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 post-run 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.
More: The Pre-Race Meal
14. Why do my toenails go black?
For regular runners, a black toenail is not a matter of if, it's when," says Dr. 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 Dr. Bright. "It builds up a lot of pressure quickly."
If the pressure is bothering you and you can handle more hurt, press the end of a paper clip or safety pin, heated with a match, through the nail. "That's a pretty painful proposition," says Dr. Bright, who recommends the gentler touch of a doctor. Do it sooner, while the blood is still fluid. If the pain decreases and doesn't bother you, no need to take action. Either way, the skin below it will heal, the nail will die and fall off. Don't worry, it'll grow back someday.
Foot notes: Are your feet signaling an injury
15. Why is it mentally so 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, author of Run: The Mind-Body Method of Running by Feel
"The brain won't actually allow a true, 100 percent effort." Robert Weinberg, Ph.D., a professor of sports psychology at 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.
"You have to train to suffer," says Fitzgerald, adding that many runners embrace one type of suffering—usually the high-volume grind—but not the lung- and leg-burning type that creates speed. He recommends intervals, hill repetitions, and tempo runs at least once a week to build your mental muscle. "Discomfort should be an explicit objective of the workout," he says. Realize you're not up for that pain? Weinberg suggests pushing yourself more moderately by running with people who are slightly speedier than you are. The peer pressure will unconsciously make you mentally stronger—and faster.
More: 4 Mental Tips for Long-Distance Runners
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 Dr. 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 Dr. Wyrick.
More: 3 Best Core Exercises for Runners