It's been a long, hard day at the office, and you need a good workout to blow off all that stress. But before you hit the free weights, the stationary bike or the elliptical machine, you spend 10 minutes carefully stretching all those stiff muscles, just as every coach, trainer and physical therapist has advised for as long as you can remember.

The question is why.

There's no evidence that you'll prevent injury. In fact, some people believe you're more likely to cause one. You won't stave off muscle soreness. You won't perform better, except possibly if you're going to do gymnastics or ice-skate. There's some reason to believe you'll do worse than if you hadn't stretched.

``There is not sufficient evidence to endorse or discontinue routine stretching before or after exercise to prevent injury among competitive or recreational athletes,'' concluded the National Center for Injury Prevention Control, part of the Centers for Disease Control and Prevention, in a 2004 study that may be the most thorough look at the research on stretching.

Research and anecdotal information attribute many benefits to stretching: reduced muscle tension, improved circulation, pain reduction and management. Perhaps most important, stretching helps us maintain range of motion as we age.

The question is whether ``static stretching'' -- the most common type, which involves holding a muscle in one position for a defined period of time -- has been misinterpreted, or oversold, as a preventive for what ails you.

``People believe all kinds of amazing things, and it changes every 10 or 15 years,'' said William Meller, a physician and associate professor of evolutionary medicine at the University of California at Santa Barbara, who sees even less value in stretching than the CDC does.

The merits of stretching are ``not based on any science. It's based on word of mouth. It's spread by coaches, spread by trainers, all kinds of different people who have an interest in pretending to be experts.''

According to Julie Gilchrist, a medical epidemiologist who helped conduct the CDC study, ``it's probably important that we maintain some norm of flexibility throughout our life spans, but I don't think anyone has really defined what that is.''

In static stretching, ``you're taking the muscle to the point where it naturally wants to go, and then you're taking it a little bit farther,'' said Meller. That produces microscopic tears of muscle fibers and does nothing to prevent injury, he said. It also may weaken the muscle slightly, increase the possibility of injury and inhibit performance, according to him and the CDC study.

For those who want to stretch, it should be done after a warm-up or at the end of an exercise routine because warm muscles are more pliable.

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