Tendons: a Sex Difference?
Statistics show that women athletes tend to have more musculoskeletal injuries than men. One example is anterior cruciate ligament, or ACL, injury at the knee, where the shin bone and hip bone meet. It occurs six times more often in women than in men. That injury is often ascribed to women’s greater “Q-angle,” which relates to the width of women’s hips in relation to their knees. Men’s hips and knees are in a somewhat straighter line.
According to a new study, however, there may be more at work than just hip and leg angles. The study notes that connective tissue is largely made of a protein called collagen. Ligaments and tendons contain estrogen receptors, which means that estrogen plays a role in connective tissue synthesis. Studies with animals, such as rats, show that high levels of estrogen decrease collagen synthesis.
In the new study researchers looked at the rate of collagen synthesis in 15 young women during either the follicular or luteal phase of their menstrual cycles; estrogen levels peak during the luteal phase. They studied the collagen synthesis levels 72 hours after the subjects did a bout of one-legged kicking exercise for 60 minutes at 67 percent of maximum workload. The results were compared to those previously obtained from men to determine any sex-related differences in collagen synthesis after exercise.
At rest and 72 hours after exercise the women had lower rates of collagen synthesis than men. At rest their rate was 55 percent of that of men. At the 72-hour mark after exercise the women synthesized only 47 percent as much as men. The lower collagen rates would explain the smaller tendons in women, as well as their higher injury rates.
Since muscle also contains connective tissue, it would seem that women are also more vulnerable to muscle injuries. However, the mechanical loading effect induced by exercise appears to overrule the effect of hormones in blunting collagen synthesis.
Since estrogen is the cause of the blunted collagen synthesis in women, how does this information affect men? Women naturally produce more estrogen than men under normal circumstances. On the other hand, many athletes use anabolic steroids that can convert into estrogen in the body. In fact, male athletes on steroids sometimes have higher estrogen levels than the average woman. While that would blunt collagen synthesis, men’s naturally larger tendons would likely overcome that.
Miller, B., et al. (2007). Tendon collagen synthesis at rest and after exercise in women. J Appl Physiol. 102:541-46.