Studies have shown that the injury rate to the knee's anterior cruciate ligament (ACL) is up to eight times higher for women than it is for men, particularly in sports requiring stopping and jumping tasks. Such injuries in young athletes often require surgery and prolonged rehabilitation.
The ACL is one of one of two ligaments that cross within the knee joint to prevent the leg bone (tibia) and thigh bone (femur) from slipping forward or backward out of the joint. Although ACL injuries are more than 70 percent sports-related, most occur when there is no direct physical contact between athletes.
Writing in the March/April issue of the American Journal of Sports Medicine, authors from the University of North Carolina at Chapel Hill point to a particular risk factor for ACL injury in women: greater anterior shear force, or forward slippage of the tibia, that is applied to the knee during sports-related stop-jump tasks.
"Data from soccer, volleyball and basketball suggest that women are two to eight times more likely to have an ACL injury. During the landing phase of stop-jump activities in those sports, significant shears occur, the tibia moving forward. And if the tibia moves too far forward of the ACL on the femur, it tears the ACL," said senior author Dr. William E. Garrett, Jr., Frank C. Wilson Distinguished Professor and chair, department of orthopaedics in the medical school.
The study involved three-dimensional videography and force-plate data to record three stop- jump tasks of ten women and ten men, recreational athletes ages 19 to 25 years. A recreational athlete was defined as someone who competes three times or less per week in sports events that require stop-jump tasks such as basketball, soccer and volleyball but does not follow a professionally-designed training regimen.
The three tasks were forward jump, vertical jump and backward jump. Subjects' movements were recorded using high-speed video cameras and force plates. Forces and moments at the knee were calculated from the videographic and force plate data.
Women showed greater knee extension (or straighter knees) and tended to be more knock-kneed than men during the landing phase of each stop-jump task. They also showed greater anterior shear force that might act to stretch the ACL.
"Our results indicate that male and female recreational athletes have quite different lower extremity motion patterns. In females, these may result in knee positions and shear forces in which ACL injuries may occur," said study co-author Dr. Bing Yu, assistant professor of allied health sciences. "Female recreational athletes tend to load their ACL more than their male counterparts in similar stop-jump tasks."
In other words, said Garrett, "when the subjects are trying to stop forward motion and to jump, the anterior shear forces are higher, and women had significantly greater anterior shear force than men."
In terms of training, Yu points out there are many programs in development. "But it's very important to have a rational basis for a program, one in which we know the risk factors for injury."
Garrett agreed, "If we get a good idea of motion patterns that predict injury, we can teach different movement patterns that cut down the shear forces, which may then offer a way to reduce the ACL injury risk. We should be able to train athletes to move more softly to prevent injuries if we can train them to move more efficiently for high performance."
UNC human movement scientists, including his group, he said, "will continue to study gender differences in lower extremity motion patterns with the aim toward finding a training program that works."
Along with Garrett and Yu, UNC School of Medicine co-authors are Drs. Jonathan D. Chappell and Donald T. Kirkendall.
media note: Contact Yu at 919-843-8643, byu@med.umnc.edu
Contact Garrett at 919-966-9072, bill_garrett@med.unc.edu
School of Medicine contact, Les Lang, 919-843-9687, llang@med.unc.edu
By LESLIE H. LANG
UNC-Chapel Hill School of Medicine