---
title: "HRV-guided training: guide, rule and limits"
description: "HRV-guided training in practice: the rule from the studies, a worked example, the limits of the method and how to tell whether it works."
url: https://www.thefitfuturist.com/en/training-analysis/hrv-guided-training/
locale: en
author: Christopher Klenk
published: 2026-10-26T22:57:25.896Z
modified: 2026-10-26T22:57:25.896Z
---

# HRV-guided training: guide, rule and limits

Your watch or ring shows you an HRV value every morning. Most people glance at it and then train whatever the plan says anyway. HRV-guided training turns this number into a clear decision: hard session or not. It won't make you fitter faster, but you will train hard on the wrong day less often.

:::tldr
HRV-guided training uses your HRV as a brake: if your 7-day average is below your personal normal range, the planned hard session becomes an easy one.

- You calculate the normal range from three to four weeks of measurement: mean plus and minus half the standard deviation.
- For VO2max, two large analyses of eight studies each show hardly any advantage over a fixed plan.
- The main benefit: fewer athletes get worse, and there is a small advantage at threshold.
- The method works best together with resting heart rate and four short questions about how you feel.
- Whether your training works, you check every six to eight weeks with a field test, not with HRV.
:::

## What HRV-guided training is

HRV-guided training uses your heart rate variability as a brake on an existing training plan. You measure daily, calculate a weekly average and compare it with your personal normal range. If the average is below it, the planned hard session becomes an easy one.\[1\]

HRV describes how much the intervals between two heartbeats fluctuate. This is mainly controlled by the parasympathetic nervous system, the part of your nervous system responsible for rest and recovery. Its most important nerve is the vagus nerve, which slows your heart beat by beat and releases it again. When this recovery system is active, the intervals vary more. After hard days, after alcohol or with an oncoming infection, they become more even and the value drops. This is exactly the reaction the method uses.

HRV is therefore mainly a recovery marker. In the long run it is linked to fitness and health: a large analysis of 16 training studies showed that regular training raises it\[22\], and a persistently low HRV is considered a warning sign. In everyday life, though, it tells you whether today is a good day for a hard stimulus, not how fit you are. That is what makes it practical: it delivers a value every morning, costs nothing and adds no strain.

## Who benefits from HRV-guided training

HRV guidance brings the most when your plan contains hard sessions. The rule from the studies swaps a planned hard session for an easy one. The more of them are in the plan, the more often it kicks in.

As a coach, I basically dealt with two types. Some you first have to motivate to exercise, others you rather have to slow down so they don't overdo it. Anyone who checks their HRV value every morning and reads articles like this one almost always belongs to the second group. For exactly these people, HRV is useful: as a marker that tells you when to hold back, even if your head still wants more.

If you only train easy in the base zone, you hardly need the rule, but HRV can still help. If your value drops clearly for several days and your resting heart rate rises at the same time, an infection may be behind it, often before you feel it. In an analysis of Fitbit data from around 2,700 people with COVID-19, HRV dropped during the illness while heart rate and breathing rate rose, in some cases a few days before the first symptoms.\[23\] A lot of stress also lowers the value. Then you shorten the easy session too or skip it. A regular look at your watch's values is enough for that, the spreadsheet with a normal range only pays off once you do hard sessions.

The guidance becomes really useful as soon as you train in a structured way: two or more intense sessions per week, blocks with increasing load, race preparation with tapering. The more hard stimuli are in the plan, the more often one of them can come at the wrong moment.

It isn't expensive. If you wear a watch, ring or fitness band that records HRV at night, you don't need anything else. Without such a device, you measure in the morning with your phone: HRV4Training via the camera, Kubios via the motion sensor with the phone lying on your breastbone. The morning measurement becomes more accurate with a chest strap for around €60 to €100. The real effort is the daily measuring discipline over weeks.

## How to use HRV for your training

### The process at a glance

HRV-guided training runs in two phases. In the first three to four weeks you only collect data and train normally according to plan. After that, you compare your weekly average with your normal range every day and decide whether the planned hard session takes place.

![Process of HRV-guided training: four weeks of baseline, calculate the normal range, then compare the 7-day average daily. If it is below the limit, train easy instead of hard, otherwise follow the plan.](/media/variants/hrv-ablauf.webp)

In concrete terms:

1. Choose a way of measuring, morning or night, and a device or app. The options are listed in the overview further down.
2. Set up a spreadsheet with date, HRV, resting heart rate, your four ratings and confounders such as alcohol or a short night.
3. Measure for three to four weeks on at least five days per week, without deciding anything from it.
4. Calculate your normal range.
5. From week five, on every day with a hard session, compare your 7-day average with the lower limit.
6. Recalculate the normal range every few weeks and do a field test every six to eight weeks.

