You already know that your videocassette recorder is great for watching movies, zapping commercials and seeing your favorite television shows at any hour you please.

Now your VCR can turn your living room into a video health clinic where you can do everything from get in shape to learn how to have better sex. Health videos offer tips on easing back pain from the American Academy of Orthopedic Surgeons, workouts with football player Lyle Alzado and a short course in cosmetic surgery with a true expert in the field, Phyllis Diller.

Once a piece of expensive electronic exotica, the VCR is now a common appliance, taking its place next to the food processor, telephone answering machine and, of course, the color television set. Currently, well over 20 percent of all U.S. households have at least one VCR, representing some 22 million machines, says the Electronic Industry Association. Another 11.5 million machines will be sold this year, EIA predicts. By the early 1990s, it is estimated that as many as 70 percent of all American homes will have at least one VCR.

Although most people buy VCRs for TV time-shifting and movie watching, there are only so many times you can watch "The A-Team" -- no matter how conveniently it fits into your schedule. Hollywood studios have nearly exhausted their vaults of old classics and can't release enough new features to feed the voracious appetite of VCR viewers.

Both the studios and scores of independent producers are rushing to fill this programming gap with special-interest, made-for-home video original productions, much of which is health-related. To be sure, it's still a tiny piece of this $3 billion dollar market, but made-for-home video programming is growing -- and fast.

Exercise tapes are, by far, the biggest area of original programming. The grandmother of them all is, of course, "Jane Fonda's Workout" (Karl/Lorimar Home Video, $59.95, 90 minutes). In fact, that tape alone helped fuel the VCR revolution.

"I bought a VCR just so I could work out to the tape," says Elynn Bernstein, a New York nurse. "With tapes, I can work out when I feel like it, it saves me money over a health club, and I don't have to worry about what I look like or having guys hitting on me." She has since purchased a half-dozen other workout tapes, which, say video producers, is typical for video exercisers.

The brainchild of entrepeneur Stuart Karl, "Workout" was "the right tape at the right time with the right star," says Pierre Loubet, director of new markets for Karl/Lorimar. Since its release in March 1982, "Workout" has sold more than 800,000 tapes worldwide, having sat atop the Billboard magazine sales charts for 166 consecutive weeks (until knocked off by "We Are the World" in early July). According to Loubet, 20,000 copies are still sold each week.

"Workout" has had its own spinoffs, also starring Fonda -- "The Workout Challenge," a tougher version of the original; "The Prime Time Workout," aimed at middle-aged exercisers; and "The Pregnancy, Birth and Recovery Workout."

Moreover, "Workout" is responsible for starting an avalanche in exercsie videos, which are now available for people of every shape, size, age and ego.

For those who find Fonda's tapes too tough and her flawless form too intimidating, there's "Do It Debbie's Way" (Video Associates, $39.95, 90 minutes), starring Debbie Reynolds. It takes a more lighthearted approach, is geared for the over-40 set and has sold some 80,000 cassettes. A similarly targeted, but not nearly as successful, tape is Sid Caesar's "Shape Up!" (Media Home Entertainment, $29.95, 58 minutes).

For the kids there's "Mousercize" from Disney Home Video ($39.95, 60 minutes), and this fall "Gymboree" (Karl/Lorimar), a program tied into a nationwide chain of franchised children's gyms, will be released.

Something more meditative? From Racquel Welch comes "Total Beauty and Fitness" (Thorn/EMI, $39.95, 108 minutes), yoga-type exercises which are a bit tough for those just starting to get in shape. If you like a little cajoling, there's "Everyday Family Fitness" (Karl/Lorimar, $59.95, 90 minutes), starring Richard Simmons, who offers a sound workout plus lots of inspirational testimonials.

For a no-nonsense approach, there's the "Bruce Jenner Workout" (Active Home Video, $39.95, 75 minutes), featuring the Olympican-turned-sportscaster and his wife, Linda Thompson, a former Miss America. It's a solid, aerobic/calisthenic program -- if you don't mind watching this grinning, all-American couple.

