Introduction
Estrogen and progesterone are the primary female endogenous reproductive hormones and have neuroactive excitatory and inhibitory effects, respectively [1]. However, research on the influence of fluctuating concentrations across the menstrual cycle on neuromuscular function and performance has led to conflicting findings ([2]ref). Increases in muscle force production are mediated by recruitment of progressively larger MUs and an increase in MU discharge rate. The ability to control force production is tightly connected with functional outcomes such as walking, risk of falls, and dexterity in humans; thus, being of clinical relevance in prevention of sports injuries, such as non-contact anterior cruciate ligament (ACL) injuries [3]. There are currently no data quantifying vastus medialis (VM) MU adaptation combined with menstrual cycle tracking across the human menstrual cycle. Therefore, the purpose of the study herein was to assess knee-extensor strength, force steadiness and VM activation using high density electromyography (HD-EMG) across the menstrual cycle.
Methods
Eight recreationally active eumenorrheic participants (age: 28 ± 7 years;; BMI: 23.9 ± 3.07) had their menstrual cycles tracked (mean cycle length: 29 ± 3 days; luteinizing hormone surge occurring on day 14 ± 2) prior to participation in the study. All attended for repeated identical assessments at the early follicular phase (EF), 24 to 48h pre-ovulation (Ov) and mid-luteal (ML) phases of the menstrual cycle. Maximum voluntary contraction (MVC) of the knee-extensors was recorded, and neuromuscular control was assessed through isometric trapezoid contractions (5s ramp up, 12s hold, 5s ramp down) peaking at 40% MVC, with real-time visual target feedback. During contractions, a 64 channel HD-EMG electrode was placed over the VM and RMS EMG was calculated as the highest amplitude within a 50ms window, and reported as a ratio of maximum RMS EMG. One way ANOVA was used to determine differences across timepoints with statistical significance accepted as p < 0.05.
Results
No statistical differences across the menstrual cycle were observed for knee extensor strength (EF:433N, Ov:418N, ML:442, p=0.500). Normalised RMS EMG did not differ across the cycle (EF:60.8, Ov:47.0, ML:56.5, p=0.139). Similarly, no statistical differences were observed in neuromuscular control during sustained phase at 40% MVC (p=0.661), during the ascent phase (p=0.292), or the descent phase (p=0.940).
Conclusion
The current pilot data highlight minimal effect of the hormonal fluctuations of the menstrual cycle in VM activation of neuromuscular performance of the knee extensors. However, additional factors of neuromuscular performance, such as individual MU features at a range of contraction levels, should be investigated to help understand the vast discrepancy of ACL injuries incidence between men and women.