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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!

 
 

Affirming DFAA-1 Aerobic Base With Vo2Master and Moxy

A New Calculation Uses Heart Rate Variability to Better Tune DFAA-1

DFAA-1 with new aerobic floor.
Garmin Connect IQ has an app called ‘AlphaHRV’. This app allows you to set custom floors for different intensities. Here, you can see my DFAA-1 (Alpha-1) is at 1.22, which is above my Aerobic floor. I’m exercising at an aerobic, comfortable, intensity. Zone ‘2’.

I’ve been a big fan of Dr. Bruce Rogers, since 2017. He is a trained MD, but publishes his work in credentialed journals. As a Masters’ level road cyclist, he knows how important heart health is. He is also fully aware of the cardiac risks to endurance athletes. This is why I am impressed with his latest work.

Dr. Rogers began to convince me that DFAA-1 had merit as a non-invasive way to calculate HRV equivalents to Ventilatory Threshold 1 (VT1) and Ventilatory Threshold 2 (VT2), around 2021, when Covid-19 was affecting afflicted athletes. As a Coach, it’s my duty to help my cyclists improve their health and fitness, but in a Holistic way.

Some of you may know that in 2016, a client and I had to perform CPR on another client (he lived). In 2023, a client of mine suffered a heart attack just after a remote class. He also lived, but both are events I never want to see repeated.

In fact, I have changed most of my protocols to include heart-healthy workouts. DFAA-1 is a new way to help everyone ride what I’ll call “Smart” aerobic base. At more intense levels of work, DFAA-1 can help identify ‘Threshold’ on a metabolic, Physiological level, instead of power. Power (watts) is just too immutable, and does not account for myriad other factors that lead to less accuracy in assessing proper intensity.

The ‘Limit’ for DFAA-1 At VT1

Now, remember – the calculation behind this technology is still nascent, but the application is fast-evolving. In 2021, research showed that a DFAA-1 of around 0.75 was fairly close to Ventilatory Threshold 1, or, the point of intensity at which our bodies begin to generate lactate at a higher rate, and thus, use more carbs in in our fuel mix. The original paradigm also set VT2, or the point of intensity at which our bodies begin to create more lactate than can be sustainably processed while staying aerobic, was around 0.5.

Well, the latest research paper from Dr. Rogers shows that there might be a better calculation for DFAA-1 at VT1.

The New Calculation for VT1 via DFAA-1

The goal of this study was to come up with a more accurate way to synchronize heart rate variability, and Ventilatory Threshold 1. What the researchers ended up doing was the following:

  1. They looked at the HIGHEST recorded DFAA-1, usually during warmup or recovery, but it could also be done at rest.
  2. Take this HIGHEST value (in my case, it’s about 1.8), and then ADD the more-traditional value of DFAA-1 at Ventilatory Threshold #2, which is 0.5. The result, in my case, is 2.3.
  3. Use this new Value (2.3), and divide it by 2. The result, again, in my case, would be 1.15.
  4. Use this last value, again, in my case, 1.15, and use THAT as the ‘more accurate’ value for VT1.

When a cyclist or a runner stays ABOVE the new value, they’re working aerobically. Thus, the majority of the gains would be synonymous with the now-ubiquitous ‘Zone 2’ or ‘Base’. Heck, I knew it as ‘LSD’, or Long, Steady Distance, from my days under the tutelage of Sally Edwards and Dr. Ken Cooper (at different times).

Testing the Metabolics of HRVT1

Garmin Connect DFAA-1 Vo2 and Respiration Rate
On this ride, I used my Vo2Master and a Moxy to get more information about the metabolic values associated with DFAA-1 at 1.15 or higher. On this chart, Alpha-1 was mostly at or above 1.2. Respiratory Rate was between 26 and 32. Vo2 was in the mid 30’s. Heart rate averaged 136 BPM.

My clients at my VQ Velocity Virtual Studio are now heavily invested in training with DFAA-1. We have mostly installed the Garmin Connect IQ Field known as “AlphaHRV“, which takes Heart Rate Variability, and calculates DFAA-1. It is accessible within the Garmin Connect ecosystem, displays in real-time, and gets saved in .fit files for post-ride analysis.

I accessed the settings for AlphaHRV within Connect IQ, set VT1 at 1.15, set VT2 at 0.5, saved the settings, and then began my ride with my clients. For 20 minutes, I rode with my Vo2Master mask and Moxy Monitor on my body, while HRV was transmitted by a Polar H10.

Here are the results:

Metric:Alpha1SmO2Vo2MaxTraditional HRWattsAlphaHRV Resp RateVo2Master Resp Rate
Average1.3439%38.561361702626

Honestly, I am MIGHTILY impressed.

