Study Affirms Differences Between Sexes - At Least When It Comes To Knees

But a University of Michigan Health System study, presented today at the annual meeting of the American Academy of Orthopaedic Surgeons here, shows that female athletes may be at an even greater risk for a certain type of knee injury than their male counterparts due to the differences in the muscle structure around the knee.

"Knee muscles are capable of protecting ligaments and preventing injury," says Edward M. Wojtys M.D., professor of orthopaedic surgery, U-M Medical School, and Director of Sports Medicine at UMHS. "Female athletes are two to eight times more likely to tear their anterior cruciate ligament because they may not able to achieve the same muscle stiffness across the knee joint."

Wojtys calls the U-M study "one piece of the puzzle" in determining the differences between female and male athletes and, consequently, providing for the training and conditioning needs of female athletes in a safe and effective way.

Injury to the anterior cruciate ligament - the ligament behind the kneecap that connects the thigh bone to the shin bone and is responsible for knee stability - is common in athletes. The U-M study measured the muscle protection, or rotational knee stiffness, present in size- and sport-matched young males and females, to determine if females are more susceptible to anterior cruciate ligament injury. In addition to gender differences, the study looked at whether the types of sports the athletes played made a difference in knee stiffness.

In the study, researchers examined 24 NCAA Division I athletes - 12 males and 12 females - who compete in basketball, volleyball and soccer. All three are "pivot" sports that, because of turning, twisting and jerking, put players at high risk for injury to the anterior cruciate ligament. Another 28 collegiate athletes - 14 males and 14 females - who participated in the "non-pivot" sports of cycling, running and crew also were examined.

Male-female pairs were matched for age, height, weight, body mass index, shoe size and activity level - making them all but equal except for gender.

Researchers used a device specifically designed for this study to measure the rotational motion of the knee when it was bent at 30 degrees and 60 degrees, both when the muscles were relaxed and when the muscles were contracted, or tensed. Knee stiffness, a gauge of joint stability and resistance to injury, was measured as a function of muscle contraction.

In both the pivoting and non-pivoting sports, males were shown to produce more knee stiffness and therefore, better protection against anterior cruciate ligament injury.

Results showed that, when an athlete's muscles went from a relaxed to a tensed state, knee stiffness increased by 258 percent for male athletes in pivoting sports and 171 percent for female athletes involved in pivoting sports. Male athletes in non-pivoting sports had an increase of 207 percent versus 198 percent for their female counterparts.

Wojtys expected the higher knee stiffness for males in pivoting sports compared to males in non-pivoting sports.

"The thought is, if you play jumping, turning, twisting sports, that you should be better prepared to protect your knee against rotational forces," Wojtys says.

That's why the measurements for the females involved in non-pivoting sports were surprising when compared to females in the pivoting sports. Females in non-pivoting sports had an increase in knee stiffness of 198 percent - 27 percent higher than the females in pivoting sports.

"Women who played jumping, turning, twisting sports actually had the poorest ability to protect themselves against rotational strains," Wojtys says.

Wojtys says that male and female athletes are generally trained the same way. But based on his recent research and that of other researchers, that could be a big mistake.

"We may need to train female athletes differently," Wojtys says. "The long term scenario is to try to find rehabilitation and training tools that will specifically help female athletes because they seem to be more susceptible to this type of injury."

Co-authors on the study were James Ashton-Miller, Ph.D., distinguished research scientist, U-M Department of Engineering; Laura J. Huston, M.S., senior health science research associate, U-M Section of Orthopaedic Surgery; James P. Boylan, B.S.; and Harold J. Schock, B.S.