20 minutes

To Stretch Or Not To Stretch?: Research now suggests that stretching before a workout isn’t necessarily a good thing, because it causes the brain to think you’re about to tear those muscles, says Reynolds. “When you stretch and hold a pose, the brain thinks you are about to damage yourself and it then sends out nerve impulses that actually tighten the muscles,” she explains. “… The result is, you’re less ready for activity, not more ready for activity.”

Don’t Skip The Warm-Up: Science suggests that a very easy warmup — a light jog, for example — may be all that most of us need. “What you want to do when you warm up is warm up the tissues,” she says. “You want to get the muscles, the tendons — all of the parts of your body — warm, and the best way to do that is to use those tissues.” Reynolds recommends jogging before a run or an intense sports match.

Running’s Rewards And Risks: Running reduces the risks of heart disease and diabetes, helps maintain your weight and improves brain health. “There’s very good science that running for even 30 minutes or so doubles the number of brain cells in certain portions of the brain related to memory,” says Reynolds. “Running is wonderful for the health of your body.” But the injury rate among runners, she cautions, is extremely high — with as many as 75 percent of runners getting one injury a year. “So running can be very hard on the body at the same time it’s very good for the body,” she says.

Humans Were Made For Walking: Walking may be the single best exercise that exists on the planet, Reynolds says. It’s low-impact and has a relatively low risk for injury. “Walking appears to be what the human body was built for,” she explains. Even 15 minutes will reduce your risk for heart disease and diabetes.

Gretchen Reynolds writes the Phys Ed column for theNew York Times.

Russell Thurston/Hudson Street Press

The Difference Between Fitness And Health: Becoming fit and becoming healthier are two different things. “You can become healthy with a much lower amount and a much lower intensity of exercise,” says Reynolds. “A nice easy walk will improve your health. If you make it a little … harder or a little more difficult for you to walk, you will become more fit and you will get more benefits. But even if you just walk lightly, you will be healthier than if you don’t do anything.”

Hydration Hype: We don’t need eight glasses of water a day to stay hydrated. “What we now know is that if you drink to thirst, if you listen to the little voice in your head that says, ‘You need water,’ you will drink as much as you need,” Reynolds says. “You don’t need to stay ahead of your thirst. Drink what you want, and you will almost certainly be fine.”

The Ultimate Post-Workout Beverage: Use chocolate milk to replenish sugars after an intense workout. Reynolds calls it an “ideal recovery beverage” because it has the right ratio of carbs and proteins to aid your body’s recovery process.