Samantha, in an episode from Sex & the City, does invisible vaginal exercises in the middle of a diner. One can imagine the mantra running through her head. Instead of the old middle school cheer, "We must, we must, we must increase our busts," Samantha's chanting "I will, I will, I will tighten my vagina."

We've heard about them since adolescence, but what are they? Kegel exercises. Kegels are contractions that strengthen and tighten our vaginal muscles like weights help tone our arms, abs and butts. Kegelling enhances your sex life by tightening up the muscles that contract when we climax. Furthermore, it prevents or reverses incontinence and strengthens the pelvic floor muscles after childbirth. Better still, there's no harm in doing them--as long as you isolate the kegel muscles. As teenagers we learn to do them merely by squeezing, but there are some devices available that facilitate and expedite your efforts.

"Even thousands of years ago tantric practitioners knew and acknowledged that a woman's vagina loosens through repeated sexual acts. But as humans developed a more sedentary lifestyle we thought less about these kinds of things," says Jay Waller, president of Kegelmaster Worldwide. "The pubococcygeal muscles have a more important purpose than sex: they support the bladder, the uterus and the rectum. Sex and childbirth may stretch the muscles and therefore lead to incontinence later in life."

In fact it was because of incontinence that kegelling first started. In the 1950s Dr. Arnold Kegel, a Los Angeles gynecologist, developed a biofeedback device known as the perineometer to help women suffering from incontinence. Weeks after using the device, his patients--housewives--reported significant orgasmic enhancement. He understood that by isolating the pubococcygeal muscles (now commonly known as kegel muscles), women could reverse incontinence. But he also recognized that merely squeezing these muscles wasn't enough.

That's why sexperts agree that biofeedback is important--though more so for women well into their middle ages. Wendy Halbert, a urological services nurse practitioner whose office assists women with incontinence and similar problems, uses a vaginal or rectal probe hooked up to a computer to determine the effectiveness of a patient's kegel contractions. The computer shows a clinician if the patient is performing the exercises correctly and to what extent--or more simply, it shows how tight her pelvic floor is. How quickly the patient experiences results depends on how weak the muscles were when they began therapy and how faithful they are with the exercise regime, she says.

While Halbert doesn't counsel her patients on the sexual aspects of kegel exercises, she says many of her patients have bragged of "improved" sex lives. Nevertheless, she says, "I am a firm believer that if women of childbearing age would do these exercises faithfully post partum and throughout their lives, we would not see the huge problem of urinary incontinence that we see now."

Karen Corkery, a gynecological nurse practitioner, agrees. Just as important as isolating the kegels is biofeedback for women middle aged or older or for women with more severe pelvic problems. Biofeedback machines measure and chart a patient's muscular development and progress, which offers both the patient and doctor the most information.

Dr. Barnaby Barratt, president elect of the American Association of Sex Educators, Counselors and Therapists, explains that tools offering resistance most effectively help isolate the proper muscles. Many people he counsels often mistake pelvic floor muscles, perineal muscles, abdominal muscles, gluteal muscles or the anal sphincter instead of vaginal muscles, rendering kegelling counterproductive. While, like Corkery, he recommends products providing biofeedback, he says using a finger inside the vagina while kegelling also gives feedback.

"This isn't rocket science," he says. "This is an easy way for women to check out the success of their kegel exercises. Insert two fingers--the index and the middle--in a V shape and see if you can close the vaginal walls over them. This tells how much pressure the muscles can exert and how in-shape the vaginal walls are."

Another way to isolate the kegels is to visualize which muscles shake and quake during orgasm. Also you can recognize them by trying to stop the flow mid-urination; the muscles that stop your flow are those same muscles you'll be working out.

The devices often resemble adult toys but using them feels more like a gynecological visit than a journey through a carnal carnival. In the long run, though, significant sexual and medical empowerment arise from kegelling, whether you squeeze or use devices like the Fria, the Kegelmaster 2000, or the Feminine Personal Trainer.

Take it from Rachel Fallaci of Carlsbad, CA. "Sometimes I [kegel] instead of working out during the middle of the day when my husband's not around," she says. She and her husband often jumpstart their lovemaking by incorporating the Kegelmaster 2000 as foreplay. "For women who have trouble getting in the mood or reaching orgasm, using the Kegelmaster as foreplay totally fast forwards things."

Fallaci admits that kegelling improved sexual relations with her husband within five weeks. Now, she says, she's empowered with a "kung foo grip" so tight and strong that her husband sometimes has to ask her to take it easy.

Also popular is the Fria machine, a plastic bulb about the size of a thumb which gives biofeedback readings. This product offers direct feedback about how much the muscle strength is improving. One model called "MySelf" automatically adjusts to your individual performance and displays each contraction level.