Have you achieved optimal pelvic-floor fitness? This is the question posed by a new medical spa which recently opened in New York. The clinic, Phit - short for "pelvic health integrated techniques" - offers everything from vulvar reconstruction to labiaplasty, all under the banner of "vaginal rejuvenation".

Run by Dr Lauri J Romanzi, Phit is all about maximum pelvic floor elasticity. Its signature treatment is a £75 gynaecological exam where the patient contracts her pelvic muscles around the doctor's fingers to determine their strength. And if you are not quite ready for "the 'other' face-lift" (non-surgical labial contouring to restore "plump firmness"), you could simply opt for the "lazy Susan" ("painless electrical stimulation" of your pelvic floor muscles).

Alongside the "mommy makeover" which features four pelvic-toning sessions, one of Phit's packages is marketed as "sexercise". This introduces an element of sexual athleticism, almost certainly so the clinic can market itself to young women as well as to the post-natal demographic.

There is something deeply disturbing about this development. Not least because not all of us aspire to the acrobatic heights reached by the Filipino stripper in The Adventures of Priscilla Queen of the Desert (who can fire ping-pong balls from her vagina).

Pelvic-floor fitness is controversial. In some ways it is ideally placed to be the latest stick with which to beat women. American surgeons, for example, have been known to offer women the grotesque reassurance that a caesarean section will keep them "honeymoon fresh". The craze for vaginal beautification simply preys on our worst fears: that we will become incontinent and sexually unattractive. But how concerned do women really need to be about the state of their pelvic-floor muscles?

Putting aside the extreme nature of the Phit clinic, pelvic-floor fitness is something every woman should be aware of, says Linda Cardozo, professor of urogynaecology at King's College Hospital, London. She recommends exercises called Kegels (tightening and releasing the muscles) from teenage years onwards.

"It's much easier to learn how to contract undamaged muscles," she says. "Schoolgirls should learn how to do pelvic-floor exercises and do them on a daily basis."

The problem with the pelvic floor for women, she says, is that it is a "design fault" in the female body. Large mammals were designed to walk on four legs. Now that we walk on two, the pelvic floor has to bear the weight of our internal organs (the womb increases this in women). Modern women also subject it to more abuse than in the past, with heavy lifting and exercise. "There is much more impact on women's pelvic floors than there used to be and women are more conscious of problems," says Cardozo.

When the pelvic floor fails, the result is stress incontinence (when the muscles come under pressure). This can occur when you laugh, cough, sneeze or exert yourself, and is often reported as a post-natal problem. "About 50% of women will have some urinary leakage at some time after childbirth," says Cardozo.

By the age of 80, however, 11% of all women will have undergone an operation for incontinence or pelvic-organ prolapse (when the womb collapses into the vagina and the bladder or bowel can slip out of place). Not all of these women will have gone through childbirth. The incidence of vaginal prolapse is increasing, says Professor Cardozo, but only because older women are now more aware of it and more likely to seek help.

Weight is a factor; Cardozo advises a body mass index of below 30 (the ideal is 20 to 25). She recommends cycling and swimming as suitable exercise because they do not stress the pelvic floor: running for hours on the treadmill or a lot of high-impact exercise can cause problems. Avoid trampolining wherever possible, she adds. ("Seventy per cent of young girls who trampoline wet themselves.") Yoga and, particularly, Pilates are "excellent", says Cardozo, because they can strengthen the pelvic-floor muscles.

Sally Imber of the Pilates Practice in Clapham, London, explains: "The pelvic floor is the main focus in Pilates. Nearly every single exercise you're doing, even if it's one working your feet, is engaging the pelvic floor. To put it bluntly, it's just like trying to stop yourself from peeing: you engage that part of your body as tightly as possible, upwardly and inwardly. The most important thing is not to hold your breath at the same time."

These exercises are not only for women who have just had children. "There are many issues with incontinence as people get older, both for men and women," she adds. (Pelvic-floor exercises for men have also been shown to help men with erectile dysfunction.)

Having a Caesarean is no sure way for a woman to avoid pelvic-floor problems.

"C-sections partially protect but not completely. Just being pregnant and having nine months of intra-abdominal pressure is enough. Women who have not had children are less likely to have pelvic-floor problems," Cardozo says.

But not even this guarantees immunity. "If you are born with weak collagen - the elastic tissue in your body - you are more likely to be exposed to pelvic-floor problems." When genes are to blame, it is hard to know whether preventative efforts will have any effect.

"There is a fine line between worrying women and looking at what they can actually do," says Bruce Ramsay, an obstetrician and gynaecologist from Peterborough. "There are so many health messages and there is enough that we would like people to be doing, without worrying about their pelvic floor."

It is also difficult to define what "normal" is."Narrowing of the vagina after childbirth is now offered as a form of cosmetic surgery," says Ramsay, "because women's expectations of sexual intercourse have altered. But if you talk to women in the older generation, their expectation is that these things are normal and it's just a natural part of getting older. Previously, if someone had incontinence, they would just buy pads and limit their activity."

Both Ramsay and Cardozo suggest seeing a physiotherapist if you are worried about your pelvic floor. If you have physical symptoms, you can get a referral from a GP. If it is just for reassurance, you will have to see one privately. Pelvic-floor toning devices such as Aquaflex (vaginal cones available from Boots for £25) can be useful for anyone who is not sure they are tightening the right muscles, says Cardozo. (If you're doing it wrong, the cone will drop out.) But I don't recommend visiting the Phit website. Unless you want to feel slightly queasy and extremely vaginally inadequate, that is.