In a previous article on Heart Rate Variability (HRV), I discussed how tracking your HRV can help optimize training by providing insights into recovery and workout readiness. That article outlined how to use the HRV data from different training devices, noting two distinct approaches, with Suunto and Garmin focusing on a 7-day rolling average, and Coros focusing on last night’s average. 

This difference isn’t just about interface design — it reflects two different approaches to HRV monitoring. Suunto and Garmin emphasize longer-term trends that filter out daily noise, while Coros prioritizes immediate, acute feedback from last night’s recovery.

With the market full of options from Suunto, Garmin, Coros, and others, this raises the question of how different devices — and different approaches to monitoring HRV — compare. To help answer this question, I did my own experiment, simultaneously tracking my personal HRV data on two devices — the Coros Pace and Suunto Vertical. 

Key takeaways from my analysis:

• Absolute HRV values differ between devices, but the trends are strongly correlated. 

• The two devices agree on recovery status 75.7% of time despite using different systems for tracking HRV.

• Most important is choosing one device/system and sticking with it to reliably track your RHR and HRV trends.

Read on for the details of the analysis and practical applications. 

Experimental Setup: Comparison of Heart Rate and HRV Data

For the experimental analysis, I collected over two months worth of data from nights of sleep wearing both a Coros and a Suunto watch simultaneously — I wore the Coros Pace 3 on my right wrist and the Suunto Vertical on my left. I specifically tracked the following data.

Resting Heart Rate (RHR)

Both the Coros and Suunto devices provide an overnight average resting heart rate.

• Overnight RHR (Coros, Suunto)

Heart Rate Variability (HRV)

Both devices provide an average overnight HRV measurement; in addition, the Suunto device provides a 7-day rolling average HRV. In my dataset, I calculated the 7-day rolling average HRV from the Coros measurements since it’s not directly provided on the device. This allowed me to compare both of these measurements across the two devices. 

• Overnight HRV (Coros, Suunto)
• 7-day average HRV (Coros, Suunto)

I also tracked my personal “normal” range provided by each device. Your personal range is based on your latest data, so there is some variability over time. This is why it’s important to consider your latest overnight HRV and/or 7-day rolling average HRV against your current “normal” range. 

• “Normal” range (Coros, Suunto)

Each device has its own system to qualitatively characterize where your HRV falls in relation to your personal “normal” range. I tracked these recovery status classifications using the native categories and labels for each device. 

Coros qualitative HRV labels: 

• Elevated
• Normal
• Reduced
• Low

Suunto qualitative HRV labels: 

• Above Normal
• Normal
• Below Normal

This approach allowed me to directly compare how these devices measure the same physiological state (mine) and whether the differences in measurement would affect training decisions.

How HRV Is Actually Measured

Both Coros and Suunto use the same underlying method to calculate HRV, called RMSSD — Root Mean Square of Successive Differences. Here’s what this means.

RMSSD measures the variations between your consecutive heartbeats. For example, if your heart beats at 60 beats per minute, you might assume each beat happens precisely every 1.0 second. In reality, there might be 1.02 seconds between one pair of beats, then 0.97 seconds between the next pair, then 1.03 seconds, and so on. This is heart rate variability (HRV).

RMSSD captures these tiny timing differences and produces a single number that represents your HRV in milliseconds (ms). Higher numbers represent more variability and generally indicate better recovery and parasympathetic (“rest and digest”) nervous system activity.

Same Measurement, Different Tracking Approaches

While both devices use RMSSD to calculate HRV, they differ in how they track and present the data.

Coros focuses on your most recent overnight average HRV reading as the primary metric. This provides immediate feedback about your recovery status based on last night’s measurement.

Suunto emphasizes your 7-day rolling average HRV (calculated from your last 7 overnight readings) as the primary metric. This approach smooths out daily fluctuations to identify more consistent patterns.

This fundamental difference in tracking approach — immediate overnight reading versus 7-day trend — influences everything from how each device defines your “normal” range to how it classifies your recovery status each day. 

Results: What The Data Revealed

Numbers Don’t Match, But Trends Do

The most obvious finding from analyzing my data was that the absolute HRV values differed between devices. 

Coros consistently read 3.5ms higher than Suunto for overnight average HRV and 3.6ms higher for 7-day average HRV measurements. For example, if Coros shows an overnight HRV of 50ms, Suunto might show a number around 46-47ms.

For RHR, the two devices occasionally disagreed by a single heartbeat in either direction, but these tiny differences balanced out, resulting in no overall difference in average readings.

Correlation Between Devices

Here is how well the measurements of the two devices aligned:

• Overnight RHR: Strong correlation between devices (0.927)
• Overnight HRV: Moderate correlation (0.789) between devices
• 7-Day Average HRV: Strong correlation (0.913) between devices

This means that while the absolute numbers differed, both devices were largely in agreement about the direction and magnitude of changes. The 7-day averages showed remarkably strong agreement, suggesting that the averaging process helps smooth out the measurement differences between devices. The correlation was extremely high (0.913) when comparing Coros’s calculated 7-day average with Suunto’s native 7-day average. 

