A cushy life: why runners need to check shoe cushioning regularly

When it comes to protecting runners, running shoes have to fulfill a number of roles, but one of the most important is to protect the joints and limbs from shock. The average 70kg or 11-stone runner strikes the ground with around three quarters of a tonne of force with every single stride or footfall. That amounts to something in the region of 1000 tonnes of cumulative impact force per mile! It’s hardly surprising then that the main task of any running shoe is to provide cushioning and protection from the otherwise damaging impact shocks that could wreak havoc with ankle, knee, hip and lower back joints.

In most running shoes, shock is controlled and dissipated in two ways, absorption and reflection. By using various shock absorbing materials such as gels, foams and different types of polymers in the sole beneath your feet, much of the kinetic energy of each foot strike can be absorbed gradually and dissipated harmlessly in the form of heat. This means far less is transmitted back up through the body. The problem however is that these shock absorbing materials deteriorate over time, which means that the cushioning ability of your running shoes decreases as the miles clock up. You might think that this wouldn’t be a problem because a reduction in cushioning is easily detectable. However, some research suggests otherwise.

How long does shoe cushioning last?

In one study, researchers sought to find out how rapidly shoe cushioning deteriorated with accumulating miles, and also to whether the loss of cushioning was easily discernable by the runners [Int J Sports Phys Ther. 2017 Aug;12(4):616-624]. More specifically, the study looked at:

• Whether runners are able to detect changes in heel cushioning properties from subjective ‘feel’.
• Whether the forces experienced by a runner’s foot increase after certain mileage (400 miles/640kms).
• If a device known as a durometer (see figure 1) can be used to objectively measure changes in the hardness of the material in the heel region of the midsole.

To do this, 15 recreational runners running at least 24km per week were provided with a brand new pair of running shoes. Each participant’s running style was observed and classified as having a rearfoot strike pattern (where cushioning is considered particularly important). All the runners were screened to ensure there was no history of unusual biomechanics and no acute injuries six months prior to the start of the study. The shoes were tested for midsole cushioning ability when brand new, and again after 160, 320, 480, and 640kms. At the same time points, the runners ran on a 42-meter indoor runway during which in-shoe plantar pressures and vertical forces were assessed. The results were then analysed to see what changes occurred with increasing mileage and this was compared with how the runners felt that the shoes felt subjectively using a standardised measure known as the Footwear Comfort Assessment Tool (FCAT).

The findings

The key finding from this study was that after running 480km, the runners experienced a 16% to 33% reduction in the amount of cushioning in the heel region of the midsole. This was reflected by durometer measurements, which showed a corresponding decrease in cushioning ability (ie increase in hardness). What was important however is that the runners were NOT able to detect these changes – ie the shoes felt ‘the same’ even though they had lost significant amounts of cushioning capacity.

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Figure 1: Example of a durometer

The lever at the bottom is depressed until it makes contact with the pin. The force required is recorded; higher forces indicate harder materials (ie less cushioning).

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What does this mean for runners?

Although there were very significant reductions in heel cushioning after running 640 km, the subjects in this study were not able to self-perceive these changes, even though they were experienced runners. This suggests that simply going on ‘feel’ or looks is not a good method of determining when your running shoes might need replacing. This in turn means that if you rely only on subjective measures, you could be unwittingly increasing your injury risk.

PRACTICAL SUGGESTIONS

Given these findings, here are some suggestions to ensure that your running shoes remain up to the job:

• Aim to replace your shoes on a regular ‘miles clocked up’ basis. Don’t rely purely on subjective feel to determine when your shoes need replacing. This means keeping a tally of your mileage over time.
• If you want an objective measure of sole cushioning, consider investing in a durometer, which can be obtained for a reasonable price.
• If you are a heel striker, your cushioning demands will be higher than a forefoot striker; be prepared to replace your shoes more often.