Film and swim star Esther Williams earned three national championships by age 16 in the breaststroke and freestyle, just before World War II began. That launched her film career and made her a favorite pinup girl for GIs - at one time, she was on the cover of 14 magazines! But swimming remained her passion. She once said, "No matter what I was doing, the best I felt all day was when I was swimming."

Now science shows why she and millions of avid swimmers feel that way: Aquatic resistance training does more than improve cardiovascular function; it actually does something that was previously thought to be impossible: It improves cartilage health.

Accepted thinking is that (if you're lucky) your joint cartilage can be well-maintained, but not improved. But a recent study out of Finland may change all that. Researchers looked at 87 post-menopausal women, ages 60 to 68, with mild osteoarthritis of the knee. Some of them did one hour of intensive aquatic lower-limb resistance exercises - 400 to 500 reps! - three times a week for four months. The results that showed up on their MRIs were surprising: That high number of repetitions of low-impact aquatic resistance exercises actually improved their knee cartilage's biochemical composition!

Now you'll have to wait to see if these results can be duplicated in another study - say, at the Los Angeles Athletic Club, where Esther Williams swam. But in the meantime, if you have arthritis in your knee(s), jump in and try your own experiment!