  • At 170 watts of power, DFAA-1 averaged 1.34. This is above the 1.15 value I calculated for myself. Ergo, this 20 minute time span was SAFELY within the AEROBIC level of intensity.
  • SmO2 on the Left Lateralis averaged 39%. This dovetails into over 10 years of data collection for myself and clients. For me, 40% is a known ‘Safe’ value for VT1.
  • Vo2 was measured at 38.56ml/kg/min. In the previous two posts, I measured my Vo2Max at 62 or 63ml/kg/min. This value, at 62% of Vo2max, is, Safely within my Aerobic Training Zone.
  • Traditional Heart Rate averaged 136 beats per minute. Earlier this year, I performed a traditional Vo2max test, and the assessment from Vo2Master’s in-house software, showed my VT1 heart rate at roughly 140 beats per minute. Again – 136 bpm is Safely within my Aerobic Training Value.
  • The last two metrics, AlphaHRV’s Respiratory Rate, and Vo2Master’s Respiratory Rate, are values that I included so that I could effectively determine whether the AlphaHRV ‘RR’ calculation was accurate. As you can see – it’s spot-on.
Complimentary physiological metrics to DFAA-1 show heart rate smo2 and garmin firstbeat values
Garmin Connect and FirstBeat metrics complement the DFAA-1 values at VT1. Wattage is in ‘Zone 2’, traditional HR is in ‘Zone 2’, Saturated Muscle Oxygen is stable (green) and high at around 40%. EPOC/Load is at 79, and both Aerobic and Anaerobic Training Effect are in the ‘maintaining’ level of volume. ‘Perf Cond’ at ‘-1’ is honestly pretty good for me, as it tends to drop further and more quickly most of the time that I ride ‘base’. It’s my least-used metric, as it seems the most vague.

A Note About AlphaHRV On Garmin Connect:

Just a quick note: AlphaHRV includes an option for the RESPIRATORY RATE of VT1 and VT2. Now that I know I can breathe at 26 breaths per minute and stay at or below the intensity for VT1, I’ll set it at, say, 27 or 28 breaths per minute, as another Affirming Metric for ‘Base’ training.

Conclusion

Vo2 on screen with a Garmin 1040
Vo2Master has a Field in Garmin Connect IQ that will link their device to a head unit or a watch. A Vo2 of 37-38ml/kg/min fits in with a DFAA-1 above 1.15.

DFAA-1 Continues to reveal more about the human heart and new methods by which measuring HRV can lead to a more scientific approach to cardiac health and fitness. My clients and I are employing AlphaHRV to ever greater effect as we settle into several months of mostly “Base/Zone 2/LSD” training, to give our hearts and bodies a rest, and prepare ourselves for another season of Tempo, Threshold, and Vo2 intervals in 2025. This should result in more fitness capacity over greater ranges and durations, which I hope will lead to more adventures on two wheels, for everyone under my umbrella at Online Bike Coach.

Thanks for Reading, and

ENJOY THE RIDE!

 

Vo2Max and Mountain Biking

Mountain Biking Requires Riding at Vo2Max!

Vo2Master outdoor data collection.
Cross Country Mountain Biking Requires working at or near your Vo2Max for an extended period of time.

CAUTION – THIS POST IS NERD-HEAVY! IT IS DEEP IN THE WEEDS! DO NOT HESITATE TO ASK QUESTIONS!!!!!

Ever since I received my Vo2Master back from my friend and code buddy in Canada, I have been collecting data on inside and outside rides. Not long ago, I rode two laps of my favorite Cross-Country Course, using my Mask, a Moxy, and a Rotor Power Meter, to learn just how hard I was working, and how I might be able to get some faster splits on the climb.

The Course

The course for this study is my all-time Peavine Favorite, which I built in Garmin Connect in a previous post. The course covers 8.6km, climbs 215 meters, and my best time on the loop is about 27:10, though I haven’t seen that time since 2020. I’m convinced that Mountain Bike Cross Country Courses change with use and weathering, and let’s face it – I’m four years older, and probably a little more skittish now than I once was. Health Insurance deductibles are always in the back of a cycling coach’s mind!

This year, my fastest time is a 29:36, and that happens to be the fastest time since a race was held on this course in 2019. It’s my home course, and I spend HOURS on the loop, trying to decipher bike fit, suspension, and tire pressure. When winter shuts me down, this trail is what I dream of for the next spring.

Vo2Max for Climbing on a Mountain Bike

The Climb section of this course starts immediately. The first section is 2.46km, and climbs 144 meters at an average of 5.7% slope.

The second segment, just a few minutes later, is 1.37km long, and climbs 54 meters at 3.9% slope.

My fastest time for this climb was set in 2020, at 10:33. My closest time this year, however, has been an 11:26. My weight, however, is about the same, at 69kg. The bike weight is similar, though it is a new bike. The difference is that I’m four years older, and maybe the trail has degraded somewhat over those four years.

