They grunt, sweat and pump iron with a fervor. But these are not Spandex-clad twenty-somethings working out at a trendy health club. This is a diverse group of exercisers in a novel environment -- a hospital-based fitness center.

They are survivors -- people who have come through heart attacks, multiple bypasses, strokes or other life-threatening conditions. Determined to get healthy, they have become converts to a fitness routine. Sometimes they are joined by "deconditioned" baby boomers who have chosen to exercise before any crisis occurs. Even active seniors have opted to exercise in this relatively new venue to combat the daily ravages of age, such as hypertension, diabetes, obesity, osteoporosis and heart and pulmonary disease.

Most come to these medical fitness centers--some in hospitals, others run by hospitals at separate sites--because they seek specialized fitness regimens and want the security of knowing that qualified medical professionals will monitor their progress and are close by if needed. Some people with disabilities come because they cannot use the equipment at commercial gyms. More than 1.2 million Americans have joined the 550 medical fitness centers around the country, according to the American Fitness Association.

"I am enjoying it," says 62-year-old Sheila Thompson, who began working out at Calvert Memorial Hospital's KeepWell Fitness Center in Prince Frederick because she is at risk of osteoporosis. She combats that through a regimen designed for her age, weight and special needs. "The exercises are not overly strenuous," she explains. "They're geared toward strengthening the overall body, not just certain parts. . . . Ninety percent of us are here for medical reasons. We're not here to make our bodies beautiful or to lose weight. We're here to improve what we have and make it better."

Thompson, like so many others who've chosen to exercise at a medical fitness centers, says she appreciates that other members run the gamut in physical condition and age. "You don't have so many young bodies around . . . When I go to commercial gyms, I'm under stress because I'm trying to keep up with people who are doing things I can't do."

Medical fitness centers vary in size, location and orientation. They range from 10,000 to 90,000 square feet, with the larger centers tending to have more specialized programming, including rehabilitation for patients who have already had in-hospital cardiac or pulmonary rehabilitation or physical therapy for osteoporosis or musculo-skeletal injuries and surgery. The more clinically oriented centers tend to offer specialty programs that serve niche communities, such as lymphedema therapy to help reduce swelling in women recovering from mastectomies.

All medical fitness centers welcome self-referred individuals and provide personalized exercise and fitness programs developed and monitored by medical and health care professionals. Monthly dues range from $36 to $50.

The line between medical rehabilitation and fitness often blurs, although some centers are clearly more rehabilitation-oriented than others. Some of the larger facilities, such as Sinai Lifebridge in Baltimore, even provide the level of cardiac rehabilitation and physical therapy normally confined to a hospital setting. Centers strong in cardiac rehabilitation are especially useful to diabetics with limb amputations and stroke patients, according to Stuart Glassman, medical director of HealthSouth Rehabilitation Hospital in Concord, N.H. Amputees and stroke patients, Glassman warns, are at high risk for having a heart attack while exercising.

Glassman also notes that exercise can "decrease the risk of respiratory failure over time and provide a better quality of life" for individuals with spinal cord injuries, post-polio syndrome and neuromuscular diseases such as Duchenne's muscular dystrophy. Yet, says Elliott Roth, senior vice president and medical director of the Medical Rehabilitation Institute of Chicago, "Many times people with disabilities will go to commercial health clubs and either the equipment is not accessible or the staff is not used to dealing with people who are in wheelchairs, have artificial limbs, and other kinds of disabilities." Besides having exercise physiologists attuned to the needs of such individuals, medical fitness centers are often equipped with exercise machines that can be adjusted to accommodate members with movement limitations.

Hospital-based fitness centers developed in the 1970s as an outgrowth of cardiac rehabilitation, the prescribed therapy for heart patients that seeks to strengthen heart and lung function. But cardiologists began to realize that their patients recovering from surgery or a heart attack needed a more complete lifestyle change than the hospital could provide in the usual three months of rehabilitation. At medical fitness centers, these patients were able to continue an exercise program under the supervision of cardiac rehabilitation specialists. Further, these facilities offered educational programs directed toward a healthier overall lifestyle, a focus not always emphasized at commercial gyms.

"The wellness center is important," says Glassman, "because aside from the exercise, you have all the other issues going on" to help prevent more heart problems, including stress management, weight reduction, cholesterol screening, blood pressure checks and smoking cessation.

Adding momentum to the growth of medical fitness centers has been a cutback in insurance coverage for inpatient rehabilitation programs.

Robin Schuette, executive director of the Medical Fitness Association, an industry group based in Evanston, Ill., says medical fitness facilities help hospitals provide a "continuum of care" and transform their image "from acute care to one of pro-active health care within the community." This image, in addition, can give a hospital a marketing edge.

In addition, medical fitness centers offer a source of revenue that hospitals can use to fund community health activities, such as free health screenings.

With the exception of Chicago, where many of the first medical fitness centers were founded, most lie outside cities, where real estate costs are lower and exercise facilities may be relatively sparse.

