We Run Ultras

(5) HRV Is a Readiness Signal Before It Is a Fitness Score

By Francesco//7 min read/Training science & durability

A response signal, not a standalone score

Heart-rate variability (HRV) contains information about both training adaptation and current stress. That makes the common binary description — fitness metric or stress metric — misleading. Resting HRV is most useful to an ultrarunner as a personal response signal: a trend to interpret alongside resting heart rate, recent load, sleep, symptoms and performance. A single high value does not prove improved fitness, and a single low value does not prove lost fitness.

HRV measures variation between successive normal heartbeats. Common morning and overnight metrics, particularly RMSSD, are strongly influenced by parasympathetic input to the heart. Endurance training can increase vagal modulation over time, so fitness is part of the signal. The same value also responds to acute training load, sleep disruption, illness, alcohol, psychological strain and environmental stress. HRV reports how autonomic regulation is changing; it does not identify one cause.

Why personal context matters

Between-runner comparison is weak because age, sex, genetics, measurement method and baseline physiology all influence HRV. The Oman Family Study found heritable components in several resting and stress-response measures. Acute altitude exposure generally reduces vagal HRV, while illness and poor sleep can also lower it. A mountain runner arriving at altitude can therefore show a falling value without any sudden loss of aerobic capacity.

Ultramarathons add a recovery problem. Studies after ultra-endurance races report substantial autonomic disturbance, sometimes lasting for days. A low reading five days after a 100-miler is compatible with recovery stress, but the number cannot tell whether the main driver is muscle damage, sleep loss, infection or accumulated life load. It is a reason to look more closely, not a diagnosis.

Fitness estimation is a separate task. Resting heart rate, exercise heart rate, pace or power, and maximal or submaximal testing have a more direct relationship with aerobic performance. Secondary analysis by Marco Altini found little additional value from HRV when estimating VO₂max after resting heart rate and basic characteristics were included. That analysis is not a clinical validation study, but it illustrates the category error: a variable useful for monitoring response need not be the best variable for estimating the size of the aerobic engine.

When a higher value can mislead

The strongest objection to "higher is always better" comes from functional overreaching. Le Meur and colleagues deliberately intensified endurance training until performance fell and observed increasing parasympathetic HRV indices in some athletes. Reviews by Buchheit and Bellenger describe both suppression and paradoxical vagal hyperactivity during maladaptation. Daniel Plews' case work in elite triathletes likewise emphasises rolling change and day-to-day variation rather than an isolated absolute value.

This does not make high HRV bad. It means direction requires context. A rising rolling average paired with improving submaximal pace, stable mood and normal recovery can fit positive adaptation. The same rise paired with worsening performance, heavy fatigue and unusually low resting heart rate may fit parasympathetic hyperactivity. The performance and symptom pattern decides which interpretation is credible.

What HRV-guided training can do

Randomised studies have used HRV to decide when to schedule harder sessions. The best meta-analyses are more modest than the marketing: compared with predefined training, HRV-guided programmes produced small, non-significant group-level effects on VO₂peak and endurance performance, though one review found better submaximal measures and fewer non-responders. The likely value is load management with fewer unnecessary hard sessions, not a superior physiological pathway.

Wearable manufacturers increasingly acknowledge this context. Garmin establishes a rolling personal baseline and notes effects from sleep and activity; Oura tells users to compare against their own history; COROS describes overnight HRV as an autonomic response to stress. Their algorithms still differ, so changing device can change the baseline even when the runner has not changed.

Measurement quality places another boundary around interpretation. Wrist sensors estimate beat-to-beat intervals optically and can be disturbed by movement, poor skin contact and ectopic beats. Overnight averages and controlled morning readings answer slightly different questions. Consistency of device, timing, posture and recording method matters more than chasing agreement between platforms.

A reading rule for training

Measure under consistent conditions and use at least a seven-day rolling average. When HRV moves outside its normal range, check resting heart rate, sleep, soreness, mood, illness symptoms and a familiar submaximal session. Modify training when several signals agree or when symptoms alone justify it. If HRV and lived performance disagree, investigate the disagreement instead of obeying the watch. That is the useful role of HRV in a durability and recovery system: it prompts a better decision without pretending to make the decision by itself.

Sources

HRV physiology and measurement standards

  1. Task Force of ESC/NASPE, "Heart Rate Variability: Standards of Measurement, Physiological Interpretation and Clinical Use" https://pubmed.ncbi.nlm.nih.gov/8598068/

  2. Shaffer, F. & Ginsberg, J. P., "An Overview of Heart Rate Variability Metrics and Norms" https://doi.org/10.3389/fpubh.2017.00258

  3. Laborde, S. et al., "Heart Rate Variability and Cardiac Vagal Tone in Psychophysiological Research" https://doi.org/10.3389/fpsyg.2017.00213

  4. Billman, G. E., "The LF/HF Ratio Does Not Accurately Measure Cardiac Sympatho-Vagal Balance" https://doi.org/10.3389/fphys.2013.00026

HRV and training monitoring

  1. Buchheit, M., "Monitoring Training Status with HR Measures: Do All Roads Lead to Rome?" https://pmc.ncbi.nlm.nih.gov/articles/PMC3936188/

