Effects of the Menstrual Cycle on Triceps Surae Properties in Women

The menstrual cycle is an important biological rhythm, whereby large cyclic fluctuations in endogenous sex hormones, such as estrogen and progesterone, are observed, which can affect performance. Evidence shows that endogenous and exogenous changes in hormone concentrations during the menstrual cycle exert many effects on the nervous and endocrine systems, in addition to the connective tissue, with consequences for the movement system. Pre-clinical studies (i.e., in animal models) show an estrogen association with the structural and mechanical properties of tendons and ligaments. Several elegant studies performed with female participants have tried to establish the mechanism underlying the effect of estrogen on collagen synthesis and its effects on exercise and functionality. Their findings suggest that the tendon properties may be affected when exposed to varying concentrations of estrogen. The largest tendon in humans, the Achilles tendon, has a direct role in functional capacity, activities of daily living, and locomotion. Studies show that the triceps surae's structural and mechanical properties may change throughout the menstrual cycle, and that these changes are related to endogenous and exogenous fluctuations in estrogen and progesterone. Musculotendinous stiffness, which is dependent on the collagen content, has been seen to vary considerably over the course of the menstrual cycle. In addition, it has been reported that females who have been taking the contraceptive pill for at least a year demonstrate lower levels of tendon strain compared to non-pill taking females, indicating a possible influence of hormonal state on tendon mechanical properties. However, the different experimental designs, the varied approaches to the evaluations and the lack of studies with high methodological quality limit our understanding of the effects of the different phases of the menstrual cycle on the triceps surae's neuromechanical properties. The aim of this observational study will be to investigate whether the morphological, mechanical (active and passive), material, and functional properties of the plantar flexor muscles and of the Achilles tendon, as well as functional parameters, change during voluntary contractions throughout the phases of the menstrual cycle in eumenorrheic women and in users of hormonal contraception. Understanding the acute effects of these properties in eumenorrheic women and users of hormonal contraception is essential so that we can adequately plan interventions and prescriptions of physical exercise and rehabilitation aimed for women.