The fastest selling discs in any record store these days are not likely to be by Pat Benatar or Rod Stewart or John Denver; it's Jane Fonda and Richard Simmons, Joannie Creggains and Carol Hensel who are hot now.

What they're pitching are not Rocky Mountain highs, but sleek and sexy thighs. And, with the exception of Simmons, these newest recording stars aren't even singing, they're barking out instructions as direct as any marine drill sergeant: SHAPE UP!

If the '70s were the Me Decade, then the '80s are the Less-of-Me Decade. With Americans toning up for summer, exercise records have suddenly become a booming business. One trade journal recently listed seven of them among the 100 top-selling albums. For retailers and record companies these projects have provided fat profit where they count the most--on the bottom line. Many exercise records have sold in the kinds of numbers--200,000 to 300,000 copies each-- that major rock groups could envy. The newest entry, "Reach" by Richard Simmons, even shipped platinum--which meant it had advance sales of 1 million copies--unheard of in today's depressed record market. By contrast, recent albums by the Rolling Stones and Queen only shipped 500,000.

Eighteen months ago, exercise records had a hard time getting into the stores, much less getting out. Most sellers remember trying to move Bonnie Pruden, Debbie Drake and Jack Lalanne in the '50s and '60s, but those records, set to old-fashioned organ or piano accompaniment, may have lacked the magic music ingredient.

Now, folks twist and grunt to the music of Kool and the Gang, the Jacksons, Billy Joel, Christopher Cross and other denizens of Top 40 radio. And while those artists expect big sales in record stores, Fonda and company can sit back and see their product sold in thousands of new retail outlets, ranging from Sears and 7-Elevens to boutiques, sporting-goods chains and food stores. Is this the beginning of an exploding trend or will the market soon reach saturation?

Richard Simmons is unconcerned. "Oh my goodness," says television's formerly fat guru of the grunts. "You know what's so exciting? There are some records you kiss by and some records that you get mellow by and some records you cry by. And THEN there are ACTUALLY some records that just totally motivate you so that you want to get out of your car and GO FOR IT!"

The irrepressible Simmons has just set his litany of loss to music. At $10.98, "Reach" is already being referred to as the "Saturday Night Fever" of exercise records.

"Jane Fonda's Workout Record" is an aural companion to her best-selling book. Most exercise albums come with 16-to-24-page booklets; Fonda's, a double record listing at $12.98, suggests users augment the record with her book . . . available for $17.50.

Other exercise ephemera:

* Disneyland's "Mousercise" album, which probably won't match the triple platinum status of "Mickey Mouse Disco" but has been moving toward gold status with very little advertising.

* Carol Hensel, the queen of exer-pop and another real Slenderella story. Her first album, the one that started the current craze, is on the verge of going platinum and only Anne Murray, Christopher Cross, Kenny Rogers, REO Speedwagon, Mickey Mouse and Pat Benatar have had albums on the current charts longer.

"Aerobic" seems to be a key word for the hit-bound. Among the winners: Marcy Muir's "Aerobic Dancing," "Kathy Smith's Aerobic Fitness," Dorian Dammer's "Aerobic Dancing," "Aerobic Dance Hits, Vol. 1," and "Aerobic Shape-up" by Joanie Greggains, an outgrowth of her popular "Morning Stretch" show. Another big seller: Judy Sheppard Missett's "Jazzercize."

Product tie-ins are also rampant, from the Simmons and Fonda albums to Nutri-Systems' "Nutricize" to the upcoming "Southampton Aerobic and Body Toner Exercise Plan, medically supervised by Stuart M. Berger, author of . . . "

The specter of specialization has crept in, as well, a sure sign of muscular proliferation. Among the offerings here: "The Classical Aerobic Woman" on the high-brow London label; "Aerobic Celebration" for born-again fatties who want to do jumpin' jacks for Jesus; "How the Waist Was Won," a country package by June La Salavia and the Lean Jeans Band that advertises itself as "From True Grit to True Fit."

Another sure sign of success: heavyweights like CBS and Elektra have suddenly moved in on a field that used to be the province of small, independent labels like Peter Pan, Muscletone and Parade. And they've entered the battle with big names like Simmons and Fonda, a ploy that hasn't always worked: skater Linda Frattiane's "Dance and Exercise" album was "a big bust" says one industry observer.

