Do you know the ins and outs of the NIRS technology? Joining me on the show today is my good buddy, Dr. Jesse Craig. Jesse is a postdoctoral research associate at the University of Utah, and he was also my post doc when I studied in his lab for two and a half years. Jesse studies oxygen delivery and utilization and its impact on exercise tolerance in health and disease. So, with the NIRS device becoming more and more popular in the fitness industry, what better time than now to bring Jesse on to share some of his knowledge and research working with the technology.
We dive into the episode giving a background on what the NIRS is and how it works. It’s becoming more prevalent that athletes and coaches don’t understand the ins and outs of the technology: what is the data giving us? What conclusions can we draw from that data? What can we use the NIRS for? Jesse then unpacks the importance of balancing oxygen utilization and supply. Without being able to match supply and demand, health and performance will deplete.
We do a deep dive into the limitations to the technology and the primary drive of change in saturation. We then unpack the importance of critical power in exercise physiology. Be sure to listen in to discover the good, the bad, and the ugly of the NIRS.
What You’ll Learn in This Episode:
- [04:37] An intro to Dr. Jesse Craig
- [07:09] Background on what the NIRS is
- [08:39] Different technology of the NIRS
- [09:54] Balance between utilization and supply
- [12:29] The NIRS role in the VIC equation
- [15:48] Limitations when looking at SAT
- [18:43] Primary driver of change in saturation
- [20:50] Typical probe distance
- [25:30] Rectus versus the vastus
- [27:44] Blood flow’s role in oxygen utilization
- [30:48] The occlusion technique
- [34:07] The importance of critical power in exercise physiology
- [38:49] Where to find Dr. Jesse Craig
James Cerbie: So let’s jump into the episode today with Dr Jesse Craig. Alright, beautiful. There we go. We are live with Doctor Jesse. Correct. Actually, I don’t think I’ve ever called you Doctor Jesse before.
Dr. Jesse Craig: Yeah, you don’t need to call me Doctor.
James Cerbie: You got to reward the work, the time and effort that was put in to get those letters.
Dr. Jesse Craig: Yeah, I get random emails and people call me Doctor, and it feels weird.
James Cerbie: That’s how you know whether or not they’re within the circle. The friend group of sending you emails when it leads with Doctor, you’re like, red flag.
Dr. Jesse Craig: Sign it real hard. Jesse.
James Cerbie: What are you going to try to sell me? I still get emails, actually, from people trying to sell lab equipment.
Dr. Jesse Craig: Is it the Texas group?
James Cerbie: It’s a whole bunch of different people. I don’t I usually just delete them right away. They have been relegated to the spam folder now, not in the market.
Dr. Jesse Craig: I never know if you want, like, a next such a neck pressure thing. I don’t know why you want it.
James Cerbie: But or one of those lower body boxes built. Well, let’s do this for everybody listening that doesn’t know who you are. Can we give them the elevator pitch of your background? Who you are, what you do. The goal of the conversation today is for us to have a very birds eye view chat about this NIRS technology that’s becoming more and more popular and prevalent and this fitness, strength and conditioning realm just to kind of give people some context what it is. What does it actually do?
Like, how do you guys use it in research? And then importantly, what are some of the limitations of the technology that we just need to keep in mind when we’re having these conversations because no one likes to talk about limitations. Everyone just likes to talk about. Look at this awesome stuff it gave me. So just kind of like, give a framework to this up and coming air quote, technology, but you lead with just who you are, your background, etc.
An Intro to Dr. Jesse Craig
Dr. Jesse Craig: Okay. So as James mentioned, I’m Jesse Craig. I did earn a PhD in kinesiology from Kansas State University. Now my research concentrates or then and now my research concentrates on oxygen delivery and utilization. So how we get oxygen down to the muscles and how the muscles use that oxygen. And then we evaluate how that impacts exercise tolerance. You might call it performance, but it’s really tolerant of health and disease. So if we can’t match the two demands with two deliveries, we can’t keep up the exercise or even daily activity in some of these disease populations.
