What do you get when you cross a video game with an exercise bike?

"An exercycle that won't sit in the corner collecting dust," claims ex-Olympic rower Stephen Kiesling, whose new invention--the Aerobics Joystick--will hit the retail shelves in August. "And the exercise will help you burn off the stress-related physiological changes that can result from intense video-game playing."

The Joystick adapter hooks a standard exercise bicycle to a video game system. The faster you pedal, the faster your missiles fire or your car goes--depending on which game you're playing. The Joystick works with several major game systems, including Atari and Sears, and with many popular games such as "Enduro" "Grand Prix" and "Defender."

Kiesling, 24, got the idea after writing an article for American Health magazine (where he is an associate editor) on the potentially hazardous stress of video games.

"A University of Nebraska researcher was testing the physical response to mental stress and used video games to simulate life stress," says Kiesling. "He found that video games can cause a pretty dramatic increase in blood pressure among about one-third of the population. For some people, playing video games is like being on the starting line of a race."

When Kiesling tested his own physical reactions to life stress he discovered "playing Pac Man got my blood pressure up higher than anything else I did all day--including seeing the movie 'Poltergeist' and leaving my notebook in the theater, getting stuck on an elevator and buying a watch on 14th Street in Manhattan.

"Then I went to a health-club show and all the owners were standing around playing video games, while their own expensive exercise machines just sat there. It seemed like a great idea to figure out a way that people could work out while playing video games."

He discussed the idea with another former Yale rower, David Potter, a 25-year-old electrical engineer. Together they created the joystick adapter, now available for $39.95 by mail from American Health, Video-Ex, 80 Fifth Ave., N.Y. 10011. It is scheduled to arrive in sports and video game stores next month. Pooling Around

Too hot to jog or take an aerobic exercise class? Not if you run or rhumba in a swimming pool.

"I foresee a day when everyone will do their jogging in water," predicts Carla Thompson, aquatics director of the BCC YMCA. "The water cushions you from injury, provides resistance to strengthen muscles and keeps you cool, too."

Thompson recently created Aqua Aerobic Action, one of the area's growing number of aerobic-exercise classes taught in a swimming pool. These aerobics-in-water programs generally follow the standard formula of warm-up, vigorous routines choreographed to music and cool-down stretch--all done up to the neck in water for about 45 minutes. The goal: to increase cardiovascular fitness, flexibility and muscle tone.

While women and men of all ages, shapes and sizes take the class, says Thompson, "It's particularly good for people who are overweight or who have leg, foot or back problems since you have a wider range of movement in the water and can exercise without straining your back.

"We even have a lady with polio who came up and kissed me afterwards because she said she'd never been able to jump around like that without fear of falling."

Water exercise is more efficient than land exercise, maintains Linda Berling, whose 4-year-old Hydro Exercise Aerobic Routines and Toning (HEART) program is now taught in about 30 YMCAs throughout the mid-Atlantic region. A former dancercize instructor, Berling created HEART four years ago in her capacity as aquatics director at the Upper Main Line YMCA in Pennsylvania.

"Working out in water aids circulation," she says, "and minimizes the effects of gravity on the joints."

Recreation departments and individual swimming pools also feature their own programs. Among them:

Water aerobics, offered through the Montgomery County recreation department (with a special class for arthritics), taught by June Andrus of the June Andrus Fitness Center.

Waterobics, created by Virginian Joan Craft, is offered at several area YMCAs--including the Silver Spring YMCA where Sally Snowberger features a version for people with bad backs--and through the Fairfax County Park Authority.

Aqua Dynamacs, a free booklet illustrating a water-exercise program developed by the President's Council on Physical Fitness & Sports, in cooperation with the National Spa and Pool Institute, is available from NSPI, 2000 K St. NW, 7th Floor, Box PN, Washington, D.C. 20006. A cassette-tape of the program costs $7.95.

Lap It Up. Swim laps--and raise money--for the American Heart Association through their "Swim for Heart" activities July 27-29 at six area pools. For information: 337-6400. Magazine Roundup

Tress Distress. "Blondes worried about picking up a greenish tinge should either wear a bathing cap or apply a low pH (2 to 4) conditioner before swimming," advises Self magazine. "Copper--which is added to pools to prevent algae--is what causes blond hair to turn green. The conditioner acts as a buffer against absorbing copper."

Killing Time. Men who retire "merely to eat and drink, may wind up not merry but dead," says American Health magazine. Citing a new government study of 64,000 people born in the even years between 1900 and 1910, the magazine notes, "American men who opt for early retirement at 62 do not live as long as those who toil till 65. By age 74, the men who had worked longer were ahead in the survival stakes by 13 percent."

Women's survival rate, however, does not seem affected by retirment age. "Perhaps there's a built-in protection factor," says the magazine. "Retired or not, woman's work is never done."

Tub trouble. You can get more than relaxed in a hot tub, says The Physician and Sportsmedicine magazine. Accompanying the increasing popularity of whirlpools, spas and hot tubs, says Dr. George A. Randt, is a rash (excuse the expression) of cases of pseudomonas folliculitis: a skin disease related specifically to hot-tub use.

Symptoms may begin from six hours to five days after bathing and may include, he says, general malaise, headache, fever, sore throat, ear pain, rhinitis, sore eyes, swollen breasts, abdominal cramping, nausea and vomiting. An accompanying rash generally clears in seven to 14 days without treatment.

To reduce the likelihood of infection, Randt advises showering with soap and water before and after entering the tub. No bathing or hair washing should be allowed in the tub and operators should follow rigorous maintenance procedures. Hot Stuff

Exercising in weather like we've had over the last week "is risky," warns Dr. Richard Dominguez in The Complete Book of Sports Medicine (Scribners), "unless you are in excellent condition and have trained in heat and humidity for more than two weeks." But even for the best-conditioned athlete, he says, "In temperatures above 90 degrees Fahrenheit with 90 percent humidity, strenuous training is not recommended."

Among the major causes of heat stroke, Dominguez lists: Being out of shape, drugs (such as amphetamines, tranquilizers, blood pressure medicine, anti-seizure medicines, diuretics), the wearing of plastic sweatsuits and embarrassment about admitting dizziness, nausea, headache or cramps.

To prevent heat stroke, he advises avoiding afternoon workouts during the dog days of July and August, and drinking lots of water--even if you're not thirsty. If someone exhibits the symptoms of heat stroke (headache, nausea, dizziness, confusion, hot, dry skin, rectal temperature above 105 degrees Fahrenheit) he advises: Get in the shade, hose down and drink water, stand in front of a fan or air conditioner.

If headache, dizziness or confusion persists with a temperature above 101 degrees, prompt hospital treatment is necessary.