Current protocols examining cerebral autoregulation (CA) parameters require participants to refrain from exercise for 12–24 hr, however there is sparse objective evidence examining the recovery trajectory of these measures following exercise across the cardiac cycle (diastole, mean, and systole). Therefore, this study sought to determine the duration acute exercise impacts CA and the within‐day reproducibility of these measures. Nine participants performed squat–stand maneuvers at 0.05 and 0.10 Hz at baseline before three interventions: 45‐min moderate‐continuous exercise (at 50% heart‐rate reserve), 30‐min high‐intensity intervals (ten, 1‐min at 85% heart‐rate reserve), and a control day (30‐min quiet rest). Squat–stands were repeated at hours zero, one, two, four, six, and eight after each condition. Transcranial doppler ultrasound of the middle cerebral artery (MCA) and the posterior cerebral artery (PCA) was used to characterize CA parameters across the cardiac cycle. At baseline, the systolic CA parameters were different than mean and diastolic components (ps < 0.015), however following both exercise protocols in both frequencies this disappeared until hour four within the MCA (ps > 0.079). In the PCA, phase values were affected only following high‐intensity intervals until hour four (ps > 0.055). Normalized gain in all cardiac cycle domains remained different following both exercise protocols (ps < 0.005) and across the control day (p < .050). All systolic differences returned by hour six across all measures (ps < 0.034). Future CA studies may use squat–stand maneuvers to assess the cerebral pressure–flow relationship 6 hr after exercise. Finally, CA measures under this paradigm appear to have negligible within‐day variation, allowing for reproducible interpretations to be drawn.

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Background: Numerous studies have examined heart rate variability (HRV) and cardiac baroreceptor sensitivity (BRS) variables during recovery both acutely (under three hours) and long-term (24, 48, and 72 hours) post-exercise. However, there is little literature examining HRV and BRS measures between these time points. Methods: Spontaneous short-term HRV and cardiac BRS measures were collected in nine participants before and zero, one, two, four, six, and eight hours after three separate conditions: moderate-intensity continuous exercise (MICE: 45-minutes at 50% heart-rate reserve), high-intensity interval exercise (HIIE: 25-minutes including ten, one minute intervals at 85% heart-rate reserve), and control (30-minutes quiet rest). Results: HRV measures in the time-domain were only affected immediately following HIIE and MICE at hour zero (all p<0.043), whereas frequency-domain metrics were unaltered (all p>0.102). These measures were highly consistent across the control day (all p>0.420). Cardiac BRS was assessed via low frequency gain, and revealed reductions following HIIE at hour zero (p<0.012). Cardiac BRS LF gain remained consistent following MICE and control interventions (all p>0.280). Conclusions: The common practice of waiting 12- to 24-hours is overly conservative as the current findings demonstrate measures return to baseline ~60-minutes post-exercise. Moreover, these metrics demonstrated high levels of within- and between-day reliability. Novelty: • Previously a 12-hour minimum restriction from exercise was required before participation in HRV/BRS studies • Recovery from moderate-intensity exercise for HRV and BRS metrics was <60 minutes; whereas, high-intensity intervals led to alterations for approximately 60 minutes • Spontaneous HRV and cardiac BRS demonstrated high levels of within-day reproducibility

Background: Ultra-short-term (UST) heart rate variability (HRV) metrics have increasingly been proposed as surrogates for short-term HRV metrics. However, the concurrent validity, within-day reliability, and between-day reliability of UST HRV have yet to be comprehensively documented. Methods: Thirty-six adults (18 males, age: 26 ± 5 years, BMI: 24 ± 3 kg/m2) were recruited. Measures of HRV were quantified in a quiet-stance upright orthostatic position via three-lead electrocardiogram (ADInstruments, FE232 BioAmp). All short-term data recordings were 300-seconds in length and five UST time points (i.e., 30-seconds, 60-seconds, 120-seconds, 180-seconds, and 240-seconds) were extracted from the original 300-second recording. Bland-Altman plots with 95% limits of agreement, repeated measures ANOVA, and two-tailed paired t-tests demarcated differences between UST and short-term recordings. Linear regressions, coefficient of variation, intraclass correlation coefficients, and other tests examined the validity and reliability in both time- and frequency-domains. Results: No group differences were noted between all short-term and UST measures, for either time- (all p>0.202) or frequency-domain metrics (all p>0.086). A longer recording duration was associated with augmented validity and reliability, that was less impacted by confounding influences from physiological variables (e.g., respiration rate, carbon dioxide end-tidals, and blood pressure). Conclusively, heart rate, time-domain, and relative frequency-domain HRV metrics were acceptable with recordings greater or equal to 60s, 240s, and 300s, respectively. Conclusions: Future studies employing UST HRV metrics, should thoroughly understand the methodological requirements to obtain accurate results. Moreover, a conservative approach should be utilized regarding the minimum acceptable recording duration, which ensures valid/reliable HRV estimates are obtained.

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Objective. Currently, a recording of 300 s is recommended to obtain accurate dynamic cerebral autoregulation estimates using transfer function analysis (TFA). Therefore, this investigation sought to explore the concurrent validity and the within- and between-day reliability of TFA estimates derived from shorter recording durations from squat-stand maneuvers. Approach. Retrospective analyses were performed on 70 young, recreationally active or endurance-trained participants (17 females; age: 26 ± 5 years, [range: 20–39 years]; body mass index: 24 ± 3 kg m−2). Participants performed 300 s of squat-stands at frequencies of 0.05 and 0.10 Hz, where shorter recordings of 60, 120, 180, and 240 s were extracted. Continuous transcranial Doppler ultrasound recordings were taken within the middle and posterior cerebral arteries. Coherence, phase, gain, and normalized gain metrics were derived. Bland–Altman plots with 95% limits of agreement (LOA), repeated measures ANOVA’s, two-tailed paired t-tests, coefficient of variation, Cronbach’s alpha, intraclass correlation coefficients, and linear regressions were conducted. Main results. When examining the concurrent validity across different recording durations, group differences were noted within coherence (F (4155) > 11.6, p < 0.001) but not phase (F (4155) < 0.27, p > 0.611), gain (F (4155) < 0.61, p > 0.440), or normalized gain (F (4155) < 0.85, p > 0.359) parameters. The Bland–Altman 95% LOA measuring the concurrent validity, trended to narrow as recording duration increased (60 s: < ±0.4, 120 s: < ±0.3, 180 s  < ±0.3, 240 s: < ±0.1). The validity of the 180 and 240 s recordings further increased when physiological covariates were included within regression models. Significance. Future studies examining autoregulation should seek to have participants perform 300 s of squat-stand maneuvers. However, valid and reliable TFA estimates can be drawn from 240 s or 180 s recordings if physiological covariates are controlled.

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