Marathon training and smartwatch monitoring: the data that separates smart preparation from injury

Why HRV is the key training metric

For the fitness-watch-wearing endurance athlete there is a little understood, unspoken catch that is rarely mentioned in any training literature. For the heart rate variability (HRV) reading of your watch to be accurate you need to have some level of constant contact between your skin, and the watch sensor. A pretty simple catch, you might say, but think about it: Where are you supposed to wear the fitness watch for your long runs every weekend? The answer is of course back in your running wardrobe locker with all your other running gear. And if you are someone that actually prefers to wear a watch other than a fitness watch, say one that looks like an Omega Speedmaster, then there is a major design conflict that needs to be balanced out. There is the natural desire to wear a more attractive, and in many cases more traditional and less “geeky”, timepiece, yet at the same time you need to consider the negative impact this could have on the accuracy of your fitness watch readings as they relate to tracking your heart rate variability, particularly if you plan on wearing your fitness watch less frequently to reflect the design you prefer, and then having to deal with any unintended consequences to your training that may arise from wearing the fitness watch less frequently, particularly if that equates to you wearing the sensor less frequently as well. Speedmaster. This is not irrational. It is completely human. The problem is that HRV-guided training requires continuous overnight and morning data, not just during workouts. If your smartwatch is off your wrist three nights a week, your training plan is working with incomplete information.choice between the mechanical watch, and the training data. Both on the same wrist. Every session. Every recovery day.

A pretty simple catch, you might say, but think about it: Where are you supposed to wear the fitness watch for your long runs every weekend? The answer is of course back in your running wardrobe locker with all your other running gear. And if you are someone that actually prefers to wear a watch other than a fitness watch, say one that looks like an Omega Speedmaster, then there is a major design conflict that needs to be balanced out.

There is the natural desire to wear a more attractive, and in many cases more traditional and less “geeky”, timepiece, yet at the same time you need to consider the negative impact this could have on the accuracy of your fitness watch readings as they relate to tracking your heart rate variability, particularly if you plan on wearing your fitness watch less frequently to reflect the design you prefer, and then having to deal with any unintended consequences to your training that may arise from wearing the fitness watch less frequently, particularly if that equates to you wearing the sensor less frequently as well. Speedmaster.

This is not irrational. It is completely human. The problem is that HRV-guided training requires continuous overnight and morning data, not just during workouts. If your smartwatch is off your wrist three nights a week, your training plan is working with incomplete information.choice between the mechanical watch, and the training data. Both on the same wrist. Every session. Every recovery day.

Why continuous monitoring outperforms periodic checks

I came across a great article from a company called Runners Connect that summarized an integrated review of 55 studies on wearable technology for runners that was published in 2024. Here are the main findings for the accuracy of location-based and biometric performance metrics: - GPS pace and distance had the highest accuracy of 97% - Heart rate and heart rate variability (HRV) are biometric metrics that can be quite useful, but they require personal baselines to be relevant. The catch to baselines is that they are a function of time in order to average out the noise. Runners Connect, found that location-based metrics (GPS pace, distance) achieve 97 percent accuracy for performance prediction, while biometric metrics including heart rate and HRV require a proper personal baseline for meaningful interpretation. That phrase carries a critical condition: establishing a baseline requires continuous data collection over time. Isolated snapshots produce noise.

The catch to baselines is that they are a function of time in order to average out the noise. Runners Connect, found that location-based metrics (GPS pace, distance) achieve 97 percent accuracy for performance prediction, while biometric metrics including heart rate and HRV require a proper personal baseline for meaningful interpretation. That phrase carries a critical condition: establishing a baseline requires continuous data collection over time. Isolated snapshots produce noise.

Recovery monitoring overnight

My heart rate as I’m falling to sleep in the dead of night as measured by my smartwatch is different to my heart rate in the morning at 8am when I’m forced to participate in the IT ritual of drills to get me in the right head space to be “ready” to work. My heart rate variability (HRV) as measured in those first minutes of the day before I’m even switched on is different again to my afternoon reading/exploring/“relaxation time” measurement.

