One issue we haven’t addressed yet, though, is a very simple intrinsic property of a shoe: how thick (or cushioned) should your running shoe be?
Running shoes and your feet
A shoe’s thickness affects how it interacts with the ground, but it’s more complicated than you might think.
A thicker or more cushioned shoe doesn’t necessarily mean you’ll hit the ground more softly.
Shoe companies and biomechanics researchers used to evaluate the cushioning of a shoe by measuring the impact force on a weight dropped from a standard height onto the heel of the shoe. But before long, scientific research demonstrated that this was an inadequate model of how shoe cushioning works while you’re actually running.
To maintain an efficient stride over a variety of terrain underfoot (whether that terrain includes muddy grass, hard pavement, cushy shoes, or a track spikes), your body will adjust the overall stiffness of your legs so that the net stiffness of the entire system supporting your weight—your legs, any insert or orthotic you wear, your shoes, and the ground—remains constant.
So, with a thick, cushioned shoe underfoot, your leg stiffness will go up to compensate, and when you are wearing thin shoes (or none at all), your leg stiffness will go down. The same applies to running on hard or soft surfaces.
Your body’s ability to actively modulate the stiffness of your legs means that impact forces are the same over a wide variety of shoe and ground cushioning conditions, because the joints and muscles of your leg can ameliorate the impact when there’s less cushioning in your shoes or on the ground.
Though the impact force is indeed distributed more or less evenly by your muscles and joints, there’s not much your body can do to regulate the localized peak forces on the bottom of your foot. These are termed “peak plantar pressures” and can be measured using force-sensing insoles placed inside the shoe.
So, when you run with a shoe that’s excessively thin, or on a surface that’s excessively hard, the peak plantar pressures on your foot will go up.
This was well-demonstrated in a 2010 paper by Vitor Tessutti and other researchers at the University of São Paulo in Brazil. In this study, Tessutti et al. measured the peak plantar pressures on the soles of 44 runners moving at eight-minute mile pace along a stretch of asphalt or natural grass.
As we predicted, even though other studies have found impact forces to be very similar on soft or hard running surfaces, the peak plantar pressures were much greater when the runners were on pavement.
Researchers have also found that shoes with a hard sole result in greater peak plantar pressures, so it follows that shoe thickness will also increase peak plantar pressures.
Until recently, there was only speculation on how this might affect injury risk. But a study published earlier this year by S.T. Ridge and colleagues looking at runners transitioning to thin, “minimalist” shoes demonstrated that there are definite risks involved with a shoe that’s much thinner than you are used to.
Some 36 runners were divided into two groups, one which trained for 10 weeks in their normal running shoes, while the runners in the other group gradually transitioned into running in a pair of extremely thin minimalist shoes over the same 10-week period.
At the end of the study, MRIs were taken of the feet of both groups of runners. Perhaps not surprisingly, the metatarsal bones of the minimal-shoe group showed significantly more signs of stress and injury—just as we’d suspect given what we know about peak plantar pressures and footwear.
Now, there are two important points to stress.
First, shoe thickness is not the only determiner of peak plantar pressures. This also depends on the density of the material used in the sole of the shoe, the ground you are running on, and the shape of the insole. But as a trend, shoes do tend to get firmer as they become thinner, so thickness is probably well-correlated with the firmness of a shoe, and likewise with peak plantar pressures on the sole of your foot.
Second, there are a few ways your body can shift how peak plantar pressures are distributed on your foot. Adopting a quicker stride frequency is one , since more steps per minute means less force overall, and thus lower peak forces on the sole of your foot. Another way to reduce peak plantar pressures is to land with a flatter foot, which spreads out the force a bit more evenly.
What does this research on shoe cushioning mean?
It is no surprise that both of these changes occur instantly when people run barefoot. But, as Tessutti et al. and Ridge et al. demonstrated, these adaptations are not enough to completely negate the higher loads on the bottom of your foot from thin shoes or a hard running surface.
So if you are thinking of switching to a new pair of shoes (especially if you’d like to be in a less-structured shoe), keep in mind that an ultra-thin shoe will very likely increase the stress on your metatarsals and the other structures in your feet.
This effect will be lessened if you run more often on soft surfaces like natural grass, and you won’t probably see your overall impact forces go up, but the increased plantar pressures associated with shoes with a very thin orvery firm sole is a big factor to consider.
In short, when transitioning to a minimal shoe, take your time and allow your body to adapt slowly. With the increased stress on your metatarsals and other foot muscles, your body needs time to adapt.
1. Ferris, D. P.; Louie, M.; Farley, C. T., Running in the real world: Adjusting leg stiffness for different surfaces. Proceedings of the Royal Society B Biological Sciences 1998,265 (1400), 989-994.
2. Tessutti, V.; Trombini-Souza, F.; Ribeiro, A. P.; Nunes, A. L.; Sacco, I. d. C. N., In-shoe plantar pressure distribution during running on natural grass and asphalt in recreational runners. Journal of Science and Medicine in Sport 2010,13 (1), 151-155.
3. Nigg, B., The Role of Impact Forces and Foot Pronation: A New Paradigm. Clinical Journal of Sports Medicine 2001, (11), 2-9.
4. Praet, S. F. E.; Louwerens, J.-W. K., The influence of shoe design on plantar pressures in neuropathic feet. Diabetes Care 2003,26, 441-445.
5. Ridge, S.; Johnson, A.; Mitchell, U.; Hunter, I.; Robinson, E.; Rich, B., Foot Bone Marrow Edema after a 10-wk Transition to Minimalist Running Shoes. Medicine & Science in Sports & Exercise 2013,45 (7), 1363-1368.
6. De Wit, B.; De Clercq, D.; Aerts, P., Biomechanical analysis of the stance phase during barefoot and shod running. Journal of Biomechanics 2000,33, 269-278.