Posted by: Ian | January 4, 2012

Endurance training with a Garmin 800

During the quiet holiday period I had a metabolic assessment. It was a procedure in two parts. First, I sat quietly for several minutes while breathing through a face mask into a tube and having my heart rate monitored. Next, I worked at a spin bike while wearing the same mask and heart rate monitor and the assessor steadily stepped up the difficulty, over a period of about 20 minutes, from very easy to very hard. At each step I gave my rate of perceived exertion on a scale of 1-10. No blood was taken and all of the metabolic findings were inferred solely from the analysis of my breath and heart rate. Immediately after the session the assessor, Richard, gave me a preliminary look at the results. Some while later he emailed me a report on each of the resting assessment and the exercise assessment. The reports lacked the cool charts that Richard had shown me on the day but there were a bunch of sciencey numbers.

Some of these, like VO2 Max – the maximum amount of oxygen I can process – provide interesting comparisons to age/gender norms and hence an index of fitness. More directly useful, though, are the specific exercise recommendations – the effectiveness of these will determine how valuable the metabolic assessment ultimately turns out to be to me. The first is simply a specification of the best zone for fat burning and endurance training. The second is a set of anaerobic interval sessions to improve fitness, essentially by nudging up the heart rate level at which I can still get a useful fraction of energy from fat rather than carbs and also improving my efficiency at processing lactate build up. The technique for this is to pinpoint and then exercise around my Lactate Threshold Heart Rate (LTHR).

One of the anaerobic intervals session that serves as a progress benchmark is to do a three minute warm-up then see how many times I can get my heart rate up to 90% of its max and then back down to 15 beats under my LTHR in 20 minutes. I gave this a go on the turbo trainer yesterday. At the outset I was undermined by stupid HR readings from my Garmin 800. Idly pedalling away with no effort, the Garmin would show a reading in the 220’s. Then, when trying as hard as I could to push up my heart rate, I’d struggle to get it over 140. The readings, as so often at the start of a session, were grossly wrong. This is a consistent problem that I’ve had with the 800 and, before I had the 800, with the Garmin 500. Emily has also had erroneous HR readings from both her old Garmin Edge 500 and her Garmin Edge 800, though in her case the readings were more prone to be too low rather than too high. Paula has had similar problems with her new Garmin Forerunner 610. I’ve replaced the battery in the strap and tried wetting the electrodes but I still get these dodgy numbers. I guess that Garmins need you to have been exercising for a while before the HR readings are trustworthy. Maybe they require a degree of vasodilation or maybe exercise changes the electrochemical properties of the skin. Even when warmed up, the 800 is prone to throw some stupid numbers – often, for example, when going very quickly downhill with no effort I’ll get a stupidly high HR reading.

Anyhow, by my third of fourth go I managed to get some sensible numbers on my anaerobic intervals session yesterday. Here’s how it went (the first two minutes are warm-up):

You can see that I managed to get in 11 cycles in my 20 minutes. The charts here are from Garmin Training Center. Golden Cheetah has the most/best analytics and Garmin Connect has the cleanest graphics but neither of these have enough precision to confirm, as I can see in Training Center, that my HR did indeed fall to 134 bpm – my recovery point – each time.

You can also see that in getting up to 160 bpm – 90% of my max HR – my HR in every case skidded some way past it before starting to fall. If I wanted to game the exercise to cram in more cycles I’d maybe ease off the power some beats earlier. You can further see that my recoveries became more ragged over the course of the 20 minutes. This is very much how it felt: much of the session was spent pedalling at an easy spin, waiting for my heart rate to return to 134. It’s very different from the power intervals that I’ve done before. We’ll see whether, over time, I can fit more than 11 cycles in the 20 minutes and whether this increases my LTHR.

Today, I tried the endurance exercise. My aim here was to do a moderate ride – I found a 29 mile circuit that is largely dead flat – and get round it keeping my HR in the range 110-149 bpm. Again, I had my Garmin 800 HR problem. While I was standing over the bike outside the house, when my actual HR was probably about 80 bpm, the Garmin was reading 170. In frustration, I pushed the strap into me a bit to settle it down against my skin – the Garmin rocketed up to 225. After a while I set off anyway and cycled steadily. Charting the Calories per minute against Average HR, both as recorded on my Garmin, for each mile after the ride gives this:

The first nine miles, during which time I don’t believe that the Garmin was showing the correct HR, are coloured in red. You can see that the rest of the dots are neatly arranged along a line. Removing the red points clarifies this:

I suspect that the only reason that the points don’t actually lie on the line is because I’m averaging over whole miles. If I could parse the Garmin activity file directly and plot each point it records, I believe that it would reveal that Garmin models calories per minute as a linear function of HR, using age, gender, weight and exercise frequency to set the slope.

If I compare this with how my metabolic assessment tells me that I burn calories as my HR changes, using the same mile points, I get this:

It’s apparent that the metabolic assessment found me burning more calories than the Garmin model posits, though with a very similar profile. This is borne out by the same analysis run on a longer ride where I was cycling at a higher intensity:

The cleanest way to reveal the essential difference is to take a plot such as this and fit Cals/min as a function of Average HR as in one of the charts above, but now for both the Garmin model and the metabolic findings:

Because the metabolic assessment was based on actual readings from actual me, it has more shape at the tails, though is still somewhat linear. The Garmin model is evidently dead linear (at least in this range). I’m due to have a debriefing with Richard sometime soon and I’ll ask him if he has any thoughts on why the Garmin might think I’m burning less energy than his assessment suggests I am.

Finally with the charts, it’s also interesting to see how the metabolic assessment believes that my calorie burn breaks down into fat cals and carb cals as my heart rate increases:

At <= 140 bpm or thereabouts I’m burning more cals from fat than carbs but then it switches over. On the ride charted here, if I have the maths right, the Garmin thinks I burned 2,231 cals (no maths required for that) while the logic of the metabolic reports suggests that I burned 981 cals from fat and a further 2,485 cals from carbs. Hmm.

So how useful is this? It’s too soon to say and I’ll write a follow-up in a few months. There are practical issues that will affect the extent to which I can act on the recommendations of the assessment (setting aside all of the diet and lifestyle stuff, which I’m not even going to start on). Doing the endurance session 1-2 times per week and the intervals 2-3 times per week (avoiding anaerobic work on consecutive days) plus doing strength work etc etc, as recommended, wouldn’t allow much time for the cycling that I do now and actually enjoy. I’m a recreational cyclist and not an athlete so I’ll do what I like. But I think there probably is a way to weave in some of the new routines into my pattern of activity and I’ll see how it goes.

One thing’s for sure – if I’m going to do more work on the turbo trainer I need a new exercise playlist. I’ve been shuffling through these tracks for a little bit too long:



  1. […] it when I got some proof that other equipment gave saner HR readings. I found this (a) when I had a metabolic assessment, which measured my HR max as 178 bpm, and (b) when I went on a gym bike that measured my HR through […]

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