Right. So most of my work is evaluating mechanistic determinants of how we match the O two delivery to utilization, which is actually really neat. That ties into the NIRS because the nears itself is providing a non-invasive indication of that to delivery utilization matching at the muscle. In my Masters, I trained with Tom Barstow. Dr Tom Barstow. He is actually one of the world leading experts in NIRS technology, at least as to how it is used in human performance or human exercise research. So a decent background in years and continue to publish with it and use it pretty regularly in research now.
Dr. Jesse Craig: Yeah.
James Cerbie: It came from the powerhouse Kansas State with Barstow and then bankie, bankie, bankie.
Dr. Jesse Craig: Bankie.
James Cerbie: Bankie. Yeah. Phenomenal oxygen physiology, cardiovascular muscle stuff that’s been coming out of there for a long time. And then I was just going to say I’ll give it a quick background of you are the post doc when I was in the lab at Utah Vasio Research Lab. And so everything that I know about the new year’s comes from you and Ryan and Joel. And the reason I want to have you on is because my knowledge and ears are significantly better than most strange coaches. But then your knowledge of the years is still white years better than mine.
So maybe with that kind of as the background and framework here, you have a lot of time and experience using nears and a lab clinical setting. And so let’s start with what we started to go down this path. Let’s start with what the NIRS is. Right. Because you began to mention that we are trying to investigate this O2 supply O2 utilization at the muscle and trying to figure out are those two things being matched? Probably or not. Right. And so the near is one way we can look at that.And I’ll let you take it from there.
Background On What the NIRS is
Dr. Jesse Craig: Yeah. So the NIRS technology as we know it, I think so. Case States and a school. So we had an interesting side introduction to the NIRS technology, where I think it was developed for the people evaluating the viability of the moisture content and seeds for agricultural purposes. And then one day someone found out, hey, if I stick this on humans, it gets a signal. And so there’s this really neat property with the chromophores in our body. So hemoglobin in the blood and my globe in the muscle.
And then cytochrome C oxidase also is in there a little bit, but we’re not going to really worry about that too much. But these chromophores, when they have oxygen, it changes their absorption properties to this light. And so this neat property of near infrared light, so it’s on that border the spectrum where it’s not visible to us, some of it and some of it might be. But this light can penetrate through human muscle and skin and even bone. And so where they first started using news was on the cerebral evaluation to see how much the oxygen saturation in the brain.
Different Technology of the NIRS
And so, as I said, whether or not oxygen is present on those chromospheres changes the absorption. And so they evaluate how much of that light that was emitted comes back. And that gives an indication of the saturation of those chromospheres in that tissue of interest. There’s a couple of different technologies in recent years. I don’t know that we really need to get into the nitty gritty of that. I do know that one of the more popular ones, particularly in the performance field, are the continuous wave because they don’t need, like, a big box.
They can do it wireless. Right. And so there are some limitations to go with that continuously that we can get into an hour later. But in essence, what it does is it shoots the light through the skin down into the muscle, and it penetrates roughly half the distance between the emitter and the detector. And so if you’re sufficiently thin, you’re getting primarily muscle signals. And what it’s picking out, the main signal from those is that saturation. So it’s just looking at it using algorithms to calculate how much of that light got through. And then based on that, it’s saying your saturation in this tissue.
James Cerbie: Yeah. So it’s really just trying to give us an idea of the balance between how much of our chromophores, hemoglobin, myoglobin, etc are loaded with oxygen and how many of them have been unloaded with oxygen. Right. Because if you think the thick equation is the easiest way to think about this balance, potentially, where we’re trying to figure out the VO2 at the muscle, how much oxygen is being consumed potentially, which is a balance between the utilization and it’s why. And so you have blood flow showing up. And then I have this AVO2 different component.
Balance Between Utilization and Supply
Dr. Jesse Craig: Yeah. So the AVO two is the extraction of that delivered oxygen. Right. And so then because of physics, the conservation of mass, anything that’s delivered and doesn’t come back out was then taken up by the muscle. And that’s our oxygen consumption.
