Showing no signs of fear, students in Batya Metalitz's advanced class demonstrate the skill and strength needed for some poses.

Yoga may have a soft and peaceful reputation, but the theme at Willow Street Yoga on a recent Monday evening was fear. A large theme, to be sure, but in this case it was focused on issues such as: If I collapse from a handstand, what happens to my face?

The students in Batya Metalitz's advanced class were no strangers to the handstand or other difficult poses, but she still encouraged them to acknowledge that some of the things she would ask them to do in the two-hour session would be unnerving.

"I want you to be okay being in that fear. Fear will encourage you to engage those muscles," Metalitz told the group.

With yoga, tai chi and other Eastern practices moving more fully into the mainstream, the question comes up: What do they actually do? Is yoga just a nice stretch, or will it make you stronger? What about cardiovascular health? How does it stack up to the activities more commonly associated (in the West) with aerobic endurance, such as running or biking?

As with most forms of exercise, the answer is, it depends -- on what goals you set and on how you organize your training. Watching the students in Metalitz's class, there was little doubt that yoga practiced at such an advanced level involved serious strength. It also takes a pretty single-minded commitment. For anyone not ready to go that route, the relevant issue is whether there is value in a less intense relationship with these disciplines, referred to as "mindful exercise" because of their mental and sometimes spiritual aspect.

My own sense, buttressed in talks with Willow Street owner Suzie Hurley and others, is that regular participation in yoga, regardless of the style or level, is going to produce at least two surefire benefits: It will identify and help strengthen weak points in your body, and it will help reawaken muscles that tend to be underused in even active people.

Whether the issue is strength or flexibility, your body's weaknesses become obvious when you start working through yoga poses, even seemingly simple ones. And the further you go, the clearer it becomes what muscle or joint is holding things up.

Clearly, you can't stand on your head unless a whole bunch of things are working right, but even a simple backbend can be revealing. During one of Hurley's less advanced sessions, I could feel the effect of a problem I have been nursing in my right shoulder. Although different styles of yoga have different emphases, proper alignment is a central tenet (if for no other reason than to prevent injury). One particular pose was done standing and involved curling the back while the hips remained perpendicular to the floor. That can be a challenge, given that the tendency, when someone says "backbend," is to throw your hips forward and start arching the legs.

When this pose is done properly, the rhomboideus and trapezius pull you backward while the hips stay still -- beneficial for me, since the issue in my shoulder involves problems getting those upper back muscles to move my shoulder blade in the proper way.

Which brings us to the other benefit of yoga: learning to use the right muscles at the right time. This might sound like something the brain takes care of without a lot of conscious planning on our part, but not necessarily. An overly sedentary life leads, unwittingly, to bad habits: We unlearn how to do things that ought to be natural: standing, sitting, walking, moving properly. The wrong muscles get used, the train of motion gets out of rhythm and problems ensue.

Yoga, as well as such disciplines as Pilates that require similar precision, forces you to concentrate on which muscles are engaged for each posture or exercise, and leads to more awareness of how we move in daily life.

That's why Hurley developed a sequence for one of her classes she calls "juice the rhomboids." The language is not quite part of the Sanskrit tradition, but it's a sentiment Hurley said is important. Those muscles of the upper back are among the more ignored, even among people who exercise regularly. Her approach modifies the plank pose -- a static position that looks like the start of a pushup, toes on the floor, arms straight -- to include a slight up-and-down motion in the upper back.

Working through the body's weak points, retraining muscles, building flexibility, teaching balance: All these flow from yoga practice, even if it's limited to the less intense styles, said Ralph La Forge, an exercise physiologist at Duke University Medical Center's Division of Endocrinology. There are other widely accepted psychological and physiological benefits as well: Yoga's emphasis on controlled breathing and its meditative aspect, for example, can help lower blood pressure and reduce stress.

What's missing?

"It is not intense cardio," Hurley said of her studio's anusara style, a more recent offshoot of the methods developed in the past few decades by B.K.S. Iyengar.

Hurley says she still goes for regular "vigorous" walks and swims to mix up her workouts and supplement her yoga.

Different styles of yoga will involve comparatively more or less motion. Some of the more dynamic, like ashtanga and vinyasa, provide "a hell of a workout," La Forge said. But in general, he said, yoga won't produce the same elevated heart rate or intense energy expenditure as more-standard aerobics.

And although yoga does develop underlying fitness (particularly important as we age), La Forge said there can be limits when it comes to strength training.

The styles that involve holding poses for a longer time build static strength, for example, as opposed to other sorts of exercise that require muscles to move weight through a range of motion or that build endurance by repeating motions under weight. In general, the gains in strength from yoga are limited by the type of resistance being used: namely, your body weight. By definition, you'll become as strong as you need to be to hold yourself in a particular pose.

That can be plenty strong, as the students in Willow Street's advanced class demonstrated. It depends on what you are trying to accomplish, as Hurley noted. If you decide yoga is your focus, be sure to find a studio that progresses from beginner to advanced levels; otherwise, you're limiting yourself from the start.

If you feel you want more or different -- whether it is a more intense style of aerobics, strength training for a particular sport or something else -- then consider yoga as a supplement.

A little can go a long way.