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* '''Steps'''. The step counting in the Garmin 935 is comparable to other devices. When you're walking or running its pretty accurate, but there are other situations where it's a genuinely ambiguous as to whether you are taking steps or not, such as preparing food in the kitchen. I found that the Garmin 935 recorded a very similar number of daily steps as other Garmin devices I wore on the other wrist, which gives me some confidence in their consistency at least.
* '''Floors Climbed'''. The Garmin 935, along with a number of other recent Garmin watches, use the barometric altimeter to determine when you're going up or down steps. This is a rather nice idea, but the accuracy seems to be rather poor. I find the Garmin 935 will report a different number of steps ascended to descended, even though I've never used an elevator on those days. There's also a significant discrepancy between Garmin watches worn concurrently.
* '''Intensity minutes'''. It's not clear how Garmin calculates "intensity minutes", but it seems to use heart rate to determine intensity, with high levels of intensity counting as a multiplier on the actual number of minutes. I've seen some strange discrepancy in intensity minutes when using the optical heart rate monitor[[Optical Heart Rate Monitoring]], which is not surprising given the accuracy issues. I suspect that changes in the heart rate zone also impact intensity minutes, but the lack of transparency in this calculation makes it hard to evaluate.
* '''Sleep'''. The estimation of sleep can be a little flaky on occasions, but overall it seems to do a reasonable job. It's hard to verify the estimation of sleep, other than checking if the bedtime and rising time seem reasonable. I'm not convinced by the Garmin estimate of how much deep and light sleep you are getting, and I tend to ignore that information as suspect. I have noticed that changing time zone can really mess up the sleep calculation, but hopefully Garmin will resolve this at some point.
* '''Resting Heart Rate'''. It uses two methods of detecting resting heart rate. If you wear the watch overnight with the OHRM enabled, it will be the lowest heart rate it sees while you're asleep. If not, then it will be the lowest one minute average it sees during the day. The value it records seems a little higher than I would expect, and I've seen some discrepancies when I'm wearing the 935 and the Fenix 5X concurrently. I keep an eye on the reported resting heart rate, as this is a potential red flag for [[Overtraining]] or other health problems, but I'd only be concerned if I see a consistent rise over several days.
Garmin claims the 935 battery will last 24 hours in normal GPS mode, which matches my testing result of 24.5 hours under optimal conditions. That's tough to achieve in the real world, as you have to avoid using the map (breadcrumb) display, backlight, or changing the display too often. This makes the 935 a great candidate for shorter races (50 miles/100k). But if you hope to be still moving during the [[Second Dawn]], then there are probably better options. You can charge the 935 on the run, but the position of the charging cable means you can't wear the watch while you're doing so (unlike the [[Garmin Fenix 3]].) See [[Best Running Watch#Watches for Ultrarunning| Watches for Ultrarunning]] for more details.
=Training Advice (Firstbeat)=
probably the biggest area of change between the [[Garmin Fenix 3]] and the 935/Fenix 5X is in the area of training advice. Garmin license the software from Firstbeat, which uses heart rate and [[Heart Rate Variability]] to model various aspects of your training. In theory, this could be of a massive value to a runner, either replacing or augmenting coaching advice. George Box once said "[[All models are wrong| all models are wrong, but some are useful]], and sadly, I'm not convinced that the advice produced is useful. Part of the problem is that because Firstbeat requires accurate heart rate information, the inaccuracies of the [[Optical Heart Rate MonitorMonitoring]] built into the Garmin 935 means that "garbage in, garbage out" is particularly appropriate. Using a chest strap heart rate monitor can at least give the Firstbeat algorithms valid data to work on, though even then I've been solidly unimpressed with the results.
<gallery widths=300px heights=300px>
File:Fenix 5X 06.png|I've found the Firstbeat estimate of VO2max is generally pretty good for me, though it's no substitute for a true test. Changes in this VO2max estimate are used by other Firstbeat training metrics.
</gallery>
===Heart Rate Variability Recording===
Unlike earlier Garmin watches, you can turn on Heart Rate Variability (HRV) recording using a menu action, though the menu is a little buried. There's no HRV recorded when using the [[Optical Heart Rate Monitor Monitoring]] (ORHM), only an external sensor, which makes sense given the issues with OHRM. (See [[Garmin Fenix 5X]] for how to navigate the menus to activate HRV logging.) The Garmin 935 will record [[Heart Rate Variability]] (HRV) and export it in FIT format. This can be processed in [https://runalyze.com Runalzye] or other similar software. There's no real-time display of HRV, but that functionality is pretty rare.
==Stryd, Footpods & Cadence==
The Garmin 935 has first-rate support for the [[Stryd]] Footpod (see above), and also supports the standard [[Footpod]]'s. A standard Footpod will give better Cadence accuracy than the built-in accelerometer but to the Garmin 935 does a remarkably good job, and it's generally within a few steps per minute. The Garmin 935 also features a metronome to help improve your cadence, another really nice feature.