Truth continuous wear fitness

The truth is that a continuous wear fitness tracker measures the heart rate and variability when they are at their most active and therefore at their most sensitive – a time when it is rarely at all convenient for me to “tune in” to the best possible state of health and wellbeing.

The first question that comes to mind is “why does this matter?” My marathon training block is approximately 100 days long (16-20 weeks). During this block of time my body gradually increases in its capacity to handle the demands I place upon it. After many hundreds of training sessions, my body becomes adapted to the demands I’m placing upon it, fatigued, recovers, and then the adaptation process begins again.

In between these periods of adaptation and recovery I don’t tend to think too much about my resting heart rate, for example, as I have no idea what a normal reading should look like in the absence of an unusual reading. If I had access to all my physiological readings for the week I’d have some idea of my normal body functioning and so when I get a value on Tuesday that’s 5 BPM higher than normal I can switch my plans for that day from high intensity to all easy.

Your training plan will dictate what needs to be done in the workout. The continuous tracking of your training metrics will then tell you if your body is actually in a state to perform the given work on this particular day – and by “this particular day” I mean literally this day, not 3 days ago when you last checked.

HRV: the single most actionable number in your training cycle

Heart Rate Variability (HRV) is the variation in time between each heartbeat. High HRV relative to your individual range is an indicator that your autonomic nervous system is able to manage the current training load optimally and is therefore a good state. Low HRV indicates that you have accumulated too much stress, are not fully recovered or that you are on the verge of falling ill. Keep in mind that this is relative to your individual HRV range, and should use your own data as reference instead of viewing it as an absolute value.

Catching fatigue before it becomes injury

Recently a 2025 narrative review was published in an MDPI Open Access Journal Sensors titled “Athlete Monitoring Using Mobile Heart Rate Variability (HRV) Devices”. This review gives an insight into current practices being undertaken by sports scientists based on current scientific understanding that an increase in long term weekly Heart Rate Variability values is an indicator of positive adaptation to a training programme and therefore that the athlete is fitter and better conditioned. Conversely a decrease in weekly Heart Rate Variability values indicates that the athlete is experiencing the accumulation of fatigue caused by either functional overreaching or overtraining syndrome. The review therefore suggests that athletes who are involved in endurance activities should provide at least 3 to 4 morning measurements of their heart rate variability per week in order to obtain a weekly average that eliminates day to day measurement noise and instead measures physiological changes. MDPI Sensors, specifically examining athlete monitoring via mobile HRV devices, established the methodological guidelines used by sports scientists: increasing weekly HRV averages over the long term reflect positive training adaptations and improved cardiovascular fitness. Declining weekly averages indicate accumulated fatigue, functional overreaching, or the early stages of overtraining syndrome. The review recommended at least 3 to 4 morning measurements per week for endurance athletes, with weekly averages smoothing day-to-day variation that individual readings cannot reliably distinguish from physiological noise.

Conversely a decrease in weekly Heart Rate Variability values indicates that the athlete is experiencing the accumulation of fatigue caused by either functional overreaching or overtraining syndrome.

Review therefore suggests athletes who

The review therefore suggests that athletes who are involved in endurance activities should provide at least 3 to 4 morning measurements of their heart rate variability per week in order to obtain a weekly average that eliminates day to day measurement noise and instead measures physiological changes.MDPI Sensors, specifically examining athlete monitoring via mobile HRV devices, established the methodological guidelines used by sports scientists: increasing weekly HRV averages over the long term reflect positive training adaptations and improved cardiovascular fitness.

Declining weekly averages indicate accumulated fatigue, functional overreaching, or the early stages of overtraining syndrome. The review recommended at least 3 to 4 morning measurements per week for endurance athletes, with weekly averages smoothing day-to-day variation that individual readings cannot reliably distinguish from physiological noise.