James Cerbie: And in the lab, what would be the gold standard then for, like, if you could do it exactly how you want? We’re not worried about being invasive or not the gold standard for investigating the balance of supply and utilization. What would the gold standard be?
Dr. Jesse Craig: Yeah. So we can do a direct fit, and we do it quite frequently here. In, in essence, you introduce the catheter. So the small plastic tubes into the we do a lot of knee extension exercise here. So you’ll put a catheter in the femoral artery and the catheter in the floral vain. And then you measure blood flow upstream of that. And so now you have an idea of how much blood flow is being given to that leg. And with blood gas sampling from the arterial and Venous side, we can see the two contents, the PH, lots of different, fun things in there.
But in essence, what we get is if you multiply the two contents in the artery times the blood flow, we know delivery. And then we can do the same thing and get the oxygen extraction by doing the AVO two differences across those two catheters, the gold standard, as you said. So we know that everything going into that leg went through that vessel that we catheter on the arterial side, and everything draining that leg came through the Venous catheter. So we do get some mixture from the cast and the foot, but those aren’t really active during the need for extensions.
We can say pretty confidently that most of that measurement that we’re making is coming from the active muscle, especially when it’s active.
James Cerbie: Yeah. Exactly. And then bringing the nears on board for that is usually going to be because not everybody really has access to the capacity to get anesthesiologists to place lines and to do all this far more invasive research similar to the study that we are running together. And I was in the lab we use years for my study because we didn’t want to have to worry about the catheters and all of that fun jazz. So what role then is the NIRS playing? If we think about the same setup, I got a Doppler alter sound on the moral artery so I can get blood flow, and then I’m putting a nears on.
So what are the nears actually doing for us? We think back to this equation concept. Yeah.
The NIRS Role in the VIC Equation
Dr. Jesse Craig: So the NIRS device we used, all of the NIRS devices give that saturation, but they calculate that saturation based on two actual measurements. So they measure the oxygenated chromophores and the deoxygenated chromophores. And again, that’s the absorption. And so then based on the proportion of oxygenated to the total oxygenated and the oxygen that gives you a saturation. With our device, we can get pretty accurate absolute values for all of those. And so depending on the experimental setup, what we care to look at that deoxy signal is a pretty good approximation to that AVO2 difference.
It reflects that kind of extraction. So at rest, your deoxyhemoglobin will be at a certain level. And as you increase your exercise intensity, you’ll see deoxy go up because you are extracting more out of blood as it goes through. And so with your study, we’re on the cutting edge of near use, and we can get into that if you want. I don’t know how much you wanted to get directly to that. We do have a little more data with it. It does seem based on what we did back in Kansas State with Barstow and Ryan Brock shamanic, you mentioned with a sufficiently advanced nears device, you can do a rough approximation of the tick calculation.
And it seems to be working, especially among young people right now. But again, that requires you to have an adopter machine to measure blood flow and one of the more fancy NIRS. But typically people in exercise testing, they use this near just to evaluate either an intervention. So whether it’s the pre post training, they want to see how the extraction or the saturation is how the body performs that given intensity of exercise, pre and post intervention, whether that’s training or D training or some sort of drug intervention, or a beat roof supplement, which has been real popular exercise physiology recently.
James Cerbie: In my study, we use primarily a deoxy signal to tell us what’s going on with extraction. So let’s Zoom back out a little bit, because a lot of the athletes that listen to this are the coaches are not going to have access to a Doppler and all these other things that we were fortunate enough to have the conversation in our realm. What people usually talk about is they just talk about sat, which is this combination of the oxygenated, chromospheres, hemoglobin and myoglobin. That’s a really important point, because it’s not just hemoglobin and then the total, which is oxygenated and deoxygenated together.
And I’m looking at that ratio. So I think this could probably be a good place to dive in and talk about what are some of the potential limitations, things we need to keep in mind with this technology because it is exciting because it does give you data. But what’s hard is, I think, always keeping in mind that we need to separate signal from noise in this. And I think that can get really difficult. This technology when you consider the different confounding variables, right. Like Maya Globen is contributing potentially six of the signals you’re picking up fat tissue.