The daily effort: one to two minutes of measurement and one minute for notes, with night-time measurement just the notes.

### Measuring under the same conditions every day

There are two ways. One is the morning measurement: right after waking up, one to two minutes, always in the same body position and before the first coffee. The other is the night-time measurement by your device, where you do nothing except wear it.

If you wear a Garmin or Polar watch, an Oura ring or a Whoop band, night-time measurement already runs automatically, and your device already factors HRV into its recovery values. What the individual devices make of it and how close that is to the rule from the studies is in the [overview of apps and watches](#apps-and-watches-that-calculate-your-normal-range) further down.

Both ways work, but not mixed. Morning and night-time values are not the same quantity and don't belong in the same baseline. Choose one way and stick with it. The same applies to the device: a constant measurement error cancels out because you only compare against your own range. After switching devices, your baseline starts from scratch.

A study with trained triathletes compared how many measurement days per week are needed. A single day showed a hard overload phase only about half as clearly as the average of three or more days. From three randomly spread measurement days on, the weekly average was practically as meaningful as with seven.\[8\] **Still plan for five**, because in everyday life a day will drop out now and then, and then you are still above the minimum.

Many outliers don't arise in the body but during measurement. How to measure cleanly, which device suits which approach and what distorts the value is covered with a checklist in our article [How to measure HRV](https://www.thefitfuturist.com/en/training-analysis/how-to-measure-hrv/).

### Baseline: the first three to four weeks

Before you derive any decision from your HRV, you need a baseline: three to four weeks of regular measurement without drawing conclusions from it. During this time you learn what your values normally look like and how much they fluctuate for you.

The manufacturers do something similar. Garmin only shows HRV status after about three weeks of night-time data, Polar compares your last night with your average over the previous 28 days.\[16, 17\] They just tell you less often why.

### Calculating your normal range

There is no good HRV value. Absolute values say almost nothing when comparing two people. An RMSSD of 45 milliseconds can be excellent for you and conspicuously low for someone else.

What counts is the deviation from your own normal range. The studies calculate it very simply: mean of your baseline plus and minus half the standard deviation.\[4, 5\] That gives you a corridor. As long as you stay within it, everything is normal.

You don't compare your daily value but your rolling seven-day average. Individual days fluctuate too much to base a training decision on them.

This is what it looks like with example numbers:

| Step | Calculation in the spreadsheet | Example |
| --- | --- | --- |
| Baseline mean | AVERAGE of the 21 to 28 baseline days | 60 ms |
| Standard deviation | STDEV of the same days | 8 ms |
| Lower limit | Mean minus half the standard deviation | 56 ms |
| Upper limit | Mean plus half the standard deviation | 64 ms |
| Today | AVERAGE of the last 7 measurement days | 54 ms |
| Decision | 7-day average below the lower limit? | Yes: easy instead of hard |

The studies mostly work with the log-transformed value, lnRMSSD. If you want to do the same, put =LN() in front of each daily value in the spreadsheet and then calculate exactly the same way. Recalculate the baseline every few weeks, because your normal range shifts with training.

:::info[tip]
**Quick explainer: RMSSD and lnRMSSD.** RMSSD measures how much the intervals between successive heartbeats vary. The more active your parasympathetic system, the higher the value. The "ln" in front just means the value is log-transformed so it is easier to calculate with. Most devices and all studies cited here use RMSSD. The Apple Watch is the exception, more on that below.
:::

### When to adjust your training

If your seven-day average is within the corridor or above it, you do the planned hard session. If it is below, it becomes an easy session or a rest day. That is the whole rule, and it has exactly one trigger.

This one-sidedness is often overlooked. The rule never tells you to train harder than planned. Load is taken away, never added. Your training plan remains the plan, HRV is only the brake.

This is where research and devices differ. Your watch reports "Readiness 82, here is your workout for today" and actively suggests a load. That is a different approach, even if the same number is behind it.

### When the value stays low for days

One or two days below the corridor are normal, for example after a hard block. If the average stays down for a week or longer although you are already training easy, swapping individual sessions is no longer enough. Then a recovery week makes more sense, and you should check what is going on outside of training.