Classification Agreement

Despite using different approaches — overnight average HRV vs. 7-day rolling average HRV — the devices agreed on my recovery classification in 75.7% of cases using the functional mapping shown below. 

• Coros “Elevated” → Suunto “Above Normal”
• Coros “Normal” → Suunto “Normal”
• Coros “Reduced” and “Low” → Suunto “Below Normal”

This agreement on three out of four occasions is actually quite good considering they’re using different tracking systems to assess and characterize recovery. 

Think of it like getting weather forecasts from two different meteorological services — one that focuses on immediate atmospheric conditions and another that emphasizes longer-term patterns. If these services agreed on whether you should expect sunshine, clouds, or rain 75% of the time, you’d likely consider either one as reasonably reliable for planning your day, even with the occasional disagreement.

Range Width

Examining the personalized HRV ranges used by each device revealed another interesting difference in their approaches. Coros had an average range width of 11.6ms compared to Suunto’s average range width of 6.0ms. This means, for example, where Coros might show my “normal” HRV range as 42-53ms, Suunto would show my “normal” HRV range as 44-50. 

This difference in range width directly reflects their philosophical approaches:

• Coros’s wider range accommodates the greater natural variability in single-night HRV measurements, preventing over-flagging of normal fluctuations while remaining sensitive to significant overnight changes.

• Suunto’s narrower range is appropriate for its 7-day average approach, where the averaging itself already reduces variability, allowing smaller but consistent shifts to be detected as meaningful changes.

This engineering design choice further demonstrates how these devices are built with different recovery monitoring philosophies in mind, right down to the width of their normal ranges.

Sample Sizes

The sample sizes provide a solid foundation for the analysis and allow for meaningful conclusions with statistical significance. 

The dataset covered 73 nights of data with three nights where I forgot to wear either one device or the other. This resulted in 70 paired data points for the HRV analysis after excluding the unpaired data. In other words, there were 70 nights where both overnight HRV and 7-day average HRV data was captured for each device.

The resting heart rate data includes 43 nights of paired data. I didn’t start to record the RHR data consistently until after the first month and I couldn’t retrieve these numbers later from the device apps. 

The correlation coefficients are likely very stable, and the classification agreement rate (75.7%) has a margin of error of ±10.1%. The core insights about device differences and their complementary approaches are unlikely to change significantly with more data even though the margin of error on the classification agreement could be reduced.

Discussion: Is One Device or Approach Better Than The Other?

Coros vs. Suunto

Based on this analysis, there’s no clear winner between Coros and Suunto for HRV tracking. Both devices are effective at monitoring recovery, but they offer different perspectives that align with different HRV monitoring approaches in the scientific literature.

Classification Disagreements

The 23.9% of days when the devices disagreed weren’t random errors — they reflected the intentional design differences between the two approaches. 

Most disagreements (58.8%) occurred when Coros showed “Elevated” while Suunto showed “Normal.” This pattern makes sense since Coros is designed to catch immediate, overnight spikes in HRV that get smoothed out in Suunto’s 7-day average. 

The second most common disagreement (29.4%) was Coros “Normal” vs. Suunto “Below Normal,” which may indicate that Suunto’s 7-day average is more sensitive to detecting prolonged downward trends.

Range Width Differences

Coros’s wider normal range means it takes a larger single-night deviation to trigger a classification change, but when it happens, it’s immediate. Suunto’s narrower range for its 7-day average means smaller shifts can trigger a classification change, but only after they’ve persisted long enough to affect the average.

Different Devices with Different Approaches

These disagreements represent different recovery perspectives. It’s not that one device is “right” and the other “wrong.” The approaches are different, but both are valid. In fact, the high correlation (0.913) when comparing the 7-day averages suggests both devices are accurately tracking the same underlying physiological changes.

While the absolute values differ between brands, both devices effectively capture changes in your recovery status over time. This is what matters most for making informed training decisions. The fact that classification systems align 75.7% of the time despite using fundamentally different approaches means that both devices are detecting meaningful physiological patterns, just through different HRV tracking systems.

Overnight Average vs. 7-Day Rolling Average

Although most endurance athletes will choose training devices for reasons other than their HRV tracking system, here are some considerations if the HRV tracking system is an important factor in your decision-making process. 

Overnight Average HRV

The approach adopted by Coros uses the overnight average HRV to guide training decisions each morning. Pioneered in early HRV research by Kiviniemi and colleagues (2007) and used in other studies (e.g., Schmitt et al. 2018), this method responds quickly to changes in your recovery status. 

Key features of using the overnight average HRV:

• More sensitive to day-to-day changes
• Provides immediate feedback on recovery status
• Better for catching acute changes in physiological state
• Wider normal range accommodates natural single-night variability
• May detect acute stressors more quickly (poor sleep, alcohol, illness)
• Often used with elite athletes who need precise daily monitoring
• May be more valuable during intensive training blocks or when monitoring responses to novel training stimuli

7-Day Rolling Average HRV

The approach adopted by Suunto uses the 7-day rolling average, where each day’s reading is the average of the current day plus the previous six days. This approach has been utilized in studies with cyclists (Javaloyes et al., 2019, 2020) and runners (Vesterinen et al., 2016; Carrasco-Poyatos et al., 2022).

