In this episode of the Flex Diet Podcast, I dive into practical ways to boost muscle growth, enhance performance, and improve body composition — all within a flexible, health-focused framework. I also talk about my Physiologic Flexibility Certification, which is closing for enrollment soon, and unpack key concepts like Human Dynamic Range (HDR) and cross-adaptation. HDR is all about building a wider physiological range — the ability to handle more stress and recover faster — while cross-adaptation looks at how training one system (like heat or hypoxia) can improve others. Plus, I dig into new research and share actionable strategies you can use right now to strengthen your cardiovascular and respiratory systems.
In this episode of the Flex Diet Podcast, I dive into practical ways to boost muscle growth, enhance performance, and improve body composition — all within a flexible, health-focused framework.
I also talk about my Physiologic Flexibility Certification, which is closing for enrollment soon, and unpack key concepts like Human Dynamic Range (HDR) and cross-adaptation. HDR is all about building a wider physiological range — the ability to handle more stress and recover faster — while cross-adaptation looks at how training one system (like heat or hypoxia) can improve others.
Plus, I dig into new research and share actionable strategies you can use right now to strengthen your cardiovascular and respiratory systems.
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Speaker: [00:00:00] Welcome back to the Flex Diet Podcast. I'm your host, Dr. Mike Nelson. On this podcast, we talk about all things to increase your ability to add on muscle, increase performance, improve body composition, do all of it within a flexible framework without destroying your health today. Important announcement.
If you're listening to this as of Monday, October 20th, 2025, the Physiologic Flexibility Certification. Closes tonight at midnight Pacific Standard Time. So if you're listening to this, as of today, on Monday, there's still time to get in if you want to enroll, which I would highly recommend of course, because I'm biased and I created the course.
You can go to the newsletter link below and then just hit reply and I'll get back to you within a couple hours. I've got a couple newsletters going out today about some more information about it. It's probably gonna [00:01:00] be the best place. If you go over to the old Instagram, you'll be able to find a link in my bio page there also.
And yeah, I'm super excited. I've got a bunch of people who said that we're gonna enroll today. Also, I know everybody waits till the last minute, which I totally understand as I'm one of them, but if that is you, your time is ticking down. The next time it'll be open is scheduled to be open in March of 2026.
So hop on now. Hop onto the newsletter. Hit me up with any questions you have on the certification there. We do have payment plans on the site there. I did send out a notice about some secret bonus items that I didn't make public, so you wanna check out those, or if you missed them, just hit reply onto the newsletter, which is free to hop on and you get all sorts of great information from me.
So today I wanted to talk about what I think [00:02:00] is probably the biggest key in the physiologic flexibility certification. I'd say there's two of 'em and not many people talk about it at all, but I find it super fascinating. So the first one I've talked about on this podcast before is what I call HDR, human Dynamic Range Simply.
The greater range you have over your physiology or that your physiology can exert, I think the better position you're going to be in, a simple example is heart rate. All things being equal, a higher heart rate and a lower heart rate. An essence gives you a greater human dynamic range if all their variables are the same.
This allows you much higher levels of performance. It allows you to upregulate to the task. And then also to heavily down regulate to the task. I know heart rate may not be the net best marker because that is the result of different adaptations. But it's just a [00:03:00] way to frame how to think about these things.
Respiratory rate would be another one. Higher respiratory rates, again, this is assuming you can move a fair amount of air and lower respiratory rates are a good thing. So the bigger range you have there. Again, all their variables being the same is gonna be better. So we'd wanna see a low respiratory rate overnight.
Ideally, my biased opinion, probably around 12, maybe 13. 14 is okay. 15. Yeah, you're on the edge. 16, 17, you definitely need to work on that. We go super deep into that on the carbohydrate, or I should say not carbohydrate, but. Carbon dioxide tolerance area, which is pillar number four in the ert, all about oxygen and CO2 regulation.