We looked at a number of scientific articles, including a 2024 PMC narrative review on HRV in strength and conditioning (PMC, 2024). This confirmed that individualising the intensity of each training session using an HRV approach (i.e. where the morning HRV reading dictates session intensity rather than a pre-determined, fixed training plan being followed) was associated with greater gains in aerobic/endurance type performance. These benefits tend to be more associated with aerobic and endurance based activities such as running a marathon, and less so with power based activities. The benefit also seems to be less location dependent with benefits also being seen in studies that looked at European and Australian athletes. PMC, 2024) confirmed that HRV-guided training, where session intensity is adjusted based on morning HRV rather than a fixed weekly schedule, led to greater improvements in endurance performance compared to predefined programming in multiple studies. The benefit is most pronounced in aerobic and endurance training - precisely the category marathon preparation falls into. The same principle applies across languages and geographies: athletes in Europe and Australia following HRV-guided protocols in the reviewed studies showed the same advantage as those in North America.

where the morning HRV reading dictates session intensity rather than a pre-determined, fixed training plan being followed) was associated with greater gains in aerobic/endurance type performance. These benefits tend to be more associated with aerobic and endurance based activities such as running a marathon, and less so with power based activities.

The benefit also seems to be less location dependent with benefits also being seen in studies that looked at European and Australian athletes.PMC, 2024) confirmed that HRV-guided training, where session intensity is adjusted based on morning HRV rather than a fixed weekly schedule, led to greater improvements in endurance performance compared to predefined programming in multiple studies.

The benefit is most pronounced in aerobic and endurance training - precisely the category marathon preparation falls into. The same principle applies across languages and geographies: athletes in Europe and Australia following HRV-guided protocols in the reviewed studies showed the same advantage as those in North America.

Framework adapted from MDPI Sensors 2025And PMC 2024.

Heart rate zones and the 80/20 rule

Elite athletes follow a weekly volume distribution that places 80% of their weekly miles in Zone 2 with the remaining 20% at a higher intensity. For recreational athletes, research suggests that this is the best place to start as well. While it is difficult to maintain this 80/20 distribution, especially if you are not monitoring your daily zones, it is a good guideline to strive for when building a base aerobic foundation.

I have been trying to run using the feel method and have been unable to bring my efforts to the required level to be in Zone 2. I have thought about it and can't help, but think that it is a motivational or discipline issue. I've read a couple of articles in running for non runners, and they seem to suggest it is an issue of perception rather than motivation or discipline.

Real-time feedback during runs

Our pace will feel faster when we are tired than when we are fresh and this can make a real difference. A heart rate zone watch on the wrist can help us to realise the difference between how fast we are running and how fast we feel. This can be a discipline of its own as the watch does the work of relating the feel factor with the actual pace.

When you upgrade to the full version of Garmin training software, it will use the lactate threshold heart rate that is estimated and calculated based on your fitness level and will change as your fitness level increases, and you develop new training zones. The Training Load function utilises the acute-to-chronic workload ratio (ACWR). Workload in sports science refers to the overall amount of stress that an athlete encounters. Sports science suggests that the ACWR should be kept between 0.8 and 1.3 to be in an optimal training zone. Under-training occurs when the ACWR is below 0.8 meaning the body is not being challenged enough as the current workload is well below the athlete’s fitness level and capacity. Conversely, an ACWR greater than 1.3 increases the risk of injury to the athlete. 0.8 to 1.3 ACWR rangeAs the safe training zone. Below 0.8: undertraining relative to established capacity. Above 1.3: injury probability increases sharply.

Sports science suggests that the ACWR should be kept between 0.8 and 1.3 to be in an optimal training zone. Under-training occurs when the ACWR is below 0.8 meaning the body is not being challenged enough as the current workload is well below the athlete’s fitness level and capacity. Conversely, an ACWR greater than 1.3 increases the risk of injury to the athlete.0.8 to 1.3 ACWR rangeAs the safe training zone. Below 0.8: undertraining relative to established capacity. Above 1.3: injury probability increases sharply.

The Polar Cardio Load Status feature and the COROS EvoLab are quite similar in terms of functionality. In general, they all require that you wear your watch on the wrist for the workout, as well as for wearing to bed and during recovery.