Key features of using the 7-day rolling average HRV:

• Reduces “noise” from daily fluctuations unrelated to training
• Provides more stable values for decision-making
• Less likely to overreact to a single unusual reading (fewer “false alarms”)
• Narrower range allows detection of smaller but consistent changes
• Better reflects trends in adaptation rather than acute changes
• Often used with recreational and well-trained athletes

Both Provide Valuable Insights

Research doesn’t definitively favor one method over the other, but there does appear to be some preference for and a shift toward the 7-day rolling average (e.g., Plews et al. 2013; Javaloyes et al. 2019, 2020; Carrasco-Poyatos et al. 2022; Figueiredo et al. 2022). The bottom line is that the immediate feedback of overnight HRV and the stability of 7-day averages both provide valuable insights that can inform training adjustments on a day-to-day basis.

Practical Applications

Regardless of which device or HRV tracking system you use, keep the following principles in mind.

Focus on the Classifications More Than the Numbers

The classification systems (normal, reduced, etc.) are fairly consistent between devices even when the absolute numbers differ. Focus on these qualitative assessments rather than fixating on specific HRV values. 

Stay Consistent with One System

While the devices generally agree on recovery classifications, switching back and forth between brands will introduce confusion in your data. Pick one system and stick with it for the most reliable tracking over time. 

Understand the Context

Remember that HRV is influenced by many factors beyond training, including sleep quality, stress, nutrition, and illness. Use HRV as one tool among many for gauging recovery, not as the sole determinant of training decisions.

Conclusion

For endurance athletes wondering which device offers better HRV tracking, the answer lies not in accuracy but in philosophy. Both Coros and Suunto provide reliable insights into recovery status — just from slightly different perspectives.

The choice between devices should depend more on which approach resonates with your training philosophy: Do you want more immediate, sensitive feedback (Coros) or more stable, trend-focused guidance (Suunto)?

Whichever you choose, the good news is that device-specific baselines effectively normalize measurement differences, leading to similar qualitative assessments of recovery status in most cases. Different devices and HRV tracking methods can effectively guide your training if used consistently and interpreted correctly.

References

Carrasco-Poyatos, María, Alberto González-Quílez, Marco Altini, and Antonio Granero-Gallegos. 2022. “Heart Rate Variability-Guided Training in Professional Runners: Effects on Performance and Vagal Modulation.” Physiology & Behavior 244: 113654. https://doi.org/10.1016/j.physbeh.2021.113654

Figueiredo, Diego Hilgemberg, Diogo Hilgemberg Figueiredo, Clint Bellenger, and Fabiana Andrade Machado. 2022. “Individually Guided Training Prescription by Heart Rate Variability and Self-Reported Measure of Stress Tolerance in Recreational Runners: Effects on Endurance Performance.” Journal of Sports Sciences 40 (24): 2732-2740. https://doi.org/10.1080/02640414.2023.2191082

Javaloyes, Alberto, Jose M. Sarabia, Robert P. Lamberts, and Manuel Moya-Ramon. 2019. “Training Prescription Guided by Heart Rate Variability in Cycling.” International Journal of Sports Physiology and Performance 14 (1): 23-32. https://doi.org/10.1123/ijspp.2018-0122

Javaloyes, Alberto, Jose M. Sarabia, Robert P. Lamberts, Daniel Plews, and Manuel Moya-Ramon. 2020. “Training Prescription Guided by Heart Rate Variability vs. Block Periodization in Well-Trained Cyclists.” Journal of Strength and Conditioning Research 34 (6): 1511-1518. https://doi.org/10.1519/JSC.0000000000003337

Kiviniemi, Antti M., Arto J. Hautala, Hannu Kinnunen, and Mikko P. Tulppo. 2007. “Endurance Training Guided Individually by Daily Heart Rate Variability Measurements.” European Journal of Applied Physiology 101: 743-751. https://doi.org/10.1007/s00421-007-0552-2

Plews, Daniel J., Paul B. Laursen, Andrew E. Kilding, and Martin Buchheit. 2013. “Evaluating Training Adaptation with Heart-Rate Measures: A Methodological Comparison.” International Journal of Sports Physiology and Performance 8 (6): 688-691. https://doi.org/10.1123/ijspp.8.6.688

Schmitt, Laurent, Sarah J. Willis, Anthony Fardel, Nicolas Coulmy, and Gregoire P. Millet. 2018. “Live High–Train Low Guided by Daily Heart Rate Variability in Elite Nordic-Skiers.” European Journal of Applied Physiology 118: 419-428. https://doi.org/10.1007/s00421-017-3784-9

Vesterinen, Ville, Ari Nummela, Ikka Heikura, Tanja Laine, Elina Hynynen, Jussi Botella, and Keijo Häkkinen. 2016. “Individual Endurance Training Prescription with Heart Rate Variability.” Medicine & Science in Sports & Exercise 48 (7): 1347-1354. https://doi.org/10.1249/MSS.0000000000000910