Advanced, Gear, Podcast, Science

Muscle Oxygen Saturation (SmO2) with Roger Schmitz | EP#85

 December 18, 2017

By  Mikael Eriksson

Muscle Oxygen Saturation (SmO2) with Roger Schmitz | EP#85

Moxy - Muscle Oxygen Saturation (SmO2) with Roger Schmitz

Roger Schmitz of Moxy, discusses next-generation physiological measurements for endurance athletes: muscle oxygen saturation, SmO2. This can already be done today at testing facilities or day to day in all training sessions, with the small and handy Moxy device.

Discuss this episode!

  • Let's discuss this episode and the topic in general. Post any comments or questions in the comments at the bottom of the shownotes. I'll be here to reply and take an active part in the conversation, so don't be shy! 
  • What's your take: does the real-time measurements of your physiology offer enough additional benefit that you would consider investing in the Moxy device? Why or why not?
  • Join the discussion here!

In this Episode you'll learn about:

  • Why measuring SmO2 to establish training zones and use in real-time is physiologically the most direct way to know what's happening in your body at different intensities any given day
  • How using SmO2 and Moxy is different from traditional lactate testing or metabolic testing
  • How using the Moxy device has helped athletes massively increase the number of intervals they can do in high-intensity workouts
  • Measuring SmO2 to determine your exact your physiological limiters - oxygen delivery, utilisation or even cardiac or respiratory function

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About Roger Schmitz

04:52 -

  • Co-founder and CEO of Moxy Monitor
  • Holds a Bachelor of Science in Mechanical Engineering from Iowa State University
  • Lives in Hutchinson, MN.
  • A long background in technology development in automated machinery and medical devices as lead design engineer.
  • Together with his co-founders at Moxy, they started development in 2010 and launched in 2013.
  • Today Moxy is the only company that develops a device that endurance athletes can use to get real-time feedback on what’s happening in there body with their physiology.

About Moxy

07:45 -

  • Moxy is a muscle oxygen monitor. It measures the percentage of haemoglobin and myoglobin that’s carrying oxygen in the capillaries and in the tissues of the muscle where the oxygen is being consumed.
  • By measuring that, we can get an indication of the balance between supply and demand of oxygen in the muscle. We do this while the athlete is actually performing their sport.
moxy monitor

The Moxy device - Click to zoom

How does the device work?

08:36 -

  • The device is a small, wearable device. It’s completely wireless. You wear it over a large working muscle. For a cyclist or a runner, that would be the vastus lateralis which is one of the four muscles of the quadriceps.
  • Runners may also wear it on the hamstrings or on the calves. For swimmers, sometimes they wear it on their back or some part of their arm.
  • The sensors are oftentimes attached with tape or sometimes just tucked under compression shorts. It just needs to be held in contact with the skin. It’s small enough that most people just forget that they have it on. It’s about the size of the bigger Garmin watches.

How is measuring muscle oxygen beneficial?

09:45 -

  • You get a rich set of information about the athlete’s physiology out of that.
  • We break this into several categories. We do assessments similar to what you do with the lactate threshold or the VO2max test.
  • The goal of the assessment is to identify what aspect of their physiology is limiting their performance.
  • However, where the Moxy is really different from other technologies is that you can use the same device in the field. You can use it to guide your daily training – interval training, long slow distance training, and recovery.
  • You can even use it in competition. We have people that are using it to help optimize their race pace.

Key takeaway

  • Traditionally, you would go to a lab and test your heart rate or VO2, and you would get heart rate or power training zones that are "surrogates" for important transition points in your physiology.
  • With Moxy, you no longer need to rely on surrogates, that may not be a true representation of what's happening in your body any given day. With Moxy you can see in real-time what is happening in your body and train accordingly on a day-by-day, second-by-second basis. 

What are some examples of the physiological limiters that Moxy can find?

12:54 -

  • The idea is that if you’re a cyclist and you transition from a power that’s below your FTP to a power that’s above your FTP, something reached its limit. There’s a reason why you can’t maintain that power above your FTP like you can when it’s just a few watts lower.
  • When we talk about the limiter, we’re figuring out which system in the body is causing that to happen. The three that we typically look for with the Moxy are the respiratory system, the cardiac system, and the muscular system.
  • We’re not saying that if it’s a limiter that it’s good or bad. Somebody who is a couch potato has a limiter and somebody that’s an Olympic level athlete also has a limiter.
  • We’re not saying it’s the level of performance but we’re saying that when you go as fast as you can go, which system hits its limit first?

What is the typical limiter of age-group athletes?

