It's known that female athletes are more likely to get knee injuries than their male counterparts, but rather than biomechanical differences as the main source of these problems, a new study suggests that menstruation-triggered changes in the nerves that control muscle activity may be to blame.
Differences in muscle structure around female athletes' knees have typically been suspected for disparities in knee injuries between the sexes, especially for athletes who play sports that involve a substantial amount of knee twisting, turning, and jerking. But study leader Matthew Tenan, from the University of Texas-Austin, says it's been unclear whether other factors, such as differences in motor unit firing patterns, might also play a part. Since female athletes' hormones fluctuate across the menstrual cycle, Tenan and his colleagues decided to investigate whether these changes affect motor unit activity.
Working with female volunteers aged between 19 and 35, the researchers asked the participants to chart their menstrual cycles using daily basal body temperature readings. Because temperature increases slightly after ovulation (the luteal phase), then dips to pre-ovulation temperatures just before the start of a new cycle (the follicular phase), it's possible to track where each volunteer was in her menstrual cycle on any given day.
The researchers also asked each volunteer to visit their facility five different times at various points of the menstrual cycle. At each visit, they inserted a fine wire electrode, no thicker than a human hair, into two muscles around one of each of the volunteers' knees. The women then did knee extensions while the researchers used these electrodes to measure the activity of motor units in those muscles.
Intriguingly, the results showed that motor unit firing patterns varied significantly across the menstrual cycle. Most notably, Tenan and his colleagues found that the rate of firing jumped in the late luteal phase compared to rates earlier in the cycle. The researchers caution that they're not sure whether these results coincide with a difference in knee injury rates at different points in the menstrual cycle, but they do say that changes in motor unit activity could make women more vulnerable to injury in general.
"Our results suggest that muscle activation patterns are altered by the menstrual cycle," Tenan concluded. "These alterations could lead to changes in rates of injury." The findings, he adds, could prompt a closer look at the neuroendocrine system as a possible cause for knee injuries in female athletes - potentially leading to new ways to help female athletes avoid these problems.
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Source: American Physiological Society