By Dr. Nicholas Shannon | Sports & Exercise Chiropractor | Shannon Clinic, Melbourne CBD
Heart rate variability (HRV) training is a buzz word among competitive athletes and active individuals. It is, however, a reading that is often misinterpreted and miss understood. This post by Melbourne sports chiropractor, Dr. Shannon covers all of it. What HRV is. How the physiology works. What the research says about its role in training, recovery, and injury prevention. How it relates to training load management. And where the evidence is strong, where it is still developing, and where you should be cautious about over-relying on a single metric.
Commonly, people understand HRV to be an indicator of physical stress, but do not know what it means or why it is an important metric for measuring athletic performance. It is therefore pertinent that individuals understand what is being measured, what that means, how to measure it properly and what its limitations are. As your wearable algorithm is not always right.
What Is Heart Rate Variability, and Why Does It Matter?
Most people think of the heart as a steady metronome where it beats at the same constant rate, beat after beat. In reality, the intervals between each heartbeat are never perfectly equal. They fluctuate, subtly but measurably, from one beat to the next. Heart rate variability is simply the measure of those fluctuations, those being the variation in the time interval between successive heartbeats, known as R-R intervals (named for the peaks visible on an electrocardiogram).
If your heart is beating at 60 beats per minute, the average interval between beats is 1 second. But those intervals might actually vary from around 900ms to 1.1 seconds from one beat to the next. That variation is HRV. Here is one important part of HRV, a heart with more variation, that being wider fluctuations in those inter-beat intervals, is generally considered healthier and more adaptable. Whereas a heart with less variation that is more rigid and clock-like, is generally associated with physiological stress, fatigue, or illness.
The Autonomic Nervous System: The System Behind the Signal
HRV is not really a heart measurement. It is a window into your autonomic nervous system (ANS), the part of your nervous system that operates largely below conscious control, regulating heart rate, blood pressure, breathing, digestion, and stress responses.
The ANS has two branches that operate in balance:
The sympathetic nervous system (SNS) often described as the “fight or flight” system which drives an increase in heart rate, redirects blood flow to working muscles, sharpens alertness, and prepares the body for physical demand. When you push hard in a training session, the sympathetic system is dominant.
The parasympathetic nervous system (PNS) is the “rest and digest” system which produces a slower heart rate, promotes recovery, supports digestion, and drives the restoration processes that allow adaptation from training. Parasympathetic activity is what allows you to actually benefit from the work you put in.
HRV reflects the balance between these two systems. When parasympathetic activity is high, your body is recovering well, managing stress effectively, and in a good state of adaptation as a result, heart rate variability is typically elevated. When sympathetic activity dominates due to high training load, poor sleep, psychological stress, illness, or accumulated fatigue, HRV tends to fall. Additionally, this drives cortisol and pro inflammatory pathways which further impede recovery.
This is the core reason HRV has attracted so much interest in sports science and performance medicine: it gives you a daily, non-invasive snapshot of where your body sits on the recovery-readiness spectrum, without needing a blood draw or a visit to a laboratory.
How Is HRV Measured? The Metrics That Matter
Walk into any sports science lab and you will encounter a bewildering array of HRV metrics derived from both time-domain and frequency-domain analysis. For clinical and practical athletic monitoring, a handful of metrics are most relevant.
Time-Domain Metrics
RMSSD (Root Mean Square of Successive Differences) is the most widely used HRV metric in sport and exercise science, and the one most commonly reported by consumer wearable devices. It measures the root mean square of the difference between consecutive R-R intervals. In practical terms, RMSSD is strongly associated with parasympathetic (vagal) nervous system activity. Higher RMSSD generally means more parasympathetic tone, better recovery, greater readiness. Lower RMSSD suggests sympathetic dominance, fatigue, poor sleep, heavy training load and/or stress.
RMSSD is preferred in athletic monitoring because it is robust to changes in breathing rate, which affects some other HRV measures, and because it can be reliably calculated from short recording windows, making it suitable for the 1–5 minute morning measurements used in most monitoring protocols.
