The Truth About Shock-Absorbing /Cushioning Insoles
Running ‘Bounce’ and Energy Management
Running is quite different to walking. Walking involves two distinct collision events with the ground, whereas running only involves just one. Impact during walking is made of two minor collisions between the heel first, then the forefoot. A separate pushing off collision occurs after the leg has rotated over the foot and just before the opposite foot makes ground contact. Running involves a single impact into the ground as the leg rotates over the foot via the ankle, then immediately pushes off the ground to launch the body into the air, well before the other foot contacts the ground. This airborne period without any foot contact is known as the aerial or float phase and it requires a lot of energy (see figure 1).
Impact collision [ weight transfers over ankle slowly ] Acceleration collision
Impact collision [rapid weight transfer] Acceleration collision Aerial phase
Running, whether in heel strike or forefoot strike position, is a ‘bounce down’ and ‘bounce up’ event that involves extensive energy management. Imagine a basketball being bounced on the floor. Bounce it straight down and it bounces straight up. You need to put more energy when sending it down to get it to bounce back up, because during the impact collision it loses energy and doesn’t bounce up as freely as it went down. This is because the ball dissipates some energy when it deforms against the ground (as heat, sound, and material strain), but it bounces back up from the energy stored in its elastic materials as it springs back into shape, greatly helped by the air pressure inside (see figure 2).
In running, because the angle of bounce off is different to the bounce down, it is more like bouncing the ball down at an angle (see figure 3). Now you can see how bouncing off the ground can send runners forward at speed. However, the same principle applies to losing energy during the impact collision downward, reducing energy from the energy-supply of the bounce back upwards.
Gravity and acceleration from running creates a downward impact force on the ground that is high in energy. This high energy could injure the body. However, bounce-off acceleration requires energy. Therefore runners dissipate some impact energy (losing it) but store some energy within the elastic connective tissues of the leg and foot to help spring away from the ground at acceleration. If too much energy is lost at impact then much greater effort must be generated for acceleration by muscle contractions that can tire muscle early (fatiguing them prematurely) resulting in an injury risk. Thus, a balance exists between dissipating energy to prevent injury and storing energy to reduce muscle effort required for acceleration (see figure 4).
Getting the ‘Bounce’ Right
Every runner has the ability to adjust their ‘bounce’ by changing hip, knee, and ankle joint angles, and by increasing or decreasing muscle activity to change the amount of elasticity within the leg and foot. On softer ground the leg and foot sets themselves stiffer. This is because on softer ground it is harder to bounce off. The anatomy must become more elastic for runners to still bounce off soft ground without too much muscle power being required.
On harder ground the reverse is true. It is easy to bounce off hard ground but the bounce down impact energy could injure the leg or foot. Therefore the anatomy behaves slightly softer, and the leg becomes more compliant. The clever bit is that humans can set the level of elasticity by predicting the impact ‘bounce’ required before we hit the ground (see figure 5).
When we predict an impact elasticity level incorrectly we get a jarring (too stiff) or giving way (too compliant) sensation. Examples are such as dropping down a curb that we didn’t see (set too hard for bigger impact), or suddenly running onto a soft surface that we didn’t notice (too compliant for a soft ground impact).
Do Runners Need Cushioning Insoles?
So what happens when you add extra cushioning to your running shoes?
First, consider the basketball being bounced onto a cushioned mat rather than on a hard basketball court. The ball is not going to bounce up very well as lots of energy has been dissipated through the shock-absorbing material deforming (see figure 6).
That loss of such energy in running terms means that the acceleration energy driving the body forward into the aerial phase has not been stored. It has to be made up for by more muscle activity or the body must be set more stiffly as for a softer impact, but in reality, it is likely to involve some of both.
How you personally handle a change to permanently softer running conditions is an unknown quantity. We know every runner is capable of adjusting to changes in terrain hardness in the short term but when this is done to or within footwear, it is difficult to know if the runner will cope with the change (see figure 7).
What is clear from the research is that cushioning insoles do not prevent injury, and that healthy fit runners do not need extra shock-absorbing insoles.
Humans are quite capable of running barefoot on ground as hard as concrete if they have been raised to do so, because their control of impact and acceleration bounce energy is so good. Injuries to joints, or muscle weaknesses may make you better or worse at running on harder or softer ground. If harder ground is a problem, limited shock-absorption on an insole can help comfort and fatigue during running. Yet adding significant cushioning tends to detrimentally increase the weight of shoes, and that is certainly bad for running performance.
HealthyStep develops its products based on the latest research and this is why we do not sell insoles based only on cushioning or shock-absorbing properties.
University fellow and international lecture in Clinical biomechanics
R&D consultant at HealthyStep Ltd.