8-Minute Intervals At “Threshold”…

In late November of 2024, I was able to perform an interval session on my indoor trainer and bike with my clients. The workout comprised of four separate 8-minute intervals. We used the VQ Velocity app, and I recorded each interval as a separate file while wearing my Vo2Master Gas Exchange Analyzer. Then, one of my clients, Ken O’Brien, actually coded an app that would allow me to compare the intervals against each other.

Traditional Interpretations of 8-Minute Intervals

When I look at the data from a ride file, I usually use graphs and charts from Garmin Connect. When I ride, I use a Garmin 1040 and the lap button, to assess real-time information. For years, I used wattage and traditional heart rate for each of the 8-minute intervals, but with advanced Heart Rate Variability Analysis, I can now include EPOC, aerobic and anaerobic training effect, and DFAA-1. These values USUALLY yield enough information to determine whether I’m achieving my training goals or not.

  

Inclusion of Gas-Exchange Analysis in 8-Minute Intervals

By including data from my Vo2Master gas-exchange analyzer, I wanted to determine whether these traditional metrics were accurate and valid. It’s one thing to get a ‘Score’ or an interpretation of training results from a software with algorithms. These are usually pulled from a EULA that traps metadata and makes suppositions about intensity, dose, and recovery. I find these interpretations to be… shallow. Hence, the need to pull data from the body. 8-minute intervals are long enough to measure metabolic changes via data that I can only get from a breath-by-breath analysis. The goal is to MELD traditional, on-bike data that I can get from my Garmin, with the knowledge gained from the Vo2Master, Moxy monitor, and AlphaHRV.

8-Minute Interval Data

Here’s the link to the 4 x 8-minute interval efforts, along with a chart showing ‘traditional’ averages for Garmin Metrics.

And here is my first summary of the information that I collected.

Lap	Ave HR	Ave Cadence	Torque Effectiveness	Ave Power	SmO2	ThB
1	168	96	85%	249	13.645%	12.408
2	167	95	84%	238	15.378%	12.369
3	165	91	85%	232	16.307%	12.364
4	168	92	84%	228	14.434%	12.371

This was where I first began to suspect that some of the ‘Traditional’ parameters for an 8-minute interval might not be as effective as decades of efforts had led me to believe. Now – here is the data from the Vo2Master… Let’s first look at Heart Rate vs. Heart Rate.

This is the graphical data for traditional HR, using the First interval as the benchmark, in black. The following intervals are in red.

For those who are interested, THIS LINK will provide access to these files. For this blog post, the files I’m using are found under ‘select dataset directory’, and are the following:

• ./Data/RW-11-28-24-Int1
• ./Data/RW-11-28-24-Int2
• ./Data/RW-11-28-24-Int3
• ./Data/RW-11-28-24-Int4

You can certainly play with any of the files, however.

Here’s the chart comparing the first interval to the third 8-minute interval.

For the fourth interval, there was something that interfered with my final 30-seconds, so the effort was cut slightly short. However, for the 6-minutes in the middle, again, traditional heart rate is similar. Refer to the above table to see just how close the heart rate averages were.

Heart Rate vs. Vo2

Now – here’s a table showing traditional on-bike metrics vs. Vo2Master metrics for each 8-minute interval.

Lap	Average Vo2	Heart Rate	Power Average	Respiratory Rate	SmO2	ThB
1	46.53	168	249	40	13.645%	12.408
2	44.63	167	238	39	15.378%	12.369
3	41.98	165	232	35	16.307%	12.364
4	41.17	167	228	37	14.434%	12.371

And here are some charts comparing different metrics from one interval to the next. Again, I’ll use Interval #1 as the benchmark.

Here’s the first interval against the third interval…

And here’s the first interval against the last interval…

Vo2 vs. SmO2 for the 8-Minute Intervals…

Now remember – we’re looking at what wearables are telling us, vs. the outcome. These intervals really were NOT that great for me, but if I want to improve both results and consistency for the time requirement and demands, I really want to dig deep and look at all of the parameters. Here, I have placed Vo2 against SmO2 in the Left Lateralis. Take a look….

Here’s 8-minute interval number 2.

I’m going to show 8-minute interval #2 again, this time with wattage instead of Vo2.

Here’s the third of the 8-minute intervals, comparing Vo2 to Smo2…

Now, the 4th 8-minute interval was wonky, and if you look at the right side axis, you’ll see that the scaling is off. You’ll need to refer to the tables, but suffice it to say – SmO2 was fairly low, wattage was compromised, and Vo2 was also compromised. My own interpretation is that I was knackered; fatigued; out of sugar; bonking. But honestly, I don’t really have an answer.

Now, here’s the Crux…

If we use some of these ALGORITHMS that claim to ACCURATELY PREDICT Threshold Power, Functional Threshold Power, VT2, LT2, Critical Power, or whatever, then we depend on them for accuracy, consistency, and repeatability.

But when you look at my tables, and then the graphs, this really is not as accurate as one might be led to believe. The two that I have used extensively in the past, which I won’t name, have my ‘Threshold’ at 270 watts, and 172 beats per minute, per traditional heart rate. These interval intensities were nowhere close. Since 2003, when I first began programming interval sessions for clients using WordPad, I’ve always understood that wattage cannot be the end-all, be-all to training. We have to look at things Holistically.

These wearables, like a Moxy and a modern Garmin or Polar Chest strap, along with 3rd-party Fields, like AlphaHRV (which I did NOT discuss in this post, but will in a follow-up), give us a price-effective way to gain greater insight into EVERY ride, EVERY INTERVAL, and even EVERY BREATH or PEDAL STROKE, down to the second.

I’ll post a follow-up to this post, with some more insights, but I’m at my limits of knowledge here. This is where the ‘Mad Scientist’ always loses out. I need a True Physiologist. Luckily, I’ve got two or ten who share my passion for these Deep Dives, and who can help me understand. I’ll share this information as soon as I can.

Thanks for reading, and

#ENJOYTHERIDE!