Original Research Influence of Menstrual Cycle and Oral Contraceptive Phase on Spinal Excitability

Background: Rates of musculoskeletal injury differ substantially between the genders, with females more likely to experience conditions such as anterior cruciate ligament (ACL) injuries than males in the same sports. Emerging evidence suggests a significant hormonal contribution. Most research has focused solely on how hormonal fluctuations affect connective tissue, but a direct link between hormonal shifts, ligamentous laxity, and ACL injury has not been borne out. There is also evidence to suggest that sex hormones can modulate the central nervous system, but how this affects neuromuscular control is not well understood. Objective: To determine whether changes in sex hormone concentrations would alter spinal excitability, measured across the menstrual and oral contraceptive pill cycle. We hypothesized that spinal excitability would fluctuate across the menstrual cycle (with increased excitability during the periovulatory phase due to peak estradiol concentration), but that there would be no fluctuation in oral contraceptive users. Design: This was a prospective cohort study. Setting: The study took place at a biomechanics laboratory at a rehabilitation hospital. Participants: A total of 30 healthy women aged 18e35 who were similar in age, body composition, and exercise-training status were included. Fifteen of the women were eumenorrheic and nonusers of oral contraceptives (nonusers), and 15 of the women were taking oral contraceptives (users). Main Outcome Measures: H-reflex (Hmax/Mmax ratio), serum estradiol, and progesterone concentrations were measured at 3 time points during the menstrual and contraceptive pill cycle. Results: The H-reflex (Hmax/Mmax ratio) remained stable across the menstrual and contraceptive pill cycle. Spinal excitability was lower in the users compared with the nonusers across all testing sessions, but this was not statistically significant. Conclusions: Our results suggest that acute fluctuations of endogenous estradiol and progesterone do not modulate spinal excitability. However, long-term exposure to exogenous estrogen and progesterone (oral contraceptives) might have an impact on spinal excitability and neuromuscular control. Further research is necessary to better understand the potential differential effect of endogenous and exogenous sex hormones on spinal excitability.