BACKGROUND: Post-activation potentiation (PAP) equates to the increase in muscle (or muscle group) twitch force (or torque) in response to prior contractile activity [(e.g., a maximal voluntary contraction (MVC)] by the investigated muscle and is thought to help offset fatigue during endurance exercise and increase the rate of force development, thus improving speed and power performance. Animal models have used post-tetanic potentiation (PTP), a measure similar to PAP that can be assessed in animals in vivo and in rodent isolated muscle preparations, to show that normal mature female mice have a higher PTP compared to ovariectomized mature female mice, indicating that estrogen impacts skeletal muscle contractility, specifically phosphorylation of the regulatory light chain (pRLC) of myosin; therefore, we aim to determine if this relationship is present within the menstrual cycle during phases of high estrogen (i.e., ovulation) versus low estrogen (i.e., menstruation) by examining PAP of knee extensors in women during these two phases. METHODS: Eleven premenopausal women with regular menstrual cycles, age 18-35, were recruited to participate in this study. After potential subjects were screened for contraceptive use and contraindications to exercise, each participant attended three sessions. Two test sessions took place when the women were ovulating for familiarization and to capture data at the high-estrogen phase of the menstrual cycle using an ovulation test kit. A third session took place on the first day of the menstrual cycle, i.e., the low-estrogen phase. Each session began by determining the subject’s optimized current which is the stimulation amplitude that facilitated peak muscle twitch force in their quadriceps muscles. This current was then used to stimulate the quadriceps muscles preceding and following a MVC of the knee extensors. Two trials were conducted during each session with ten-minute rest periods between each. A paired t-test was used to compare PAP values at the two phases of the menstrual cycle.RESULTS: Women at ovulation versus menstruation demonstrated a mean (±SD) PAP of 82 (±24)% and 73 (± 20)%, respectively but were not significantly different (p=0.25). CONCLUSION: In ovulating women, PAP was 13% greater than in menstruating women; however, this change is not statistically significant, and the clinical implications of this finding warrants further exploration.

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