Smartlet One Shadow Compatible with: Garmin Fenix Polar Vantage COROS Apex Apple Watch + all other training focused smartwatches up to 18-24mm lug width

Sleep, and the data you generate before every session

Sleep is the most important component of marathon training. During sleep your body repairs and rebuilds damaged muscle fibres by synthesising protein. Your brain also solidifies learning in your motor control system, which includes your muscles, nerves, and the spinal cord, and your body stores glycogen.

A single poor night of sleep is not a huge deal and may not significantly affect the following day’s performance; rather, it is more of a subtle underlying factor. In reality, the effects of a poor night of sleep are less about impacting performance on a given day and more about the environment in, which you will be running and how your body is able to recover in advance of the next night’s sleep.

The 2024 Smartwatch Sleep Edition update contains our 2024 smartwatch sleep data analysis. We have verified, using Apple Watch sleep data, that our sleep duration estimates made by smartwatch match wrist actigraphy - the gold standard clinical sleep testing method in adults. The estimates were very accurate for total sleep duration and somewhat variable for sleep stage identification. For the purpose of sleep training, where you want to know you have had enough sleep to be able to cope with the scheduled activity for the day, the important metric is total sleep.

If your heart rate exceeds your average resting heart rate by 5-7 BPM, it may indicate that your body is not feeling well, or that you are experiencing some level of dehydration or fatigue. This is mentioned in the EvoLab platform documentation by Garmin, and has been backed up by studies. COROS also mention that a climbing resting heart rate can be a sign of body stress. A huge amount of data is captured by your watch and is at your fingertips with a full breakdown in the morning report, straight from the sleep tracking section. All you need to do is go to sleep with your watch on.

The overtraining warning system you are not using yet

Overtraining syndrome is the most damaging and probably the most preventable negative consequence of marathon training. Overtraining syndrome is a physiological condition, which is brought on by overloading the body with too much physical stress. Weeks of stress must exceed weeks of recovery to create this condition.

By the time the negative effects of overtraining start to become apparent, the athlete will have already been in a state of non-functional overreaching for 2 to 3 weeks. The only way to cure overtraining syndrome is with a long period of rest in, which no training takes place and therefore no training benefits are derived.

The MDPI Sensors 2025Concept: Overtraining Syndrome Progression The original work described three levels of overtraining that progress from reversible to more permanent and clinically serious states. Functional overreaching (FOR) is associated with temporary performance decrements that are reversible upon recovery and restoration of fitness via supercompensation. Non-functional overreaching (NFOR) involves more permanent and obvious changes in performance as well as significant alterations in mood and bodily function. Overtraining syndrome (OTS) is the end stage and can potentially take months to fully recover. It has also been shown that you may transition from functional to non-functional overreaching weeks to months before you realize you are actually dropping in performance (as measured by decreases in HRV months before you actually notice you are performing poorly).

Non-functional overreaching (NFOR) involves more

Non-functional overreaching (NFOR) involves more permanent and obvious changes in performance as well as significant alterations in mood and bodily function. Overtraining syndrome (OTS) is the end stage and can potentially take months to fully recover. It has also been shown that you may transition from functional to non-functional overreaching weeks to months before you realize you are actually dropping in performance (as measured by decreases in HRV months before you actually notice you are performing poorly).

This phase of exercise adaptation is more fully captured with the type of training described in the HRV guided training section of the PMC 2024 review. It is an important phase of adaptation that needs to be caught before it becomes too expensive. The data is there, you just have to remember to wear your smartwatch on your wrist for: - Training sessions - Through the night - During rest days PMC 2024 review, produces better endurance outcomes because it catches this transition before it becomes costly. The data is available. The only requirement is that the smartwatch is on your wrist throughout the full training cycle, during sessions, overnight, and during recovery days.

The data is there, you just have to remember to wear your smartwatch on your wrist for: - Training sessions - Through the night - During rest daysPMC 2024 review, produces better endurance outcomes because it catches this transition before it becomes costly. The data is available. The only requirement is that the smartwatch is on your wrist throughout the full training cycle, during sessions, overnight, and during recovery days.

