Estrogen-induced effects on the neuro-mechanics of hopping in humans

Estrogen receptors in skeletal muscle suggest a tissue-based mechanism for influencing neuromuscular control. This has important physiological implications for both eumenorrheic women with fluctuating estrogen levels and those with constant and attenuated estrogen levels, i.e., women using the monophasic oral contraceptive pill (MOCP). This study examined the effects of endogenous plasma estrogen levels on leg stiffness (K (LEG)) and foot center of pressure (COP) during hopping. Nineteen females (Age = 28.0 ± 4.2 years, Ht = 1.67 ± 0.07 m, Mass = 61.6 ± 6.8 kg) who had been using the MOCP for at least 12 months together with 19 matched, female, non-MOCP users (Age = 31.9 ± 7.3 years, Ht = 1.63 ± 0.05 m, Mass = 62.5 ± 5.9 kg) participated. Non-MOCP users were tested at the time of lowest (menstruation) and highest (≈ ovulation) estrogen whilst MOCP users were tested at Day 1 and Day 14 of their cycle. At each test session, K (LEG) (N m(-1) kg(-1)) and foot COP path length (mm) and path velocity (mm s(-1)) were determined from ground reaction force data as participants hopped at 2.2 Hz on a force plate. Statistical analysis revealed no significant (p < 0.05) differences for K (LEG). In contrast, significantly higher COP path length (30%) and COP path velocity (25%) were identified at ≈ ovulation compared to menstruation in the non-MOCP users. Whilst there was no evidence of an estrogen-induced effect on K (LEG); significantly elevated estrogen at ≈ ovulation presumably increased extensibility of connective tissue and/or diminished neuromuscular control. Consistent lower limb dynamics of MOCP users demands less reliance on acutely modified neuromuscular control strategies during dynamic tasks and may explain the lower rate of lower limb musculoskeletal injuries in this population compared to non-MOCP users.