The enormous success of many of these tapes has just about every entertainer getting into the act. Which is an unfortunate for the consumer who buys a tape before renting it to check it out. Irene Mandrell (Barbara's sister) stars in "Texercise" (Embassy Home Entertainment, $39.95, 34 minutes), an insipid corn pone mishmash with routines like "The Thighs of Texas." Sandahl Bergman, who costarred in "Conan the Barbarian," is the centerpiece of "Sandahl Bergman's Body" (Monterrey Home Video, $39.95, 60 minutes), which is soft-core porn in the guise of an exercise tape.

While celebrities dominate exercise videos, they don't have an exclusive hold on this continually growing area of programming.

"If you want to get in shape, why not work out with somebody who's an expert in exercise?" asks David Catlin, the president of JCI Video, producer of "Kathy Smith's Ultimate Video Workout" ($39.95, 60 minutes). It's a good question, because this tape, starring veteran aerobics instructor Smith, offers a high-energy, sound aerobics program that rivals the best put out by the big names. Of course, the absence of a celebrity is no guarantee of quality. Witness "Ethnicise" (Ethnicise Inc., $39.95, 60 minutes), a collection of inane routines set to multicultural music.

Although women have been the major purchasers of exercise videos, in the last year producers began pouring out a stream of tapes that they hope will appeal to men. Not surprisingly, they've turned to macho role models. One of the first of these was "No Sweat!" (Karl/Lorimar, $39.95, 60 minutes), starring Lyle Alzado of the Los Angeles Raiders football team. The latest is "Bubba Until It Hurts" (Continental Video, $39.95, 88 minutes), featuring a former football player, 6-foot-7, 247-pound Bubba Smith. Alzado's is a slicker, better-designed workout, while Smith's offers the advantage of less bouncing, which is easier on people with tender or damaged knees.

Also geared for men is "Ultimate Fitness" ($39.95, 90 minutes), based on the book of the same name from Esquire magazine and featuring a solid, no-frills program. On tap from the new Sportsvideo label, a division of International Video Entertainment, is a tape hosted by boxer Ray "Boom Boom" Mancini called "Knockout Workout."

Some of the newest fitness tapes target specific body parts. Richard Simmons hosts "The Stomach Formula" (Karl/Lorimar, $29.95, 60 minutes), an entire program devoted to helping you loose that gut. And from USA Home Video there's "Thin Thighs in 30 Days" ($39.95, 60 minutes).

There are also tapes for specific physical conditions. The American Academy of Orthopaedic Surgeons and Feeling Fine videos recently released "Recover From Back Pain" ($39.95, 49 minutes) and "Prevent Back Pain" ($39.95, 46 minutes). These are excellent, no-nonsense programs that come with instruction booklets. Feeling Fine videos also offers "The Pregnancy Exercise Program" ($39.95, 51 minutes) and "The Postnatal Exercise Program" ($39.95, 55 minutes), based on guidelines by the American College of Obstetricians and Gynecologists, and hosted by Dr. Art Ulene, medical reporter for NBC's "Today" show.

And talk about esoteric exercise tapes. From Cyclevision Tours comes "Videocycle" ($49.95, 60 minutes each), a series of video tours of national parks -- including Yellowstone and Grand Teton -- to watch while pedaling an exercise bike.

"There should be no surprise about the success of exercise tapes," says Suzie Peterson, director of new product development for MCA Home Video. "They're a natural first step for home video because they take advantage of what video does well -- giving information more effectively than books. What you rather do, read about exercises or see them demonstrated?"

Video is a particularly helpful medium for health information because it can show as well as tell. For example, this fall Embassy will release "The Baby-Safe Home," a joint venture with McGraw-Hill that stars author and television consumer activist David Horowitz. The tape will show how to child-proof a home using vignettes with real children getting into trouble.

