Pharmacist Brenda Schrier promotes this simple prescription for health: "Shake Well Before Using."

"But I'm not talking about shaking a medicine," she stresses. "I'm talking about shaking your body."

The best way to do a "health shake," she says, is by jumping on a miniature trampoline-like device called an aerobic rebounder. One year of rebounding "nearly every day," says the 42-year-old "certified reboundologist" has improved her stamina, strength, coordination and jean size -- down from a 9 to a 7.

Rebounding is different from--and better than--other forms of excerise, she claims, because it involves moving vertically instead of horizontally. Going with and against gravity, she contends, creates an acceleration-deceleration phenomenon that "gives every cell in the body a super workout" and makes a daily dose of jumping "the single best thing a person can do to get in shape and maintain good health."

Schrier is so hopped-up on rebounding that in December she opened what she says is the area's first "rebound center"--Rebound for Life in Alexandria. At the center, she and several other rebound enthusiasts offer rebound instruction, aerobic dance classes on rebounders and sell several models of rebounders--ranging in price from $119 on up to the crushed velvet-trimmed "cadillac of rebounders" that costs $200 and comes with optional accessories such as a slant board and a safety bar at extra cost.

First manufactured in 1975 as an indoor jogging device, rebounders began to catch on among fitness enthusiasts in the late '70s, largely through the efforts of ex-professional trampolinist Albert E. Carter, who chronicled the joys of jumping in 1977 in his self-published book, Rebound for Better Health.

"Today rebounding has become a billion-dollar industry," claims Carter, 40, now head of the 2-year-old National Institute of Reboundology and Health in Edmonds, Wash. More than 100 companies manufacture rebound units, he says, and NIRH has trained and certified more than 250 people in the physiology, mechanics and practice of rebound exercise.

What jogging was in the '70s, he claims, rebounding will be in the '80s. "People who've been jogging awhile are developing injuries from the shock to the body caused by running on hard ground. So they're turning to rebounding, which eliminates that stress on the weight-bearing joints and on the entire skeletal system."

Today, everyone from Olympic athletes to the Philadelphia Eagles football team are confirmed 'bounders. Carter attributes the rebounding boom to the physiological benefits of the exercise and "the fact that you can do it in the convenience, privacy and safety of your own home."

Rebounding, Carter claims, can improve everything from weak vision to the body's resistance to disease. Among the "miracles" he attributes to the exercise: "circulating the lymphatic system," "stimulating every cell in the body," "strengthening internal organs" and improving problems from cerebral palsy to a lousy golf game.

"Visual therapists and special education teachers have been using trampolines for about 30 years," Carter notes, "to improve vision, balance and coordination in children. When I was teaching trampoline, a number of eye doctors often sent their patients to me. Rebounding is a simple, logical exercise."

While few medical experts dispute rebounding's virtues as a shock-absorbing cardiovascular exercise, they appear less than convinced of its other professed capabilities.

"A rebounder is one of the safest things in the world to jog on," says Washington sports medicine specialist Dr. Gabe Mirkin, "and it can give you absolutely as good a cardiovascular workout as any other aerobic exercise. But the rest of the claims are absurd."

To Mirkin's knowledge, "There is not a single controlled study to show that rebounding has any more benefits than any other cardiovascular exercise performed at the same intensity. All that stuff about getting lymph flowing, and contracting and expanding cells is ridiculous."

The key to training your heart, he says, is getting your pulse rate up to 120 beats per minute, for 30 minutes, three times a week. The specific method used to achieve this--bicycling, jumping rope, swimming, jogging, rebounding--is irrelevant, he says. The cardiovascular result is the same.

One expert, Dr. J.E. Greenleaf of NASA's Aims Research Center, says Carter's continued use of paragraphs from his report on an agency fitness study amounts to "total misuse of our findings" to promote rebounding. "When I discovered he was quoting me," says Greenleaf, "I wrote him about how I felt. I never received an answer."

Carter says he never received the letter and counters, "I make it clear that the NASA study was not done for the purpose of proving or disproving rebounding's effectiveness. But it was a comparison between jogging on a treadmill and bouncing on a trampoline. I admit I'm not a scientist, but I am a trampolinist and I have drawn my own conclusions based on their results."

Despite the dispute, one thing most people seem to agree on is that rebounding is great fun.

"Jumping is nature's most natural exercise," says Schrier, who got her rebounder to improve her skills as a member of the Mighty Mean Mothers soccer team. "Think of a baby. The first thing he does when he can stand up is hold onto the crib and jump. Remember as a kid how fun it was to jump on your bed till your mother came and made you stop?"

When Schrier gets home after a day on her feet at the pharmacy she rebounds "to get the blood flowing again in my legs. I practice soccer moves on it. It's helped my game."

"I've always considered myself a pretty good athlete in very good condition," says ex-flight attendant David Vanderpoole, 29, who rebounds five days a week for 30 minutes or more. "But since I started rebounding a year ago I've lost 10 pounds and reduced my body's fat level. I see muscles I've never seen before."

Even after a lifetime of dancing, says Dot Yaness, 27, "a year of rebounding has made my legs even stronger and helped my asthma." Yaness, who holds a master's degree in dance and teaches aerobic dancing at Rebound for Life.

"I've taken rebounders to nursing homes, and I remember one 93-year-old woman got on and bounced gently while holding onto a chair. She had problems with circulation, and said it made her feel warmer."

As coach of Georgetown University's cheerleaders, Yaness says her next project is rebound cheerleading. "Since cheerleaders boost the team," she says, "it should help to boost the cheerleaders."