As part of my training plan, my speed session this week was for a progressive 45 minutes. 15 minutes in HR zone 2, 15 minutes in HR zone 3 and 15 minutes in HR zone 4. The plan that I have selected is predominantly HR based which, having always strictly worked to pace in the past, has been quite refreshing. It’s adaptive and works far better on mixed terrain. Trying to maintain pace up and down hills often results in you working too hard or not hard enough for what you’re trying to achieve. I appreciate that HR is not instant and lags versus effort, but for 15-minute segments I’ve found it a better gauge to work to than pace.

Anyway, I set my watch up for the workout, meaning that it would guide me through the steps, auto-lapping when necessary and alerting me to if and when I was working in the incorrect zones. A couple of minutes into the run it alerted me to the fact that I was in zone 0.9. This could be correct I thought, as I was still warming up and was travelling downhill. A couple of minutes later, I received another alert that I was in zone 1.4. This didn’t seem right, and I wondered if I had ‘licked’ the probes on my heart rate strap. I put my hand on my chest to see if it needed repositioning and realised that I wasn’t actually wearing it, I had forgot. I briefly debated heading back but, given that I was time pressed anyway, I decided to continue and do without.

I did all the right things to improve the accuracy of the wrist-based sensor. I moved the watch back to just behind the wrist bone, and tightened the strap another notch, as opposed to its normal comfortable position right on the bone itself. From experience, and recent threshold sessions, I knew that Z2 was around 09:30/mile pace or slower and Z4 was around 7:30/mile. I assumed therefore that Z3 would be around 08:10-08:30/mile. I decided to continue with these paces as my guide and see where I was at in terms of HR zones according to the alerts from my watch.

Getting towards the tip of the first (modest) incline around 6 minutes in, the sensor finally kicked in to provide a believable reading, going from 132 (still Z1) to 152 (just into Z3). Despite the lag, again this was believable, so I slowed the pace down slightly in an attempt to drop it back into Z2. I pulled back to slower than 10:00/mile before it eventually signalled that I was in the correct zone. Next up was a gradual decline as I picked the pace back up to around 09:30. A few seconds later I got a notification that I was back in Z3 so I slowed again to bring it back down.

Following this was a jump into the next progressive split, so I increased my pace to around 08:20. Ironically, I was now struggling to get into Z3 at all, although I was heading down a long decline, so I didn’t want to force it too much at this stage. At the bottom of the ‘bowl’ I started to head back up a sharp hill, climbing just over 20 metres in the space of 4 minutes. Still struggling to get into Z3 I maintained pace of 08:10-08:20/mile. I was working relatively hard at this point to get up the hill, and yet I was still receiving alerts that I was in the wrong zone, working in Z2 as opposed to what felt more like Z4.

At this point, I decided to ignore the readings and stick to the paces that I had determined early in the run. For the final segment, I only briefly tipped into Z4 a few times before dropping back into Z3. I knew, given how hard I was working, that this was incorrect. I also knew the day after, when it felt like I had completed a hard interval session as opposed to a progressive threshold session. In truth, I had pushed too hard.

Elevation, heart rate and pace overlay

I find the optical, wrist-based HR sensor to work near perfectly when resting, and also for day-to-day general use. But activity wise, the harder you work, the less accurate that it is, in my experience at least. I was working in Z3 when travelling down a slight hill at 09:20/mile pace and in Z2 when travelling up a tough hill at 08:20/mile pace. I understand the variables and potential errors, but this is too much of a disconnect, coupled with an inability to reach Z4 when pushing hard in the final segment. This is Garmin’s Elevate heart rate sensor technology. Optical sensors still have some way to go to reach the accuracy of chest straps. One should bear in mind that, if a chest strap isn’t used, it is these questionable readings that Garmin uses for it’s lactate threshold and V02 Max predictions, not to mention the Training Peaks feature which advises on recent training progress and recommended rest periods after workouts.

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