Weird VO2 Max test results - drop in calorie usage just below LT2. Ideas?

Hello all. Long time lurker, first time poster! So much knowledge on here I thought it would be the best place to post this odd question…

I did a couple of ‘VO2 Max’ tests at a local triathlon store recently, two months apart. The guy who runs the shop has been an IronMan competitor since the year began with a 1, and has carried out a lot of these tests. He couldn’t explain something weird in my results though, which happened in both cases.

The test is a standard ramp on a WattBike Pro, with resistance increasing at 1 minute intervals until failure. The red line in the plots below is heart rate (ignore this on the second graph, I felt terrible that day!), with the grey line being calorie expenditure as measured / calculated by a medical-grade gas analyser. First test was start of April, second test the end of May. For info, my LT1 is around 125bpm and LT2 / Threshold around 155bpm.

April test:

May test:

So breathing rates continue to climb as you’d expect, with ventilation to match, but there is a reduction in calorie requirement just below LT2, highlighted in yellow… weird. As I mentioned, the chap running the test said he’d not seen it before and couldn’t explain it.

Has anyone seen / experienced this before, or have any ideas why this could be happening? I have a theory, but will keep that under wraps for now…

Thanks in advance!

Never seen this before, but could be related to substrate utilisation.

Do you have the Respiratory Exchange Ratio (RER) data?

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Have you focused on a lot of sweetspot? No clue here to be honest but maybe your body has gotten really efficient specifically at that…?

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I wonder is it something to do with your breathing rate. You reach peak exhalation per breathe at that point. If during that minute your were taking deep breaths (something I find myself doing as I come under pressure in ramp tests), it would reduce the CO2 getting cleared, pushing down RER and making it looks like you’re expending less KCals.

In the next minute your respiratory rate starts to increase pretty rapidly afterwards which would help clear CO2, pushing up your RER - appearing as higher KCal burn.

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Look at the relative vo2 column, numbers drop for the same stages. Look at the absolute vo2 column, numbers show normal increase. This means the weight calculation changed. Bet the tech leaned on the keyboard. Can be safely ignored, or “correct” the numbers and add an asterisk for future reference.


At the exact same point, in two tests with a month between them?

Probably lost your seal on the mouthpiece when you were starting to breathe harder.

Thanks for the comments and suggestions everyone!

@d_diston - this was my theory too. Sadly no RER data, but this is something I am now keen to investigate having researched it a little (thank you for drawing my attention to this metric - I’m new to all of this!).

To try and see if there was some sort of substrate usage, I completed a (total of) 20 minute @ 120% FTP VO2 Max session to make sure I was pretty dead, then tried some 30/30s under-overs with a target average HR of 150 and trying to maintain RPE. The first set produced 226/332w, and the last (8 minutes total) was 236/353w. Obviously this isn’t conclusive, but perhaps it does add a little weight to the theory that I am using substrates somehow.

@Yepyep - I think I may have accidentally done a lot of sweetspot, yes. Trying to shorten this as much as possible… I’ve been riding (almost exclusively) single speed MTB for the past 3 years or so. It’s pretty hilly round here (will see at least 1200ft of climbing for every 10 miles) and obviously there’s no ‘going easy’ without gears. After I started ‘training’ and began using a HR monitor 4 months ago, I noticed that without thinking I ‘dropped in’ to 150bpm as my go-to for pretty much every climb. So, I will have spent a lot of time there in the last few years.

Other things that may be factors, but I don’t understand this stuff enough yet to say if they could influence the above charts: I have been trying to train my body to use fat as fuel (lots of fasted rides and eating a high % of calories from fat), and I also think I have developed a ‘different’ balance of muscle fibres to the average rider due to the singlespeed riding.

With cadence being so low (average for a ride hovering around 60), every ride will see a peak torque of over 300Nm and sustained efforts at more than 50, 60 or even 70Nm. I’m not sure if these things could contribute to substrate utilisation, but thought I would mention it in case anyone has come across correlations of this type before.

Again, thanks for everyone’s input. I guess it doesn’t really matter if I can understand this - I just need to learn how to exploit it! - but it’s good to known what’s going on with your body.

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Regarding long efforts at low cadence and high torque… doing these has resulted in fitness gains and seemingly increased fat utilization (because I didn’t bonk on long climbing days).

However I have no lab data, and anything beyond “my fitness benefited from doing aerobic low cadence efforts” is speculation.