As most runners will know, the shoe manufacturing industry has been pumping out increasingly cushioned running shoes in recent times. It is getting more and more popular to wear shoes that have rigid arch support, are motion-controlled, and most importantly, provide more cushion for your foot to land on to reduce the impact.

This has been in response to a rise in running injuries. The thought is that if more cushion is provided, the load you have to bear and impact you take on will be less of an issue because each foot strike will be accompanied by some brace for your landing.

While this makes logical sense, it has yet to be the case as running injuries have not decreased with the introduction of more cushioning, according to a study from nature.com.

The study wanted to see exactly why the cushioning has not been helping so they took 12 healthy males with an average age of 27 and had them each wear a pair of conventional shoes and a pair of maximalist shoes (highly cushioned) while running at two different speeds, 10km/hr and 14.5km/hr.

What the researchers found right away is that the maximalist shoes increased impact loading compared to the conventional shoes. Not only that, but as the speed increased from 10 to 14.5km/hr, the difference in impact loading between the shoes became even more apparent.

“We found significant main effects for impact LR [loading rate] (p = 0.049) and IP [impact peak] (p = 0.001) rate; both showed greater values of loading when running with MAX shoes versus CON shoes.”

When measuring the impact peak of each shoe, the maximalist shoe measured at 6.4% higher than the conventional shoe at the slower pace. At the faster pace, this is where things became more blatantly in favor of the conventional shoe. The faster pace proved to be 10.7% higher in the impact peak and 12.3% higher in the loading rate for the maximalist shoe.

These are both metrics that you would want to show up as low as possible, especially when thinking about yourself wearing the shoes while you’re running.

In all metrics measured, the maximalist shoes had greater numbers than the conventional shoes in this study, the figures I listed above were just the most significant ones.

It’s clear to see that the highly cushioned shoes do the exact opposite of what they intended. They create greater overall ground reaction force and increase impact loading compared to the conventional shoes, which is to the detriment of the runner wearing them. 

Why Is This the Case?

While using a spring-mass model to check the stiffness of each runner’s legs in both shoes, the researchers found a noticeable increase in leg stiffness when the runners wore the maximalist shoe.

Just like the first experiment in the study, the results were accentuated as the speed increased from 10 to 14.5km/hr. The leg stiffness increased even more while wearing the maximalist shoe at a faster speed.

“The results revealed a significant main effect (p = 0.007) for shoe conditions, showing greater leg stiffness for the subjects running with MAX shoes…The analysis of slow and fast running speeds revealed that the increase in the leg stiffness in the MAX shoe was more pronounced at faster (p = 0.009) than slower (p = 0.189) running speeds.”

They then found that the runners compressed their legs significantly less when wearing the maximalist shoe compared to the conventional shoe at both speeds. They also found that runners in the maximalist shoe showed a speed-dependent response in the peak vertical ground reaction force.

This led them to conclude that the increased leg stiffness in maximalist shoes at faster speeds was due to differences in ground reaction force rather than leg compression.

“This indicates that the more pronounced increase in leg stiffness observed with MAX shoes at faster running speeds was due to differences in the ground reaction force rather than changes in leg compression.”

So the maximalist shoes made the runners’ legs stiffer, but not because they were compressing their legs. It may have been because of the increased ground reaction force with the increase in speed.

My Take On Why This Is

With what these researchers found, I can give a pretty good estimate of why highly cushioned running shoes increased leg stiffness in runners and created more impact (ground reaction force). 

When a shoe is highly cushioned, most of that cushioning is put in the back of the shoe, under the heel. This is because they expect you to be striking the ground heel-first. So yes, the entire sole of the shoe will be bulkier, but especially the heel, giving the runner the effect of having their heels unnaturally elevated inside the shoe.

The body naturally doesn’t want to heel-strike, as it’s a very inefficient and high-impact way of moving. It would rather strike the ground mid-foot or forefoot first, then bring the heel down behind it after most of the initial impact has been absorbed. The impact would be absorbed by the front of the foot and be assisted by the leg muscles because your ankles, knees, and hips would be slightly bent.

With the heel-strike however, it guarantees you take more impact because when your heel is striking the ground, your legs will always be straighter (stiffer) in that position. This makes you absorb most of the impact through your heels, ankles, knees, and other joints.

The size of the sole, particularly the heel, forces you into a heel-strike with all this extra cushioning and this is very detrimental for long-term running usage. The legs will be stiffer just by holding the position that this shoe encourages, think standing on your toes with your heels raised as the default position in the shoe.

Then, the way you have to strike the ground (heel-first) is only going to expose your already vulnerable legs through the volume of load and impact you have to manage in this poor position. 

This study proves that running is not about cushioning, it’s about technique. No amount of cushioning can make up for poor technique. Our bodies already have a system of impact absorption while in motion, we just need to utilize it correctly.