I could take HDR of sprint performance so the faster you can go from zero to top end speed, [00:04:00] that is gonna be a benefit. The faster you can run and almost all sports is a benefit. Again, there's specificity of sports. Is it linear? Is it change of direction, et cetera. So that'd be concept number one. How fast can you change direction?
And what are the ends? This is very similar to the concept of metabolic flexibility on the right end of the spectrum, how well can you use carbohydrates? How far can we push that to the right? And think of a kind of a barbell type model. How well can you use fat on the other end? Or can you down regulate and use fat to a high degree?
Part three is how well can you switch back and forth between these two? Very similar with the concept of human dynamic range. How high is your high? How low is your low? How fast can you switch back and forth between them? So this is also the basis for the flexible cardio courses. So in there we do a lot into the depth of the different cardiovascular [00:05:00] performance.
But there is some ways you can test this. For example, you could do a one minute or two minute heart rate recovery test, which I think is a great test that is vastly underutilized by a lot of people. What this is telling you is what was your max heart rate? What is your resting heart rate? And then transitions.
How fast can you get back to baseline? The faster you can be, back to baseline is a benefit, and I would argue for both performance, recovery, and health. This is a sign of a high amount of parasympathetic tone, or what's called parasympathetic reactivation. So the braking portion. So if we use the analogy of a fancy car, we'll take an F1 car.
'cause I got to present to those guys a couple weeks ago, which was amazing. It was awesome. The F1 cars, not only are they great for acceleration handling, have huge horsepower. They have massive brakes so that you can control. That, and you can decelerate as fast as [00:06:00] possible also. So the second part is what's called cross adaptation.
Very simply, you can think of this in terms of training as transfer. So for example, if you want to deadlift more, then the said principle specific adaptation to impose demand would say that, shocker, you need to deadlift. And if your goal is to do one RM three RM, or five rep max, the more specific you can make it to that goal.
That is going to get you there faster. Now, the downside of this, anyone who's done lifting knows that you're not gonna go to the gym every day and try to do a one around on your deadlift. Now, you might get by with it. If you're new, you may get by with it a few days. Again, I would not recommend that you do this.
So there is a limitation to how often you can do very specific practice. Now again, the more you can do that, I do think you will get to your goal faster, but there's some limitations on that, whether it's structurally [00:07:00] energy, nervous system wise, et cetera. But you can do other things that may transfer to your deadlift and have it go up.
They may not look like a deadlift. So if you're a big fan of kettlebells and you've read lots of paval stuff, you may say, whoa, kettlebell swings. So I'm gonna do kettlebell swings. And see if that increases my deadlift. If it does, then the kettlebell swing transfers to increase your deadlift performance.
The kettlebell swing doesn't look anything like a deadlift, but increasing your kettlebell swing performance transfers to increasing your deadlift performance or what the fancy words are. It gets into di dynamic correspondence and all that kind of stuff. If we take those concepts now and we transfer them to physiology at large, we do see something in the physiology literature, which is called cross [00:08:00] adaptation.
You're doing something that improves said thing and then transfers to something else. So in the physiologic flexibility certification, we talk a lot about. This in the best examples are actually in the temperature regulation area of heat to hypoxia, cross adaptation. So here's a study from Dr.
Rodriguez and exercise physiology 2025 heat to hypoxic cross adaptation effects of six weeks post-exercise, hot water immersion on exercise performance in acute hypoxia. So again, this has lots of really fancy words. So hypoxia is just a fancy word for low levels of oxygen. And what was cool on this study is that they did it for six weeks.
So they're looking at post-exercise, hot water immersion. So what I call this in the cer tubby time, you're doing some type of [00:09:00] hot tub type thing. And we are gonna look at how does that affect chronic, I'm sorry, in this case, acute hypoxia. So exposure to a low level oxygen condition, which in humans is very threatening.
Now this could be from a long breath hold. This could be from fancy type equipment you could use in the lab or also just simply exercise induced, right? Because at some point when you max out aerobic exercise, you've maxed out the body's ability to get oxygen. So you are gonna have a slight hypoxic condition there.
