Women who work out regularly should consider taking it easy in the week before their period is due - as they could be at greater risk of injury, say scientists.
Researchers found that the nerve fibres around their knee muscles fired more often during this week than earlier in their menstrual cycle.
They said this difference in firing rate could affect the stability of the joint, potentially making it more susceptible to injury.
Sportswomen typically suffer from more knee injuries than their male counterparts, especially in sports such as football that involve knee twisting and turning. Previous studies found women were also more likely to experience ligament tears and chronic pain.
Researchers set out to see if hormone levels could be part of the problem, by affecting muscle-controlling nerves.
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.
It is possible to track where a woman is on her menstrual cycle because body temperature increases slightly after ovulation and dips to pre-ovulation temperatures just before the start of a new cycle. Hormonal levels also fluctuate during the cycle, with progesterone and estrogen levels falling in the final week before menstruation.
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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'.
Research leader Professor Matthew Tenan, from the University of Texas-Austin, said: 'Our results suggest that muscle activation patterns are altered by the menstrual cycle.
'These alterations could lead to changes in rates of injury.'
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.
Read more: http://www.dailymail.co.uk/health/article-2218520/Why-jogging-week-period-damage-knees.html#ixzz29WqmIoKo
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