14:04 -

  • It’s across the board. It amazes me how different athletes really are when we look at these plots.
  • Some people who go as hard as they can go in one of these assessments, won’t desaturate. They’ll be up at 75-80% saturation and no matter how hard they go, they can’t get it to drop below that. This is an extreme case of muscle limitation. Their muscles can’t use all of the oxygen that the rest of their body is capable of delivering.
  • On the flipside, you’ll see somebody who is desaturated down to 10-20% when they’re right at their FTP or even a little bit below it. This is an indication that they have really good muscle oxidative capacity in their muscles but their body can’t deliver as much oxygen as what their muscles can use.

How can you see the respiratory system limitation in the saturation data?

15:16 -

  • The Moxy measures two parameters. It measures the saturation (SmO2) and the total haemoglobin (THb).
  • When it’s an oxygen delivery issue, that means either the cardiac system or the respiratory system can’t get enough oxygen to the muscles. In both of those cases, we tend to see low SmO2 (muscle oxygen saturation).
  • The THb (total haemoglobin) is an indication of the blood volume in the muscle. That’s how we use it to interpret the data.
  • When we’re seeing low saturation (SmO2), we look to the THb data. And if we see indications of vasodilation where the blood vessels are opening up and the blood volume is expanding in the muscle, this is an indication that the body has sufficient blood flow but you have vasodilators building up. This is because you’re not getting rid of CO2 fast enough.
  • On the flipside, if we see vasoconstriction where the blood volume is going down, that’s the body taking action to restrict blood flow so that it preserves blood pressure for the brain.
  • The body is really protective of blood flow to the heart and brain. It will shut down everything else in order to keep blood going to the heart and brain.

How does muscle oxygen saturation work at a high level?

17:11 -

  • The simplest way to think about saturation is the measure of the balance between supply and demand.
  • If your body can deliver more oxygen to the muscles than what the muscles can use, which means supply exceeds demand, then the SmO2 goes up.
  • Vice versa, if the muscles are wanting to use more oxygen than the body can deliver, then the SmO2 goes down.
  • Essentially, the amount of oxygen that you’re using in the muscle is how much is extracted times the blood flow rate.
  • It’s actually more efficient in some aspects to have low oxygenation in the muscle because then for every heartbeat you’re taking a lot of oxygen off of the blood and using it to do work.
  • We typically like to see that low oxygen in the muscle because that means your muscles are able to use it and it’s an efficient use of your blood flow.
moxy muscle oxygen saturation chart

Example SmO2 chart during a ramp test on the bike. Notice how SmO2 (green line) rises at first, then reaches homeostasis and stays constant, and then at a higher intensity starts to drop. Notice also the four Moxy training zones. AR = Active Recovery, STEI = Stuctural Endurance Intensity, FEI = Functional Endurance Intensity, HI = HIgh Intensity. Click to zoom

Key takeaway

  • When you work out at a low intensity, your body starts supplying oxygen to the muscles, but your muscles don't need to use all of that oxygen because the intensity is low. Oxygen supply increases, and SmO2 goes up.
  • At some intensity, the oxygen that the muscles require matches the supply by the body. But even if you increase your intensity further, the body is capable of increasing its supply by raising your heart rate or the work rate of your respiratory system. Therefore, up until a certain point, oxygen supply and demand remain evenly matched, and SmO2 stays constant. The body is in homeostasis.
  • At a certain intensity, your body can no longer supply more oxygen, but as the intensity increases further, the oxygen demand by the muscles keep increasing. This is where demand exceeds supply, and SmO2 starts decreasing.

When you see a saturation curve go down, is that when you start to get to a point where it’s unsustainable and your performance drops?

18:37 -

  • This varies from athlete to athlete. There is a point where you learn how you respond when you get to that unsustainable point.
  • The common response is that you get to a point where the SmO2 is going down a little bit and then suddenly it goes down a lot as you increase the load.
  • This is one of the indications where you’re at a point where you can’t sustain the load anymore.

How using SmO2 and Moxy is different or better from using training zones from a training perspective?

19:24 -

  • The way that it is better is mainly that the information is much richer. You get more useful, actionable information when using the Moxy than what you would get from a lactate threshold test.
  • People measure lactate because they’re trying to figure out what’s going on with the oxygen in the muscle. In the past, before devices like Moxy existed, it wasn’t possible to measure that directly.
  • People measured lactate as an indirect indication of what’s going on in the muscle. It’s like reading about the sporting event the day after it happened rather than watching it live.
  • With the Moxy, you can see live what’s going on in the muscle and not have to wait for the lactate response which is much more difficult to interpret.
  • There are really two delays in this lactate testing. One is that there’s the physiological delay of how long does it take the body to reach homeostasis at a new load, which is typically 3-5 minutes.
  • The next delay is that the lactate has to circulate through the body so that you can measure in the finger and try to infer what happened back in the muscle.
  • With the assessments that we do for Moxy, we recommend a 5-minute step length which is longer than what a lot of the other assessments recommend. This is to really help us get to that homeostasis point.
  • What’s nice is that you’re not drawing blood and you’re able to get data continuously through the entire test. You’re not just getting it at discrete points. It’s more practical to have those longer load steps when we’re doing those kinds of assessments.
moxy muscle oxygen saturation chart - long slow distance training

Long slow distance training with Moxy. Notice how SmO2 (green line) remain stable over time in the STEI = Stuctural Endurance Intensity zone. Click to zoom

moxy muscle oxygen saturation chart - interval training

Interval training with Moxy. Notice how SmO2 drops quickly in the high-intensity intervals, but is then allowed to recover between intervals. Click to zoom

How does Moxy compare to metabolic testing?