Limitations When Looking at SAT
I know that you can totally change the Sat Mark just by increasing temperature and getting more blood flow through the skin. So you have all these different considerations. So I’d love to hear you kind of talk about what are some of the limitations when we start talking about just looking at Sat as our measure.
Dr. Jesse Craig: So looking at Sat by itself is actually one of the safer approaches you can do with the spectrum of NIRS technology. So as I mentioned, the continuous wave ones don’t adjust for scattering. And so just quickly scattering as those photons go through the tissue, they hit things and bounce. So you lose some of them that weren’t absorbed, they just bounced and never made it back to the detector. But a majority of them will get there eventually. And so it kind of delays them. Sometimes the continuous wave doesn’t adjust for that.
But the Saturation that it gets is still pretty accurate. All things considered, when you get down to the specifics, like the deoxygenated or the oxygenated or the total of the two, it’s not necessarily super accurate with those pieces of equipment, but the changes during exercise in that set seem to be pretty, pretty consistent with the equipment. But like you mentioned, there are several confounding factors. So heat, which would increase the skin blood flow. You can kind of dilute your Saturate that way at that point. So the skin is not really metabolically active.
It’s primarily that they’re just the coolest. Right. And so you can artificially increase your Saturation by getting a greater skin profusion. Ideally, if you’re using the NIRS to evaluate either your training effect or your performance, you would try to keep, like, your conditions similar as you could across. So in the lab, we can control that really, really well, which isn’t necessarily ecologically valid. So if we go out and do a field test, we’re not going to be able to say, hey, it’s exactly 72 degrees there.
I think so. That for people wanting to use nears to evaluate certain things, they should try to, like, keep consistency across their evaluation days, at least. But changes in Saturation, I think you’re safe with that with most of the technology that people can have easy access to. I was just like you mentioned, we don’t know necessarily, though, what’s driving that Saturation signal. So if they have a greater blood flow, Saturation would be higher. If they had a lower metabolic demand, Saturation could be higher. And so you don’t know what’s driving that difference day to day, necessarily.
James Cerbie: That was going to be an important one I wanted to bring up because the change in fat does seem to be accurate enough for us to use. But then being able to parse that apart, parse that apart, I don’t really like that, but we’ll go with it in order to parse that to figure out. Okay. Why is it changing? Is it a supply based thing or is it a utilization based thing? Which of these two components is really driving the Delta? And I think the issue is with just Sat, we can say it’s definitely changing, but we can’t go to the next level and say it’s this variable or this variable that’s the primary driver of the change. Correct?
Primary Driver of Change in Saturation
Dr. Jesse Craig: Yeah. I mean, you could use some inference, like, we wouldn’t expect dramatic changes in uptake at, like a given workload unless you went from being, like, really untrained to really training. But again, there are lots of things that can impact that delivery. You are at a different altitude that day. You can interfere with it a little bit, or if you drink enough water the day before. And so you’re a little bit dehydrated, you’d say, you know, and so that can interfere with delivery a little bit.
And also your Matic or your hemi concentration in the blood itself. Yeah. I think as long as people try to be really consistent with how they’re using it in the day and the test that they’re doing, hopefully it’s consistent across those. But there are lots of little tidbits in there that get away for sure.
James Cerbie: I think another important thing to bring up here is when we put the nears on. So let’s say you put it on a Rec. Tim, for example, in our world, I haven’t finished reading that Nike paper you sent me yet. I’m just part way into the method right now, but they put it on a quad instance. They put it on a quad and have them go run on it’s a self-power tread. Alright. One of those curved treadmills. And so what’s interesting here is if we’re putting this on and this more fitness strength conditioning realm, you’re putting it on to probably want to make some more globalist type claims, but with the technology that can actually get really difficult because we know the signal is not homogeneous, it’s a hetero signal.
So just because I’m seeing Sat do one thing directly underneath this probe in the middle of my Rec film doesn’t mean that I could come to a totally different conclusion if it was on a different part of my Rec film, if it was on a different muscle entirely. So I’d love to hear you unpack that a little bit, because that’s for me, one of the hardest things to figure out what this technology is. I can only make claims about what’s going on directly underneath this, like several inch long probes.
