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The next time you’re at the starting line of a race, look around and consider that the majority of nearby runners will likely experience an injury in the following year. Despite innovations in shoe cushioning, training, and sports science, the rate of running injuries hasn’t budged. One of the reasons for this unchanging rate is likely that each runner is their own laboratory, with a specific set of injury do’s and don’ts that depend on gender, genetics and a whole host of other factors. Part of that runner-specific individuality is the speed and running surfaces you choose.
Some love trails and some pound the concrete in dense urban jungles. But what surface is best, and how fast should you run to stay healthy? The answer to those questions isn’t as obvious as one would think, largely due to the fact that many of the commonly held notions about the causes of running injury don’t actually make the scientific cut.
Take running surface, for instance. Though popular belief holds that running on trails or softer surfaces is easier on the joints, well-established scientific evidence says otherwise. It turns out that the brain has its own version of a car’s road sensing suspension—something termed “muscle tuning.” While running, the brain constantly anticipates the stiffness of the surface—using data from past experience and information from the previous stride—and “tunes” how strongly the leg muscles contract before the foot hits the ground.
So when the trail gets softer, the leg becomes stiffer, leaving the net impact to the leg roughly the same. It’s how the body maintains the overall stiffness of the surface/shoe/leg combination and it’s the reason why running on softer surfaces doesn’t necessarily result in a lower rate of injury. The overall impact to the leg remains virtually the same whether running on trails, a beach or concrete.
But there’s an asterisk. “We know how the body adjusts to different surfaces in the short term, but what we don’t know are the long term consequences of running on a particular surface,” says Dr. Brian Heiderscheit, Director of the University of Wisconsin’s Runners’ Clinic.
Of course, the cushioning of the shoe impacts the equation as well, and could be part of the reason why ultra-cushioned shoes haven’t solved the injury conundrum. Just like a softer surface, the legs will adjust to a softer cushioned shoe by increasing leg stiffness. In fact, one of the few studies to evaluate shoe cushioning and impact forces found evidence to support the soft shoe, stiff landing theory.
What about the treadmill? The dampened surface of a treadmill has long been believed to be beneficial to the joints. But impact represents only one of the stresses to the body with running; also important is the stress to soft tissue structures like tendons and muscles. An example of this is running uphill—though it imposes less impact to the joints, the muscles of the calf, hamstring and hip have to work harder, increasing the stress to the hamstring and Achilles tendons.
In fact, in a recent study comparing loads to the kneecap and Achilles tendon during treadmill and overground running, researchers found a 14 percent greater overall stress to the Achilles tendon as compared to overground running (load to the kneecap was roughly equal during both). While the results of the study shouldn’t spur wholesale abandonment of treadmills, it should serve as a note of caution for those that use them regularly, especially those with a history of Achilles injury.
To minimize the risk of injury, Heiderscheit believes that runners should vary running surface, much like they vary their training plans. “Just like a runner would try runs of different intensities—tempo and interval training for instance—my advice is to incorporate a little bit of all the different surfaces into training,” Heiderscheit says.
Just as the finer points of running style and foot landing have been scrutinized by experts, so too has the question of optimal running speed. With the link of speed work to overuse injury, many would assume that running faster equals a greater risk of injury.
But, again, every runner is different, and slower may not always be better. “The majority of forces generally scale up with increasing speed, but running faster isn’t necessarily uniformly more demanding to the entire body,” says Heiderscheit. The structures that face the greatest increase in demand are the muscles and tendons tasked to supplying that extra speed—hamstrings, calf and glutes—with other structures realizing a less pronounced demand.
Several recent studies illustrate that point. A 2015 article in the Journal of Orthopedic and Sports Physical Therapy sheds a little light on the role running speed plays in the amount of impact the knee experiences when running. Researchers from the Department of Public Health at Denmark’s Aarhus University asked a group of runners to run 1 kilometer at three different speeds: 5 mph, 7.3 mph and 9.8 mph.
Although the impact stress to the knee with every stride increased with faster running, the total stress to the knee was 30 percent less at the faster speed because of the lower number of strides needed to cover the same distance. On the basis of these findings, running longer distances at slower speeds, especially when fatigued, may contribute to overuse injuries of the knee.
Before you push the accelerator, consider again that injury risk can’t simply be boiled down to impact. Other research—conducted by the same Danish group and presented in Clinical Biomechanics—determined that the extra energy supplied by the muscles of the calf and foot with an increase in speed predisposes the Achilles and plantar fascia to injury.
The bottom line is: There isn’t one surface or speed that is right for everyone. For runners looking to avoid injury, cross-training shouldn’t just involve the elliptical or bike, but also running on different surfaces and at varied speeds.