THURSDAY, March 6 (HealthDay News) -- The thousands of Americans who undergo knee replacement surgeries each year may want to put away their tennis rackets and get back on their bikes, a new study shows.

Biking and treadmill walking appear to be two of the gentlest exercises for those who have had a knee replaced with an artificial joint, researchers found, while higher impact sports such as jogging and tennis generated higher forces on new knees, according to study author Dr. Darryl D'Lima, director of the Orthopaedic Research Laboratory at Scripps Clinic, in La Jolla, Calif.

More surprisingly, golf swings were also tough on the knees, although D'Lima was quick to point out the swings make up just part of the exercise in a round of golf.

He is scheduled to present his research Thursday at the American Academy of Orthopaedic Surgeon's annual meeting in San Francisco. The study was initiated by Dr. Clifford W. Colwell, director of the Shiley Center for Orthopaedic Research & Education at Scripps.

An estimated 478,000 total knee replacement surgeries, called total knee arthoplasties (TKAs), are done in the United States each year, according to the researchers. In the surgery, the original joint is replaced by one made of various materials, such as plastic and metal.

Surgeons typically advise patients to resume physical activity when they are able. But advice about which activity is best has been subjective, D'Lima said. So, his team decided to measure forces on the knee while patients took part in a variety of exercises.

In the study, D'Lima's team measured forces on the tibia, or shinbone, in four patients who had undergone TKA. These four patients had a specially designed joint that allowed forces to be measured from inside the implant.

A year after their surgery, each of the four patients jogged, played tennis, did golf swings, walked (on a treadmill and on level ground) and biked while the forces to their knee area were measured.

Golf swings produced forces of 4.5 times body weight on the forward knee and 3.2 times body weight in the opposite knee, the researchers found.

D'Lima noted that the forces produced by the golf swing, however, occur in an instant, while the forces produced by jogging are constant. "I think golf should be OK," he said. "It's more of scientific interest."

There were some other surprises, D'Lima said. "We expected that walking on a treadmill, which is more controlled, would be better than biking [in terms of low impact to the knee]," the researcher noted. But biking actually won out, he said.

Other details on the results:

• Biking generated the least force, producing impact of about 1.3 times the person's body weight.
• Treadmill walking was next best, producing forces of 2.05 the body weight.
• Walking on level ground generated forces of 2.6 times the body weight.
• Tennis produced forces of 3.1 to 3.8 times the body weight, with serving producing the highest impact.
• Jogging produced forces of 4.3 times body weight.

Modification of certain high-impact exercises could help, he said. For avid golfers, "it's possible you could modify your swing," D'Lima says. Golfers could get a high-tech evaluation of their swings, offered by many golf club makers, or ask their pro about modifying the swing to exert less force on their knee, D'Lima suggested.

However, for more strenuous high-impact pursuits such as jogging, people who've undergone knee replacement should make a permanent switch to another form of exercise, the expert said.

Dr. Daniel Oakes, a staff orthopedic surgeon at Santa Monica-UCLA & Orthopaedic Hospital in Santa Monica, Calif., said the study confirms advice he generally gives to his knee-replacement patients.

"We strongly dissuade [them from] jogging," he said. "I tell them activities that are OK are walking, biking, hiking, riding an exercise bike, riding an elliptical trainer and walking on the treadmill."

He suggests doubles tennis, not singles, since it is not as aggressive. He generally allows downhill or cross-country skiers to resume that sport when they are healed. And he thinks golf is fine.

While the study confirms the experts' suspicions about which activities are more stressful on the new knee, it can't be determined from the study if the increased forces will lead to a higher failure rate in the joint, said Oakes, who is also an assistant professor of orthopedic surgery and chief of the joint replacement service at the University of California Los Angeles' David Geffen School of Medicine.

More information

To learn more about knee replacement, visit the American Academy of Orthopaedic Surgeon.

SOURCES: Darryl D'Lima, M.D., Ph.D., director, Orthopaedic Research Laboratory, Scripps Clinic, La Jolla, Calif.; Daniel Oakes, M.D, staff orthopedic surgeon, Santa Monica-UCLA & Orthopaedic Hospital, Santa Monica, Calif., assistant professor, orthopaedic surgery, and chief, joint replacement service, University of Southern California Los Angeles, David Geffen School of Medicine; March 6, 2008, presentation, American Academy of Orthopaedic Surgeons annual meeting, San Francisco

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