Author(s): NEVANPERÄ, S., PIIRAINEN, J.M., WALKER, S., MIKKONEN, R.S., Institution: UNIVERSITY OF JYVÄSKYLÄ, Country: FINLAND, Abstract-ID: 1380

INTRODUCTION:
A menstrual cycle (MC) is characterized by fluctuation of ovarian hormones, progesterone (P4) and estradiol (E2). The influence of sex hormones in neural responses at the cortical level has been demonstrated in several studies, where voluntary activation has been observed to increase together with higher concentrations of the neuroexcitatory hormone E2 during the late-follicular phase (LF) [1], and increased central inhibition measured with conditioned MEP has been observed during the mid-luteal phase (ML), coinciding with high concentrations of neuroinhibitory P4 [2]. However, the influence of ovarian hormones on spinal excitability is less investigated, studies reporting decreased presynaptic inhibition concurrent with higher concentrations of E2 at ovulatory phase [3] or no changes at all [4]. Importantly, most studies have been conducted in prone or seated positions, failing to capture the principles of bipedal motor control. The purpose of this study was to examine the influence of MC-related fluctuation of ovarian hormones on neural responses during dynamic balance perturbations.
METHODS:
Ten normally menstruating females (31±7 yrs) were measured at the EF (early-follicular), LF and ML phases of the MC. Venous blood samples were taken on the morning of each test and analyzed retrospectively. A custom-built dynamic balance platform was used to induce perturbations on a horizontal plane in anterior-posterior direction. Spinal excitability and neural drive were measured from soleus-muscle at four latencies from the onset of perturbation (40, 70, 100 and 130 ms) using H-reflex and V-wave techniques, respectively.
RESULTS:
A significant main effect of MC was observed for both H-reflex (F=3.559; p=0.032) and V-wave (F=3.917; p=0.023) responses. H-reflex was at its lowest at ML, pairwise comparison showed significant overall differences between EF and ML (p=0.027) and LF and ML (p=0.020), but not between EF and LF (p=0.903). Similarly, V-wave was lower at ML compared to EF (p=0.015) and LF (p=0.020), but no differences in V-wave were observed between EF and LF (p=0.900).
CONCLUSION:
This study suggests that spinal excitability and neural drive during dynamic balance perturbations are influenced by MC phase, which may be related to fluctuation in ovarian hormones. Consistent with previous studies [1,2], higher concentrations of P4 were observed to accompany reduced neural drive, but also decreased spinal excitability, which may reflect the neuroinhibitory mechanism of P4.

1. Ansdell P, Brownstein CG, Škarabot J, Hicks KM, Simoes DC, Thomas K, Goodall S. J Appl Physiol 2019; 126(6), 1701-1712.
2. Smith MJ, Keel JC, Greenberg BD, Adams LF, Schmidt PJ, Rubinow DA, Wassermann EM. Neurol. 1999;53(9):2069–72.
3. Hoffman MA, Doeringer JR, Norcross MF, Johnson ST, Chappell PE. Scand J Med Sci Sports 2018; 28:2009–2015.
4. Casey E, Reese M, Okafor E, Chun D, Gagnon C, Nigl F, Dhaher YY. PM&R 2016; 8(9), 860–868.