Because HRV doesn't only react to training. Alcohol the evening before lowers night-time values noticeably, as does an oncoming infection, heat in the bedroom or a hard training day. In women, HRV also fluctuates with the menstrual cycle and is usually somewhat lower in the second half of the cycle. A short night, travel or a late meal affect some people clearly and others hardly at all, only your own log will show you. Knowing this, you won't interpret a single outlier as a training problem.

### Apps and watches that calculate your normal range

You don't have to calculate the normal range by hand. Many watches, rings and apps compare your values with your average themselves, but not all of them the way the studies do:

| Option | How you measure | What it does for you | What to watch out for |
| --- | --- | --- | --- |
| Garmin, HRV status \[17\] | At night with the watch | 7-day average and baseline range, after about three weeks | For the rule, only the average compared to the range counts, not the workout suggestions. |
| Polar, Nightly Recharge \[16\] | At night with the watch | Comparison of the last night with 28 days | Only rates one night, you calculate the weekly average yourself. |
| Polar, orthostatic test \[18\] | In the morning, four minutes lying and standing, with an H9 or H10 chest strap | Comparison with your normal range from four weeks | Only on Polar watches with this feature. |
| Oura \[19\] | At night with the ring | 14-day average compared to three months | Reacts more sluggishly than the rule from the studies. Subscription required. |
| HRV4Training \[20\] | In the morning, one minute with phone camera or chest strap | Normal range, daily recommendation and questions about how you feel | One-off €9.99 (as of October 2026). Discloses its method. |
| Kubios HRV \[21\] | In the morning lying down with a chest strap or the phone on your breastbone | Personal readiness value | Free, doesn't work with the Apple Watch. |
| Apple Watch | At night and during breathing exercises | Only SDNN, no normal range | HRV4Training can analyse the values from Health. |

HRV is also already part of your device's daily values: Garmin uses HRV status for training readiness, Oura uses HRV balance for its readiness score, Whoop uses HRV for its recovery value. These scores mix HRV with sleep, load and other values, though, with a weighting the manufacturers don't disclose. For the rule from the studies, you therefore look at the HRV value itself, not at the score.

If you already wear a Garmin or Polar watch or an Oura ring, you don't need anything new: enter the night-time values into your spreadsheet and apply the rule. If you prefer measuring in the morning, HRV4Training or Kubios with a chest strap will serve you well. How differently apps implement the same concept is shown by the [study of 206 HRV apps](https://www.thefitfuturist.com/en/news/hrv-apps-study/), and how little manufacturers reveal about their metrics is covered in our review of the [fourteen readiness scores on the market](https://www.thefitfuturist.com/en/news/is-there-a-readiness-score/).

## Combining HRV with resting heart rate and how you feel

HRV guidance brings the most together with resting heart rate and a short self-assessment. The clearest evidence for this comes from a 2025 study with 28 experienced cyclists over 40 days.\[6\] They were split into three groups, each training according to different values:

| Group | Guided by | Participants | Clearly improved |
| --- | --- | --- | --- |
| 1 | HRV | 8 | No performance test |
| 2 | HRV and self-assessment | 12 | One- and five-minute power |
| 3 | HRV, self-assessment and resting heart rate | 8 | Five- and twenty-minute power, FTP absolute and per kilogram |

The groups were small, they were men only, and there was no control group with a fixed plan. Also, HRV as the only guiding value has worked in other studies, for example in well-trained cyclists.\[4, 5\] So the study doesn't show that HRV alone is ineffective, but that the combination gets more out of it. This fits the research on overtraining: there, too, you need further values to interpret changes in HRV correctly.

### Measuring resting heart rate as well

Resting heart rate is the most underrated number in this field, and it is the most reliable. In a comparison with ECG over 536 nights, four of the five devices tested were on average only one to just under two beats per minute off for night-time resting heart rate. A deviation only becomes medically relevant from about five beats. For HRV, the same devices were much further apart.\[13\]

It helps exactly where HRV alone is not clear. Resting heart rate also reacts to circulatory strain, oncoming infections and dehydration. A clearly raised resting heart rate is a reason to dial it back, even if HRV looks unremarkable. Why this metric is so much better than its reputation is explained in more detail in our [assessment of the FDA's relaxed rules for wellness wearables](https://www.thefitfuturist.com/en/news/fda-wellness-wearables-oura-classification/).