Typical Probe Distance
Dr. Jesse Craig: Yeah, like the typical probe distance. I think people are probably getting, like, an inch deep into whatever they’re measuring. So, like you mentioned before, if you do have a little bit of fat on your quad, any of that fat is going to reduce how deep it’s penetrating into the muscle. But we’ll say it’s an inch on most people. There’s been some really neat work out of Japan and Kobe, Japan, with Shintaku Koga, who collaborated a bunch with my mentors as I was going through grad school. And even now in my postDoc stuff, he has a really powerful NIRS device that they got approved over in Japan.
And so they can do an extra deep kind of penetration. And they’ve shown really, really interesting stuff that you mentioned. So, like, the vastest ladder, Alice, the rectus femoris, the vastus medialis, they all
have separate kinds of oxygenation characteristics during activity. But then if you go down the leg, so you go distal on that muscle versus proximal up near your hip, you can change how it behaves. And then the deep versus shallow. There’s also quite a heterogeneity, as you said, of how that o to delivery and demand is being matched just due to different properties of the muscle fibers themselves and how they’re integrated with the blood vessels and the cab Larie.
And, you know, maybe that relates to the function of those particular muscles. Again, they’re not all activated equally when we’re doing an activity, like running or when we’re doing something biking or even a squat. You know, that is an important consideration where one muscle may tell you something completely different from the other.
James Cerbie: That’s the hardest one for me to try to wrap my head around because I have one of those Moxi monitors that the Nike paper I used I played at some time to time. I need to charge it and break it out and start playing with it again. Actually, that’s when the hardest one for me is to try to rationalize my way through. Like, I’m looking at it and I’m trying to draw some conclusions. But like, the minute that, you know, that something’s heterogeneous, it gets really hard to be able to make any type of large enough claim about what’s going on, like, whole body per se.
Right. And the point about the depth is really important also, because, like, the Moxy monitor, for example, which will be the one that most people in our industry are using, like, the depth you’re going to get is not going to be anything substantial. Right. Yeah.
Dr. Jesse Craig: And the rough estimate with the Beer Lambert law that they based all this on is half the distance between your emitter and detector is how deep penetrate. So I don’t know that equipment specifically, but you can take the measurement and get a rough idea of how deep it is.
James Cerbie: It’s not very deep. It’s smaller than the probe that we use in the lab. So you’re not getting very deep. And let’s just go ahead and say, if you’re working with people in our realm, there probably can be, like, slightly bigger, more muscular humans that have, like, if we use the quad example, it’s a really popular place to do this. If you’ve got a really big meaty quad, you’re only going to be getting data on, like, the super, the top superficial layer of what’s happening, and you’re missing everything in the deep muscle.
And I remember Dr David Pool asking me that part of the defense was, hey, have you considered the heterogeneity between superficial and deep because it looks like the majority of the action is potentially taking place in the deep muscle and not the superficial muscle. Yeah.
Dr. Jesse Craig: And the reason he brought that up. So David does a lot of animal research and a lot of humans as well. But like, my dissertation with David was primarily animal. In the rats, we can see, like, a dramatic heterogeneity of muscle fiber percentage. So, like type one, type two in the rat, they, like, have entire muscles that are primarily type two, and entire muscles that are primarily type one. And so we’re not that I don’t remember the term they use for it, but we’re not that dramatic in humans.
But even in humans, we do have a kind of, like, gradient of fiber typing within a given muscle. And they’ve shown this with cadaver studies where it’s a little easier for them to do it because they can take the whole cross sectional area of a quad. We can’t ethically do that in the living human. Right. So even though we have a pretty homogeneous fiber typing in the human quads, you do see that the more superficial portions of your muscle will be more type two, and then that deeper muscle, closer to the blood flow and delivery, they are more type one.
