"No pain, no gain." That classic phrase, heard from gym teachers and coaches, is the instructor's way of saying that exercise takes exertion to produce benefits.

But those same teachers will also caution against exercising through sharp pain -- which could lead to serious injury.

The tough question is: How do you separate the inevitable aches of exercise from the pain that is a warning sign of real trouble, such as a pulled muscle or damaged tendon?

It's usually fairly easy to distinguish between harmless discomfort and destructive pain in weight training, says Ben Moss of the Vermont Avenue Nautilus Center in the District. On the one hand, Moss says, there is the "good" pain some people feel after lifting weights, which "comes from the lactic acid buildup you get from going to muscle failure" -- working so hard that you can't exercise the muscle anymore. This produces what lifters sometimes call a "pump," a burning sensation that is felt throughout an entire muscle.

The sign of a tendon, ligament or muscle pull, on the other hand, is a very sharp, isolated pain. And that's a bad pain. "You can almost always tell the difference," Moss says.

Is "no pain, no gain" true in the weight room? Yes and no, Moss says. "You can get gains without going to muscle failure, but it's not going to be as quick and noticeable as when you go to muscle failure."

Jnanam MacIsaac, an instructor at the Integral Yoga Institute in Arlington, never advises students in her classes to stretch until they feel pain. "We don't strain," she says. "We are into gentle stretching. You stretch to your capacity and if you feel pain, you stop and go no further than that."

MacIsaac says, however, that yoga practitioners must work hard to get the full benefits from their stretching routines. "You have to take it slowly and take it easy, but you can't be lazy. You have to stretch to your capacity."

Each individual alone can determine the point at which potentially constructive discomfort becomes potentially destructive pain, says Sissie Twiggs, an aerobics instructor at the Fitness Studio on Wisconsin Avenue. Aerobic dancing, she says, always involves some discomfort.

"You have to push yourself to get the endurance," Twiggs says. "When you're working a muscle in a small range in a contracted state, you're going to feel pain."

Some rules of thumb have been developed to help determine when exercise has become too strenuous. Chuck Crocker, a cardiac rehabilitation specialist with the cardiology exercise program at Georgetown University Hospital, has devised the talk test for those who run, swim, cycle, dance or walk for aerobic training.

"If you can't talk while you're exercising, then you're working too hard," Crocker says. "You ought to be able to hold a normal conversation" while performing aerobic exercises. "If you're sucking air, as we say, you should ease up because you're not only working too hard for the moment, you're also probably going to get a little sore. You're not going to be having fun and you may not continue it."

There are also a number of intensity scales. Crocker uses a scale that requires the exerciser to rate his exertion from 1 to 15 -- with 1 being the easiest and 15 being the hardest. In this system Crocker advises those doing aerobics to maintain an average intensity of 8 to 10. "You can run hard for a bit and then ease back. You may run to 11, then drop back to 7 and then go at 7 for a while."

Dr. Robert P. Nirschl, an orthopedic surgeon and medical director of the Virginia Sports Medicine Institute in Arlington, uses a 1-to-5 scale that measures discomfort. At level 1, the person feels mild discomfort during exercise. The discomfort from the first three levels usually is nothing to worry about, Nirschl says. But at level 4, the pain is so intense that it forces the person to alter his exercise, and level 5, in which the pain is constant, signals the possibility of significant injury.

"You need to push yourself beyond your present boundary if you're going to make any progress," Nirschl says. "However, pushing yourself beyond your present physiological boundary doesn't necessarily mean that there has to be pain as such. You have to pay a little price, but it's not a heavy price."