As a former University of Iowa athlete and professional softball player, Dr. Kristin Johnson has always been interested in how control of movement affects athletic performance and injury. This fascination led her to a career in physical therapy and a post-doctoral position doing female sports injury prevention research in the UI Department of Physical Therapy and Rehabilitation Science.
“It was during my time in the clinic that I began to realize the lack of high-quality research focused on female athletes,” she said.
Johnson received a UI Injury Prevention Research Center pilot grant in 2021 to study women’s ability to regulate areas of their central nervous systems that are important for movement and the potential influence of sex hormones (such as fluctuations in estrogen and progesterone) on this capacity. In addition, she studied how female athletes’ knee movement control, which can affect anterior cruciate ligament (ACL) injuries, changes during different parts their menstrual cycles.
“Most athletes report that either their menstrual cycle or hormonal contraception interferes with their competitive performance,” Johnson said. “However, our understanding of hormonal influences on sport performance and injury is currently limited by a lack of robust research in this area.”
Healthcare treatment and sports training strategies are largely informed by studies of male athletes, she said, and only 6% of studies focused exclusively on female athletes. Johnson hopes to help remedy this by continuing to focus on female athlete health and injuries in her new position as a post-doctoral research scholar at Colorado State University.
“While a person’s sex is not relevant to every performance outcome, there are key sex differences in injury rates which highlights the need for sports medicine research to include female athletes,” she said.
Here, Johnson talks about her pilot research results, why female athletes are at risk for injuries, and where more research is needed.
Q & A: How do sex hormones affect ACL injuries in female athletes?
The main findings from my pilot study (manuscript in review) were that athletes learned to improve knee control during active movement regardless of their menstrual cycle phase, but athletes who were in the latter half of the cycle (mid-luteal phase) performed the best. These findings may be consistent with ACL injury data in which less injuries occur in the luteal phase of the menstrual cycle.
The ACL is a structure located inside the knee joint that helps to stabilize the leg when we move. An ACL injury can occur when the knee is contacted by another person or when moving in such a way that excessive stress is placed on the ligament. ACL injuries commonly occur when landing or making sudden changes in direction on a single leg.
It is also common for these injuries to follow an unexpected event, or a perturbation (e.g., when a person’s foot slips on grass because it is wet while the person expects the grass to be dry). Several years ago [my mentor] Dr. Richard Shields developed a device that could deliver perturbations during performance of a single-leg squatting task. In this controlled scenario we can assess the influence of various factors (muscle fatigue, biomechanics, cognitive challenge requiring mental effort) on knee control during perturbed active performance. In a prior study with the device, we showed that people can learn to improve knee control during the perturbation by training.
In my pilot study we measured collegiate athletes’ ability to control knee motion while they performed an active squatting task. We focused on the neural strategies used to control knee motion before and after an unexpected event as these are common at the time of ACL injury. We assigned the athletes to perform the task during a hormonally distinct phase of the menstrual cycle so we could determine whether these neural strategies and whether the control of knee motion depended on this hormonal phenomenon.
ACL injuries garner a lot of attention because they can be quite costly to an athlete’s competitive career, and they can exacerbate the development of degenerative joint disease. Females suffer anywhere from 2-9 times higher rates of ACL injuries than males.
Q & A: How do sex hormones affect muscles and the nervous system in female athletes?
To better understand athletic injury, we must understand the ways in which the nervous system regulates movement. There is a growing body of evidence that indicates the neural areas important for movement are sensitive to fluctuations in estrogen and progesterone. These findings support the potential for female sex hormones to influence injury rates by disrupting neural control of movement.
Main findings from my pilot study were that most females quickly learned to adapt neural pathways that are essential for movement and the females who were most successful had lower levels of estrogen.
We assessed whether young, healthy females could learn to train the H-reflex [Hoffman reflex]. The H-reflex is a measure of the responsiveness of muscle to neural input (e.g., when a nerve in the body is stimulated by a tap on the tendon or electric current, this sends a signal to the spinal cord and triggers a response in the muscles connected to that nerve). H-reflex training may relate to improved motor skills.
Importantly, the ability to train the H-reflex does not depend on other factors like muscle strength or ligament laxity (flexibility, looseness), factors that may also be influenced by female sex hormones. Therefore, this study was a way to hone in on a potential hormonal influence on the neural contribution to movement.
Most of the research to date has focused on females while they are in a passive, resting state. Our studies took the next steps to determine whether sex hormones alter females’ capacity to actively regulate these neural regions.
Q & A: What research is needed on this topic?
We cannot draw any conclusions about injury prevention from my studies as they were both small, pilot studies. But these studies were the necessary next steps based on the current state of the literature and these findings provide the physiological grounding for future large scale prospective studies that determine the potential hormonal influence on athletic injury.
There is a lot more we need to know. My findings provide theoretical support for a hormonal contribution to sports injuries yet a hormonal mechanism [how it works] for injury is still unknown. Before we can implement injury prevention strategies, we must first clarify the magnitude and the mechanism of the hormonal effect on injury. It may occur via alterations in ligament laxity, muscle stiffness, or neural excitability.
We also need to better understand the factors that determine the magnitude of hormonal side effects. Some athletes don’t experience any adverse side effects during their menstrual cycle or hormonal contraception cycle and some athletes experience severe adverse emotional and physical side effects. The number of studies currently available to inform this phenomenon are mostly of low quality.
About 50% of high-level athletes use hormonal contraception. It is commonly assumed that hormonal contraception eliminates adverse physical side effects of the menstrual cycle and that they even prevent injury. These assumptions are incorrect as hormonal contraception, like the menstrual cycle, causes adverse side effects for many athletes and current evidence does not support a protective effect from injury. Hormonal contraception will remain an important and necessary treatment for many female athletes for many different reasons, but we need to better understand the varying effects of different types of contraception (shot, pill, implant) on performance and injury.
I think an important next step in the evolution of women’s sports is to improve the quantity and quality of research that informs the health and performance of female athletes. I’m looking forward to partnering with several other researchers who are working in this sphere to help make this a reality.
Published July 27, 2023
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