Not so, say two studies appearing in the February issue of the British Journal of Sports Medicine.
One study found that professional soccer players have an increased risk of osteoarthritis in the hip, while researchers in the second study discovered that women who run long distances are at risk for lower bone density, which can lead to osteoporosis.
For the first study, researchers in the United Kingdom compared 68 professional soccer players to 136 comparably aged "controls" who hadn't played professional sports. The soccer players answered questionnaires about their health, while information about members of the control group was obtained from X-rays used in a previous study.
"The study showed that the footballers (soccer players) had a tenfold higher prevalence of osteoarthritis of the hip than the general public," says one of the study's authors, Dr. Gordon Shepard, a consultant orthopedic surgeon at Royal Bolton Hospital in Lancashire.
Dr. Charles Ruotolo, director of Sports Medicine at Nassau University Medical Center in New York, says he's not surprised by the findings.
"It's not an uncommon thing to hear about professional athletes after 20 or 30 years of competition developing osteoarthritis," he says.
He did, however, say the study would have been more useful if the researchers had compared hip X-rays of the soccer players to the X-rays of the control group, rather than relying on questionnaires for the soccer players.
For the second study, researchers from the University of East London looked at 52 premenopausal women, between the ages of 18 and 44, who ran between five and 70 kilometers a week. That's about the equivalent of three to 43 miles.
The study volunteers kept food diaries so the researchers could track intake of vital nutrients, such as calcium, magnesium and zinc. The scientists also included information on menstrual cycles and contraceptive use in their analysis.
They found that the more a woman ran, the greater her chance of low bone density. The researchers noted a 2 percent drop in bone density for every 10 kilometers (6 miles) run a week.
Diet didn't seem to play a large role in the bone density of the spine in this study. However, higher intake of magnesium coupled with lower zinc intake was associated with higher bone density in the thigh.
"Load-bearing exercise is assumed to increase bone density," says Ruotolo, "but for women, if you do too much, it can actually decrease bone mineral density."
Does all this mean you should hang up your gym shoes? Not at all.
Shepard points out his study was on professional athletes, and says he doubts that people who play soccer recreationally would get arthritis from it.
Ruotolo suggests that women who like to run long distances should talk with their doctors about their diets, to be sure they're getting enough nutrients to keep their bones healthy.
And, while there's no "magic number" of miles that women should run each week, Ruotolo says if you find you're not getting your menstrual period as regularly as you used to, that's often a sign that you're running too much.
Here's some more information, from the Arthritis Foundation, on how moderate exercise can help people with arthritis. And this article from The Physician and Sportsmedicine discusses what exercises are helpful for preventing osteoporosis.