SDNN (Standard Deviation of Normal-to-Normal Intervals) reflects overall HRV from both sympathetic and parasympathetic influences. It is a broader measure of autonomic variability and is more commonly used in cardiovascular health research than athletic performance monitoring.
Frequency-Domain Metrics
Frequency-domain analysis decomposes the HRV signal into different frequency bands:
- High Frequency (HF) power reflects parasympathetic activity and is closely tied to respiratory sinus arrhythmia, the natural rise and fall in heart rate with breathing.
- Low Frequency (LF) power reflects a mix of sympathetic and parasympathetic activity.
- LF/HF ratio has historically been used as a proxy for sympathovagal balance, though its interpretation is more contested in the current literature.
For daily athletic monitoring, RMSSD (often log-transformed as lnRMSSD to normalise its distribution) is the recommended metric. The frequency-domain metrics add value in research and clinical cardiology settings but are less practical for day-to-day training decisions.
How Long Do You Need to Measure?
Traditional clinical HRV analysis used 24-hour Holter recordings. Sports science research then moved to standardised 5-minute recordings in a controlled supine has become the “gold standard” for measuring short-term HRV. More recently, research has validated ultra-short recordings of 1–2 minutes as sufficiently reliable for monitoring RMSSD in athletic populations. Most wearables will use the RMSSD to measure HRV during sleep.
The key methodological requirements for reliable daily HRV recording are consistency: same time of day (morning, immediately upon waking), same body position, same duration. Variability in any of these factors introduces noise into the data and undermines trend interpretation.
Consumer Wearables: What Does the Research Say About Their Accuracy?
A 2025 peer-reviewed validation study published in Physiological Reports compared five popular consumer wearables; Garmin Fenix 6, Oura Generation 3, Oura Generation 4, Polar Grit X Pro, and WHOOP 4.0 against a simultaneous ECG reference across 536 nights in 13 healthy adults. Although the sample size is small, the findings are consistent with other similar studies and are relevant to anyone using these devices to inform their training decisions.
For HRV accuracy, the results were meaningfully different across devices. The Oura Gen 4 demonstrated the highest concordance with ECG reference (Lin’s Concordance Correlation Coefficient of 0.99, Mean Absolute Percentage Error of 5.96%), followed closely by Oura Gen 3 (CCC = 0.97, MAPE = 7.15%). WHOOP 4.0 showed moderate agreement (CCC = 0.94, MAPE = 8.17%). Garmin Fenix 6 and Polar Grit X Pro showed poorer HRV concordance (CCC of 0.87 and 0.82 respectively, with Polar showing particularly wide limits of agreement).
What does this mean in practice? Oura and WHOOP are currently the more reliable options for nocturnal HRV measurement. Garmin devices, despite their dominance in the Australian running market show meaningful error in HRV specifically (though their activity and GPS functions remain excellent). Polar’s chest strap remains the gold standard reference device for short-term morning recordings when used with a validated app such as Kubios or HRV4Training.
The differences lie in the underlying mechanics of how the devices use photoplethysmography (PPG) sensors to derive their HRV reading. Each manufacturer’s algorithm’s input data varies from how signal artifact is filtered, to how signal quality is interpreted, to the different weighting of the data which all impact the accuracy of the HRV reading.
One critical limitation: these wearables derive HRV from optical (PPG) sensors, not electrical (ECG) signals. ECG remains the gold standard way for measuring HRV. PPG is affected by skin tone, device fit, movement artefact, and perfusion, which can introduce noise particularly during periods of illness, alcohol consumption, temperature extremes, or changes in medication. No wearable algorithm perfectly replicates the ECG gold standard.
What HRV Tells Us About Training Status, Recovery, and Readiness
The Big Picture: What Higher and Lower HRV Mean
A single HRV reading in isolation has limited value. What matters is your individual baseline and trend over time. The established approach in sports science is to calculate a rolling 7-day average of daily RMSSD (often expressed as lnRMSSD) and use daily deviations from that baseline as the actionable signal.
