What Does Running Do to Your Body?


As runners, we understand what training does for our legs, lungs, and heart. We’re also intimately familiar with the other, less attractive ways running impacts our bodies. But we don’t necessarily know why we have to pee even though the shrubs got watered just two miles ago. Or why our knees crackle and pop as we go downstairs.

So we consulted doctors, physiologists, nutritionists, and other experts, and frankly asked them the most quirky and perplexing questions about the bodies we know and love. We also asked for practical advice about how to deal with our issues. Here’s what the experts had to say about what running does to your body.

1. What’s the reason behind how fast or slow people run? There are plenty of reasons why this person can leave you in the dust. Some people may be more experienced than others regarding speedwork and racing. Not everyone matches up in terms of VO2 max, mental toughness, and injury history, according to Kristen Dieffenbach, Ph.D., associate professor of athletic coaching education at West Virginia University. 

Many performance components, such as endurance, pace, turnover, and mental toughness can be improved with planned, systematic training—except for one very significant one: genetics. “Muscle-fiber type and VO2 max are genetic,” says physical therapist Jay Dicharry, director of the SPEED Clinic at the University of Virginia. “That’s how some people who don’t even train can blow by you on race day.”

Running Rx: You can’t change your genetic destiny, but you can greatly influence your performance by training smart, adding speedwork, tempo runs, running-specific drills, and strength training to your routine. Plus, remember there’s a reason it’s called a PR: Beat it—not yourself—up.

2. Why does my GI tract act up when I’m running?

Some people get headaches when they’re stressed. Runners get the trots. Studies generally suggest that 30 to 50 percent of endurance athletes experience GI-related complaints during exercise.

“The GI tract is very sensitive to stress, and running—or the anticipation before a race—is definitely stressful,” says Darrin Bright, M.D., family physician and sports medicine specialist in Columbus, Ohio. When you run, your intestines take a double hit: The motion jostles their contents and speeds things along. Plus, blood, essential for your tract to stay on track, is rerouted to vital organs and muscles in your lower half, disrupting the sensitive balance your body has for fluid absorption and possibly causing dehydration, which can lead to cramps that force you to beeline for the bathroom.

Running Rx: Bright recommends avoiding bathroom-inducing high-fiber and high-fat foods 24 hours before a race or long run, and fueling up on benign, already-tested, plain meals.

3. Why do I get so antsy during a prerace taper?

That two-week span where you cut back training volume by about 50 percent gives you time to recover and to become mentally and physically stronger. You probably haven’t felt well-rested in weeks.

“Runners typically aren’t used to having all that energy,” says Larry McDaniel, former associate professor of physical education at Dakota State University in Madison, South Dakota. “The body gets accustomed to fatigue as a ‘normal’ state.” Your mind is probably on overdrive, too, thinking about your highly-anticipated race day. “A fresh body, coupled with nerves and excitement, can drive you—and those around you—crazy.”

Running Rx: Take 10 minutes to visualize the race, and then try not to think about it for the rest of the day. See a movie (avoid Chariots of Fire); read a book (stay away from Born to Run); grab a beer with a nonrunning friend; do some gentle exercise if you must.

4. Why do your nipples sometimes chafe?

Sweat is a mix of water, salt, and a handful of other minerals. When the water evaporates, you’re left with abrasive salt on your nipples, which are front-and-center in a high-sweat zone.

“After a few hours, a shirt rubbing against that salt feels like sandpaper,” says Bright, adding that beginner male runners are most susceptible because men typically sweat more than women. The abrasion causes chafing, which causes bleeding, which causes red stripes down the front of a white shirt, especially near the end of marathons. Women aren’t immune. Even nursing moms can be affected. “The skin around your nipples isn’t capable of thickening and getting stronger,” says Bright. The few women he has seen with bloody nipples were wearing no bra, a poorly fitting bra, or a cotton one.

Running Rx: Stay hydrated. “When you stop sweating, all you have left on your skin is salt,” says Bright. “The liquid takes the edge off the salt.” You can also buy circular Band-Aids or NipGuards and/or wear a moisture-wicking, properly fitted sports bra.

5. Why does the inside of one ankle get bloody from being hit by the opposite heel, but not the other?

That red tattoo is called a heel whip, and it’s from excessive rotational motion of your foot. Instead of your foot traveling in a forward plane, it makes an arc, causing your heel to nick your anklebone. It doesn’t have to be gory: Heel whips can also just dirty your inside shin.

“The extra torsion can be caused by anything from the alignment in your ankle to a hip issue,” says Dicharry, who adds that one side usually bears the bloody brunt because of muscular imbalances.

Running Rx: Think about pushing off through the big toe, not the pinky toe, so that your foot swings cleanly forward, and you’ll whip your ankle less. If you need more than just a Band-Aid after a run (e.g., ice packs and Advil for various parts of your lower body), a visit to a physical therapist will help you determine whether you have muscle imbalances that can be corrected with single-leg exercises.

