Women who go jogging a week prior to their period are at higher risk of injury during that time, warn scientists.
Researchers found that the nerve fibers around their knee muscles fired more often during this week than earlier in their menstrual cycle, the Daily Mail reported.
They said this difference in firing rate could affect the stability of the joint, potentially making it more susceptible to injury.
Numerous studies have shown that female athletes are more likely to get knee injuries, especially anterior cruciate ligament (ACL) tears and chronic pain, than their male counterparts.
While previous research has focused on biomechanical differences as the main source of these problems, a new study suggests another distinction that could play a role: changes across the menstrual cycle in nerves that control muscle activity.
The finding may eventually lead to new ways to prevent knee problems in female athletes.
Working with female volunteers aged between 19 and 35, the team from the University of Texas-Austin and University of North Carolina, charted their menstrual cycles by taking body temperature measurements every morning.
The scientists also measured the women's motor activity in their knees at five different points during the study. They inserted a fine wire electrode into two knee muscles and took readings as the women performed simple knee extensions.
The results from the seven women revealed that the rate of nerve firing in these muscles jumped in the third week of the menstrual cycle, known as the 'late luteal phase'.
"Our results suggest that muscle activation patterns are altered by the menstrual cycle, research leader Professor Matthew Tenan, from the University of Texas-Austin, said.
"These alterations could lead to changes in rates of injury," he added.
He said further investigation was now needed to see whether these results coincide with a difference in knee injury rates at different points in the menstrual cycle.
The study was presented at The Integrative Biology of Exercise conference held in Colorado.