22:42 -

  • There’s actually some complementary information and a lot of our more serious training centres use both VO2-testing and the Moxy.
  • You can get more detailed information about what’s going on in the respiratory system when you also measure it with VO2.
  • The limitation there is it’s just not practical to wear a mask during regular training, even with the portable VO2-machines.
  • A lot of the information that you get out of the VO2 can be inferred out of the Moxy but of course, it’s better to get it direct from the metabolic cart if you can.

How do you establish training zones based on oxygen saturation and Moxy assessments?

23:55 -

  • We try and measure the zones directly rather than calculating them from a fixed point. A lot of the time, someone will calculate an FTP and then they’ll say your zones are some percentage of your FTP.
  • We typically break it into three zones. The low-intensity zone which we call the oxygenating zone. This is where the harder you go, the higher your oxygenation gets.
  • This is where supply exceeds demand. It’s like you’re still warming up. If you’re at a low enough intensity, you won’t fully warm up. You won’t get to a point where your blood vessels are fully dilated and you won’t fully maximise your SmO2. You might only get up to 80% and you won’t go any higher. If that’s the case, then you’re still in that oxygenating zone
  • At some point, you get to an intensity where your body will fully warm up and you’ll reach your peak. Typically that is 85-95% SmO2. This is a typical high point for most people. This will sustain over a range of loads.
  • You can increase the load and your oxygenation doesn’t change much. We call this homeostasis because your body can just deal with it. There’s a range of loads at which you’re in homeostasis. 
  • Then you get to a point where if you go harder, your body has to extract more oxygen to keep supplying the load. This doesn’t mean that you’re over your FTP necessarily. It does mean that your body is changing how it adapts to that increasing load. We call this the deoxygenating zone.

Key takeaway

  • Roger talked about three training zones that correspond to increasing SmO2 (oxygen supply exceeds demand), stable SmO2 (supply and demand are matched), and decreasing SmO2 (oxygen demand exceeds supply. These zones are the oxygenating, homeostasis, and deoxygneating zones.
  • However, in their training material, Moxy also identify a fourth training zone, where SmO2 has started dropping but then plateaued at a lower level before it will drop again with increasing intensity. This material also uses slightly different terminology.

    "An increasing trend at the beginning identifies the Active Recovery (AR) zone, as signified by the colour green (if you do not have an increase at the beginning of your test, you should redo the test but start at lower intensity). The plateau at the highest point of SmO2 identifies the Structural Endurance Intensity (STEI) zone, as signified by the colour yellow. A second SmO2 plateau is possible at a lower SmO2 value, but may not necessarily be seen; this would be the Functional Endurance Intensity (FEI) zone, identified by the colour orange. A clear and continuous decrease in SmO2 identifies the High Intensity (HI) zone."
moxy muscle oxygen saturation chart

How do you use those zones in training? Do they somehow relate to the aerobic and anaerobic thresholds that are often used to base workouts around?

26:31 -

  • We tend to use them directly because there’s a lot of baggage with the terminology of the thresholds.
  • One example is if you’re going to do a recovery ride or run, we would typically do that at the very low end of the homeostasis zone – the transition between oxygenating and homeostasis.
  • This gets you in a situation where you are minimising the stress. You’re doing the lowest intensity that you can but still get the oxygenation as high as you can.
  • This is perfect for recovery. You’re getting a really good blood flow and lots of oxygen to the muscles but you’re not inducing any more stress than is necessary.
  • If you’re going to do a threshold pace or power where you want to be right at the edge of as fast as you can go, then you’d be at the other end of that homeostasis.
  • This is where if you go any harder, your oxygenation goes down. If you back off a little bit, your oxygenation will pop back up.
  • If you’re right on that edge, we can use that to guide that intensity as well.
  • What’s really nice about both of those is it may not be a fixed power. If you’re not well recovered or if it is a hot day, you might have that transition at a lower power than what you would if you were fully recovered.

How would you do VO2max or high-intensity intervals with Moxy?