"My bins are bulging," says Sheldon Michaelson of Record and Tape Ltd. in Georgetown. Those bins and charts are crowded with a new class of stars looking to shake and rattle the rolls right off an overweight America. They come not from smoky nightclubs but from brightly lit television studios (Barbie Allen, Creggains, Simmons), best-selling books (Simmons, Fonda) or exercise parlors (Fonda, Simmons, Dammer, Muir, Hensel, Smith).

They are all cashing in on a confluence of crazes--exercising and disco. Olivia Newton-John prophetically suggested "Let's Get Physical," and in the last year, home exercise equipment (mats, weights, chest pulls, etc.) has become the hottest category of sporting goods; and nowadays you have to add to that list a stereo system or a tape deck . . . Diana Ross contributed a new theme song on her most recent album: "Work That Body."

Carol Hensel was an unknown dance instructor at Akron University with a reputation for exciting and effective classes, at the heart of which were routines built around current pop hits. Word eventually reached producer Joey Porrello, who went into the studio with a band playing hits while Hensel rapped out her instruction in an insistent voice. Hensel and Porrello had surveyed students and housewives to find out what they wanted, which was simple instructions and lots of music. (Most people who buy exercise records are women between the ages of 25 and 40.) Hensel took it a step further, designing routines that could be done in small apartment spaces; she even eliminated excessive jumping in consideration of neighbors.

"All the other exercise albums are a repercussion of our album's making it," says Ben Rzepka of Mirus Music, which produced the Hensel record. "Nobody thought an exercise album could be commercially successful." Mirus broke the album through a combination of old-fashioned hustle and a few fortunate breaks. Rather than confining it to record stores, they placed it in health spas, boutiques, sporting-goods departments and food stores. Then some morning deejays started playing cuts as a wake-up service to their listeners. The record took off like pounds before a rampaging Nautilus machine. It became the first of its ilk to enter the Top 100, first to reach Top 30, first to go gold.

Hensel has since become a star and has signed with the William Morris Agency. Soon, there will probably be a new line of Hensel sportswear. Cross-merchandising is another clue to the big bucks involved in the world of exer-pop. Barbie Allen is putting the finishing touches on her own "Dance/Exercise" book to compete with the Fonda and Simmons tomes.

And then there's video, a natural for those exer-pop stars already involved with television or film. Paramount Home Video came up with the first program, "Aerobicize: The Beautiful Workout," but Allen will release an RCA video and Fonda will undoubtedly become a triple winner when her "Workout" video is released soon on videodisc and tape. There will be a Mickey Mouse video using re-timed cartoons set to new soundtracks. Simmons, of course, has a program already carried in more than 200 markets.

Simmons' entry into the marketplace has most worried the smaller companies. "He's a threat to everyone because of his book and a TV program that reaches every major market," says Mirus' Rzepka. "We hope to hold our own but the penetration of his name and personality is at a level far beyond any other exercise person in America." Oddly enough, the only other "personality" that exercise record manufacturers seem scared of is Miss Piggy, whose album is scheduled for fall release.

There is one factor that might help Mirus and the others: While most exercise albums feature spoken commands over hits or copy-versions of hits, Simmons is singing (and commanding) over all original music, co-authored with veteran songwriters Bruce Roberts and Allee Willis. "All he had to do was put out something that said 'Aerobics' on it and he would have owned the world," says Rzepka. "But the artist in him got the better of him."

Simmons admits that he "never sang in my life. I mean, we all sing in cars with the windows up where people stare and we sing in the showers where the echo is great and we try out for chorus but we sing lightly and let the other voices get bigger." So he studied with Nate Lam, Hollywood's cantor to the stars (including Burt Reynolds for "Best Little Whorehouse"). "I do scream a lot and talk a lot; he told me it was just an extension. So now every morning, I get up and do exercises for my voice."

Simmons reports that super manager Jerry Weintraub (Frank Sinatra, John Denver, Diana Ross) has asked him to go on tour . . . as a singer. "I told him I'd do it with Frank Sinatra but that might not work since Frank doesn't exercise and . . . he smokes."