Imagine yourself trying out a new running trail, and you come to a crossroads. On the left, a paved sidewalk circling a park. On the right, a dirt path patted out around a pond. Which path would be easier on your body?
The answer, it turns out, is not as easy as you might think. “Many runners believe that they’re less likely to sustain a stress injury if they run on softer paths instead of harder ones,” says Dr. Bob Baravarian, podiatric surgeon and Director at the University Foot and Ankle Institute. But the truth is, this is mostly a rumor based on faulty “common sense,” and isn’t actually borne out by the facts.
Running on paths can cause acute injuries as runners may trip or twist their ankles running over the bumps and holes in the dirt.
“There isn’t enough scientific evidence to support one way or the other.” Dr. Baravarian explained that it’s difficult to design a study with large enough numbers of participants that would be able to give us a clear picture of the types and rates of injuries acquired on different surfaces.
You would have to recruit hundreds of runners, split them randomly into groups, and each group would only run on one type of surface. Then you would track their injuries over the next year or so and conduct a statistical comparative analysis.
This is what’s known as a randomized controlled trial. “Randomized controlled trials are scientists’ best tool to knowledge about the effects of something in comparison to something else,” says Dr. Baravarian. Unfortunately, they are also expensive and difficult to carry out on the scale needed to generalize to the broader population.
And let’s be honest: would you be willing to put your safety on the line and hand over control of your exercise routine to a team of scientists?
Although conventional wisdom might reason that softer surfaces, like grass and dirt paths, would put less strain on runner’s knee and ankle joints, it could also be reasoned that these paths cause more acute injuries: runners may trip or twist their ankles running over the irregular bumps and divots of dirt paths compared to smooth pavement.
Other factors that don’t seem to have a strong association with injuries include the runner’s age, gender, BMI, season, or time of day. Similarly, running on hills versus flat roads, or participation in other sports don’t appear to have an impact.
One type of study that offers a clue actually approaches the question from a different angle. Instead of running on softer or harder surfaces, participants run on plates that are capable of measuring the force with which their feet strike the ground.
What Scientists Had to Say
Researchers approximate the durometer (hardness) of the surface by varying the amount of padding in the participants’ sneakers. Softer cushioning mimicks softer surfaces.
The result? The human body automatically adapts to the running surface. The force with which the foot strikes the plate is virtually the same regardless of the amount of cushioning in the shoes. Your body does this unconsciously to keep the amount of force constant.
It’s important to understand that every running shoe is not meant for every surface, different surfaces have different requirements.
You can try the experiment yourself. Jump from the top of a small step ladder to the ground: you’ll bend your knees to absorb the shock. Now, jump from the ladder onto a mattress. You will hold your legs much more rigidly. The mattress absorbs much of the shock for you.
Running on a soft surface works the same way. The dirt or grass yields to your feet with each step, so your joints stiffen in order to conserve energy during push-off. On harder surfaces like asphalt, your knees, ankles, and hips bend slightly more on each impact to reduce stress.
Of course, these studies only model the biomechanics of running on hard or soft surfaces. We can only guess at what this might mean for rates of injury. Dr. Baravarian says that, given current understandings, runners should just hit whichever surface they prefer. Better to focus on choosing the right shoes and insoles for your feet and your activity to reduce the risk of injury.
That said, if you’re a racer, you should run on asphalt. Most races are held on roads, so you might as well practice on them. Whenever you make a change to your routine, work it in slowly. Don’t just quit hard surfaces for soft ones cold turkey. Gradually shift the balance so that your body has time to adjust.
As a teaching institution, University Foot and Ankle Institute’s Fellowship Program is among the most advanced in the nation.
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Great Post!