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.
10 Responses
Good Read – Question – Is a runner either upper leg stiffness or lower leg stiffness or both at different points in the gait cycle?
I switched from road to trails years ago and won’t go back. My legs just feel better running on trails. The other benefit I’ve found is that I’m concentrated less on distance and more on time and, paradoxically, I’m running longer and enjoying it more because I let the vagaries of the trail dictate my pace rather than try to force a pace that may not be appropriate for my condition, the conditions or other variables. Each run, even on the same trail, is different because of the constant changes in terrain and this tends to keep me more mentally engaged in the activity and away from the ‘pain’ of the activity whereas the boredom of a long run over paved roads sends my mind to the “when will this be over” place that dampens my enthusiasm and enjoyment.
Great article! The higher the impact force, with good coordination, the less time is spent on the ground. Soft tissue injury comes from not having this coordination, which then puts you back to square one running with bad form, which is associated with longer times spent on the ground. Excessive time on the ground increases the risk of injury for the biomechanics associated with different speeds of motion. With high impact forces the first thing to go is soft tissue, luckliy. Flip the coin and run on softer surface such as football pitches or motion controlled cushioned trainers and the muscles have to produce higher isometric contractions to produce the right amount of elastic recoil to keep you in motion. It is this isometric contraction that becomes chronic – it is the only skill the tissue has and eventually it wins over the lever systems and causes damage such as the ones mentioned with stress fractures. This will be heavily sided with excessive joint torques from bad running form. Like a football player, they not only run they sprint to. This allows their tissues more resources in types of action. A rule of thumb is to have a quiver of running shoes all for different terrains, fast shoes for fast terrain, flats for perfect flat conditions, gripper thicker soles for ruggered terrain and as you mentioned above mix it up, you will get huge benefits from it as your skill will increase for all eventualities, like the football player. (Don’t get me wrong here a lot of football players have the same problem – they need to mix it up on different terrains to!)
Nice article. But it’s “role”, not “roll”. This definitely sheds some insight into why my body is so much happier with more minimalist shoes — no support and not a lot of cushioning.
Good catch!
Nice article. I read it twice, but I don’t get one thing…
If the surface/shoe/leg system is a constant (our brain is trying to keep it constant), then how can we really change the strains on our bones, let’s say.
An example: Surface is a constant, we always run on soft ground. We suffer from tibial stress fractures, so we probably have stiffer legs than average. We want to losen them up, so we change our shoes to stiffer ones. This should make the muscles more lose, right? But the thing is, that by logic, the muscles will losen up only to absorb the stress that earlier on was absorbed by the shoe, and the strain on bones (tibia) will stay the same.
The same will happen if we change the surface to stiffer. Again, muscles will absorb the energy which was earlier absorbed by the terrain. Is there any reason to think, that it will absorb more?
Where am I getting it wrong?
Thanks for the answer! 🙂
Hi Tomek, thanks for reaching out. This may help to answer your question as it talks about running shoes and how they affect the way your body is affected based on different shoes. https://runnersconnect.net/running-training-articles/choosing-the-right-running-shoes/ Hopefully this answers your question!
Thanks Tina for the link. I’ve read it, but it doesn’t answer my question. My question was not about shoues but about how the leg stiffeness works with balancing the force of the impact between the soft tissue and the hard tissue in our legs.
It makes sense that the muscles are stiffer when running on softer ground, but I don’t see any evidence for saying, that changing the surface (and changing the muscle stiffeness at the same time) will reduce the force absorbed by bone structures.