For lots of people, running is synonymous with knee pain. The repetitive motion of literally pounding the pavement can do a number on your joints, especially if you have any weaknesses in the muscles of your lower body. When your muscles aren’t able to take on the brunt of the pressure, your joints are subjected to more wear and tear.
There are so many factors that can add to discomfort and pain in your knees; everything from hip tightness to weak glutes can make a difference and oftentimes need to be addressed by adding strength and flexibility workouts to your routine. It can take time to see improvement. But one of the factors that you can immediately control is where you run.
We decided to dig into the research and chat with a few experts to find out how much the surface you run on really matters. Here’s what we found and the general guidelines you should follow to give your joints some relief.
Studies on how running surfaces stress the feet can tell you a lot about how they may impact the knees.
First, a disclaimer: There’s no definitive evidence to say which terrain is the winner, Armin Tehrany, M.D., orthopedic surgeon and founder of Manhattan Orthopedic Care, tells SELF. That’s because there aren’t that many studies done on the topic. Plus, the studies that have been done are pretty small, usually involving about 10 to 20 people. With this type of research, though, that’s to be expected. This type of experiment is typically very time-consuming, expensive, and involved, so studies end up having a small sample size.
A 2010 study done on 44 adult runners and published in the Journal of Science and Medicine in Sport compared running on asphalt and natural grass. The runners wore special insoles that measured the pressure as the foot struck the ground and could record the pressure at different points on the foot. The results showed that grass put less pressure on the rearfoot and forefoot. The peak pressure—that is, the maximum amount of pressure on the foot at any point in time—was about 12 percent greater on asphalt compared with grass.
A smaller study on 15 runners published in 2012 in the journal Research in Sports Medicine followed a similar protocol, with participants wearing pressure-sensing insoles while running on concrete, grass, or synthetic rubber. That study also found that grass surfaces put less pressure on the foot compared with concrete.
A treadmill is considered more of a midrange surface, falling somewhere between soft and hard, Timothy Miller, M.D., sports-medicine specialist and orthopedic surgeon at the Ohio State University Wexner Medical Center, tells SELF. A 2013 study published in the journal Gait & Posture analyzed 27 runners and found that, compared with running on the ground, running on a treadmill reduces the maximum pressure on the foot—especially the heel of the foot—by about 26 to 32 percent (depending on the speed).
The research can only tell us so much and doesn’t take into account differences among individual runners.
The research leaves many unanswered questions, however. Is running on grass better for everyone, or are there some people who should avoid it? What about runners who are less experienced, heavier, or older than the runners studied?
It’s also unclear just how the forces and pressure on the foot translate to soreness and injury. Plus, there’s no evidence to conclude whether certain surfaces lead to fewer joint injuries, Miller says. “The theory is that [softer surfaces] can slow down progression of things like arthritic changes or other joint pain,” he says. “But it hasn’t been proven, and there has been some debate.”
The general consensus is that soft surfaces are better, for a couple of main reasons.
The studies that are out there—and experts who treat knee pain—typically recommend patients run on softer surfaces if their knees hurt. Why? Everything from your foot up to your knee is connected. “The stress and impact of pressure from the foot and heel striking the ground affects the knee joint,” Tehrany says. “The softer the surface, the lesser the impact.”
In addition, soft surfaces give your body a little bit of extra time to adjust to the surface. Studies almost conclusively show that running on grass increases the amount of time your foot is in contact with the ground. “When your foot is on the ground for an extended period of time, it can distribute the force across a larger surface area, and also the [hips and knees] can flex more,” Miller explains.
Both of these things allow the impact to be shared and absorbed—by different parts of the foot, plus the joints, bones, and soft tissues—so no one thing has to take on all the impact. Over time, this might decrease the potential for injury. When you’re running on a hard surface, your foot is in contact with the ground for a split second, and therefore your hips and knees don’t have as much time to bend and absorb the impact.
But running on sand—though it’s soft—is another beast entirely.
When it comes to running surfaces, sand is kind of in its own category. It’s considered a soft surface compared with asphalt or concrete, but it’s also very unstable, which comes with its own problems.
A recent study done on 10 women and published in the European Journal of Sport Science compared markers (in the blood) of muscle damage and inflammation in women when they ran on soft sand and grass. The results showed less damage on the sand, and the researchers concluded that running on softer surfaces may be better for the body. A 2014 review of studies on sand training in the Journal of Sports Sciences also found that the low impact of sand may help reduce soreness.
While sand seems to be the least impactful surface, Miller says that running on soft sand can actually put more of a strain on soft tissues like the Achilles tendon and parts of the calf. “The ground is so unstable, so it’s hard for the foot and ankle to really control themselves,” he says. This instability can also increase the risk of rolling your ankle, Miller adds. That’s not going to make your running experience very enjoyable in the end.
Your body also has to work harder when running in sand because the ground absorbs a lot of the force. Think about how much harder it is to just walk in sand: Your ankles and legs don’t have something strong and stable to push off, so your body has to work much harder to lift off the ground.
Some terrains may just feel more comfortable for some people.
Experts make recommendations based on research but also on what is most comfortable for patients in practice. And typically, that means that very hard surfaces tend to cause more discomfort than surfaces that have a little bit of give.
Tehrany suggests running on grass or even a track that’s got a little bounce to it. “There are some tracks that are very soft, and those can be even better, potentially,” he says. Miller adds that dirt trails are a good option too, though you have to be careful to avoid things like roots and rocks.
The biggest benefit of running on a treadmill, Tehrany says, is that it’s an even surface; there’s no risk of accidentally hitting an uneven spot or tripping over a stick. “Most treadmills are designed for long-distance running,” he adds, “so if a person is comfortable running on one and doesn’t feel pain, it’s fine.” One potential downfall is that the moving belt forces you to keep up a certain pace, “so the ground is kind of being forced at you,” Miller says. When your body is being told what to do, you can’t adjust your pace and form according to what feels natural, so it can be a little uncomfortable to some people.
If switching up your terrain doesn’t reduce discomfort, or if your pain gets worse or becomes sharp, you might actually be injured. Stop running and see a doctor to make sure there’s nothing more going on.
This article originally appeared on self.com