I trusted the training advice on my running watch – what happened next?

Putting Polar and Garmin’s training and recovery tools to the test
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Running watches are getting smarter. The pricier watches now take tracking to a whole new level, offering up a raft of training insights, coaching advice and recovery recommendations, all designed to help you train smart, rest properly, avoid injury and hopefully bag a PB or two.

The latest and greatest running watches including the Fenix 6, Suunto 9 and Polar Vantage V all create an incredible amount of advice, based on data from the heart rate sensor and other algorithms. And even cheaper watches like the Polar Unite and Polar Ignite use features like Fitspark to help guide your training.

Just a quick look at our guide to how to understand the data from your running watch shows what's on offer: Training Load, recovery times, Lactate Threshold, heart rate zones. Each analysed and designed to offer insight into how we can become better runners, get more from training sessions and avoid over-training.

But as these recommendation tools become more prevalent, there’s also increasing debate about whether or not what they’re telling us is reliable. If you’ve ever looked at your watch and wondered why it says your training is ‘unproductive’ despite the fact you’ve logged what feels like a solid 50-mile week, then you’re not alone.

But how do all these training effect features work? And can you really rely on the assessments these clever tools offer up?

We put two of the leading watches – the Polar Vantage V and the Garmin Forerunner 945 – to the test following 31-days of a marathon training plan to find out. Here's what happened.

What the running watches track

Before we get stuck into the test and our results, here are the main things each device offers.

Polar Vantage V

I trusted the training advice on my running watch – what happened next?

The Vantage V is Polar’s most advanced training tool to date and it has some very serious training load, running fitness and recovery features. Here’s what we looked at in our test.

Training Benefit

After every run, the Vantage V uses your heart rate data to estimate the impact that the session had on your fitness. It tells you whether it was Recovery, Basic, Steady State, Tempo, Maximum or Maximum training+ along with an explanation of the key benefits.

Training Load Pro

This feature set is designed to provide detailed insights into how your training sessions strain your body. It monitors a combination of three things:

  • Cardio load – your cardiac response to a training session, or in other words how much strain a training session puts on your cardiovascular system
  • Muscle load – the impact on your musculoskeletal system, automatically estimated from power data gathered during your runs.
  • Perceived Load – a subjective assessment you make about how hard you felt a run was.

Using this data over time, Polar creates Strain and Tolerance scores. Strain shows how much you have strained yourself in training, based on your average daily load from the past seven days. Tolerance shows how prepared you are to endure cardio training based on your load from the past 28 days.

By looking at the ratio of strain to tolerance over time, the Vantage V judges whether you are Detraining, Maintaining, being Productive or Overtraining. Too much strain and overtraining and it’ll flag that you’re an injury and illness risk and suggest you back off.

Recovery Pro

Recovery Pro tells you how well your body copes with the strain of training. It’s measured by doing an orthostatic test (three mins lying down, three mins standing up with a chest strap on) that looks at heart rate variability and your autonomic nervous system. It also looks at your training history based on cardio load and your answers to some simple questions about how you feel.

What you learn from this is whether your cardio system is Recovered or Not Fully Recovered, plus a recommendation on whether you should Train Light & Rest, Train More, Today is Good For Cardio or a combinations of these suggestions. Unlike on the Garmin, the Polar doesn’t give an estimated recovery time.

Garmin Forerunner 945

I trusted the training advice on my running watch – what happened next?

The Forerunner 945 is Garmin’s equivalent to the Vantage V. It also packs a wide range of innovative features, designed to help you train smarter, recover intelligently and optimise your running performance.

Training effect and primary benefit

Garmin’s version of Polar’s training benefit comes after each run. You'll get a readout in the post-run data of the impact of your efforts on your aerobic fitness. This includes whether the run was a Base, Tempo, Anaerobic, Threshold or VO2 Max session. It can be used to make sure you’re getting a good mix of sessions.

Training status

This lets you see if you’re training effectively by tracking your training history and fitness level trends. Readings are based on changes to your training load and VO2 Max estimate over an extended time period.

What you get on the watch is top-line feedback on the overall trajectory of your training: Detraining, Recovery, Unproductive, Maintaining, Productive, Peaking and Overreaching.

You also get a steer on whether your fitness is going up, flatlining or going down and if your cumulative load is up, flat or down.

VO2 Max and race predictor

Both watches offer estimated VO2 Max. Garmin’s is displayed on the watch while Polar’s is called Running Index and can only be viewed in the Polar Flow app and web tools

They also offer race time predictions based on recent workouts and your VO2 Max estimates, for distances from 5k to the marathon.

The training test

I trusted the training advice on my running watch – what happened next?