So what did they do? This study evaluated the effectiveness of a six week program of hot water intervention on exercise performance in hypoxia. So here hypoxia was an O2 of 13%, which is pretty darn low. So they took 20 healthy volunteers that were 28 years old, and they had a VO two peak of 47. So pretty good.
This is 12 males, eight females. [00:10:00] They completed some fun interval cycling. This is a classic Norwegian four by four, so four rounds of four minutes at 90% of max heart rate, and they did this three times a week, so they really beat the crap out of these people. And then they followed that by hot water immersion, either at 34.5 degrees Celsius the control, or 42 point.
Yeah, 42.0 Celsius. So they've got just a neutral water condition and they've got the warm water, and they do that for 40 to 50 minutes, five times per week. So what did they find? So following the six week intervention, post-exercise, hot water immersion group exhibited lower resting heart rate and a core temperature, and they actually had elevated hemoglobin concentrations.
Now this matches some of the other literature with sauna showing expansion of hemoglobin, sometimes plasma volume. If you increase plasma volume in general, you'll see a [00:11:00] lower resting heart rate, and I would theorize that's probably what's going on here. Also compared to the control group, the hot water immersion group showed greater improvements in time to exhaustion during a trial under hypoxia, but not in aerobic peak or peak oxygen consumption.
What was interesting is that only under the hypoxic condition do they see those changes. Now remember, both groups here are doing these same exercise intervention. Just one is doing hot water immersion. The other is just doing neutral water immersion. What's also interesting is that during the hypoxic steady state exercise, which they did at 60% of EO two peak, the hot water immersion group exhibited lower core temperature and higher peripheral oxygen saturations, even despite the hypoxic condition.
What they also served was no difference in group was observed between mean VO O2 respiratory exchange ratio, heart rate or perceived exertion, nor in VO two peak and aerobic peak under normal oxygen [00:12:00] conditions. So what I find here is super interesting that the benefits we did see a little bit lower heart rate in some other things, but most of the benefits showed up only when they were very stressed under the hypoxic condition.
So in this study, they concluded. Post-exercise, hot water immersion enhances maximal exercise performance under acute hypoxia, likely due to increased hemoglobin concentration, lower core temp, and improved respiratory efficiency. So, super fascinating. And what you find is a lot of the changes due to CO2 and O2 regulation, some of those do actually acutely increase performance.
Some do not. And some are very hyper specific to certain cases. So when I was doing the creation of the phys flex certs, the O2 and CO2 regulation portions, oh man, that took me the longest by far. 'cause I was trying to condense, basically respiratory [00:13:00] physiology, changes in oxygen, changes in pressure, changes in concentration.
Try and take all this stuff and to make it both usable based on the research and also accurate 'cause there is a ton, in my opinion, of misinformation in that area. So one other part I wanted to talk about here is this. Another, there's some really cool studies in this area. This one here is a cross adaptation to heat stress, to hypoxia, systematic review and ex exploratory meta-analysis.
This is by Wilmont, and this was published in the Journal of Thermal Biology February, 2024. And so they did pull nine studies, did a whole bunch of stuff, and what they found was that quote, cross adaptation may be appropriate for individuals such as occupational military workers. Whose access to [00:14:00] altitude exposure prior to undertaking sub max activity and hypoxic conditions is restricted.
Right? This is back to the transfer effect. So maybe they can't get specific adaptation practice, so maybe we can do this other thing and have it transfer. Methodological variations exist within the current literature, and females and well trained individuals have yet to be investigated where the research should focus on these cohorts and explanatory mechanisms underpinning cross adaptation.
So. There isn't a ton of data in cross adaptation right now. There is more and more data coming out all of the time, and it is, I find just a fascinating area. So they've got a really cool graphic here that looked at everything from cardiovascular to respiratory to thermal regulatory perception and performance.
And most of the benefits, if I were to summarize this show up with a specific practice and [00:15:00] potentially under maximal exercise performance. Now, again, that data is limited. Not all studies agree on that, but that would make sense because during maximal exercise, if you're gonna have any sort of hypoxic condition that is probably when it's gonna show up the most.
