Heart Rate Variability, Blood Pressure Variability, and Baroreflex Sensitivity in Overtrained Athletes

The study investigates how abrupt increases in training load affect autonomic cardiovascular control in athletes, a domain implicated in overreaching/overtraining. While moderate endurance training generally increases parasympathetic and decreases sympathetic activity at rest, overtraining is hypothesized to disturb this balance. Overtraining/overreaching remains prevalent among athletes and lacks a definitive diagnostic test. The authors hypothesized that HRV, BPV, and BRS metrics can detect autonomic changes induced by sudden training increases and serve as markers for overreaching/overtraining.

Prior work shows endurance training influences autonomic control, with HRV/BPV/BRS being predictors of adverse cardiac events (post-MI, heart failure, cardiomyopathy). Overtraining has been associated with autonomic imbalance, with an early sympathetic-dominant phase and later parasympathetic dominance. Studies on overtraining effects on HRV are mixed: some report reduced HRV in overtrained vs trained athletes; others (e.g., overnight HRV) find no significant changes, potentially due to nonstationary recording conditions. Gender differences in HRV exist (women with lower LF and stronger vagal modulation), and training status may modulate HRV responses. BPV responses to overtraining have also been inconsistent. These findings underscore the need for standardized, resting-condition assessments like those used here.

Design and participants: Retrospective longitudinal study of 10 healthy experienced athletes (5 men, 5 women; track/field and triathlon). All had normal medical exams and cardiopulmonary exercise tests, were medication-free, and reported normal mood state profiles pre-camp. Anthropometrics and VO2peak were recorded. Training intervention: A 13-day training camp with one rest day after 6 days. Daily program included a stepwise-increasing cycling test (lab-based; starting load 50 W for women, 100 W for men; +50 W every 3 minutes to exhaustion; objective exhaustion criteria from respiratory data), plus 40 minutes running and ~80 minutes cycling at 85–90% of individual anaerobic threshold. Training logs captured duration and intensity. After the camp, athletes recovered 3–4 days with none or very low intensity training (≤30 minutes at ~70% of threshold). Measurements: Autonomic recordings at three time points: M1 (1 week pre-camp), M2 (after 1 week of camp), M3 (3–4 days post-camp). Supine 30-minute recordings under standardized resting conditions: high-resolution ECG (1600 Hz) and continuous noninvasive finger arterial blood pressure (Portapres M2). Beat-to-beat intervals and systolic blood pressure time series were extracted; artifacts and ectopy were removed and interpolated. Mood: Profile of Mood States (POMS; German version) collected at each time point (subscales: depression, anger, vigor, fatigue; global score). HRV/BPV analysis: Time and frequency domain measures per Task Force standards. Frequency domain via FFT with Blackman-Harris window on equidistantly interpolated time series (500 ms resolution). Computed parameters: meanNN, sdNN, rmssd, LF (0.04–0.15 Hz), HF (0.15–0.4 Hz), LFn for HRV; smeanNN, dmeanNN, bsdNN, brmssd, bLF, bHF for BPV. Baroreflex sensitivity: Sequence method detecting sequences of rising/falling systolic BP with corresponding beat interval changes (1-beat delay); average regression slope used as BRS estimate. Statistical analysis: Nonparametric statistics (medians, IQR). Friedman test for repeated measures; post hoc Wilcoxon paired tests for M1 vs M2, M2 vs M3, and M1 vs M3. Significance at P<0.05. Due to cold-finger vasoconstriction, one M1 and two M3 BP recordings were excluded; BPV/BRS of two women were excluded from Friedman test and of one woman from Wilcoxon tests.

- Performance: Peak cycling performance decreased during camp and rebounded above baseline after recovery (median watts: M1 301 [219–350] vs M2 279 [217–317] vs M3 309 [234–338]; significant M1–M2, M2–M3, and M1–M3 differences). - Mood: Overall POMS global score remained stable; subscale vigor decreased significantly during camp and further at M3; fatigue tended to increase during camp (not significant). - HRV: Mean heart rate increased during camp (meanNN decreased) and normalized after recovery (meanNN medians: 1042 → 933 → 1055 ms; P<0.01 overall). sdNN showed no significant changes. rmssd decreased during camp and was higher after recovery (68 → 52 → 61 ms; significant differences noted, especially M2–M3). LF and HF powers did not show significant changes; LFn showed no significant change. - BPV: Mean systolic/diastolic pressures unchanged across time points. Time-domain (bsdNN, brmssd) and spectral (bLF, bHF) BPV showed no significant changes. - Baroreflex sensitivity: BRS significantly reduced during camp and returned toward baseline after recovery (25.2 → 17.0 → 25.7 ms/mmHg; significant M1–M2 and M2–M3). Overall, findings indicate a shift toward sympathetic activation and reduced vagal modulation during intensified training, with recovery after 3–4 days.

The study demonstrates that abrupt increases in training load in trained athletes produce measurable autonomic shifts at rest: increased heart rate (reduced meanNN), reduced short-term HRV (rmssd), and decreased BRS, consistent with sympathetic activation and vagal withdrawal. These changes largely resolve after short recovery, aligning with overreaching rather than established overtraining. The absence of significant changes in BPV and HRV spectral bands may reflect individual variability, small sample size, and methodological differences observed in prior literature. Recording under controlled, stationary resting conditions likely improved sensitivity over nonstationary overnight measures reported elsewhere. The Portapres finger BP method and the indirect nature of HRV/BRS as autonomic proxies are noted, but collectively the metrics proved useful for capturing training-induced autonomic modulation. These findings support the utility of HRV and BRS monitoring as practical, noninvasive markers to detect early maladaptation (overreaching) and guide training adjustments to prevent progression to overtraining.

Abruptly intensified training over two weeks led to increased resting heart rate, reduced short-term HRV (rmssd), and reduced BRS in athletes, indicating a shift toward sympathetic dominance with vagal inhibition. These alterations largely normalized within 3–4 days of recovery, suggesting overreaching rather than full overtraining. HRV and BRS assessed under standardized resting conditions appear to be suitable markers for monitoring autonomic responses to training load and may help in early detection and prevention of overtraining. Future research should include larger cohorts to enable gender-specific analyses, and integrate intraindividual relationships among performance metrics, autonomic measures, and blood-based markers of overtraining.

- Small sample size (n=10) limited statistical power and precluded gender-specific analyses. - Noninvasive measures provide indirect estimates of autonomic tone; Portapres captures peripheral (finger) rather than central arterial pressure. - Some BP/BRS datasets were excluded due to cold-finger vasoconstriction, potentially biasing BPV/BRS analyses. - Variability in baseline performance (particularly among men) may have attenuated some effects. - Free-breathing conditions may introduce respiratory variability, though chosen to reflect natural cardiorespiratory control. - Retrospective longitudinal design without a control group limits causal inference.