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The Physiological Perspective of Four Separate 8-Minute Intervals

8-Minute Intervals At “Threshold”…

4x8 minute intervals online bike coach richard wharton coach wharton garmin connect
Wattage, heart rate, and Saturated Muscle Oxygen do not always agree. In this blog post, I dive into the physiology of EACH INTERVAL, which gives me greater insight into how to improve workouts for myself and you.

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

Vo2Master App Metrics
This is just one page of data fields that I can use on the Vo2Master app. You can also run these fields as graphs. All of the data is stored in a detailed .csv file that can be downloaded and shared for analysis.

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.

LapAve HRAve CadenceTorque EffectivenessAve PowerSmO2ThB
11689685%24913.645%12.408
21679584%23815.378%12.369
31659185%23216.307%12.364
41689284%22814.434%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.

Vo2master heart rate vs heart rate chart 1
Heart rate for the first 8-minute interval is in black. Heart rate for the second interval is in red. You can see a similar pattern.

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

o2master heart rate vs heart rate chart 2
Again, once I get set into the effort, traditional heart rate follows a similar path.

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.

LapAverage Vo2Heart RatePower AverageRespiratory RateSmO2ThB
146.531682494013.645%12.408
244.631672383915.378%12.369
341.981652323516.307%12.364
441.171672283714.434%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.

Online Bike Coach Vo2Master 8 Minute Intervals Compared n1
This is a Vo2 comparison of my first and second 8-minute Interval. Black is the first interval, red is the second. The averages are on the table above.

Here’s the first interval against the third interval…

Online Bike Coach Vo2Master 8 Minute Intervals Compared n2
Again, the first interval is in black, while the second is in red. I lost Vo2 intensity and efficiency for roughly equivalent heart rate.

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

Online Bike Coach Vo2Master 8 Minute Intervals Compared n3
And finally – here’s the last of the four 8-minute intervals. Vo2 was suppressed, as was wattage, but heart rate was not.

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….

Online Bike Coach Vo2Master Moxy Monitor 8 Minute Intervals Compared n1
Rember to use the table a few paragraphs up. There, you’ll see my average SmO2 for each interval. In my opinion, I went WAY too hard for most of the intervals, and ended up fatigued out, which affected the other three efforts. Let’s continue…

Here’s 8-minute interval number 2.

Online Bike Coach Vo2Master Moxy Monitor 8 Minute Intervals Compared n2
This interval is unique because you can see how ‘backing off’ on the intensity, lowered my Vo2, and raised my SmO2, right around Minute 4 to 5. I think this is an important value, which I’ll point out in the following image….

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

Online Bike Coach Moxy vs wattage 8 Minute Interval n2
I love looking at SmO2 from my Moxy, because it’s consistent, sensitive, and is a fair proxy for VT2 and/or LT2. As you can see, when power (Vo2 is in the previous image), drops as a result of less effort, SmO2 rises, allowing me to witness and recover without losing too much power or intensity. When I rally, Smo2 drops again, but I am better able to find a ‘sweet spot’ of intensity that leads to a sort of ‘Plateau’ for saturated muscle oxygen. By the way – the drop in power at the end is from a hard shift that forced me to stand. It messed up the readings. Don’t Do This!

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

Online Bike Coach Vo2Master Moxy Monitor 8 Minute Intervals Compared n3
By the third interval, I had kind of nailed down the intensity that I was going to be able to sustain more appropriately. Vo2 is more consistent, while SmO2 range is also more consistent.

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.

Online Bike Coach Vo2Master Moxy Monitor 8 Minute Intervals Compared n4
Not much to say here, other than I believe the SmO2 sensor CAN give a clear picture, on a Garmin head unit, of those liminal states of intensity, below which (higher SmO2 %) you’re safe, and above which (lower % SmO2), you’re on ‘borrowed time’. I pretty much hit the ‘sweet spot’ via Smo2 (demand) at about the 5-minute mark.

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!

 
 



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