You know running can boost your endurance, protect your heart, and build strong legs. You might also know that the sport we love can cause interesting and perplexing things to happen to the body, like cause you to use the bathroom way more often or make your toenails turn black.
But maybe you don’t know why all these side effects happen or what else might be in store for you as you clock miles. So we consulted doctors, physiologists, nutritionists, and other experts, and frankly asked them the most quirky and confusing questions about what running does to our bodies. We also asked for practical advice about how to deal with the issues.
Here’s what the experts had to say about what running does to your body.
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1. What’s the reason behind how fast or slow people run?
There are plenty of reasons why a 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., executive director at the Center for Applied Coaching and Sports Sciences 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, author of Running Rewired. “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.
Check out these four important tips for increasing your speed
2. Why does my GI tract act up when I’m running?
One annoying thing running does to the body: causes 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.
Learn about training your gut to avoid bathroom issues
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 and tapering gives you energy that maybe you don’t know what to do with. But sit tight.
“So many runners train hard right up to the day of the marathon because they’re desperately afraid of losing fitness if they don’t,” Patti Finke, former run coach who lead many runners through marathons, tells Runner’s World in another article. “What they don’t realize is that in those last few weeks, it’s the rest more than the work that makes you strong. And you don’t lose fitness in three weeks of tapering.”
Your mind is probably on overdrive, too, thinking about your highly-anticipated race day.
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); hang out with a nonrunning friend; do some gentle exercise if you must.
Follow this guide for how to taper correctly
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.
Learn more about how to prevent chafing here
The Best High-Impact Bras
Reebok Reebok PureMove+ Bra Motion Sense
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Knix Catalyst Sports Bra
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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.
Find out the full scoop on the heel whip
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., C.S.C.S., senior clinical professor in the Department of Sport Science and Physical Education at Huntingdon University.
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 Rx: Warming 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.
Learn more about the benefits of warming up
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, coffee 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.
Get more info on how caffeine can actually help your runs
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.”
Learn more about running-related nausea
4 Great Energy Gels for Long Runs
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GU Energy Gel, 24 Count
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Honey Stinger Organic Fruit Smoothie Energy Gel
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UCAN Edge Energy Gel
This gel can be easier on the stomach.
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-long run (without drinking). Each pound lost equals 16 ounces of fluid you should drink per hour.
Learn more about why you get headaches after a workout
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.
Read more about the reason for cracking joints
11. Why is it easier for me to run in the morning and so hard to rally at the end of the day—or vice versa?
Your natural bird persona—lark or owl—is partly determined by genetics. Housed in the hypothalamus, the portion of the brain that also controls sex drive and appetite, your biological clock is difficult to alter.
“Natural morning people seem to hit their lowest body temperature earlier in the night than evening people do,” says Chris Kline, Ph.D., assistant professor in the Department of Health and Human Development at the University of Pittsburgh, who specializes in sleep research. “Their body temperature is warmer when they wake up, so they’re much more ready to go.”
But even early birds aren’t primed to perform at sunrise. “Typically, aerobic capacity is slightly lower in the morning because of a lower core temperature and lower levels of hormones that affect performance,” says Matt Fitzgerald, author of 80/20 Running. In the late afternoon, the body is naturally the strongest and most flexible it’ll be all day, plus your aerobic capacity is at its highest.
“Emotional moods and motivation have been shown to peak in the late afternoon,” says Kline. “Nobody really knows why, but people are typically more willing to push themselves harder in the afternoon.”
Running Rx: If you want to hit the track at 6 a.m.—and not hit anybody there over the head with a coffee cup—expose yourself to light, the easiest way to wake up your body, as soon as the alarm goes off. Also, realize that as you age, you naturally become more of a lark. Want to extend your staying power? Exercise either outside, if the sun is still out, or in a bright room two to four hours before bed.
Steal tips on how to become a morning runner
12. Why am I so sore after a marathon, when I’ve done 22-mile training runs?
Did you do your training runs with crowds yelling at you and competitors around you unconsciously prompting you to run faster? Thought not.
Whether you’re a 2:30 or a 5:30 marathoner, your race-day pace tends to be at least a smidge—and possibly lots—faster than training days. That’s the difference, says Bright, between being pleasantly and painfully sore. “You accumulate lactic acid in your muscles by pushing the pace, which brings on premature fatigue,” says Bright. “Plus, the extra mileage—very few people do a 26-mile training run—causes more micro tears in your muscles, and it’s likely your muscles haven’t totally healed from your training. Race day, they get even more beat up.” The combination nets marathonitis, an acute condition that demands stairs be taken backward and the size of a stride be cut in half.
Running Rx: A huge fan of ice baths, Bright recommends the anti-inflammatory plunge postrace, for at least five to 10 minutes. Don’t bother taking NSAIDs like ibuprofen. “The newer studies show they really don’t do that much for inflammation,” says Bright. “And they can potentially put your kidneys at risk.” You can also try some of the newest at-home recovery methods, such as self-massage or compression boots.
Learn more about recovering after a marathon
Best Recovery Tools
NormaTec Normatec Pulse 2.0 Leg Recovery System
Roll Recovery R8
13. Why do my legs twitch in bed at night after I’ve run that day?
If your legs are still moving when you’re under the covers, chances are you skimped on a postrun meal. “When you work hard and sweat, you excrete a lot of sodium and calcium, two electrolytes that are responsible for muscle relaxation,” says Olson. “Being iron deficient, especially for women, can also contribute.
