Influence of sex on the reliability of cerebral blood velocity regulation during lower body negative pressure and supine cycling with considerations of the menstrual cycle.

This study examined cerebral blood velocity (CBv) regulation over four testing visits within a week between supine cycling with/without lower body negative pressure (LBNP) to evaluate the reliability of absolute and relative CBv during an LBNP protocol. Approach: A total of 19 participants (11 females; aged 20-33 years) completed five testing sessions, occurring on 7-day intervals. Visit 1 was a maximal-ramp-cycle test to ascertain peak CBv wattage. During visits 2-5, supine cycling protocol occurred at individualized peak CBv wattages with progressive decreases in LBNP from 0 Torr to -20, -40, -60, -70, and -80 Torr. Menstrual cycle day was self-reported via the Rhinessa Women's Questionnaire. Transcranial Doppler ultrasound (TCD) insonated bilateral middle cerebral artery velocity (MCAv). Two-way ANOVA assessed potential day- and sex-differences at each LBNP stage. Reliability was determined using intraclass correlation coefficients (ICC) and coefficient of variation (CoV). Main results: For all physiological measures, no main-effects were present for day- or interaction-terms (p>0.067; negligible-to-small effect sizes), while sex differences were noted for MCAv, blood pressure, and heart rate (p<0.046). Across visits, males and females displayed excellent and good-to-excellent levels of reliability for MCAv metrics, respectively (ICC range: 0.745-0.999; CoV range: 0.33-9.90%). Significance: During the current investigation, both relative and absolute CBv demonstrated high reliability in both male and female participants during a supine LBNP cycling protocol. An exploratory analysis revealed increased variance was found in female participants dependent on contraceptive use. Despite this, results indicate future LBNP studies may include females at any menstrual cycle stage.