They don't require a monthly payment plan and their own room in your house. In fact, you can get 'em for around $20 and when you're not using them they tuck away in a drawer or under your bed.

Resistance bands - elastic tubes with plastic handles on either end - are the plain Jane secret of a billion-dollar fitness industry that thrives on pricey equipment to quick-fix your physical shortcomings.

And while these specialized exercise machines often target specific muscle groups - witness the plethora of ab machines on the market - resistance bands offer a full-body attack.

"You can target all the major muscle groups with them," says Lisa Phipps, who oversees group exercise programs at Charlotte's Dowd YMCA. "You can also work on the smaller stabilizer groups" that help with maintaining balance. That makes the bands particularly popular with older exercisers.

"It amazes me how quickly these people can improve their balance," says Lauren Llewellyn, who incorporates bands into a Seniorcise class she teaches at Raleigh's Lake Lynn and Greystone Community Centers. "Within two weeks they can tell a difference."

Resistance bands are relatively new in fitness circles, making the scene in the past dozen years or so as a cheap, effective strength-training option.

Their popularity has been boosted by increasing evidence that strength training, according to the Mayo Clinic, can play a key role in everything from strengthening bones, controlling weight and managing chronic conditions to boosting stamina and reducing risk of injury.

That may explain why resistance bands have played a role even longer in physical therapy, earning the endorsement of the American Physical Therapy Association in 1995.

Resistance bands have become popular for strength and conditioning for a variety of reasons:

They're inexpensive. You can buy them at Target or Wal-Mart for less than $20, close to half that online.

They're versatile. They target all the major muscle groups.

They're mobile. They take up little space so you can take them on the road, and all but the smallest hotel rooms will accommodate a resistance band workout.

They work for any ability level. The bands come color-coded, reflecting their thickness. Start with a thinner, more elastic, easier-to-pull band and advance through thicker levels.

Phipps says resistance bands have become incorporated into a number of the YMCA's strength and conditioning courses. "We use them in athletic conditioning classes, circuit training ..."

Which isn't to say you can't get a full-strength workout out of resistance bands alone.

"You could take them through a full 30-minute workout," says Phipps.

And while many bands come with a CD demonstrating proper use and various exercises, Phipps recommends taking a class to learn proper technique - not only to get an optimal workout but also to avoid unintentional backlash.

"Kids in particular sometimes have a problem with letting go of one end and getting snapped in the face."