### A short questionnaire in the morning

The self-assessment is the cheapest part of the system. Four questions are enough, each on a scale of one to seven:

- How well did you sleep?
- How tired are you?
- How stressed are you?
- How sore are your muscles?

Write down the answers before you look at your device. If you see the number first and then rate how you feel, you are no longer assessing independently.

If you also record after training how hard the session felt, RPE for short, the rating of perceived exertion, you get the second half of the picture. HRV tells you how your body reacted to yesterday. RPE tells you how costly yesterday was for you. Together they show whether load and response match.

### When HRV and feeling contradict each other

Your HRV is in the basement, but you feel great. Both can be true, because feeling and HRV measure different things. Your feeling reflects motivation, mind and muscles. HRV shows how busy your nervous system is right now and often reacts earlier, for example to an infection you don't feel yet. Conversely, a large review concluded that self-assessments often indicate training load more sensitively than measured values.\[15\] So both signals count.

This is how to proceed in such a case:

- Check whether only today's value is low. A bad morning with an unremarkable 7-day average triggers nothing.
- Ask yourself whether there is a confounder: alcohol, a late meal, a short night, a measurement done differently than usual.
- Add resting heart rate. If it is also clearly raised, the hard session is cancelled, no matter how good you feel.
- If resting heart rate is normal, it is a borderline case. In the study with the 28 cyclists, hard training only took place when HRV and self-assessment were both within the normal range.

In a borderline case, you start the session with a stop criterion. If the first interval feels as usual and heart rate and power match, you carry on. Otherwise it becomes an easy session. If you tend to be one of those who need slowing down, the threshold for stopping should be low.

The opposite case is simpler: HRV looks good, but you feel flat. Then your feeling counts, because with overload HRV can even rise.

## What the studies show

Two large analyses from 2021 each summarised eight studies in which HRV-guided training was compared against a fixed plan.\[2, 3\] The result is sober, but very useful for practice.

### Effect on VO2max

For maximal oxygen uptake and endurance performance, neither analysis found a notable advantage over a fixed training plan. The differences were so small that they could just as well be chance. Anyone selling HRV guidance as a shortcut to more VO2max doesn't have the evidence on their side.

### Effect on performance at threshold

It looks different below maximal performance, in the range of your threshold. Here one analysis found a small to moderate advantage for HRV guidance that stood out clearly from chance.\[2\] The other saw the same direction, but not clearly enough to be sure. The only clear finding there was that HRV itself improved more under HRV guidance.\[3\]

So the method doesn't make you more capable at maximum. It rather helps where most recreational athletes actually perform: in the range of the threshold.

### Fewer athletes without progress

The best-supported benefit is not a performance gain, but that fewer athletes get worse. Under HRV guidance there were fewer participants who made no progress at all and more who improved.\[2\] In a study with well-trained cyclists, one in seven got worse in the HRV group, three in eight in the fixed-plan group.\[5\]

That sounds unspectacular, but it is the most important point for practice. Anyone who rigidly follows a plan will at some point train hard at the wrong moment. That is exactly what the method catches: the hard sessions land on days when your body can handle them. It doesn't reliably protect against real overtraining, though, because with severe overload HRV can even rise. More on that in the next section.

## Rising HRV with overload

A rising HRV can mean good adaptation or beginning overload. Every recovery value that only shows you one number has to deal with this, and it has been described since 2013.

A study with overreached triathletes found an overly active parasympathetic system in them.\[9\] A study of eleven runners confirmed this: one week of light training, two weeks of heavy training, ten days of tapering. RMSSD after an identical test run rose precisely in the heavy phase.\[10\]

The authors name the problem themselves. The same rise also indicates successful adaptation. They suspect that the overly active parasympathetic system prevents you from fully ramping up in competition. Your performance drops while your number looks good.

You therefore need a warning pattern instead of a threshold value. You only see the difference when you put HRV next to performance, feeling and the fluctuation of your daily values:

| Signal | Good adaptation | Warning sign of overload |
| --- | --- | --- |
| HRV | Rises | Rises |
| Daily values | Fluctuate less | Fluctuate more |
| Performance | Stable or better | Drops |
| Feeling | Fresh | Flat |

The rule from the previous section has a gap here. It only brakes when HRV is low. In the overload case it doesn't kick in, because the value points upwards. Only looking at performance and how you feel closes this gap.

## HRV in strength training

After heavy strength training, HRV is back to baseline after about 24 hours, your muscles need up to 48. That is why the method has to be assessed differently in strength sports than in endurance sports.