So the more oxidative muscle is deeper. So you may be biasing your interpretations based on a not very oxidative glycolytic muscle that’s more superficial. I do think another important consideration is so you mentioned the rectus versus like, we typically study the vastest because the vastest does have less adipose tissue. I published a paper for my Masters where we did just four sites, and you can see quite a dramatic heterogeneity even at a post thickness on a given muscle. So, like, the rectus had more fat typically than a vast ladder, Alice, than most people.
The calf has less adipose tissue than both of those. So you get a bit of heterogeneity in the adiposity, but you also need to consider it. So the rectus itself is I’m sure you’re familiar. It’s a double joint, so it crosses the hip and it connects down on the knee. And so then if your activity is like running, maybe the rectus is a good choice. If you’re looking at something like squatting or kicking and other things, maybe a muscle that only innervates cross the knee joint where you’re getting your primary motion is potentially better to be, like, the fastest.
James Cerbie: Sun is coming up here in Salt Lake, we finally got some decent air quality today. We got to walk this one. And I was like, God, this is so nice. I’m not just, like, getting bombarded with all this fire smoke from California.
Dr. Jesse Craig: You fly to California, get away for a minute. Follow you.
James Cerbie: That’s a thing. The people in California, unless you’re like, right by the fire, obviously. I want to be careful. I say this because the fires are a true tragedy. Yes. My father and mother in law are in Santa Cruz, California. Geographically, they are way closer to these fires than we are. They have perfectly clear weather because of all the smoke we just get hammered over here. Okay.
Dr. Jesse Craig: We actually aside, we actually had to cancel several studies this week because we’ve had, you know, four or five days with really high, please.
James Cerbie: I was actually talking to cells about that the other day. She said, what app should I use? I was like, well, we need to use Utah Air. Like, that was what we were using the lab. And I told her, I bet they’ve had to cancel some studies because the average 24 hours on the PM 2.5 has been really high.
Dr. Jesse Craig: Orange is better for, like, five days now.
James Cerbie: High score wins, right? Yeah. For the air quality in the world last Friday.
Dr. Jesse Craig: Yeah.
Blood Flow’s Role in Oxygen Utilization
James Cerbie: So one of the things I do, what I talk about here as well is we’ve talked about flow and blood flow being such an important part of this supply utilization, this balance of supply and demand. Can you unpack a little bit, because this is one of the things I think messed up the most across the board. The NIRS is not telling us supply, because what people will try to do is they’ll take the total component and then they’re going to try and say, well, total is telling me essentially supply.
And like, no, that’s not how it works. They’ll track together, right. But you don’t really have any idea what the supply function is. You don’t really know blood flow for years.
Dr. Jesse Craig: Right. So I’ll touch on the total a little bit specifically. So there’s some network that integrated the animal research David Pool did in the human research that Tom Barsa did and Shintaku Koga, they all kind of collaborated on this. And what they found was that the hematocrit in the capillaries at rest is way lower than our systematic thematic rate. So our systematic, maybe 45 on the typical person, you know, but they showed that in those capillaries and the animals, it may be as low as nomadic, but when you start the muscular contractions that very quickly approaches a systemic level of Matica, it never quite gets to the full amount.
But then they followed this up with a study in humans, and we found we didn’t find it, but they found that that total signal behaves very similarly to that hematocrit at the capillary level. So we take a step back a little bit. The nears will detect the chromospheres in tissues, including the manuals and the arterials and the capillaries. But once it gets over, I think they say about a millimeter. So you’re getting into your bigger diameter vessels to that point, it’s fully absorbing everything because there’s so many red cells in there.
And so most of your signals come from the microvasculature and the muscle. But what they found is that when you go from rest to exercise, that total signal increases in a similar proportion that we see when we do the rat studies, the super invasive ones. So that total signal may actually represent more closely the hematocrit in that microvascular space rather than the flow. Because while, yes, they increase, you know, proportionally, they’re not necessarily equivalent. And so the function of the near is measuring the volume or the concentration of these chromospheres per volume of tissue.
So in essence, you’re getting hematocrit assessment, especially during exercise, because we don’t expect my globe to increase. So the only increase should be driven by an increase in red cell content below the probe. So. Well, yes, it tracks with flow. It’s not the flow at all.
