighter and more responsive shoes because their mode of support produces a lower impact force on their limbs and therefore injuries are less frequent for them.

Practical examples

  • RFS weighing 70kg driving at around 5h30/km
  • CFF weighing 70 kg driving at about 5h30/km

The two runners, in the landing phase, will be subjected to an impact force which is on average about 2.2 times their weight (ie 70 x 2.2 = 154 kg). The runner who relies on the heel will load it on the limb in much less time than the runner who relies on the forefoot. As a result, the first will stress your body much more than the second.

Article designed and supervised by:
Sandro Mastrolembo
Physicist and biomedical engineer

Friend of LBMsport and runner
See Sandro’s notes

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