29:04 -

  • A lot of the time people talk about these as anaerobic zones but they don’t have to be and you can manage them really well using Moxy to guide it.
  • If you’re going to do high-intensity intervals – let’s say you’re going nearly all out – and you want to be able to do a lot of them over and over again.
  • If you watch your SmO2 as you start out doing the high-intensity interval, your SmO2 is going to drop fast because you’re way over your FTP.
  • As soon as your SmO2 reaches its low plateau, which might be 20% for some people or 50% for others, as soon as you get there, stop and then recover until your SmO2 gets back up to its recovery point which is 80-90%. Then once you’re recovered, you can go again.
  • If you use this method, you’re actually staying aerobic because the whole time you’re doing the high-intensity interval your oxygen is going down.
  • This means that you’re using all of the oxygen that your body can deliver plus the oxygen that’s stored in the muscle which is the myoglobin aspect of what we measure.
  • By doing this, you can avoid getting into the situation where you go anaerobic. You stop right before you have to use your oxygen-independent sources of metabolism.
  • We’ve seen where people can do many more intervals than they could otherwise because they’re not burning the matches. They’re not using up they’re oxygen-independent stores of metabolism. They’re able to stay aerobic while doing this.
  • This is a way to guide your interval training to maximise your oxygen utilisation capacity of your muscles.
  • Another aspect is that if your goal is to stress your cardiac system, you would want to set your interval intensity so that you could maintain it for 5 minutes. Tweak your intensity so that you would go from peak SmO2 to your low plateau over a period of 5 minutes rather than dropping in 30-45 seconds.
  • This gives your cardiac system enough time to get fully up to speed and you’re going over your FTP so you’re working your cardiac system as hard as you possibly can. Then you stop and let that recover.
  • So again, you’re staying aerobic because you’re not continuing to do work after you’ve reached that low plateau but you’re giving your cardiac system enough time to make it work as hard as it possibly can.
  • This is a way that you can target a particular system. Whereas, if you’re doing all-out intensity, your heart rate never gets up to the full intensity. By the time you’re done with the interval, your heart rate isn’t even maxed out yet.
  • This is another way where you can dial the training in to accomplish what you want to rather than just leaving it to chance.

Is there a pattern in how your users tend to change their interval training when starting to use the Moxy device? 

33:10 -

  • It depends a lot on the individual. The interval training is great for helping people to improve their muscle oxidative capacity. The people who really need to do interval training are the ones who can’t desaturate well.
  • We have people who did the interval training for a few months and they go from not being able to desaturate at all to be able to start to desaturate.
  • As mentioned earlier, a lot of people start finding that they can do many more intervals than previously when they control this training with Moxy.

How can triathletes get a Moxy or get tested for it?

34:56 -

  • Moxy is worthwhile for athletes who want to get faster efficiently or improve their performance more efficiently.
  • Also, if you want to learn more about yourself and your physiology, if you want to be more involved in your training and understand how your body is reacting and learning to adapt when you’re fatigued, Moxy is important.
  • To some extent, if someone is brand new to sport, one who is going from being a couch potato to just starting to exercise, it’s probably not as useful.
  • As far as where to go, if you go to our website, we have a Training Centres tab there. We have certified training centres all around the world. A lot of them do remote training.
  • For athletes who want to get hooked up with a training centre, contact us and we can get you referred to a training centre that would be a good fit for you.
  • We also have some self-trained athletes. We have some training materials that are available on the website.

Do you have any case studies of athletes who have been using Moxy and the improvements they have seen from it?

37:04 -

  • One of our training centres in Canada has 11 Ironman triathletes, 9 of them had personal records after they started using Moxy.
  • Another interesting one is Nike. They have been working on a project to try and achieve the first sub 2-hour marathon. They just had a documentary posted on that recently. If you look closely, you can see that they’re using Moxy.

Rapid-fire questions

40:16 -

Favourite book, blog, or resource related to endurance sports:
DC Rainmaker

A personal habit that helped achieve success:

  • Focus and tenacity

What you wish you had known or done differently at some point in your career:

  • I wish I had done more work with programming.

Links, resources and contact

Connect with Roger

Links and resources mentioned

Connect with host Mikael Eriksson


Hi! I'm your host Mikael,

I am a full-time triathlon coach and an ambitious age-group triathlete. My goal is podium at the Finnish national championships within the next few years.

I first started the website Scientific Triathlon in autumn 2015 as a passion project to share my learnings with a larger triathlon audience. Later on, in early 2017 I started the podcast That Triathlon Show. 

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Let's discuss this episode and the topic in general. Post any comments or questions in the comments below. I'll be here to reply and take an active part in the conversation, so don't be shy!

What's your take: does the real-time measurements of your physiology offer enough additional benefit that you would consider investing in the Moxy device? Why or why not?

Mikael Eriksson

I am a full-time triathlon coach, founder of Scientific Triathlon, and host of the top-rated podcast That Triathlon Show. I am from Finland but live in Lisbon, Portugal.

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