Menstrual Cycle Associated Alteration of Vastus Lateralis Motor Unit Function

Area of Science:

• Neuroscience
• Human Physiology
• Endocrinology

Background:

• Female sex hormones, estrogen and progesterone, influence neuronal function.
• Menstrual cycle hormone fluctuations may affect neuromuscular function, but human data is limited.
• Previous research relied on animal/cell models, lacking human motor unit data with hormone levels.

Purpose of the Study:

• To investigate the effects of menstrual cycle phases on human motor unit function.
• To quantify circulating hormones and assess neuromuscular performance across the cycle.
• To bridge the gap between animal models and human motor unit responses to hormonal changes.

Main Methods:

• Intramuscular electromyography recorded motor unit potentials in the vastus lateralis of nine females.
• Data collected during early follicular, ovulation, and mid-luteal phases.
• Neuromuscular performance (MVC, jump power, steadiness, balance) and motor unit activity were analyzed.

Main Results:

• No significant differences in knee extensor MVC, jump power, force steadiness, or balance across menstrual phases.
• Low-threshold motor unit firing rate was reduced during ovulation and mid-luteal phases (p<0.001).
• Motor unit potentials showed increased complexity during ovulation and mid-luteal phases (p<0.03), with no change in neuromuscular junction instability.

Conclusions:

• Neuromuscular performance remained consistent throughout the eumenorrheic menstrual cycle.
• Reduced low-threshold motor unit firing during high progesterone phases suggests potential inhibitory effects and altered recruitment strategies.
• Contraction intensity-dependent modulation of motor unit function occurs across the menstrual cycle, without impacting overall performance.