The Polar Grit X: Features and first impression

I've been using the Polar Grit X since the beginning of June 2020, nearly 3 months. Things have moved on a fair bit since my original Garmin Forerunner in 2013 and one of the reasons I opted for Polar was for the HR sensor integrated in the Polar Grit, which I believe Polar are leading the way on.

Polar Grit X

As well as my previous Garmin, I have used Suunto watches in the past, which at the time was an upgrade to better assist with recovery (the Suunto Ambit 3 has it's own recovery features). I also bought the Suunto for it's ability to track swimming lengths which was handy - it's not the easiest counting your own lengths amongst other things. The Polar Grit X does both of this, but has its specialism in heart rate sensors I think and if you're training for a triathlon, or multisport event, it's not always possible to train for each sport based on pace alone. For example, a heavy cycle the day before may mean you're working relatively harder the following day on a run. That's where I think the Polar Grit X does particularly well; here I mention some of the features I have found useful and that have improved my training. They're not necessary specific to the 'Grit' and most can be found in some of the other Polar range, such as the Ignite and the Vantage, however still I'm going over them here.

HR sensors until recently have only worked effectively as a part of chest strap setup; the Polar series being one of the first watches that provides a realistic replacement with quality wrist-based HR.

A good HR sensor I felt would enable me to train more according to HR, and also importantly, gauge recovery.

Heart rate sensor, sleep tracking and recovery

Since purchasing the watch it's been quite interesting and useful to have a measure of my sleep quality. It has made me much more conscious to get a good sleep and try to get an earlier night. It does this through the optical wrist based heart rate sensor to detect the various stages of sleep and build a log of the time spent in deep sleep, REM and light sleep, every night. 

In addition, the HR monitor has been very useful for interval training. Previously I was not organised to dig out the chest HR monitor on my old watches, however with wrist based HR it's dealt with. I can therefore gauge my intervals based on HR and my own perception of effort to avoid overtraining. 

Progress monitor

Training Load Pro

With every activity, the Polar Grit determines the overall cardio load of the activity and essentially keeps track of this to build a picture overtime of your training. It calls this suite of features Training Load Pro and it's very useful. After every session, it calculates the cardio load, based on duration of the activity and HR (relative to max HR).

There two metrics used to monitor progress, 'strain' and 'tolerance'. Strain is the average cardio load from the previous 7 days, whilst tolerance is the average from the previous 28 days. Strain divided by tolerance gives your current cardio load status. Based on this it can be seen that a cardio load above 1 shows progression, the watch reports these as 'Detraining', 'Maintaining', 'Productive' and 'Overreaching':

Detraining – ratio below 0.8
Maintaining – ratio between 0.8 – 1.0
Productive  - ratio between 1.0 -1.3
Overreaching – ratio over 1.3

I think the key to getting the best out of these is having a good baseline of reasonably regular training. That way it's more obvious that you are steadily ramping up your training and adapting. For more information see the official Polar guidance on these features:

Here is an example view of the Cardio load buildup. The red bars on the daily cardio load, the blue the 28 day moving average (tolerance) and the purple the 7 day moving average (strain).

Running Index

The Polar Flow app also keeps track and displays information on your current 'Running Index'. This is effectively a measure, based on heart rate, of your current running fitness level. A lower heart rate for the same paced run, indicates a higher level of running fitness and therefore running index.

The following is an example Running Index chart from the Polar Flow app. The blue line indicating the moving average running index over time, whilst the grey dots are the calculated Running Index for each individual session. In my, case things appear to drop off and this could be down to a number of factors including the heat of summer and an increase in cycling during the summer months resulting in higher heart rates during equivalent runs.

I might be mistaken, but but one way to really benefit from the Running Index metric and to properly gauge your progress would be to stay more consistent over time with other activities which might impact running fatigue. Long term this would probably even itself out. On the whole I see the Running Index as a good way to measure progress and help judge what training works in the long run.

Note as a separate measure, muscle load is also calculated for each activity, as a measure of musculoskeletal strain, which might be higher from hills and interval sessions but not necessarily as apparent from the HR numbers.

Training Plans

Polar provide training plans that integrate with the watch. At the moment I'm following a 10K programme for 12 weeks. The  plan is synced with the watch, so when you start an activity on any given day, it asks if you want to follow the planned activity. Today I did the scheduled 4 x 4 minutes in HR zones 4-5 with 3 minutes rest. I didn't look at my pace once, mostly just listened for the beeps to keep myself with the correct HR zones. This was also important on an extremely warm day having cycled the day before, naturally my HR would be higher for a given pace, therefore gauging effort off pace alone would have been a stretch and probably left me feeling like I was missing the targets.

There are other training plans available, all based on your current level and target race distance in mind.


Overall the Polar Grit X is good looking watch that allows athletes to play closer attention to heart rate, and therefore sleep and recovery. The Polar Flow app takes a bit of getting used to, but once there it provides plenty of data to monitor progress. My favourite feature of the watch is the cardio load monitoring through Training Load Pro, I think this really helps simplify the process of incrementally increasing training load and giving an objective element to recovery and progress. 

One thing that has been a bit frustrating is the connectivity. I have a slightly old Android phone, so that might explain some of the delays and issues syncing over bluetooth. 

The HR sensor seems pretty good to me, some occasionally unusual readings and perhaps a few bits shy of my true heart rate, but we'll see with further testing.

Finally, I've still to work out how to smooth the GPS readings whilst out and about. I find it can give readings at any given moment varying by up to 1 or 1.5 minutes per mile, which is a lot and makes pacing more difficult. Overall the GPS comes in accurate at the end of the run, it's just moment to moment readings. Other watches were much more consistent in this regard. It might be I need to update the polling frequency or adjust a setting somewhere.

For more help and information on Polar watches see the official support pages at