Fenix 5 High HR Discrepancy

I think the app records the highest and lowest 1 minute (or it might even be 2 minutes) HR. Short duration HR spikes can be above this during HIIT, but if the aren't sustained, they won't show up in the HR app won't record them. The low-res graph in your screen shot suggests the peaks from your HIIT were fairly short.

The exercises were 40 secs on, 20 secs off. I need a chest strap really because my HR was reading 89bpm when I was absolutely going for it. I could feel my heart pumping faster than that! Any recommendations for a chest strap for this kind of exercise? 

I’ve always used the included chest strap that comes with Garmin watches (HRM or Performance bundle). They’ve all worked great for exercises like HIIT. 

Optical pulse reasing sucks for everything but daily pulse, especially if you have a high MaxHR.

Thanks Zebban. I kind of knew this before I bought my Fenix 5. The benefit of Garmin is that you have the option to use an external HR sensors for more accurate readings. The majority of my workouts are HIIT & Bootcamp with runs in between. So I think I'll definitely be getting one. I'll also be able to use the Lactate Threshold and HRV functions too.