That HRV reflects strength load is established. A study with 37 well-trained athletes ran over four days of baseline, six days of overload block and four days of recovery.\[11\] After the strength block, resting heart rate rose and HRV dropped, and in the recovery phase both reversed. The link between these changes and performance, however, was weak and inconsistent. The number moved, but said little about what the athlete could achieve that day.

### Why HRV doesn't show muscle fatigue

The reason is shown by a study with ten strength-trained men after six sets to muscle failure in squat, bench press and lat pulldown. HRV was back to baseline after 24 hours. Strength needed 48 hours, and the subjective feeling of recovery was still not back even then.\[12\]

![Recovery after heavy strength training: HRV is back to baseline after 24 hours, strength after 48 hours, the feeling of recovery takes longer than 48 hours.](/media/variants/hrv-kraft.webp)

**Your HRV reports recovery while your muscles are not there yet.** In endurance training these recovery curves run largely in parallel, in strength training they don't.

This fits a study with 70 untrained adults over eleven weeks. Both training groups were guided by HRV, one self-guided via an app, the other by a personal trainer. For endurance there was no clear difference between the two groups, for strength the trainer group was clearly superior.\[7\] That is probably due less to HRV than to the coaching, for example with technique and weight progression. But it shows that HRV guidance alone is not the decisive piece in strength training.

### What else to watch in strength training

Two additions make sense:

- A performance measure from the training itself: the velocity of your first rep at a fixed weight, or simply how the warm-up weight feels.
- The orthostatic test, where you measure first lying down and then standing. In the study with the 37 athletes, the load from interval training only showed up in the standing measurement, lying down it remained invisible.\[11\]

## How to tell whether it works for you

Check the effect with a field test, not with your watch's estimated VO2max. An independent test of the Garmin Fenix 6 found an average error of around seven percent\[14\], while a real improvement in trained athletes is often only a few percent. Why that is so is explained in [Can AI estimate my VO2max?](https://www.thefitfuturist.com/en/training-analysis/can-ai-estimate-my-vo2max/)

A useful test is one you can repeat:

- On the bike, your twenty- or five-minute power.
- For running, your threshold pace over a fixed distance.
- In strength sports, the velocity at a fixed weight or a test with a set number of reps.

Do the test every six to eight weeks under comparable conditions. If you train with a chest strap, you can also determine your aerobic threshold with our [DFA α1 calculator](https://www.thefitfuturist.com/en/tools/dfa1/).

If you already wear a watch or ring that records HRV at night, you don't need anything new: measure for four weeks, write down resting heart rate and your four ratings, then apply the rule. And if your app doesn't tell you how it calculates, use it as a data source and make the decision yourself.