6. Why do my legs shake after a hard run?

If your rubbery, burned-out legs had a fuel gauge, it would be firmly on “E.” For beginners, the needle may arrive there as a result of sheer effort.

“If your muscles aren’t familiar with a new movement, they become inefficient at contracting and can’t work in a coordinated manner, which results in shaking,” says Michele Olson, Ph.D., senior clinical professor in the Department of Sport Science and Physical Education at Huntingdon University.

Starting your workout too fast too soon could also be the culprit, according to McDaniel.

“When you go out too hard, the oxidative system doesn’t kick in as smoothly as it does when you warm up and work up to a pace,” McDaniel says. “It’s like shifting gears too quickly in a car. You deplete your energy levels prematurely.”

The other cause is simply that your muscles are depleted of electrolytes and glycogen—easily accessible fuel on which they run—and the shaking is their way of telling you to fill ‘em up.

Running RxWarming up prerun is key for beginners and vets. Start slow, and ease into your ultimate goal pace. If you’re running hard for more than 45 minutes, drink 8 ounces of sports drink about 20 minutes before you run; the carbs will keep your muscles humming. Postrun, if you’re trying to shake the shakes, walk around, stretch gently, and grab quick fuel, like a sports drink.

7. Why does coffee speed up more than just my legs?

A prerun prereq for many runners to clear the system on their own terms, java stimulates the muscles in the GI tract faster than Mother Nature; some reports say coffee jolts your system in as little as four minutes. Once you’re out on the road, proceed with caution: Many energy gels have caffeine in them, which may cause your intestines to move as quickly as your legs.

Running Rx: In the weeks before an important run or race, determine how much coffee you need for an evac, then sip and lighten your load accordingly. Also, figure out if you can tolerate caffeinated gels. Plan B: Pick a route with a few public restrooms along the way, so you can properly do your business.

8. Why do I feel nauseated after a long run?

You put in 18 miles to feel ready for your upcoming marathon, not to feel nauseous afterward. Blame the decreased appetite on chemistry; a study published in the American Journal of Physiology found that a 60-minute session of treadmill running increased the amount of the gut hormone peptide YY, an appetite suppressant, and suppressed acylated ghrelin, an appetite stimulant. Full-on nausea? “There’s a good probability you haven’t fueled properly during the run,” says Ilana Katz, M.S., R.D., a sports nutritionist in Atlanta. A lack of fuel in your body sends it into a stressed mode, that fight-or-flight mentality where survival is key.Running Rx: Try to prevent the problem by taking in about 60 grams of carbs per hour, either through a sports drink, gel, or regular food during your run. “The body can process about 1 gram of carbs per minute,” says Katz. Postrun, try to knock back something easy, like a recovery drink, within 30 minutes. If you can’t eat right away, don’t worry too much. “Appetite loss is typically short-lived,” says Katz. “Within an hour or two, suddenly you’ll have a major one.”

9. Why do I get headaches during or after a run?

It’s not just because you know you’re returning to the mess you ran away from. Headaches stem from a range of causes, from simple (a too-tight hat) to complex (a proclivity for migraines). Two of the most common reasons are tight muscles and poor hydration. “The trapezius attaches high on your scalp, so if you hold a lot of tension in your upper body as you run, your head could ache,” says Bright. Headaches are also a symptom of both underdrinking and overdrinking.

Running Rx: Shake out your arms and hands and teeter-totter your neck as you run. At home, hold your left ear toward your left shoulder, right toward your right; repeat with the chin. Nail your beverage needs by weighing yourself before and after an hour run (without drinking). Each pound lost equals 16 ounces of fluid you should drink per hour.

10. Why do my bending knees sound like Rice Krispies when I walk down the stairs?

Snap, crackle, pop? Crepitus, the medical term, happens when cartilage, the connective tissue between bones, starts to age, says John Wyrick, M.D., orthopedic surgeon and professor at the University of Cincinnati in Ohio. We all start life with quiet, smooth cartilage between our kneecaps and thigh bones, but over time, it becomes gray and old and doesn’t regenerate; most people older than age 30 have some mild crepitus. Weak quads or a tight IT band can pull the kneecaps out of alignment and exacerbate the wear and tear. Your knees pipe up when they bend past 30 degrees because the kneecap tracks into a groove in your femur—that is, cartilage-weak bone grinds into cartilage-weak bone. “The intensity of the pressure and the different contact points in the groove make the noise,” says Dicharry.

Running Rx: Cracking knees may lead to problems down the line, like arthritis,” says Dicharry. Minimize that chance by strengthening the muscles that control the hips and knees, and keep your lower half in alignment with exercises such as clamshells for the hips and squats for the knees.

Resource: runnersworld