Sex, Hormones, and Use of Contraceptives on Muscle Strength and Activation

Abstract

Women are more likely to sustain knee injuries (e.g., ACL tears) than their male counterparts. The mechanisms responsible for this disparity are unclear. However fluctuating hormones during the menstrual cycle may be an influencing factor since more ACL injuries have been observed preceding ovulation when estrogen levels increase. Previous research shows females to have increased muscle strength and altered neuromuscular activation prior to ovulation. These findings have not been replicated in females using oral contraceptives (OC). To date, no study has examined all of these factors simultaneously. The purpose of this study was to determine the effect of sex, hormones, and contraceptive use on neuromuscular function at 3 points during a menstrual cycle. A prospective cohort design with independent variables of group [male (n=10), female no-OC (n=10), female OC (n=10)], and testing session (1,2,3) was used to assess knee function (i.e., tibial translation, isometric strength, vertical leg stiffness, and neuromuscular activation) on physically active college-aged participants. Three testing sessions were scheduled throughout a typical menstrual cycle. Tibial translation was measured at the start of each testing session to assess ACL laxity, for handgrip, knee extensors and knee flexors strength. Area EMG activity of the rectus femoris (representing quadriceps) and biceps femoris (representing hamstrings) was recorded over 3 46cm drop jumps, and vertical leg stiffness was calculated based on measurements obtained by a force plate. A 3 (group) x 3 (testing session) MANOVA (p ≤ 0.05) was used to assess knee function, as defined by tibial translation, strength, EMG activation and vertical leg stiffness. A significant difference was found with respect to strength, as males overall displayed greater strength than both female groups. No other significant differences were found. Although this study attempted to explain the effect of estrogen on strength and neuromuscular function with an improved design, no conclusive evidence was found to further explain this relationship. Future studies should use more sensitive and objective measures to explore this dynamic on a greater sample size over multiple menstrual cycle phases.