Running surface and injuries: The role of leg stiffness in running injuries
This week, we’ll be looking at one of the funny ways the body adjusts to a variety of running surfaces and conditions, motivated by a basic question: “Where should I go running?”
Hitting the trails, or at least getting off of paved roads, is one of the most common recommendations to runners who can’t seem to shake a streak of injuries.
However, large-scale scientific studies haven’t found any connection between how much of your running is on hard vs. soft surfaces and injury rates.1
One of the possible explanations for why that is comes from an understanding of leg stiffness, a way in which biomechanics researchers model how the leg interacts with different surfaces encountered while running.
What is leg stiffness
At its core, running is based around propelling the body forward in more or less a straight line. To this end, the body does its best to keep its center of gravity level during the running gait. During impact with the ground, the muscles and tendons of the leg act much like a spring, absorbing energy and releasing it later in the gait cycle. But that’s not the only factor in the equation.
Your shoes and the surface you run on also function as springs, absorbing and releasing energy. And each of these components has its own unique stiffness—much like a bungee cord is a stiffer spring than a rubber band, so too is a concrete road a stiffer “spring” than a soccer field. So, given that we’ll be running over a variety of surfaces in the same pair of shoes, it’s easy to see that our body is going to have to change the stiffness of our legs to keep our body level.
How does leg stiffness change based on running surface
Fascinatingly, the leg’s stiffness is “pre-tuned” before impact for each individual footstride. Combining feedback from the previous stride and information stored in the brain, the body anticipates the stiffness of the surface you’re running on and adjusts how strongly the leg muscles contract before impact. If you put your hand on your quads while running, you can actually feel them tighten up before your foot hits the ground.
And perhaps the neatest tidbit about this phenomenon is that the body changes the stiffness of the legs before the first impact on a new surface!2 So your body pre-tunes the legs differently in mid-air as you step from concrete onto grass.
But here’s the odd part: if you are running on a hard surface, your legs are less stiff than they are on a soft surface. It’s a consequence of the necessity of keeping the stiffness of the surface/shoe/leg system constant, and this has some surprising implications when we consider injury prevention.
What role does leg stiffness play in running injuries
The role of impact in running injuries is not clear—while work by Irene Davis at the University of Delaware has linked high impacts (or more properly, high impact loading rates) with plantar fasciitis3 and tibial stress fractures,4 two common running injuries, other research done by Benno Nigg at the University of Calgary has found that overall injury rates are slightly lower among runners with high impact loading rates!5
This apparent paradox has no clear resolution in sight, as both Davis and Nigg are highly respected biomechanics researchers with good data to back up their claims. It’s possible, though, that they could both be right. Some injuries may be linked to stress from high leg stiffness and others may be linked to stress from low leg stiffness.6 However, right now there’s only evidence for half of this puzzle.
Leg stiffness and stress fractures
Davis’ research has fairly convincingly linked leg stiffness, impact, and stress on the tibia. On average, runners who have suffered a tibial stress fracture have higher impact forces, stiffer legs, and greater forces traveling up their shins.4
If, then, we propose that runners with a history of tibial stress fractures take steps to lower their leg stiffness (which is a big leap—we cannot yet say for sure that stiffer legs cause tibial stress fractures), we arrive at a surprising conclusion: they should wear thinner shoes with a firmer midsole, and run on harder surfaces.
Again, since the overall stiffness of the entire surface/shoe/leg system must remain constant, if we make the shoes and surface stiffer, the leg is forced to be more compliant! The logic is that the muscles and tendons of the leg will absorb more of the impact, taking some strain off the bones; the trade off being that the risk of tendon/muscle injury may increase.
Leg stiffness and muscle injuries
Using that logic, we might propose that runners who have suffered from soft tissue injuries wear more cushioned shoes or train on softer surfaces, again to move stresses away from injury-prone areas.
As of yet, the only solid research linking higher leg stiffness to reduced injury risk is Benno Nigg’s work, which links higher impact loading rates with lower injury risk overall. If my theory is right, and some injuries are aggravated by higher leg stiffness/impact loading rates and others are aggravated by lower leg stiffness/impact loading, there’s a lot of work to be done to classify which injuries belong in which category.
What surface should you run on
Until then, we’ll have to return to the tired but reliable mainstay of moderation: try to do some running on softer surfaces and some running on harder surfaces. It’s unrealistic for most people to do all of their running on grass and dirt anyways, and if anything, I think most runners are “underexposed” to softer surfaces like trails, grass, and gravel roads.
But if you have a history of plantar fasciitis or tibial stress fracture and are willing to go out on a limb, you might think about switching to a firmer shoe and sticking to the roads. Likewise, if you’ve had a lot of muscle and tendon injuries, you might think about hitting the trails.
Finally, there’s one more boon to running on softer surfaces, and that is that they tend to be more irregular. If you run ten miles on an asphalt road, every step is about the same as the last. But if you run the same distance on a trail, or even on a golf course or gravel country road, there are small variations in each step. Intuition suggests that this might alleviate some injury problems by switching up where the forces are going inside your body. While I doubt that the benefits of this would be large enough to be deemed statistically significant in a scientific study (not to mention the practical difficulties of determining how “irregular” a particular running route is!) I still think it’s a small help. My college team, which had access to a large system of trails and gravel roads in the fall and spring, but had to run on paved roads during the winter, seemed to suffer a few more injuries when we were out on the roads in the winter than when we were hitting the trails in warmer weather.