Regular running, which some experts once feared would harm the joints of America's 15 million joggers, appears instead to strengthen bones without leading to arthritis, according to two new studies.

The reports, published today in the Journal of the American Medical Association, provide reassurance that jogging's current popularity is not likely to lead to a rise in osteoarthritis or degenerative joint disease, the most common form of arthritis.

Long-distance runners studied by Stanford University researchers also were found to have 40 percent more mineral in their bones than nonrunners, which probably protects them against fractures, according to Dr. James F. Fries, an associate professor at Stanford's School of Medicine.

Fries said the finding of increased mineral "was not what we anticipated. We were very pleased to find it."

The Stanford study compared 41 male and female long-distance runners between 50 and 72 years old with 41 "controls" similar to the runners except in their exercise habits. The joggers had run regularly for an average of nine years and exercised for an average of almost five hours a week.

Researchers found no difference between the two groups in frequency of arthritis symptoms or in loss of cartilage -- an indicator of early osteoarthritis -- as shown by X-rays of knees, spine and hands.

A second study by University of Florida scientists compared 17 male runners with 18 male nonrunners and also reported no difference in the frequency of arthritis.

The discovery of increased mineral in runner's bones is encouraging because loss of mineral from bones can lead to osteoporosis, a condition in which bones become fragile. Osteoporosis affects 15 million to 20 million Americans, most of them women, and costs an estimated $3.8 billion a year -- mostly for treatment of fractures of the hip, spine and wrist.

Most researchers believe that running strengthens bones by subjecting them to gravitational stress, but it may also influence nutrition, according to Peter D. Wood, associate director of Stanford's Center for Research in Disease Prevention.

He said other studies have shown that runners "eat more food" and thus may take in more calcium, the major mineral in bone.

Dr. Bertram Zarins, chief of the sports medicine unit at Massachusetts General Hospital, said the studies do not prove running safe for everyone because they may have excluded people who gave it up because of joint symptoms.