Jack LaLanne was 21 when he opened his first health studio in Oakland, Calif., in 1936.

Doctors at the time did not advocate the program."They said working out with weights would give people heart attacks and that they would lose their sex drive," LaLanne says. "Women would be like men."

Today, LaLanne and his wife, Elaine, travel about 150,000 miles a year preaching the gospel of exercise and physical fitness.

Elaine LaLanne writes books, including "Fitness After 50" and "Dynastride!" Her latest book, "Fitness After 50 Workout" (The Stephen Greene Press, Pelham Books, $12.95), combines illustrated exercises and worksheets.

The aging process can be controlled through exercise, Elaine LaLanne says.

"There are numerous examples of people who got into exercise relatively late in life, paced themselves in getting fit and who accomplished the seemingly impossible," she writes. "Almost anyone, at any age, can get fit, stay fit and by doing so change their lives for the better."

Before starting an exercise program, 50-plus men and women should keep these points in mind, she says:

- Have a complete physical, including a stress test. "Then, each year, as you are retested and continue your fitness plan, you should see some startling changes."

- Decide just what you want from a fit lifestyle: better endurance, more energy reserves, weight control or better overall health.