The advertisements look spectacular. Lithe, scantily-clad women and muscular, square-jawed men exercise on space-age fitness equipment while an announcer promises health, happiness and beauty to those who join the health club.

This pitch can be most compelling in January -- month of post-holiday flab and idealistic resolutions -- when outdoor exercise may be nil and indoor workouts tend toward hoisting six-packs and unwrapping Twinkies.

But before you rush out and spend several hundred dollars on a health-spa contract -- hoping to be poster material by bikini season -- beware that the pursuit of your dream body could turn into a financial nightmare.

The fitness industry is big business, and -- with America's ever-increasing concern about health -- it's getting bigger all the time. Today there are an estimated 5,000 fitness facilities operating nationwide, ranging from reputable, well-designed establishments run by qualified fitness specialists, to shoddy operations run by salespeople trained to trim wallets, not bodies.

Numerous complaints about deceptive practices and false representations in the multi-million-dollar health-club business, prompted the Federal Trade Commission in 1975 to propose regulations for the industry. Among some of the testimony they heard:

* An Indiana spa member said he had been given a longer session on the machines than was proper. He suffered severe headaches and five days later had a stroke. The spa refused to cancel his contract.

* A prospective spa member said he wanted more time to consider enrollment. He was persuaded to sign a contract "for convenience sake" and was told it would not be processed until he called to okay it. When he phoned a few days later to say he decided not to join, the salesman denied that he made that representation.

* A 60-year-old woman told spa personnel when she enrolled that she suffered from arthritis, rheumatism and migraine headaches. After two visits and three payments she discovered the exercises brought on the migraines. She invoked her contract's disability cancellation provision, but was refused on the grounds that her disability was a pre-exisitng one.

* A Minnesota spa member became aware of a heart condition three months after she joined. Although she presented a letter from her doctor, the spa refused to cancel and refund her money.

* A Florida consumer moved to an area in which there was no reciprocal spa to attend. Cancellation of her contract was refused; she continued paying to maintain her credit rating.

* A Connecticut consumer said that, although the spa she joined was closed, she was still required to pay out the contract she had charged on her Master Card.

Basically, the proposed rule would give consumers a three-day "cooling-off" period in which they could change their minds and get a full refund. After that, members could cancel their contract at any time and receive a pro-rated refund, minus a cancellation fee of up to 5 percent of the total contract price.

The rule was designed to counter "unfair practices" cited by the agency such as:

High pressure sales tactics, offers of fictitious bargains, misrepresentation of facilities and qualifications of employes, false claims about effectiveness of weight-reduction and figure-shaping programs, unfair cancellation and refund policies and the closing of facilities without arranging to meet contractual obligations.

The agency held hearings on the proposed rule in 1977, and presiding officer Roger J. Fitzpatrick issued his report in 1979. The FTC staff report is expected to be out this summer, after which there will be a 60-day period for comment. The commissioners could then modify the rule and vote on whether or not to issue it. If issued, congressional approval would be required. The industry, then, would be likely to appeal.

So even if the rule is passed, says an FTC spokesman, it will be "a good while" before the consumer protection provisions would take effect. One reason the process is taking so long, says a consumer activist, is because "the entire issue of the FTC's rulemaking authority has become such a political football."

While consumer groups have expressed support of the proposed rule, the fitness industry -- not surprisingly -- is vehemently opposed.

"It's poorly-prepared and already out-dated," claims industry spokesman Jimmy Johnson, executive director of the Association of Physical Fitness Centers. "It would be disastrous economically, for the industry and for consumers. If enacted, prices would go up 100 to 200 percent."

Johnson said his group's 600 members are "all for full disclosure in contracts" and have "no objection" to instituting a 3-day "cooling off" provision. The major problem with the proposed rule, he says, is the pro-rated refund provision which is "not aligned with the economics of the industry.

"Fitness equipment today is very sophisticated and expensive, as is real estate, since you have to put a fitness center in the middle of a densely-populated area. And think of the enormous energy bill if you're operating a pool, sauna and steam room. There's no way clubs could operate with that kind of refund system.

"Spas use the membership contracts as a valuable asset. These contracts are either pledged as collateral or factored as a source of working capital loans, expansion capital, renovation funds and other uses. If they are unilaterally cancellable no bank or lending institution will permit investor's resources to be loaned against worthless paper."

Also, says Johnson, "It doesn't address the number-one problem within the industry today -- the fly-by-night operators who close facilities after taking consumer's money. Over the last two or three years, almost 200,000 people have lost their money completely to these operators."

Consumer's Union attorney Luana Martilla, who testified on behalf of consumers, rejects the idea that liberal cancellation policies would destroy the industry: "Many reputable spas operate on a monthly membership basis, so members aren't locked into a lengthy commitment.

"The staff may be more responsive to members if they could cancel any time they became dissatisfied. The real heart of the (proposed FTC) rule is the consumer's right to pay only for services they receive."

Although the fitness industry stresses that consumers who have knowingly entered into a contract should honor it, Martilla claims, "Consumers often are not given thorough and correct information by sales staff... so they enroll without a full understanding of what is being offered."

Repeated consumer complaints about health clubs have led about 20 local jurisdictions across the country to enact special consumer-protection laws regarding health spas. The District has such a law; Maryland and Virginia do not.

Under the D.C. Health Spa Consumer Act, new members have 15 days in which to cancel a health spa contract for any reason. After that period they can cancel as a result of illness, death, injury or change in residence, and receive a refund for the time they have not used, minus a registration fee of 5 percent of the contract price, not to exceed $25.

One of the most common questions health spa members bring to the D.C. Office of Consumer Protection, says general counsel Matthew J. Green, is "how in the world to get out of the contract. They may not read all the fine print, then find that because they've knowledgeably signed a contract they're kind of stuck."

Another complaint, says Green, "is nasty conditions." How could a place say it promotes health and fitness, he asks, when it's dirty and smelly?

Arlington had a problem about 10 years ago with a club that changed ownership several times, says Jean Galloway, director of the county's Office of Consumer Affairs. The club is no longer in existence.

Complaints about "pressure selling" are heard frequently by the Maryland Consumer Protection Office, according to Wendy Ginsberg, who heads the complaint division. Although "consumers often feel like they're forced into signing," she says, "if the consumer speaks English and is of sound mind there's usually nothing the agency can do."