James Cerbie: Yeah. Very different.
The Occlusion Technique
Dr. Jesse Craig: People can evaluate flow with the NIRS devices. But again, you need to do periods of occlusion. You need to measure the changes in these signals either during or immediately after collusion.
So, like, you can’t really evaluate flow with the NIRS device without doing a bunch of additional work trying to evaluate or control for these things.
James Cerbie: But even with the occlusion technique, if I remember this correctly. So say you wanted to measure it on the arm wherever it is like an easy place to place a cuff. If I include, then I released that cuff and I’m going to basically see total increase because it should be pretty minimal when I’m fully included. I’m going to watch that trend line. If I remember correctly, you’re only really you, like, those first one to two cardiac cycles, I think, is what Bar saw in his paper.
You can only use that data for flow within a very short window of time immediately after releasing the cuff, because once you’re outside of those first one to two cardiac cycles, then you can’t really still do that. Is that right?
Dr. Jesse Craig: Yeah. You get some confounding factors now happening right? There’s very specific protocols with the occlusion and releases and the measurements to try and evaluate flow, and they’ve been validated. I don’t remember if you’re familiar with the Venus occlusion plethysmography where they used to. People still measure blood flow and limbs with a little strain gauge around the limb, and then they do periods of occlusion or lease, and they measure the change in circumference of that limb. And that kind of tracks with blood flow. And so, in essence, you have to do the same thing with the NIRS device.
If you want to evaluate flow, you can’t do it during contractions. That’s a big limitation, because, again, the movement of the muscle and the blood vessels will confound what you’re getting. So you have to do it like during a period of rest between the contractions. So there are lots of limitations to it.
James Cerbie: Yeah, for sure. So if we wanted to Zoom back out and talk more about the Sat measurement, this balance between oxygenated and total, I’m not super familiar with this, but if we wanted to think more applied, I know there’s a lot of what the guys at Nike did, so I need to get through that paper. But historically, have any people been able to essentially use something like Sat or even on the device that we had where we could get a strong, reliable deoxy signal? Could we use that?
And things like critical power assessment, where we’re trying to figure out this moderate heavy severe, maybe do a little bit of prediction to time to exhaustion, like, I’m not going to be able to parse apart and say, well, this person supply limited and this person’s utilization limited, whatever a lot of that means, despite the fact that people on the Internet love that talk. Right. But if we want to think because critical power has a very strong basis in the literature with the Sat measurement, or even just a deoxy, good enough, essentially to be able to start seeing and declaring moderate heavy severe, indicating, like, we have an inflection point here. Things in that realm, if I’m making any sense, they’re like, Wait, more applied performance usage.
The Importance of Critical Power in Exercise Physiology
Dr. Jesse Craig: This has been gaining a lot of steam in the last decade, and Ryan Broxson myself actually have got it in some scientific debates back and forth with other groups about whether this is appropriate or not, because there has been a big push where people are saying, hey, your deoxy signal will continue increasing as you increase workload. So, like an incremental test, it’ll keep going up, up, up, up. And then it hits a kind of ceiling, and it flat lines. And they’re like, hey, when this thing flat lines it can’t go up anymore; that’s your critical power.
And critical power is a big, big, important thing in exercise physiology because there’s some pretty strong evidence of that demarcates. It’s the threshold for the highest sustainable metabolic rate, where they’re showing that the marathon runners are running at the critical speed. And so it seems to be one of our stronger predictors of the upper threshold is a tricky one. But the upper level of sustainable aerobic activity. So a lot of people want to find ways that say, hey, we can isolate this with this non-invasive, super easy thing to stick on a leg.
There’s lots of nuances to that, for sure. But the saturation signals and the deoxy signals, they do behave like we should. They behave like we would expect with oxygen. Update, where you’re different intensity domains. You mentioned the moderate. So the moderate, you very quickly reach a steady state, oxygen uptake, and you can sustain that activity for three 4 hours. You’re even longer. You can do that for days as long as you feed appropriately during the activity. Like the ultra-guys, they’re probably working down in the moderate inch and eating tons of food between their hours of running.