The aim of this experiment was simple, I wanted to see if the information my watches told me during a 31-day marathon training block made sense and matched up to how I felt and performed.

To test this, I wore the watches 24/7, except for when they needed to be charged, and followed 31 days of a marathon training plan.

Each morning I logged all of the insights my watches offered up, including the training status, training load assessments, recovery time and training recommendations.

I used the same heart rate monitor chest strap during each training session so the heart rate data was the same, and I made sure my heart rate zones in both watches matched.

After each training session, I also recorded the immediate training effect feedback, compared the VO2 Max estimates and the Race Predictor potential race times.

I also kept a training log of how I felt each during each session and each morning. Plus, we logged the sleep data for good measure.

What did we discover?

I trusted the training advice on my running watch – what happened next?

First, a disclaimer. I went into this experiment knowing that I’d almost certainly find anomalies in the data, recommendations that didn’t quite add up and things I might not fully understand. But I was also hoping to find some kind of logic to explain how the watches might occasionally come to a conclusion we might not expect.

What I found was that despite using the same heart rate data, the watches often contradicted each other. The Vantage V would say I was fully recovered and today was a good day to “Go For It” with a cardio workout while Garmin would advise me to train at an “Easy Effort”. To complicate things further, there wasn’t one watch that always matched how I felt in real life.

Despite using the same heart rate data, the watches often contradicted each other

Despite Polar and Garmin's clear efforts to make the very complicated science of training simple and protect us from needing a sports science degree, feedback discrepancies quickly became hard to decipher and explain away, without serious background knowledge that most users just don’t have.

Here’s an example. Two days into training I ran a tough threshold session (60min progression run as 20/20/20 with the last 20mins@threshold). The following day the Vantage V told me I was Overreaching (with a 1.5 ratio of strain to tolerance) and an “Injury & illness risk”. In itself that was fine, I went into training full tilt and was probably pushing too hard, too soon.

I trusted the training advice on my running watch – what happened next?

Contradicting training status data from the Polar and Garmin watches

However, it also said “Today is a good day for cardio training” and that my cardio system was “ Fully Recovered”. That felt contradictory. Should I rest? Do more cardio? How hard should I train?

The answer here is potentially that I needed to do a light cardio session, a recovery run or perhaps drop in a mobility workout. But the recommendation lacked precision and that hits your confidence in what the watch is saying.

On the same day, the Garmin tied itself up in knots too. After my hard threshold run, the half marathon race predictor improved my estimated time by 40 seconds but the watch insisted my training was “Unproductive” and my fitness was decreasing. Which raises the obvious question of just how I could be losing fitness but getting faster?

The same happened the following day when my Garmin VO2 Max dropped from 56 to 55, my fitness showed to be decreasing again, but the half marathon prediction got faster once more.

I trusted the training advice on my running watch – what happened next?

I uncovered discrepancies in my training load data too on the Forerunner 945. When it came to my training load, Garmin’s 7-day load estimate said the “Load is ideal for maintaining & improving” but on the 4-week load it warned “Your overall load is high, try scaling back the duration and frequency of workouts.” So it’s ideal and high at the same time?

According to Garmin, looking at a single week is different to a training block, in this case four weeks. So while the week might be in the right load range based on prior activity, the cumulative impact of the total stress load over four weeks could be too much. Translated: what I’m doing this week might be spot on to improve right now, but do this load long term, and I could be storing up problems.

Read this: Garmin Forerunner 945 v Fenix 5

To the untrained – or less trained – eye, that’s pretty confusing. What’s my training response, ease off this week? Carry on and have an easier week next week? Garmin’s recommendation on that day was to “Train As Usual”. Most people would take that as clearance to follow whatever’s on their plan.

Add to that the fact that the Forerunner also said my training status was Unproductive and my fitness was down and you’ve got a mix of information that leaves you wondering what on earth is going on.

Polar struggled with this too. On another day, after I took the required orthostatic test, my Vantage V told me my training was Productive. I then took a rest day as planned in my training schedule. The following day the watch told me I was ‘Overreaching’.

In this instance, my orthostatic test on the morning after my rest day may have picked up signs of stress from other areas of my life and that could have potentially triggered the watch to suggest I was overreaching but there’s no reference to that in the feedback. The watch knows I didn’t train but fails to offer up anything to explain the unexpected shift.

Witnessing the fitness

I trusted the training advice on my running watch – what happened next?

When it came to VO2 Max – a good indicator of fitness progress – the Polar’s estimates ranged from 59 to 72 while the Garmin had a much smaller range from 55 to 58. That’s a significant difference between the two.

There were further anomalies here too. For example, after a really good training session where I hit faster than marathon pace for three blocks of 15 minutes, my Polar Running Index got worse. My half marathon prediction time also got slower, in complete contradiction to the fact that I felt I ran light, strong and faster.

