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,” has 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.
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.
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.
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. Though
they’re not sure whether these results coincide with
a difference in knee injury rates at different
points in the menstrual cycle—a topic for future
research. Tenan notes 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,” he says. “These
alterations could lead to changes in rates of
injury.”
The findings, he adds, could prompt a closer look at
the neuroendocrine system in addition to
biomechanics as a possible cause for knee injuries
in female athletes—potentially leading to new ways
to help female athletes avoid these problems.
For more information
The American Physiological Society aps
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