  2. Plews, D. J. et al., "Heart Rate Variability in Elite Triathletes: Is Variation in Variability the Key to Effective Monitoring?" https://pubmed.ncbi.nlm.nih.gov/22367011/

  3. Plews, D. J. et al., "Training Adaptation and Heart Rate Variability in Elite Endurance Athletes: Opening the Door to Effective Monitoring" https://doi.org/10.1007/s40279-013-0071-8

  4. Schmitt, L. et al., "Monitoring Fatigue Status with HRV Measures in Elite Athletes: An Avenue Beyond RMSSD" https://doi.org/10.3389/fphys.2015.00343

Overreaching and parasympathetic hyperactivity

  1. Le Meur, Y. et al., "Evidence of Parasympathetic Hyperactivity in Functionally Overreached Athletes" https://pubmed.ncbi.nlm.nih.gov/24136138/

  2. Bellenger, C. R. et al., "The Impact of Functional Overreaching on Post-Exercise Heart Rate Variability" https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2020.614765/full

  3. Meeusen, R. et al., "Prevention, Diagnosis, and Treatment of the Overtraining Syndrome: ECSS and ACSM Joint Consensus Statement" https://doi.org/10.1249/MSS.0b013e318279a10a

HRV-guided training

  1. Kiviniemi, A. M. et al., "Endurance Training Guided Individually by Daily Heart Rate Variability Measurements" https://doi.org/10.1007/s00421-007-0552-2

  2. Vesterinen, V. et al., "Individual Endurance Training Prescription with Heart Rate Variability" https://doi.org/10.1249/MSS.0000000000000770

  3. Düking, P. et al., "Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis" https://doi.org/10.1016/j.jsams.2021.04.012

  4. Manresa-Rocamora, A. et al., "HRV-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Systematic Review with Meta-Analysis" https://doi.org/10.3390/ijerph181910299

HRV and VO₂max prediction

  1. Venter, R. E., "A Comparison Between Heart Rate and Heart Rate Variability as Indicators of Cardiac Health and Fitness" https://pmc.ncbi.nlm.nih.gov/articles/PMC3834240/

  2. Altini, M., "VO2max Estimation: Non-Exercise Data, Resting Heart Rate, HRV, Sub-Maximal Heart Rate — What's the Difference" https://www.hrv4training.com/blog2/vo2max-estimation-non-exercise-data-resting-heart-rate-hrv-sub-maximal-heart-rate-whats-the-difference

Genetics, stress, sleep and alcohol

  1. Muñoz, M. L. et al., "Heritability and genetic correlations of heart rate variability at rest and during stress in the Oman Family Study" https://pmc.ncbi.nlm.nih.gov/articles/PMC5991177/

  2. Kim, J. et al., "Reduced Heart Rate Variability Is Related to Fluctuations in Psychological Stress Levels in Daily Life" https://onlinelibrary.wiley.com/doi/10.1002/smi.3447

  3. Koskinen, P. et al., "Acute Alcohol Intake Decreases Short-Term Heart Rate Variability in Healthy Subjects" https://doi.org/10.1042/cs0870225

  4. Hottenrott, L. et al., "Utilizing Heart Rate Variability for Coaching Athletes During and After Viral Infection: A Case Report in an Elite Endurance Athlete" https://doi.org/10.3389/fspor.2021.612782

  5. Altini, M. & Flatt, A., "There's More in Life Than Training: Non-Training Related Stressors and Recovery" https://www.hrv4training.com/blog2/theres-more-in-life-than-training-non-training-related-stressors-and-recovery

Ultra-endurance and HRV

  1. Paech, C. et al., "Influence of a 100-Mile Ultramarathon on Heart Rate and Heart Rate Variability" https://pmc.ncbi.nlm.nih.gov/articles/PMC8126321/

  2. Fazackerley, L. A. et al., "The Effect of an Ultra-Endurance Running Race on Heart Rate Variability" https://pubmed.ncbi.nlm.nih.gov/31321510/

  3. Gifford, R. M. et al., "Recovery Time and Heart Rate Variability Following Extreme Endurance Exercise in Healthy Women" https://doi.org/10.14814/phy2.13905

Altitude and HRV

  1. Li, H. et al., "Effects of Acute High-Altitude Exposure on Heart Rate Variability: Systematic Review and Meta-Analysis" https://pmc.ncbi.nlm.nih.gov/articles/PMC12812737/

  2. Plews, D. J. & Altini, M., "Adaptation and HRV in Elite Triathletes During Altitude Training" https://hiitscience.com/hrv-altitude-training/

Wearable documentation

  1. Garmin, "HRV Status on Compatible Garmin Devices" https://support.garmin.com/en-GB/?faq=HnFAR4oFRF4kHeqYme3bU6

  2. Oura, "Heart Rate Variability" https://support.ouraring.com/hc/en-us/articles/360025441974-Heart-Rate-Variability

  3. COROS, "Overnight HRV" https://support.coros.com/hc/en-us/articles/20959044334612-Overnight-HRV

Related work