The ride we’re studying occurred on August 15th, 2024. Here’s the link to the file:

Physiological Demands

I really wanted to see how much Strain was required to perform the climbs on this course, so I gave it all I had. I wanted to witness more than just power and time. I wanted to observe Vo2 demands, Saturated Muscle Oxygen values, Respiratory Rate, and more.

Here are a few graphs of the first climb. I think the results are pretty interesting.

Vo2Master Vo2 and SmO2 while MTB Climbing.
This first image highlights Vo2, elevation, and SmO2. You can see how SmO2 stayed suppressed below 20% for a good chunk of the first climb, roughly corresponding with the high Vo2. I believe my Vo2max is about 62ml/kg/min, and for this climb, I am estimating I climbed it at about a 55ml/kg/min, or 89% of Vo2max. Note also, just how quickly SmO2 recovers when terrain changes, as opposed to Vo2, which takes slightly longer. The legs provide the ‘Demand’ side of the equation, while the lungs in this case reveal the metabolic cost of this demand. 
Vo2master Garmin Connect Vo2max Vo2 Threshold heart rate mountain biking climbs
Traditional heart rate definitely seems to correlate with Vo2 on this effort. However – it does seem like the heart rate ‘drifted’ higher with the effort, while Vo2 was roughly steady, especially around the 14:45 mark. I tend to throw out the short spikes of Vo2, and in after-action, I always try to export the file and extract the highest Mean Max Vo2 for the duration I’m observing.
Vo2Max Mountain Bike Watts Stochastic Power
This is the same time span, showing Vo2 against power on the climb. Look at how incredibly stochastic the wattage is! This is created by terrain and changes in direction, but the metabolic costs (see the above charts showing SmO2, Heart Rate, and Vo2, all remain more consistent. Wattage dictates lots and lots of small, tiny gains, measured in single meters.
Vo2Master Vo2 DFAA1 DFAA-1 DFA Alpha 1 Alpha HRV AlphaHRV
I know there’s debate about the efficacy of DFAA-1, but I thought I’d show it anyway. Compare Alpha1 to SmO2. I usually dictate SmO2 at Threshold to be about 17-20%, and I usually use 0.4 -0.5 for Threshold at DFAA1. For the majority of this climb, once the calculation caught up, DFAA-1 was below 0.5, and was below 0.4 at times as well. It IS useful, especially for longer climbs.

Results

Okay – this is where things get crazy.

I have to believe that some of my results are wind and temperature aided. However, glances on www.wunderground.com history, just do not provide the details I believe are necessary to provide proper context. I also believe the trail conditions change over the course of a season, and over the course of years. I know I’m getting older, and with that age, a lower Vo2max and diminished power at Threshold, but I also believe that I’m giving this climb my best effort. This year alone, I have climbed this hill 24 times, with a best result of 11:26. August 15th’s ride was, however, a lowly 12:31, my seventh-best time this year.

Ironically, on August 18th, I climbed this hill in 12:04, and on August 25th, just 10 days later, I finished in 11:28. Unfortunately, I was not wearing my Vo2Master for those rides. I may follow up this post with an analysis of those rides, since I was using the Moxy, and AlphaHRV on my Garmin 1040.

That’s the Bad News. The Good News is that it’s the fastest time on this segment on Strava for the year, and it’s the fifth-fastest time ever. I now have the Vo2, SmO2, HR, and power metrics needed to determine how to actually IMPROVE my time through increased fitness!

Conclusion

Mountain Biking demands a lot of effort at, or near, your Vo2Max!

I now know just what the physiological parameters are for this climb. Knowledge comes from data, and the data revealed so much. I’m going to work on my 10-minute average power values, and power at roughly 54-55 Vo2. I’m going to work on leg strength, and overall body strength. I still don’t believe I have the best bike/body position for climbing, and I’m going to study that and make changes.

The beautiful thing about mountain biking is that the same course, the same segment, the same trail, is NEVER, EVER, EVER the same. Rocks move. Moisture comes and goes. Wind erodes. Some areas get softer, some areas get more packed. These are all things that GPS trackers cannot measure, and on-bike and on-body metrics can’t perceive or analyze.

I’ve spoken before about a term I learned when I was flying sailplanes. It’s called “Whipadilling”, and it’s sort of a 6th Sense. In aviation, it’s the ability to ‘read the air’ and ‘see the sky’. For me, on a mountain bike, it’s about reading the terrain, picking lines, making mistakes, rolling over those mistakes and remembering the next obstacle. There are SO MANY VARIABLES. The goal, however, is to optimize what I can on my body and my bike, and then practice, practice, practice, until the trail shuts down for the season, due to rain, snow, and short days.

 

Until then…

ENJOY THE RIDE.