## Sources

1. Schaffarczyk M, Sperlich B: Heart rate variability-guided endurance training: evaluating strengths, weaknesses, opportunities, and threats for load prescription and adjustment. Frontiers in Sports and Active Living, 2026. [doi 10.3389/fspor.2026.1858271](https://doi.org/10.3389/fspor.2026.1858271)
2. Düking P, Zinner C, Trabelsi K, Reed JL, Holmberg HC, Kunz P, Sperlich B: Monitoring and adapting endurance training on the basis of heart rate variability monitored by wearable technologies: A systematic review with meta-analysis. Journal of Science and Medicine in Sport, 2021. [PubMed 34489178](https://pubmed.ncbi.nlm.nih.gov/34489178/)
3. Manresa-Rocamora A, Sarabia JM, Javaloyes A, Flatt AA, Moya-Ramón M: Heart Rate Variability-Guided Training for Enhancing Cardiac-Vagal Modulation, Aerobic Fitness, and Endurance Performance: A Methodological Systematic Review with Meta-Analysis. International Journal of Environmental Research and Public Health, 2021. [doi 10.3390/ijerph181910299](https://doi.org/10.3390/ijerph181910299)
4. Javaloyes A, Sarabia JM, Lamberts RP, Moya-Ramon M: Training Prescription Guided by Heart-Rate Variability in Cycling. International Journal of Sports Physiology and Performance, 2019.
5. Javaloyes A, Sarabia JM, Lamberts RP, Plews D, Moya-Ramon M: Training Prescription Guided by Heart Rate Variability Vs. Block Periodization in Well-Trained Cyclists. Journal of Strength and Conditioning Research, 2020. [doi 10.1519/JSC.0000000000003337](https://doi.org/10.1519/JSC.0000000000003337)
6. Alfonso C, Clarke D, Capdevila L: Individual training prescribed by heart rate variability, heart rate and well-being scores in experienced cyclists. Scientific Reports, 2025. [doi 10.1038/s41598-025-13540-z](https://doi.org/10.1038/s41598-025-13540-z)
7. Casanova-Lizón A et al.: Impact of heart rate variability-based exercise prescription: self-guided by technology and trainer-guided exercise in sedentary adults. Frontiers in Sports and Active Living, 2025. [doi 10.3389/fspor.2025.1578478](https://doi.org/10.3389/fspor.2025.1578478)
8. Plews DJ, Laursen PB, Le Meur Y, Hausswirth C, Kilding AE, Buchheit M: Monitoring Training With Heart-Rate Variability: How Much Compliance Is Needed for Valid Assessment? International Journal of Sports Physiology and Performance, 2014. [doi 10.1123/IJSPP.2013-0455](https://doi.org/10.1123/IJSPP.2013-0455)
9. Le Meur Y, Pichon A, Schaal K, Schmitt L, Louis J, Gueneron J, Vidal PP, Hausswirth C: Evidence of Parasympathetic Hyperactivity in Functionally Overreached Athletes. Medicine and Science in Sports and Exercise, 2013. [doi 10.1249/MSS.0b013e3182980125](https://doi.org/10.1249/MSS.0b013e3182980125)
10. Bellenger CR et al.: The Impact of Functional Overreaching on Post-exercise Parasympathetic Reactivation in Runners. Frontiers in Physiology, 2020. [doi 10.3389/fphys.2020.614765](https://doi.org/10.3389/fphys.2020.614765)
11. Schneider C, Wiewelhove T, Raeder C, Flatt AA et al.: Heart Rate Variability Monitoring During Strength and High-Intensity Interval Training Overload Microcycles. Frontiers in Physiology, 2019. [doi 10.3389/fphys.2019.00582](https://doi.org/10.3389/fphys.2019.00582)
12. Flatt AA, Globensky L, Bass E, Sapp BL, Riemann BL: Heart Rate Variability, Neuromuscular and Perceptual Recovery Following Resistance Training. Sports, 2019. [doi 10.3390/sports7100225](https://doi.org/10.3390/sports7100225)
13. Dial M et al.: Validation of nocturnal resting heart rate and heart rate variability in consumer wearables. Physiological Reports, 2025. [doi 10.14814/phy2.70527](https://doi.org/10.14814/phy2.70527)
14. Carrier B, Marten Chaves S, Navalta JW: Validation of Aerobic Capacity (VO2max) and Pulse Oximetry in Wearable Technology. Sensors, 2025. [PubMed 39797066](https://pubmed.ncbi.nlm.nih.gov/39797066/)
15. Saw AE, Main LC, Gastin PB: Monitoring the athlete training response: subjective self-reported measures trump commonly used objective measures: a systematic review. British Journal of Sports Medicine, 2016. [PMC4789708](https://pmc.ncbi.nlm.nih.gov/articles/PMC4789708/)
16. Polar: Nightly Recharge White Paper, 2019, and Polar Support: Nightly Recharge recovery measurement.
17. Garmin: HRV Status, Health Science.
18. Polar Support: Orthostatic test, user manuals Vantage and Grit X. [support.polar.com](https://support.polar.com/e_manuals/vantage-v3/polar-vantage-v3-user-manual-english/orthostatic-test.htm)
19. Oura Support: HRV Balance. [support.ouraring.com](https://support.ouraring.com/hc/en-us/articles/360057791533)
20. HRV4Training: FAQ on measurement, devices and integrations. [hrv4training.com/faq](https://www.hrv4training.com/faq.html)
21. Kubios: Kubios HRV app and smartphone measurement. [kubios.com/hrv-app](https://www.kubios.com/hrv-app/)
22. Amekran Y, El hangouche AJ: Effects of Exercise Training on Heart Rate Variability in Healthy Adults: A Systematic Review and Meta-analysis of Randomized Controlled Trials. Cureus, 2024. [PMC11250637](https://pmc.ncbi.nlm.nih.gov/articles/PMC11250637/)
23. Natarajan A, Su HW, Heneghan C: Assessment of physiological signs associated with COVID-19 measured using wearable devices. npj Digital Medicine, 2020. [doi 10.1038/s41746-020-00363-7](https://doi.org/10.1038/s41746-020-00363-7)