On one occasion, the Forerunner 945 training effect also reported that my run session had highly impacted VO2 Max, but weirdly my VO2 Max score went down.

Essential reading: Understanding VO2 Max on your wearable

The race predictor didn’t fare much better for either device. My half marathon time predictions went from 1:26:30 (Polar) and 1:42:4 (Garmin) before the training block, to 1:28:00 (Polar) and 1:32:34 (Garmin) at the end. The Polar did drop to 1:25:30 at one stage and the Garmin hit a low of 1:30:32. It’s possible that Polar took some time to get to know my fitness and adjust, though I’d been using the V for many months prior to this experiment.

What’s perhaps more puzzling though, is that during training I ran a 1:30 half marathon. It was within a full marathon training run at the New Forest Marathon, which quite clearly suggests I’d be capable of a much faster half marathon time certainly than the Forerunner – and perhaps even the Polar – predicted. Even after clocking that time, Garmin’s race predictor still had me at a 1:31:46.

Run a marathon, be a worse runner?

For that same run, I completed the 26.2 miles slower than my Polar race predictor time and my Running Index score went down compared to the previous run. Again, most people would be surprised by the idea that it’s possible to complete a marathon and be a worse runner at the end. So what was going on?

According to Polar, adrenaline and an elevated heart rate that might come with the stresses of race day can push down Running index and create a false read. As can cardiac drift during a marathon due to dehydration and lower blood capacity.

Essentially the conditions affect the physiological readings and make it look like that you’ve performed below par.

Rapid recovery

I trusted the training advice on my running watch – what happened next?

Two days after the New Forest Marathon the Forerunner and the Vantage recovery tools told me I was fully recovered and fine to train as usual. In fact, Polar suggested I was “Detraining” and Garmin said my 7-day load was “Low” and I’d be unlikely to see improvement.

In all honesty, I felt recovered so physiologically it’s possible my cardio system had bounced back and the watches were right. But I wonder whether it might be useful to enable runners to add context, to tell the watch you just raced a marathon and for it to factor that into its recommendations, for example, “You ran a marathon two days ago, it might be smart to train light or rest today”.

Read this: How to use heart rate variability to up your running game

On another occasion, immediately after a really hard interval session Garmin’s recovery recommended four days before I’d be fully recovered. Then two hours later that had dropped to 70 hours. Garmin says that the recovery readings are fluid but again it feels odd to drop a whole day’s recovery in just two hours. What’s changed to make that so?

That said, it is important to note that there were times when both watches called my recovery status really accurately. The readings matched the way I felt and I was able to adjust my plan sensibly and with good effect.

Can you trust your running watch?

I trusted the training advice on my running watch – what happened next?

I went into this experiment hoping to prove that the advanced information we’re now getting from our watches was largely reliable. Even when it appeared wrong, I wanted to be able to provide a physiological or technological reason for what might appear an odd result. So that we could all feel confident in the tools.

Thirty-one days later and I have really mixed feelings. Once you build a detailed overview of the readouts, you start to see discrepancies and contradictions that at best need detailed product knowledge to unravel, and at worst just feel like errors and bugs.

There were plenty of times where the watch matched how I felt the morning after a heavy session, or after a few days build up, and I was able to respond to those and avoid injury. But there were also far too many times where things didn’t quite stand up to scrutiny.

What also struck me is that the language used is crucial. Often the terminology isn’t quite as clear and refined as it might be and that can lead to misunderstandings about what the watch is actually telling you.

And that’s the biggest issue here, it only requires a few readouts to appear out of kilter and you start to question the validity of the rest of the data.

Having said that, neither Garmin nor Polar claim these devices are designed to replace good coaching. The recommendations are there to offer additional guidance and help you qualify how you feel, rather than lay down the training law.

So, having worn two of the most cutting edge running trackers money can buy for 31 days, my advice would be to use the watches as a sense check. Dig deep into the data and ultimately use it to get more in tune with your body.

Oh and don’t trust the race predictors. They’re nonsense.

How we test

Kieran Alger


Kieran is a world record-setting runner and one of the UK's most experienced running journalists.

A constant tester of the latest fitness technology, he's always hunting for innovations that can make him run faster, further and generally be in better shape.

Kieran is often found wearing four GPS running watches at once. And to date he's tracked more than 50 marathons, 13 ultras and countless half marathons - including the Marathon Des Sables.

In 2022, he became the first person to run the river Danube from sea to source, a measly 1,830 miles in 66 days. And still had time to test running gear.

Kieran regularly takes running tech to the extremes for Wareable and the likes of Runner's World, Mens Health and Wired.

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