Whether chasing a new personal record or simply trying to stay active, many women may not realize how their menstrual cycle can affect muscle health and performance. While working in Dr. Robert Hyldahl’s lab, Brandon Pfeifer (EXCS ‘22) became interested in how estrogen deficiency may affect female athletes. During his initial research, he found minimal information concerning young female athletes. While hormone manipulation is common in studies with mice and post-menopausal women, it’s considered unsafe in young women. This left him asking: Does estrogen deficiency negatively impact muscle growth and recovery in young female athletes?
Together, Pfeifer and Hyldahl developed a study to help young women better understand how hormonal fluctuations influence muscle injury and recovery. With Hyldahl’s experience in muscle biopsies, they incorporated the procedure into their study to examine tissue changes. By collecting biopsies before and after the muscle damage protocol at different menstrual cycle phases, they could observe estrogen’s effects without manipulating hormone levels.
Pfeifer and Hyldahl recruited 28 female participants—half in the low-estrogen phase (early cycle) and half in the high-estrogen phase (mid-cycle). Participants first tracked their menstrual cycles for two months, then used ovulation tests to confirm their cycle phase before being randomized into early or late follicular groups. After collecting baseline data, Pfeifer and Hyldahl used electrical stimulation to induce a reversible muscle injury similar to post-workout soreness. They monitored recovery through soreness and blood hormone levels, then performed a final biopsy one week later to assess muscle changes.
Pfeifer and Hyldahl initially hypothesized that high estrogen levels would protect against muscle damage and promote faster recovery, while low estrogen would increase vulnerability and slow healing. Contrary to their expectations, women in the late follicular phase, when estrogen is elevated, were more prone to muscle injury. Yet, this phase also showed a reduced inflammatory response following injury, suggesting a complex and phase-specific role of estrogen in muscle recovery.
This research highlights the importance of health in female athletes, from reducing muscle injuries to enhancing muscle growth and recovery. “The goal is to provide further evidence and encouragement for female athletes to be healthy in terms of eating enough food, not over-training, and ensuring good menstrual cycle health,” explains Pfeifer. For Hyldahl, the most unique part of the study was that it was led almost entirely by an undergraduate. He often encourages undergraduates to propose and execute independent projects, so when Pfeifer came to him with his question, he was excited and knew they had the resources in the lab to conduct the experiment.
The Process of Faith and Science
Pfeifer recalled experiencing several difficulties throughout his study, including a slow recruiting process and his graduation. Despite those challenges, he continued to work with Hydahl to complete his research. “Faith is a process of asking questions and trusting that the work we do will get us answers,” Pfeifer says. He feels lucky to have worked with such good people and grateful that he had “faith in the process.”
To learn more about Pfeifer’s research, click here.