Even inveterate couch potatoes can turn back the clock on aging by lifting weights as few as two times per week, researchers said Tuesday.

In a study that proves to be an enthusiastic endorsement for exercise at any age, Tufts University researchers have found that high intensity strength training with weights helps post menopausal women develop denser bones, stronger muscles and better balance."No other program has shown improvement in all theses areas that are associated with osteoporotic bone fractures," said physiologist Miriam Nelson of the Human Nutrition Research Center at Tufts. "Realistically, we turned the clock back for these women by 20 years."

Bone fractures as the result of osteoporosis are a common affliction of elderly women. One-third of all women who live to age 90 will break a hip, while a quarter will suffer crushed vertebrae from the progressive bone loss that often accompanies menopause. And, fractures among the elderly can be fatal.

Nelson and her colleagues decided to test whether strength training could help women between the ages of 50 and 70 to avoid the pitfalls of growing old.

For one year, they studied 40 women who were "quite sedentary." Twenty were assigned to a weight-lifting exercise program designed to strengthen abdomen, upper and lower back, thighs and buttocks. The other 20 were to carry on as usual.

The weight-lifting women lost fat, gained muscle mass, increased the strength of their muscles by 36 percent to 76 percent and gained 1 percent more density in their bones, the researchers reported in this week's issue of the Journal of the American Medical Association.

The 20 women who remained sedentary had no strength gains and lost 2.5 percent of their bone density over the year.