The watch collector's training dilemma

Watch collectors and serious endurance athletes know all too well the unrelenting trade-off that they face in their passion for running and fine horology. That is, whether to leave their Rolex Submariner or Omega Speedmaster behind, to spare it potential damage, or to carry it along and have nothing, but the possibility of the watch being trashed for the cost of the exercise of running with it. Going for a 25-kilometre run with an actual watch therefore amounts to a waste for the athlete, as it is without tangible benefit. The damage to the watch is irreversible. It is a sacrifice that is utterly void of reward. Rolex SubmarinerOr an Omega Speedmaster on a 25-kilometer training run exposes an irreplaceable investment to sweat, impact, and vibration while providing zero biometric data. Every kilometer is risk without upside.

Going for a 25-kilometre run with an actual watch therefore amounts to a waste for the athlete, as it is without tangible benefit. The damage to the watch is irreversible. It is a sacrifice that is utterly void of reward. Rolex SubmarinerOr anOmegaSpeedmaster on a 25-kilometer training run exposes an irreplaceable investment to sweat, impact, and vibration while providing zero biometric data. Every kilometer is risk without upside.

The common solution to this problem is to leave the mechanical watch at home and to wear only the sports smartwatch on training days. While this works to address the problem, it creates a significant source of disruption to the rhythm of watch-wearing throughout the training period: for many watch collectors with expensive timepieces, there are 16 to 20 weeks of training days each year when these watches are left in their drawer with little opportunity to be worn.

All know mechanical watches are

And we all know that mechanical watches are meant to be worn. Leaving a mechanical watch in the drawer on a training day is a day in, which the promise of the watch is unfulfilled.

Going back to the two wrist two watch method, all of the issues we had with access are resolved, but all of the design decisions we made to keep the number of straps to a minimum are lost. Also, the arm that calculates dominant or non dominant and is used to determine step and pace is on the wrong wrist for a large majority of runners.

If you are a serious collector or athlete the real cost of not having a Smartlet is 20 weeks of choosing between wearing your favourite watch or the tool that your training requires.

How Smartlet solves it

This smartwatch is worn on the inner wrist with the Smartlet Pod and the mechanical watch is worn on the outer wrist. Both on the same wrist and for all training sessions, recovery days, nights and hours when tracking sleep and HRV.

A running geek with a large collection of watches may see the “Smartlet” from a number of angles. While running in a couple of watches doesn’t look especially fashionable, that isn’t what this watch combination is designed for. Here though, it’s about having a watch that is on the wrist enough to build a reliable HRV baseline to enable accurate Recovery scores. I have a Garmin Forerunner 945, and my Apple Watch Ultra. Typically I have about 3 – 4 weeks of data on each before my Recovery scores start to accurately reflect the state of my body.

However, that’s a couple of months before it’s really time to change my watch to my Seamaster for swimming on a Saturday morning. If I leave my Garmin on the wrist for a couple of weeks longer (so that it’s on my wrist for Saturday morning’s swim), and then leave it off at the weekends, my baseline is never created. My Seamaster has to be taken off the wrist and put on a Smartlet, a strap that also holds the Garmin Garmin HR sensor close to the skin of my wrist, giving much better heart rate variability readings, whilst also keeping the watch charged.

While the wearable can be used as a PPG sensor when worn on the inner wrist, it is still in contact with the body continuously, which is required for our HRV baseline, athlete resting heart rate, as well as our sleep and zone data for runners. The accelerometer and gyroscope in the wearable still work under the laws of 3D physics whether it is worn on the wrist or in any other location and step counting, sleep analysis and fall detection all work as expected.

