Does Force Absorption Exist?

How many times we've heard about force absorption within the strenght and conditioning industry?

Usually, when the expression "force absorption" is being used, the intent is either to describe the process of attenuating the impact during foot strike or the neuromuscular capability to handle and manage the amount of forces produced during a given movement.

However, if we want to properly employ biomechanics in our own language we should be aware that force absorption does not exist.

In fact, force cannot be absorbed but to understand this concept, we will have a look back at some essential mechanics notions such as energy, force, shock, damping, and momentum.

In order for human beings to move and perform finely-tuned skills with the desired speed of execution and higher level of force production, they need to do work. And the capacity to do work is called energy.

Energy exists in various forms, such as heat, light, sound, and chemical energy. In biomechanics, our main focus is mechanical energy, which can be divided into two types: kinetic and potential energy.

Kinetic energy refers to the energy an object possesses because of its motion, while potential energy describes the energy stored in an object due to its position.

So energy quantifies how much work can be done in a system. But how does energy relate to force?

First, force is a straight-line push or pull and it is an interaction between two bodies that results in the change of motion of an object.

A force applied over a distance transfers energy to or from a system. When we accelerate, we apply a high level of force backward through the interaction between the foot and the ground: the result of such interaction is a change in motion of the body due to the production of energy, which is the currency that gets transferred when the pushing action moves the body forward.

Differences between Force and Energy

When we strike the ground during sprinting, running, or changing direction, the impact cause a rapid acceleration or impulse defined as shock.

Shock is transient, as it lasts microseconds to milliseconds, and is characterised by an high amplitude, oftern extreme compared to regular forces.

Having said that, how the notions of energy, force and shock relate to the fact that force cannot be absorbed?

Force is a steady or slowly applied interaction, while shock is a sudden and transient event, like a pulse of acceleration or force. A constant push (i.e. a steady force) needs to be counteracted and resisted, and this is the reason why it cannot be absorbed.

Shock, on the other hand, since it represents a transitory and traumatic event, needs to be absorbed in order to dissipate the energy produced.

Thus shock absorption is the correct expression from a biomechanics and physics point of view.

Shock absorbent materials in running shoes change the momentum of an impact over a longer period of time

But how can we define shock absorption? To do so we need to review the last 2 notions, i.e. damping and momentum.

Damping is a complex, typically nonlinear process present in any system where energy dissipation occurs. Therefore, shock absorption is defined as "the progressive damping of an applied force" (Rodgers and Cavanagh, 1984).

The mechanism of action of shock absorbent materials used in running shoes is to change the momentum of an impact over a longer period of time. Momentum is calculated by multiplying an object's mass by its velocity; greater mass or speed results in more momentum.

In conclusion, force absorption simply does not exist because a steady force requires to be counterbalanced and resisted, while a sudden and extremely brief shock needs to be absorbed and energy dissipated and converted into heat and deformation.

Antonio Robustelli is the mastermind behind Omniathlete. He is an international high performance consultant and sought-after speaker in the area of Sport Science and Sports Medicine, working all over the world with individual athletes (including participation in the last 5 Olympics) as well as professional teams in soccer, basketball, rugby, baseball since 23 years. Currently serving as Faculty Member and Programme Leader at the National Institute of Sports in India (SAI-NSNIS).