Kelly O’Mara—For years, runners have lived by the adage that pounding the pavement is a surefire route to injury. The common prescription was to head to the trails for a softer running surface.
Dr. Hirofumi Tanaka, an exercise physiologist at the University of Texas, was told to do just that while recovering from knee surgery. He ended up twisting his ankle on uneven soft ground and became the subject of a New York Times article last fall that prompted a debate over the value of soft surface running.
While there has been research showing an increase in the force on the legs while running on harder surfaces, there are no studies that connect that to increased rates of injury or a change in performance.
“It’s basically unknown,” said Tanaka.
“What it boils down to is it’s really hard to do the gold standard test,” said Daniel Ferris, PhD., a professor of movement science at the University of Michigan School of Kinesiology. That gold standard test would have to follow hundreds of runners doing the exact same workouts over a year, with one group on pavement, one on dirt, and one on the track. It’s not surprising that few runners want to be those test subjects. In fact, most in-the-field studies on running are conducted by the military on new recruits at boot camp.
With all the different surfaces available, it’s best, said Ferris, to mix it up. Hit the trails one day and run on the road the next.
“The variety is going to stress different parts of your body,” he said.
But, Stuart Warden, director of the Center for Translational Musculoskeletal Research at Indiana University, emphasized that for experienced runners injury comes from drastic change. While variety helps strengthen leg muscles, it’s important to ease into running on new surfaces.
“Increase the complexity as you progress,” Warden said.
As race day approaches, you should up the amount you’re running on the same surface as the race, both Warden and Ferris said. Trail races demand training on trails. But, if you only run on trails, you won’t be prepared for the pounding of a road marathon.
When deciding where to run, Ferris said runners should consider the properties of each surface. Not all dirt, after all, is created equal.
Stiffness v. Compliance
The main surface property is stiffness. Concrete is very stiff; a track is very compliant. If you dropped a bowling ball on a world-class track it would bounce back up, said Ferris.
Studies in labs and on gymnasts landing on mats have found that on a stiff surface your leg bends more to absorb the shock, but on a softer surface your leg stays stiffer.
“Your body sort of knows if it’s on a hard or soft surface,” said Warden.
That slight bend in your leg is like the difference between standing straight or standing in a squat. Stiff surfaces that cause your leg to bend more put more force on your legs. Though there are no studies that connect that to an increase in injuries, common sense, said Ferris, is that it can have an effect.
“It’s enough that if you do it 10,000 times for a run, it’s going to matter,” he said.
The amount a surface dissipates energy are the damping effects. Sand is very damping; a bowling ball dropped into the sand would lose all its energy and not bounce. Surfaces with higher damping properties – like sand, thick grass, or dirt – are harder to run on, because of the loss of energy. Those surfaces can tax the muscles in the calf and knee.
However, there are times in training when it makes sense to do some sand running, for example, because it strengthens those different parts of the leg, said Ferris.
“It’s more like lifting weights than regular running,” he said. While it’s not known if regular periodic running on sand helps with injury, it’s clear it helps with performance, he said.
Any runner who has tried to do strides on wet, slick grass knows it can be particularly challenging. Trying to avoid falling can force a runner to change their stride and work harder.
“It becomes an injury problem if it’s too slick,” said Ferris.
The unevenness of trails is what got the best of Tanaka when he twisted and sprained his ankle. But, beyond tripping, the effects of irregular surfaces are only starting to be understood. Ferris recently built a treadmill with minor unevenness and bumps in order to study the effect that uneven terrain had on runners.
It causes changes, he said, in the muscle activation and distributes some of the work to the knee. You, as a runner, put a premium on foot placement when the surface is uneven, which forces your hips and knees to work harder.
What this means for runners is unknown right now, but Ferris hypothesizes that it could be beneficial to people with shin splits to run on uneven surfaces once a week to activate the shin muscles differently.
With all these different properties, what you should run on depends on your experience, training, and injuries. Is there one best surface?
“That’s the million dollar question,” said Tanaka.