The most useful Smartlet-compatible training devices for marathon preparation:

All three Smartlet versions are suitable for marathon training. The Titanium version is made of Grade 2 titanium and is at the lighter end of the weight scale. It is therefore recommended if you are concerned about the weight of the wrist during a marathon. The Shadow has a design that is more in line with traditional training watches. The Classic is the entry level version that has the same modular design as the other two. Titanium (599 EUR, Grade 2 titanium) sits toward the lower end of the weight range, making it the natural choice for runners sensitive to total wrist mass during long runs. The Shadow (449 EUR, matte PVD black) suits the aesthetic of most training watches. The Classic (349 EUR, brushed SS316L) is the entry price with identical modular architecture.

The Classic is the entry level version that has the same modular design as the other two. Titanium(599 EUR, Grade 2 titanium) sits toward the lower end of the weight range, making it the natural choice for runners sensitive to total wrist mass during long runs. TheShadow(449 EUR, matte PVD black) suits the aesthetic of most training watches. TheClassic(349 EUR, brushed SS316L) is the entry price with identical modular architecture.

Smartlet One Classic Wearable Upgrade Smartlet One Classic compatible with 18 to 24mm lug width watches including most commercial fitness watches, and all major training smartwatches such as Garmin, Polar, COROS, Apple Watch and Whoop.

Frequently asked questions

How many times per week should I measure HRV during marathon training?

A 2025 MDPI SensorsThis paper was published in the journal Sensors, which is part of the MDPI group of open access journals. It is described as a narrative review. Recommendation: Record at least 3-4 blood oxygen level (SPO2) readings each morning, 7 days per week. Use weekly averages in your training program. All training smartwatches currently do this automatically, over night.

Is Apple Watch accurate enough for marathon training zone management?

Yes, for zone management. An 2024 integrated review of 55 studies examined the Apple Watch heart rate accuracy and determined that the Apple Watch heart rate is within +/- 2 to 3 bpm when standing still and +/- 5 bpm 87% of the time during exercise. For running power metrics and other multidimensional biomechanics data, a training watch specifically designed for this purpose will provide more data. See the Garmin Fenix and Polar Vantage Compatibility pages for compatible watches. Garmin FenixAnd Polar VantageCompatibility pages.

Does wearing the training watch with Smartlet affect GPS accuracy?

No. The GPS antennas are in the watch case. Not in the sensor array in the case back. And the location of the watch on the inner or outer wrist does not have an affect on the GPS signal. So all GPS watches including Garmin, Apple, Polar and COROS will maintain full GPS functionality.

Can I race with Smartlet?

Yes. Smartlet can be worn all day long during sports, training or work. On competition day, you will be able to wear your watch on the outer wrist to be read at the finish line and your training monitor on the inner wrist to receive your speed, heart rate and time in real time. See the vibrations and water resistance of your mechanical watch, if you have a high value watch for racing:Rolex compatibility pageTudor Pelagos pageSport version. Rolex compatibility pageOr the Tudor Pelagos pageFor sport-oriented configurations.

Is HRV-guided training beneficial for all runners?

Sources that are peer-reviewed and focus on endurance athletes with an aerobic training component (all marathon training programs). The PMC 2024 review on this topic concluded that the benefit of tracking HRV is largest in the context of aerobic training. Experienced athletes who know their resting HRV and its response to training and competition may not benefit from tracking daily HRV values; it is the less experienced runner who will benefit from this knowledge because they do not yet have a working knowledge of how their body responds to increased training and competition loads. PMC 2024 reviewFound the benefit most pronounced in aerobic training programs. For elite athletes or very experienced runners whose baseline is well-established, the marginal benefit of single-day HRV readings may be lower than for recreational runners who are still learning their physiological responses to load.

Experienced athletes who know their resting HRV and its response to training and competition may not benefit from tracking daily HRV values; it is the less experienced runner who will benefit from this knowledge because they do not yet have a working knowledge of how their body responds to increased training and competition loads.PMC 2024 reviewFound the benefit most pronounced in aerobic training programs.

For elite athletes or very experienced runners whose baseline is well-established, the marginal benefit of single-day HRV readings may be lower than for recreational runners who are still learning their physiological responses to load.

HRV-guided training requires data from every night, not just the nights you remember to wear the smartwatch. Smartlet keeps it on your wrist. Every session. Every recovery.