Sex disparity in anterior cruciate ligament injury is multifactorial. Sex hormones, such as relaxin, may play a role in the increased risk of injury in female athletes. In order to fully investigate this relationship, optimal strategies for capturing serum relaxin concentration across the menstrual cycle must be determined. The aim of this study was to describe the variability in the timing and magnitude of relaxin concentration changes across the menstrual cycle.
Serum samples from 26 recreationally active women from one menstrual cycle (6 days of menses and 8-10 days of the luteal phase) were analyzed with enzyme linked immunosorbent assay. Line plots were used to analyze the relationship between the relaxin or progesterone peak concentration and day of the menstrual cycle, day of the luteal phase, and the start of menstruation. A repeated measures ANOVA at the 95% confidence interval was used for this data. Z-scores of the average daily means were calculated in order to compare the variability between progesterone and relaxin concentrations on the same scale .
The mean peak relaxin concentration was 49.6 ± 30.5 pg/mL. The peak values occurred later than expected with 65% of subjects demonstrating peaks on luteal days 9 or 10 and a mean of 3.03 ± 1.66 days after peak progesterone concentration.
Like other sex hormones, there is significant inter-subject variability in the magnitude and timing of peak relaxin concentration across the menstrual cycle. Future investigation of the role of relaxin in anterior cruciate ligament injury should employ serial sampling, particularly throughout days 8-12 of the luteal phase to increase the likelihood of accurately capturing the peak relaxin concentration.