What Running a Marathon Does to the Body

by Sam Jones

With every stride that hits pavement, the bones in a runner’s legs break down. Every muscle fiber is damaged from the effort it takes to propel the body forward. Yet when the runner is no longer running, when they’re sleeping or eating or resting, the bones and muscles repair themselves to become stronger than they were before, so they can handle the next run better than the last.

Shane Davis, M17, an assistant professor at Tufts University School of Medicine and a sports medicine physician at Tufts Medical Center, says a long-distance runner’s body is always adapting to the demands of the sport. To run the distance and with the intensity that a marathon requires, the body adapts to do so efficiently, pumping blood and oxygen at a lower heart rate with less resistance. Lung capacity expands. The immune system fortifies. Insulin sensitivity and kidney health improve. The marathon runner’s body is a new and improved version of its former self in nearly every way. That is, if they can get through training without an injury derailing their hard-earned progress.

The Road to Race Day

The trick for any marathon runner is making it to the race without pain. It’s a delicate balancing act of building strength and endurance without pushing the body beyond its limits. Runners often get injured when their bones, muscles, and tendons break down more than they’re able to rebuild, whether from a lack of recovery or going too far or fast too soon.

“The biggest thing is prevention,” Davis says. “Running is a very repetitive activity, so I see a lot of repetitive overuse injuries. It's just the same thing over and over for thousands upon thousands of times.”

Tendinopathy, or painful conditions in and around tendons, is one of the most persistent and classic overuse injuries Davis sees among long-distance runners, typically in their knees, feet, and ankles. The connective tissue straining against the push and pull of the muscles and bones breaks down, too, and needs time to recover and rebuild strong enough to handle the training of a marathon runner.

Stress fractures are the other major injury Davis sees in long-distance runners. He explains that bones respond to stress, meaning that if you load them too much without giving them the chance to recover and rebuild, they’ll break down to the point of fracturing. But if you load them, then allow them to recover, you're actually strengthening the bones and preventing injury, just like with tendons.

While a runner’s form can cause some of these injuries to manifest, Davis says that ramping up training too quickly is usually the culprit of tendon issues and stress fractures. And because running is such a repetitive activity, Davis emphasizes the need for runners to incorporate other forms of exercise in their training plans.

“One thing that runners sometimes overlook is not doing other exercises, like strength training or mixed forms of conditioning. Running should never be the only thing a runner does,” Davis says. “The variability will strengthen other muscles and bones while decreasing the load on the same parts of your body. If you swim some days, it's going to be a lot less stress on the bones than running would be, for example.”

He also says that while runners tend not to get muscle injuries like tears and strains, they do tend to have very tight hamstrings. Maintaining flexibility by stretching regularly will loosen the strain of those overused leg muscles and the tendons that tether them to bone.

Ready for Race Day

While long-distance running has many long-term benefits for heart health and metabolic disease prevention, Davis says that race day presents its own set of acute risks of which participants should be aware. Pushing your physical limits in a marathon presents short-term risks to nearly every organ system, including the heart, lungs, and kidneys. This is particularly true in those with pre-existing conditions or inadequate training.

The more preparation a runner has, the more exertion their body can handle on race day, so training properly for a marathon is not just important for their overall time, it’s essential for safety.

Davis has provided medical assistance at several endurance competitions like the Ironman World Championships and the Boston Marathon. He will be awaiting athletes in the medical tent at the finish line of the Boston Marathon again this year, where he says the health concerns he sees are actually more weather-dependent. 

If a runner has made it the entire 26.2 miles, he says they typically end up in the medical tent because of heat exhaustion and dehydration on warmer race days or hypothermia on colder, rainier race days. Nausea, vomiting, cramping, fatigue, and mental status changes from electrolyte imbalances are also a common problem he sees at the finish line. If runners feel like they need help along the course, there are medical specialists at the ready.

“It’s impressive how many volunteers and medical staff are at the Boston Marathon, even people with no ties to sports from very different areas of health care,” Davis says. “This race is a once-in-a-lifetime opportunity for anyone who participates and I love seeing how it brings our community together.”