A comprehensive 2025 narrative review published in Sensors (MDPI), examining HRV monitoring via mobile devices in athletes, provides a clear framework for interpreting RMSSD trends in the context of training status:
- Stable RMSSD within individual normal variation → this represents a normal training week where the the body is tolerating training load and recovering appropriately. Training can proceed as planned.
- Elevated RMSSD above baseline → typically indicates positive adaptations to training with excellent recovery and readiness for higher training stress. A green light for quality sessions.
- Moderately suppressed RMSSD (Functional Overreaching) → this would include a training camp where the training load is elevated for a short period of time leading to a suppressed HRV signalling the athletes needs to reduce intensity and prioritise recovery. Lower training load, aerobic base work, or a rest day.
- Persistently suppressed RMSSD across multiple days (Non-Functional Overreaching) → this would be an extended training block without minimal recovery periods signalling the athletes training load exceeded their adaptive capacity leading to insufficient recovery and accumulated fatigue. Increasing the rest of an athlete heading towards overtraining syndrome and requires more prolonged recovery.
This framework is not a rigid algorithm, it is a pattern recognition tool. Single daily readings are inherently “noisy” and should not dictate training in isolation. The 7–10 day rolling average is the signal; the individual daily reading is context.
HRV and Overtraining: What the Evidence Shows
A 2025 systematic review published in Physiological Reports, examining 19 studies and conforming to PRISMA guidelines, investigated the relationship between HRV parameters and overtraining symptoms in soccer players. The findings indicated there is a correlation between HRV parameters and overtraining syndrome in soccer players, suggesting HRV can be a marker of overtraining syndrome.
There were correlations between HRV parameters with VO2max (measure of aerobic performance/fitness), fatigue, muscle soreness, sleep and pain scores. Suggesting a rolling HRV reading can help coaches and athletes identify when overreaching is more non-functional and the athlete is a risk of overtraining syndrome before the more overt markers arise and performance declines.
However, the review also noted that the methodological quality of included studies was “fair” on average, with heterogeneous HRV measurement protocols and varying definitions of overtraining across studies. The directional finding (lower HRV = higher overtraining risk) is robust, but HRV alone cannot diagnose overtraining syndrome, which is a complex condition also involving hormonal, immunological, and psychological components.
HRV-Guided Training: Does Adjusting Based on Your Score Actually Work?
This is the critical question for anyone wondering whether paying attention to their Oura readiness score or WHOOP recovery score will actually change their performance trajectory.
The published evidence is cautiously encouraging, with some important nuance.
A systematic review and meta-analysis published in the Environmental Research and Public Health compared HRV-guided training against predefined training programs (ie periodized training schedule) for aerobic fitness improvements. This paper found that HRV-guided training produced a small but statistically significant improvements in aerobic capacity and endurance performance. HRV-guided training maybe more effective for maintaining and improving vagal-mediated HRV. The caveat here being that improvement is small when compared to predefined training. For the average weekly park runner or tennis player this is likely to be insignificant. For an elite athlete, these are the 1% improvements you look for.
A randomized controlled trial of 12 professional endurance runners further underscores these findings, where runners in the HRV-guided training program over 8 weeks, resulted in better cardiovascular adaptations as they were able to train at a higher intensity (higher volume and higher speeds) compared to the predefined training group.
Furthermore, a systematic review and meta analysis of randomized controlled trials of HRV training in endurance athletes found that HRV is a good indicator of physiological responses to training in endurance athletes. That is, using HRV scores in individualized training programs is an effective way to optimize performance, which was reflected in an improvement in the VO2Max in the HRV group.
The practical limitation to acknowledge: most of these trials are relatively short (6–12 weeks), involve small samples (often 10–25 athletes per group), and use highly standardised HRV measurement protocols that do not perfectly replicate what most recreational athletes do with their consumer wearables. The translation from research protocol to real-world wearable use involves additional sources of variability and error.
The evidence is sufficient to say: HRV-guided training is a legitimate and physiologically grounded approach to individualising training load, with a meaningful advantage over identical-for-everyone periodisation. It is not magic, and it is not a substitute for good programming, but it is a genuinely useful signal when measured consistently and interpreted within the broader context of training load, life stress, sleep, and subjective wellbeing.
