I believe that it is better to warm up and not stretch before tennis. I think vigorous stretching will decrease resistance and power. I do believe in stretching when not playing and after play. Too much flexibility is bad for sports performance. An article in US News and World Report confirms my opinion: Science & Society No bending or twisting By Emily Sohn Can't touch your toes? Don't sweat it. For decades, flexibility has been considered a key element of fitness. From the professional football field to the local health club, trainers have advocated stretching as part of a regular workout. But new research is showing that stretching does not prevent injuries or make you any less sore the next day. On top of that, it doesn't appear to improve performance. Stretching might, in fact, cause more problems than it solves, say a growing number of researchers. This is especially true for women, who tend to suffer from knee, ankle, and other soft-tissue injuries far more often than men do. Extra flexibility might be to blame. "It is so hard to believe that stretching could somehow be the enemy," says exercise physiologist Stacy Ingraham of the University of Minnesota-Twin Cities. "But there has never been any science that actually put stretching into the athletic world." Indeed, a recent review underscores how little support there is for the value of pulling, reaching, and twisting. Researchers from the Centers for Disease Control and Prevention found only six studies designed to isolate the effects of stretching, and none showed a link between stretching more and getting hurt less, says lead researcher Stephen Thacker. In fact, says Ingraham, more-pliable muscles might cause more injuries than they prevent, especially if you stretch before you exercise. Baseball players are notorious for stretching before games, Ingraham says, yet they tear muscles and ligaments all the time. "One study showed that marathon runners who stretched had a higher rate of injuries than those that didn't," says fitness expert Jay Blahnik, author of Full-Body Flexibility . But why? One explanation is that stretching muscle fibers makes them less stable and less able to resist the jarring impact of running or jumping. Stretching may hinder performance because a stretched muscle can't produce the same kind of force as an unstretched one. And as muscle fibers lengthen, it takes longer for the brain's messages to tell the muscles to move. Rather than stretching, Blahnik and other experts encourage people to warm up by jogging slowly, lightly swinging a golf club, or doing whatever else they need to do to achieve the range of motion required for their activity. Strengthening and conditioning exercises are also helpful, Ingraham says, because fatigue and muscle weakness cause most injuries. The only reason stretching feels so good, she says, is that it results in tiny tears in the muscle fibers. The body then releases hormones that mask the pain. Flexibility might even help explain why women are up to 10 times as likely as men to tear the anterior cruciate ligament in their knees, and four times as likely to have serious ankle sprains, among other injuries, Ingraham says. She studies a hormone called relaxin, which increases flexibility in women. Unlike men, women have receptors for the hormone in their ACL s and in their pubic ligaments. Levels of the hormone change throughout a woman's monthly cycle. In a study of 28 women over three months, Ingraham found, participants were more likely to get injured when levels of relaxin were highest. Stretching might have the same effect, she says. Not all experts agree that stretching is all bad all the time. Light stretching is OK as part of a cool-down after a workout, Blahnik says, because it won't affect performance and won't cause injury. The key is to do stretches that stay within your normal range of motion. Most important of all, Thacker says, is that people keep moving. "We strongly believe that we want to have people exercising. We just don't want people going out and getting hurt."