There are no medical fitness centers in the District, but several exist in the metropolitan area. Almost two years ago Prince William Hospital in Manassas created its wellness center, which has about 800 members. Howard County General Hospital and the University of Maryland Medical Center established the Central Maryland Rehabilitative Center in Columbia as a collaborative venture about three years ago. On Nov. 1, Calvert Memorial Hospital opened its KeepWell Fitness Center.

While most people who join hospital-based fitness centers are acting on their physician's recommendation or making a transition from inpatient rehabilitation, others "are coming off the street just as they would with a commercial club," according to Schuette.

These people want to exercise "in an environment where they know they have someone who is monitoring them and checking on their progress," says Doug Meadows, fitness operations coordinator at Prince William Hospital's Wellness Center, where nearly half the members come in without a physician's referral. Other self-referred clients might include teenagers seeking strength training and conditioning so they can excel in sports and even elite athletes recovering from sports injuries.

What distinguishes hospital-based fitness centers from commercial gyms is the medical expertise of the staff, which generally includes an exercise physiologist, a registered nurse and medical professionals experienced in physical therapy and cardiac rehabilitation. All exercise physiologists or athletic trainers at medical fitness centers have at least a four-year degree in exercise physiology and are certified by the American College of Sports Medicine. Likewise, most centers require their specialists, such as physical therapists, cardiac RNs and physiatrists (specialists in physical medicine and rehabilitation) to maintain national certification. All staff, often even the receptionists, must know how to perform cardiopulmonary resuscitation, and at least one staff member knows how to administer advanced cardiac life support. And most of these centers have defibrillators, devices that deliver an electric current to help restart the heart after cardiac arrest.

No such training is required at most commercial gyms. Further, few gyms have defibrillators or anyone medically qualified to administer drugs in case of emergencies.

New members at medical fitness centers are required to fill out a comprehensive questionnaire on their medical history and undergo a monitored fitness assessment. Those at risk must get approval from their physician, who assists the exercise physiologist in developing an exercise routine for the patient.

Jimmy Page, director of the Sinai Lifebridge Health and Fitness Center in Baltimore, reports that "70 percent of our members require some level of physician clearance." While exercise programs are customized to the specific needs of the individual, all include cardiovascular and resistance training on the center's equipment.

To make lifestyle changes, a member's fitness program might include stress management, weight reduction, smoking cessation or counseling by a nutritionist, dietitian or diabetes educator. Progress reports are sent to physicians of high-risk members, and reassessments are performed at regular intervals during each workout session.

After six weeks of physical therapy for osteoporosis, Frances McMillan joined Prince William Hospital's Wellness Center, where she was put on a thrice-weekly, 90-minute strength-training program. Almost two years later, 72-year-old McMillan is still at it, even though her back pain disappeared and her posture markedly improved seven months into her exercise program.

"I'm like everyone's grandmother," said McMillan, after establishing that she routinely lifts a total of 6,470 pounds on seven weight machines.

Padded floors and specialized equipment are among the measures many centers take to be "joint-friendly" and to reduce the risk of injury. Treatment for people with arthritis or chronic pain often involves aquatic therapy.

Membership is not required to access the many outreach services offered by medical fitness centers. The centers typically publish newsletters, mount health fairs with free screenings and present educational programs ranging from luncheon seminars on diet and nutrition to weekend Lamaze classes.

The opportunity to exercise with others who share the same health concerns keeps many fitness center members committed to their exercise program. The Zipper Gang, a group of open-heart-surgery survivors at United Hospital's ExerCare in St. Paul, Minn., has been exercising and socializing since 1993. Its members have grown so close that they have their own phone directory with 41 names and Zipper Gang T-shirts.

"I don't think any of us would be doing much exercise if it weren't for the other people, because if I miss a day, there'd be a phone call saying, 'Where are you?' " says Pat Casey, an early member. In 1996, Casey went into cardiac arrest while exercising at ExerCare. Staff members resuscitated him with a defibrillator within 10 to 15 seconds. While their lifesaving efforts earned Casey's unwavering loyalty, the camaraderie he shares with his fellow "gang" members is important, too. "One of the things that helps heart patients heal is laughter, and we do a lot of that," says Casey.

By all appearances, members of the 11-to-noon cardiac rehab group at Central Maryland Rehabilitative Center seem likewise to have bonded. "We have a very congenial group," says 77-year-old Tony Siani, who had a triple bypass in 1997.

"The reason I like coming here, these guys are trained, the emergency equipment is here, and you would get immediate help," says his friend Riaz Rana, a 64-year-old survivor of two bypasses. "At a gym, you would get someone yelling, 'Who knows CPR?' and by the time help arrives, you're done."

He has one suggestion, however. "They should provide vodka and orange juice after the exercise to keep us all oiled up."

CAPTION: Riaz Rana checks the heartbeat of his friend Tony Siani, who recently had a pacemaker implanted in his chest.

CAPTION: Trainer Kurt Larsen checks settings on treadmills during a class at the Central Maryland Rehabilitation Center.