Searching For the Missing Link in Female Knee InjuriesIt's a known fact that women have more anterior cruciate ligament (ACL) injuries than men. This difference is seen most often in young athletes participating in noncontact sports.
There's been some suggestion that perhaps the reason for this is the stiffness factor in the knee. In this context, knee joint stiffness refers to how much torsional spring there is.
How well does the joint respond to loads when the foot is planted firmly on the ground? Is there a difference in spring between men and women that could account for the differences in injury rates?
In this study, researchers measure the stiffness values in the knees of normal, healthy, college-aged men and women. The idea was to see if women have less stiffness than men at each point in the range of motion when load was applied.
A special device called the Vermont Knee Laxity Device (VKLD) was used to measure how much laxity (slack or looseness) was present in the knees front-to-back and side-to-side.
The amount of stiffness was remeasured as increasing amounts of torque were applied to the joint. Two types of force were used: varus/valgus and internal/external rotation. Varus and valgus force goes across the joint from one side to the other. Internal and external torque is a twisting motion inward and outward.
All measurements were taken with the leg in a non-weightbearing position. The same measurements were repeated with the foot in contact with a surface. A 40 per cent weightbearing load was applied through the foot to the knee. This load is similar to the amount of weight applied through the knee when the athlete was standing equally on both feet.
The authors report that women had lower stiffness values compared with men when low loads were applied to the knee. As the load increased, the joint stiffness also increased. This is different from men who have the same stable stiffness value no matter what load was applied.
This variable response to changes in load may help explain the knee biomechanics that lead to ACL injuries. Women may be especially affected when moving from a nonweightbearing to weightbearing stance. Women activate their muscles sooner than men when weight-shifting. Less joint stiffness combined with differences in muscle activation and recruitment may be important factors as well.
It's possible that joint stiffness may be less important in men than it is in women. For women, changes in joint position, stiffness, and muscle function combined together may put the female knee at increased risk for ACL injury. More study is needed to sort this all out and find better ways to protect the female athlete from this type of injury.
Randy J. Schmitz, PhD, ATC, et al. Varus/Valgus and Internal/External Torsional Knee Joint Stiffness Differs Between Sexes. In The American Journal of Sports Medicine. July 2008. Vol. 36. No. 7. Pp. 1380-1388.
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