HRV and the Acute:Chronic Workload Ratio: The Bigger Picture on Injury Prevention
In sports medicine and performance, HRV does not operate in isolation. It is most useful when understood alongside training load monitoring, and specifically the acute:chronic workload ratio (ACWR), a framework that has become central to modern injury prevention.
The ACWR compares the training load from the most recent week (acute load) against the average training load from the preceding four weeks (chronic load). The ratio provides a measure of how much the current week’s training demand deviates from the athlete’s established training base.
Research from the Norwegian School of Sport Sciences, using data from two football populations, demonstrated clearly that injury risk on any given day is highest for athletes with low chronic load who are exposed to high acute load. The interaction between acute and chronic load matters enormously. Spiking training volume or intensity relative to what your body has been consistently exposed to is where injuries happen, not simply from training a lot. Athletes with a well-established chronic load (a strong “fitness base”) can tolerate higher absolute acute loads because their tissues have adapted to training stress.
A 2021 paper looked at the relationship between the ACWR and HRV in collegiate soccer players. In that study, they found the ACWR as a measure of time (ie training / match session duration for a player) was a significant predictor of HRV. Where players who had higher acute session times were likely to have lower HRV. Although this is only one study, it indicates the complimentary nature between the ACWR and HRV in monitoring athletic load, the physiological adaptability window and fatigue levels.
ACWR helps to understand the week to week loading changes and subsequent injury risks. While HRV creates a window into the athletes physiology state; are they responding well to the current training load, are their external factors impacting their ability to tolerate the current training load, such as life stress, poor sleep / recovery etc. Used together, they can help to create a better overall physiological picture of the athlete and adaptability / injury risk profiles.
The combination of objective load monitoring and HRV trend tracking is more powerful than either alone. This multi-signal approach is what elite sports programs now use; GPS load data, session RPE, HRV, and subjective wellness scores integrated to form a daily picture of athlete readiness. The same principle applies to the Melbourne CBD runner fitting training around a demanding job, or the competitive cyclist managing commute kilometres on top of structured sessions.
What HRV Does Not Tell You (And Where Clinicians Add Value)
For all its utility, HRV has important limitations that are not always acknowledged in the enthusiast and coaching communities.
1. HRV is non-specific. A suppressed HRV reading does not tell you why it is suppressed. High training load, poor sleep, alcohol from the night before, psychological stress, early illness, dehydration, and altitude can all suppress HRV. The number alone does not differentiate between these causes that requires clinical context.
2. Individual variation is enormous. There is massive inter-individual variation in baseline HRV, making absolute numbers meaningless for comparison across people. A lnRMSSD of 4.2 may represent excellent recovery for one athlete and chronic fatigue for another. Your own trend relative to your personal baseline is all that matters.
3. HRV may miss overreaching in some training modalities. In high-intensity strength and power training, the relationship between HRV and performance or overreaching is less clear than in endurance sport. Do not assume your HRV-guided approach transfers identically from your running to your powerlifting.
4. HRV cannot replace clinical assessment of injury. A rising HRV score does not mean a healing tendon is ready to return to full load. Tissue healing has its own timeline that is independent of autonomic recovery. This is one of the most important clinical distinctions in sports practice — autonomic readiness and tissue readiness are not the same thing.
5. Wearable algorithms are proprietary and variable. The “readiness” or “recovery” scores shown by consumer devices are not the same as raw RMSSD values — they incorporate sleep data, activity history, and proprietary algorithms that may not align with the published research protocols. Use these scores as directional guidance, not definitive prescriptions.
How Shannon Clinic Integrates HRV Into Athlete Management
At Shannon Clinic, we work with a broad range of active individuals across Melbourne, from elite track and field athletes and professional tennis players to CBD professionals training for their first marathon and Pilates-going office workers managing recurrent back pain alongside an active lifestyle.
We help our athletes and active individuals manage their training loads and injury risks using the ACWR. Importantly we use time as our preferred metric from measuring load, which as was highlighted earlier, is a strong predictor of HRV. When HRV data is available, it is tied together with the ACWR to create a more holistic performance picture. Which will also include discussions around nutrition, hydration, sleep, and recovery.
