Nerve and muscle activity vary across menstrual cycle

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.

Matthew Tenan, Yi-Ling Peng, and Lisa Griffin, all of the University of Texas-Austin, and Anthony Hackney, of the University of North Carolina-Chapel Hill, measured the activity of motor units - nerve fibers and the muscles they control - around the knees of female volunteers at various points of their menstrual cycles. They found that these bundles had firing rates that were significantly higher in the late luteal phase, about a week before a woman's next period, compared to earlier in the menstrual cycle. This difference in firing rate could affect the stability of the joint, potentially affecting its susceptibility to injury.

Their poster presentation entitled, "Maximal Force and Motor Unit Recruitment Patterns are Altered Across the Human Menstrual Cycle," will be discussed at The Integrative Biology of Exercise VI meeting being held October 10-13 at the Westin Westminster Hotel in Westminster, CO. This popular meeting is a collaborative effort between the American Physiological Society, the American College of Sports Medicine and the Canadian Society for Exercise Physiology. The conference is supported in part by the National Institute of Arthritis and Musculoskeletal and Skin Diseases, an institute of the National Institutes of Health, GlaxoSmithKline, Inc., Stealth Peptides, Inc., and Seahorse Biosciences.

More Than Biomechanics?

Differences in muscle structure around female athletes' knees have typically gotten the blame for disparities in knee injuries between the sexes, especially for athletes who play sports such as soccer or basketball that involve a substantial amount of knee twisting, turning, and jerking, says study leader Tenan. However, he adds, 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 seven female volunteers, all between the ages of 19 and 35, the researchers asked these study participants to chart their menstrual cycles using basal body temperature. This method involves taking body temperature every morning upon waking over the course of the menstrual cycle. 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.