Biomarkers for rapid H-reflex operant conditioning among females.

Operant conditioning of a spinal monosynaptic pathway using the Hoffman reflex (H-reflex) is well established in animal and human studies. There is a subset within the human population (∼20% nonresponders) who are unable to up train this pathway suggesting some distinct or unique identifying characteristics. Importantly, females, who have a nine times higher rate of injury during human performance activities than men, have been understudied in areas of CNS neuroplasticity. Our long-term goal is to understand if innate ability to rapidly up train the H-reflex is predictive of future performance-based injury among females. In this study, we primarily determined whether healthy, young females could rapidly increase the H-reflex within a single session of operant conditioning and secondarily determined if electro-physiological, humoral, cognitive, anthropometric, or anxiety biomarkers distinguished the responders from nonresponders. Eighteen females (mean age: 24) participated in the study. Overall, females showed a group main effect for up training the H-reflex ( P < 0.05). Of the cohort, 10 of 18 females met the criteria for up training the H-reflex (responders). The responders showed lower levels of estradiol ( P < 0.05). A multivariate stepwise regression model supported that extracellular to intracellular water ratio (ECW/ICW) and H-max/M-max ratio explained 60% of the variation in up training among females. These findings support that females can acutely upregulate the H-reflex with training and that electro-physiological and hormonal factors may be associated with the up training. NEW & NOTEWORTHY Young females who acutely increase their H-reflexes with operant conditioning had lower levels of estradiol. However, the best predictors of those who could up-train the H-reflex were baseline H-reflex excitability (H-max/M-max) and extracellular to intracellular water ratio (ECW/ICW). Future studies are warranted to understand the complex relationship between operant conditioning, human performance, and injury among active young females.