"It should work because visually it gets the message across better than a book," says Robin Montgomery, senior marketing vice president for Embassy Home Video. Another example: MCA Home Video will be coming out with "You Can Win: Negotiation for Power, Love and Money," a video based on the book "Winning Through Negotiation" by Dr. Tessa Warshaw.

Publishing houses are making video spinoffs of some particularly successful books. One book-born video currently available is the recently released "Eat to Win" (Karl/Lorimar, $39.95, 72 minutes) based on the best-seller by Dr. Robert Haas. But if it's indicative of what's coming, this genre could be in trouble. While it's well-produced -- some of the still life food shots are downright elegant -- it fails in the one criterion that all publishers and producers agree is requisite: It really doesn't add anything to the book. And where the book is filled with the recipies that make Haas' program work, on the video there are fewer of them, and you have to copy them down off the screen.

The industry, however, is keeping a close eye on this tape because the Red Lobster restaurant chain helped sponsor it in return for mention on the packaging and in the production. Many predict sponsor-support will emerge as a force in home video.

Not all health videos start as books. Increasingly, despite limited commercial success so far, original productions of life style, self-help and diet tapes are proliferating, albeit with decidedly mixed results. Like exercise tapes, celebrity hosts, who offer more marquee value than credibility, have become an integral part of this form.

Here's a quick, opinionated rundown:

*"Looking Good Through The Art of Cosmetic Surgery" (Lamb Productions, $49.95, 97 minutes). One of the more bizarre offerings, hosted by Phyllis Diller, this production gives a good idea of what plastic surgery can do. But it treats some rather serious surgical procedures, like liposuction (surgical removal of fat), with little mention of the dangers.

*"Less Stress in 5 Easy Steps" (Video Associates, 60 minutes, $39.95). Ed Asner humiliates himself as the star of this California psycho-babble production. You'd be better served spending your time and money talking to your local bartender.

*"The Pritikin Promise: 28 Days to a Longer, Healthier Life" (Media Home Entertainment, $29.95, 60 minutes each). This two-tape set hosted by Lorne Greene is based on the diet and exercise principles developed by the late Nathan Pritikin. Though well-intentioned and filled with practical advice, this is an overproduced effort that gets bogged down trying to turn information into entertainment.

*"Love Skills" (MCA Home Video, $39.95, 56 minutes). This guide to better lovemaking is one of the more interesting made-for-video productions. Though a little hokey and explicit at times, this tape offers solid, down-to-earth advice on fostering a healthy sex life.

*"Weight Watcher's Magazine Guide to a Healthy Lifestyle" (Vestron Video, $39.95, 56 minutes). Lynn Redgrave uses her own weight-battling experience to host this program of solid advice and testimonials about how to lose weight.

*"Living Lean" (Stouffer's Lean Cuisine, $129.95, seven hours). This three-tape production is unusual and ambitious. One cassette is an exercise program that offers different groups of exercises for people at different levels of fitness. The other two are designed to be viewed 10 minutes at a time over a 30-day period, following the lives of a family of actors who participate in the regimen in real time. You actually watch them lose weight -- and supposedly do the same as you follow the program. Interesting, informative and engaging, it is a bit too expensive and lacking in a certain credibility because the sponsor, frozen food maker Stouffer's, plugs its own product as part of losing weight.

*"First Aid: The Video Book" (CBS/Fox Home Video, $39.95, 95 minutes). A comprehensive but somewhat belabored guide to dealing with medical emergencies.

*"Stop Smoking" and "Weight Loss" (Hypnovision, $29.95, 20 min. each). Two tapes that claim the use of subliminal suggestions, which flash imperceptibly across the screen, will help control behavior.

*"How to Win at Life Extension" (Direct Broadcast Programs, $49.95, two hours). An interactive tape that features a test to help predict how long you will live, with hints on how to get back what the good times have taken away.