For individuals who are managing their work loads correctly but still feel fatigued, have suboptimal performance or are struggling with their training, we help educate them on the important role of HRV, how to take a reading and interpret their results. To help them further understand why they are experiencing fatigue and/or performance drop off.
What we are ultimately doing is combining the objective physiological signal from HRV with a clinical understanding of tissue load, injury history, training history, lifestyle context, and physical assessment findings. The wearable gives you one piece of the puzzle. The clinical assessment completes it.
Practical Guidance: How to Use HRV Effectively
For athletes and active individuals who want to start using HRV data intelligently, here is a framework based on the current evidence:
Measure consistently. Same time (morning, immediately on waking), same position (seated or supine, choose one and stick with it), same duration (1–5 minutes). Consistency is the key with HRV readings. The Polar H10 chest strap paired with the Elite HRV, HRV4Training, or Kubios app remains the most validated approach for morning short-term recordings. If using a wearable, Oura and WHOOP currently show the strongest nocturnal HRV accuracy in the published literature.
Establish your individual baseline first. Do not act on single readings for at least 2–3 weeks. The 7-day rolling average is your reference point. Individual days are noisy.
Use trend, not single values. A single low reading is not a rest day mandate. Three to five consecutive suppressed readings below your established baseline is a meaningful signal.
Layer HRV with subjective data. Your perceived fatigue, sleep quality, mood, and motivation are independent signals that complement HRV. Research consistently shows that the combination of HRV and subjective wellbeing scores is more predictive than either alone.
Log your training load alongside your HRV. The relationship between HRV and load is the insight, not the HRV number in isolation. If your ACWR is spiking and your HRV is falling simultaneously, that is a much stronger signal to reduce load than either alone.
Do not over-prescribe based on HRV. An elevated HRV reading is a green light for hard training, not a mandate. Planned recovery is still planned recovery.
Seek clinical guidance when patterns persist. A sustained downward trend in HRV that is not resolving with reduced training warrants proper assessment — not just more rest. It may reflect illness, relative energy deficiency in sport (RED-S), accumulated psychological stress, or an underlying musculoskeletal issue that is adding to physiological load in ways that are not obvious from training data alone.
The Bottom Line
Heart rate variability is a physiologically meaningful, non-invasive biomarkers that is accessible to athletes and active individuals. The science supporting its use in monitoring autonomic recovery, guiding training load decisions, and detecting early overreaching continues to grow.
It is also a metric that is easy to misinterpret, easy to over-rely on, and subject to meaningful measurement error depending on the device you use and the consistency of your measurement protocol.
When used correctly, as one signal among several, measured consistently, interpreted against your own baseline, and understood within the context of your total training load, life stress, and physical health; HRV can genuinely change how intelligently you train and recover.
At Shannon Clinic, our sports chiropractor Melbourne help athletes and active individuals in Melbourne CBD make sense of the data their bodies and their devices are generating, integrate it with clinical assessment, and build training and rehabilitation plans that account for the full picture. Whether you are managing a niggling injury, building toward a target event, or simply trying to train consistently without breaking down, the combination of good clinical guidance and objective physiological monitoring is a powerful one.
If your HRV is telling you something and you are not sure what to do with it, or if you have been pushing through fatigue and wondering whether your body is trying to tell you something, we are here to help.
Book an appointment with Dr. Nicholas Shannon at Shannon Clinic, Suite 9.16, Level 9, 220 Collins Street, Melbourne CBD. Easily accessible by tram and train from across Melbourne. Call us or book online.
Shannon Clinic is located at Suite 9.16, Level 9, 220 Collins Street, Melbourne VIC 3000. Dr. Nicholas Shannon is one of Australia’s most qualified sports and exercise medicine chiropractors and has been providing evidence-based chiropractic and sports care in Melbourne CBD since 2007.
The information in this article is educational and does not constitute individual medical advice. For assessment of your specific circumstances, please book a consultation.