Running Rx: Get up and head to the kitchen for a glass of milk and some pretzels. To stave off future problems, make sure to include dairy, salt, and iron—found in lean red meat and spinach—in your meals after a run.
Learn more about how to prevent cramps
14. Why do my toenails turn black?
“For regular runners, a black toenail is not a matter of if, it’s when,” says Bright. Three causes of the black badge: a too-short shoe, a toenail that comes into contact with the roof of the shoe too often, and a runner who uses his toes to grip too hard. However it happens, the result is the same. Blood vessels under the nail break open, which spill blood (which looks black under the opaque nail) into the area between the toe bed and the toenail. “That area isn’t accommodating to blood collection—it’s rigid and restrictive,” says Bright. “It builds up a lot of pressure quickly.”
Running Rx: The first thing you want to do is get a new pair of better-fitting shoes. In mild cases, no treatment is needed, and the black nail will simply grow out. But if the pressure is bothering you, head to a doctor right away. “A doctor can poke a few holes into the nail to drain the blood, which relieves the pressure and will also help save the nail,” Runner’s World previously reported. It’s important to note that this should be done within the first few days of experiencing the black nail.
Read a full guide on how to prevent black toenails
15. Why is it so mentally tough to push myself?
There is, alas, no simple answer to the million-dollar question. Experts confidently proclaim two basic things: The brain controls the amount of pain to which you willingly subject yourself, and the human body inherently does not like pain.
“Our brain discourages us from running to the point of disrupting the physiological homeostasis that our bodies depend on to preserve life,” says Fitzgerald.
“The brain won’t actually allow a true, 100 percent effort.” Robert Weinberg, Ph.D., a professor in the Department of Kinesiology and Health at the Miami University in Ohio, adds that one’s goals may not be aligned with what one is truly willing to physically endure. “You may think you want a sub-three-hour marathon, but you may not be interested in doing the hard work it takes,” he says.
Running Rx: Fitzgerald recommends doing intervals, hill repetitions, and tempo runs at least once a week to build your mental muscle. If you’re not up for that kind of intensity, Weinberg suggests pushing yourself more moderately by running with people who are slightly speedier than you are—this can subconsciously make you mentally stronger and faster.
Find out more ways to build mental toughness
16. Why do I get side stitches?
That pain that rips through your midsection, usually on the right side? Chalk it up to the act of breathing. Or, more accurately, to your diaphragm, the muscle that controls your breathing motion. “It attaches to the liver on the right side,” says Wyrick. “When you run, the attaching ligaments stretch, which stresses the diaphragm and causes pain.”
Running Rx: Slow down or walk so you can take deep, full breaths. Grabbing your right side and squeezing it to support the liver may also end the pain. Another option: When your left foot hits the ground, exhale, which causes your diaphragm to rise; inhale on your right foot, and it falls down, which decreases the stretching. Finally, keep training—side stitches typically happen to beginners. “Over time, the ligaments become conditioned to the stress,” says Wyrick.
Learn more about how to treat and prevent side stitches
17. I use the bathroom right before I start, so why do I have to pee midrun?
The urge to detour into the bushes can happen for a couple of reasons, says Craig Comiter, M.D., associate professor of urology at Stanford Medical School. As your heart pumps blood more rapidly around your body, your kidneys may produce more urine, especially if you were well-hydrated prior to your run and you drink during it.
You may also be dehydrated, and the concentrated urine in your bladder may give you that got-to-go feeling. Or, because of a slightly weak sphincter combined with the jostling of running, a bit of urine may leak through the bladder and stimulate the urethra, making you wish you could cross your legs while running. (Pregnancy causes the need for more pitstops, too.)
Running Rx: Take a pee break, says Comiter. If it happens a lot, schedule a pitstop at a urologist’s office.
Find out everything you need to know about staying hydrated
18. Why do I feel like a genius after a run?
Perhaps the biggest benefit of a great 10K is that postrun, you’re sure you could score 1,600 on the SATs (2,400 if you’re under 30)—or at least improve.
“Running increases levels of positive neurotransmitters—endorphins such as norepinephrine, which is responsible for alertness, and serotonin, which helps regulate mood,” says Fitzgerald. “Plus, running puts the brain in an ‘alpha-wave’ state, which is associated with feelings of calmness and well-being.”
A handful of studies have documented that moving your feet correlates with improving your brain. Two conducted at the University of Illinois found that 30 minutes of exercise resulted in up to a 10 percent improvement in cognition, or being more effective in processing a problem or situation. Other, newer research of more than 2,500 participants says regular exercise can boost cognitive function
Running Rx: Go for a run!
Learn all about how exercise benefits your brain
19. Why does my nose run as fast as my feet?
A runny nose, a condition called exercise-induced rhinitis, is most likely because of the increased air flow—as your breathing rate increases, your nose kicks into hyperactivity. “Cool and dry air—or both—have been shown to increase secretions, similar to what we see in exercise-induced asthma,” says James Sublett, M.D., an allergist in Louisville, Kentucky.
If you’re self-conscious about your drippy nose, know you’re not alone: A study published in the Annals of Allergy, Asthma & Immunology, surveyed 164 exercisers and found that 40 percent had a runny nose while exercising inside, and 56 percent had one outside.
Running Rx: If your runny nose is a serious issue—it continues to run long after your workout—you might consider taking an antihistamine, such as Claritin or Zyrtec, or using an over-the-counter saline nasal spray prior to your run. Otherwise, stuff your pockets with tissues, and perfect your farmer’s blow.
Here’s what to know if you’re actually trying to run with a cold