If you go above that heavy, so during the heavy, you do reach a steady state, but it’s delayed a little bit because you are recruiting the higher order type two fibers and you are seeing changes in the perturbation in the muscles. So you’re getting a little bit more acidic. But you will reach a study saying you can maintain that activity for a while, several hours. Again, if you’re appropriately hydrating, feeding and cooling. But then if you go above critical power, you predictably fatigue.
Like we’re talking 20 minutes or even down to a minute that you can sustain this activity. So it’s been a real important point that people try to pinpoint. As with everything, there’s a lot of Gray to it. So specifically with the continuous wave, I’m not sure. Without a big, complicated setup like the Nike paper, which we can talk about again, if you want to bring it back or bring me back and talk about it. But it requires lots of testing, repeated testing. And then if you do it appropriately, it seems like it does predict athletic performance, at least during these aerobic events.
So they measured it on runners with quite a range of distances, for sure.
James Cerbie: Yeah, I need to get through the rest of that paper, and then maybe we can come back and jam on that a little bit. But what I’ll do is I’ll throw links in the show notes to the bar Stove review. I think that’s still required reading. If anybody is going to use the technology, that review is fantastic. And then I’ll throw a link to the Nike paper, the bar so one you can get for free. I know, just off PubMed or Google Scholar the Nike you may need.
Dr. Jesse Craig: I think it’s still under embargo because it’s so new.
James Cerbie: Yeah. Okay.
Dr. Jesse Craig: A year from the date of publication, people should be able to get it for free.
James Cerbie: Beautiful.
Dr. Jesse Craig: They might be able to email the author and say, hey, can you send me the paper the author is allowed to share?
James Cerbie: Okay, that works, but a quick take home.
Dr. Jesse Craig: So we mentioned the different intensity domains, and that the SAT signal does behave differently based on the intensity domain. And so if you know what you’re looking for, you can kind of parse out. You say, hey, my Saturation plateaued very quickly. So you’re probably not working too hard. If your Saturation takes a little longer to Plateau, you’re probably working in that heavy domain. And if you never quite get a Plateau, it said that SAT just keeps dropping at a slow rate until you fail, you’re definitely in that severe domain above your critical power.
So, again, with lots of repeated testing, you can kind of get an idea of where that threshold is, and you can kind of set up your training around it for sure, because there’s lots of evidence now that W I keep hammering on this aerobic kind of activity that training at or around that critical speed. Critical power is probably going to give you quite a good benefit on your training adaptations, since you know where that threshold is and you can kind of push and design your training around it.
But again, it will take quite a few tests to try and figure that out.
James Cerbie: I actually find those demarcations.
Dr. Jesse Craig: If you’re doing all those tests, you can just calculate your critical power, your critical speed on its own without the NIRS device.
James Cerbie: That’s true. That’s very true. Excellent man. Well, Jesse, thank you so much for coming on to do this. If everybody listening, found this informative, incredibly helpful. So if anybody wanted to follow up with you, if you would like to be found and you can say no, no one has said no yet, but I’m waiting. Someone will eventually say no. Where is the best place for them to reach out, contact or find you if they’re really interested in this topic.
Where to find Dr. Jesse Craig
Dr. Jesse Craig: So my University email is available. If you find a paper that I’m on that I wrote, you can put it in your shots. I’m not too worried about my email getting out there, but yeah, people can shoot me an email if they want to talk more about it. I tend to get back to people fairly quickly. Depends on how busy we are in that day in that week. But yeah, I look forward to talking to more people about it. I do love nears it’s just it is one of those black boxes that people over interpret.
James Cerbie: Sometimes literally, literally and figuratively. A black box. Beautiful. Well, thank you so much for coming on. We’ll have to do this again and talk through that Nike paper because I think that’s such a really cool applied way of using this in a performance setting and actually trying to predict fatigue markers and things like that. Time is Austin etc. Yeah, man, thank you.
Dr. Jesse Craig: No problem.
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