The Scientist: Stephen Rice, M.D., is a sports medicine specialist at the Jersey Shore Sports Medicine Center in Neptune, N.J.The Answer: You may have heard a lot of debate recently about whether stretching is good for your muscles or hurts performance, prevents injury or causes it, is a necessity or a no-no. Well, that’s because the sports medicine community is intensely divided. Some say, “Do it! Every time!” while others question its benefits entirely. Here’s what you should know to make the best decision for yourself and your goals.The longheld idea is that stretching should loosen your muscles so they’re pliable and less likely to strain or tear. As your muscles contract and relax, molecules in the muscle fibers slide along one another. If muscles are too short, they won’t glide as smoothly. Stretching helps fibers reach their maximum length and improves their ability to snap into action when the muscle is contracted.There are two main types of stretching. There’s classic static stretching, in which you hold a pose. Think middle school gym class—bending down to touch your toes for your hamstrings, holding one foot (and your friend’s shoulder for balance) to target the quads. Then, there’s dynamic stretching, or stretching with body movement, such as swinging your leg perpendicular to your torso, which also stretches your hamstrings, and lunges. There’s some evidence that dynamic stretches are more functional and useful, but they can be difficult to learn on your own, so ask a trainer to show you proper technique. If you stick with static, hold for between 30 and 45 seconds to get the benefit without overstretching. Don’t bounce up and down in the stretch as this triggers muscles’ reflexes to contract, which is the opposite of what you want to do.Some studies have shown that static stretching before a workout decreases strength and stamina. Theories suggest that if you expend energy on it before your workout, you’ll have less left over, and that stretching muscles makes them too lax and loose to perform optimally. This could be a problem for competitive athletes and serious weight lifters. But if you’re going for a jog or playing a friendly game of tennis, you probably don’t need to worry about that.Since muscles are most flexible when they’re warm, the best option is to do a bit of a warm-up before you stretch. Do some jumping jacks, or run in place or around the room. You want to get your heart pumping and your muscles warmed up, but not to exercise anywhere near the point at which you might get hurt. That would defeat the purpose, wouldn’t it?MORE EXERCISE SCIENCE FROM YOUBEAUTY.COMThe Best Way to Beat Sore MusclesDo Minimalist Running Shoes Work?Why ACL Injuries Are More Common in Women