Another table. This is from heat to hypoxic Cross. Acclimation experimental data just gave a brief overview of some of the studies, and I'm not gonna go through all of them here. But Hal did one 2012. They did a heat acclimation protocol, 12 days, 120 minutes per day. And then they used walking at 30% of EO two peak.
What they found was, in terms of adaptation, the heat, peak heart rate dropped by 12 beats per minute. And peak time temperature wise dropped also. That makes sense. Right? So that is a specific adaptation to heat. We go over a lot of the heat adaptations and the f exert. The short answer there is, depending on what you're doing, you can probably [00:16:00] get relatively close to maximal heat adaptation within two to three weeks.
Now again, are you gonna hit a hundred percent adaptation in that time period? No, but you can get pretty darn close. So they did a hypoxic protocol and what they found was improvements response to hypoxia. Heart rate at certain levels was much lower. So another study by Lee 2014 heat acclimation period was only three days 75 minutes per day, including 15 minutes.
Preliminary rest they did heat tolerance increase by three minutes. Sweat rate increased by 23%. Plasma volume went up by 4.6%. So in only three days, we're already seeing a relatively large, let's say 5% change in plasma volume. This is also why in the ERT we talk about the benefits of Asana from both physiology in terms of what you can expect to see.
And the big one is [00:17:00] plasma volume changes. If you acutely increase your plasma volume, you will increase your aerobic capacity. Cyclists used to do this by doing infusions. Again, it's very limited 'cause your body on that point, you're gonna overload your system, which you have to be careful with because you can run into issues.
Again, I'm not recommending that, but just shows that the principle works. Again, if you go high overload, you do run some risks for sure, and your body will dre or it will get rid of some of that plasma volume relatively fast. Where sauna or heat acclimation those plasma volumes stay around as long as you are acclimated to the heat and your body will not just basically piss them out right away.
A toxic protocol, they did 15 minutes at rest and then 60 minutes at 50% of VO two peak by changing the amount of oxygen and what they saw in terms of improved response to hypoxia. Mean heart rate dropped about nine beats per [00:18:00] minute and a bunch of other temp regulations were better. So anyway, I won't go through all of these, but some very cool stuff showing that there is cross adaptation effects between the temperature regulation, how your body is doing that and its ability to function in a hypoxic environment.
So how does this work? Again, we're not a hundred percent sure. Most of this, if you go down to the cellular level, is that these heat stresses will trigger the production of protective proteins like heat shock protein 72. These can help the body cope with cellular damage caused by hypoxia at a systemic level.
Both heat and hypoxic stress can activate similar adaptive pathways leading to systemic improvements and how the body handles these different environments. What we notice with effects of heat acclimation on the hypoxia, [00:19:00] there tends to be a reduced strain. Individuals show lower core body temperature, lower heart rate, and lower blood lactate accumulation in sub max exercise in the hypoxic environment.
And this was only after heat adaptation. Again, this also works with some cold adaptation also. They did show increased performance, right? So the body ability to exercise longer and duration of a hypoxic condition and actually improved oxygen saturations. So peripheral oxygen SATs can lead to increases during exercise in hypoxia.
Again, if you want to get super fancy, you can even measure blood levels or what's called O2 with nears devices such as the NOx device or the Mxi device, and I have both of those. So it's been fun to play around with this stuff. The cool part is that a lot of these changes happen relatively fast.
They've shown repeated heat exposure over several days to a few weeks does show some effect. And like I said, the [00:20:00] study we showed there did show effects from six week intervention. Now is this gonna lead to amazing performance and be the number one thing you should do? By all means no. In all honesty, I'd probably sell a lot more courses if I told you yes, but obviously doing exercise is gonna be number one, proper nutrition and sleep.
Those are all covered in the flex diet cert. Those are the basics, but if you're getting it, I'd say at least a B in there, then I would explore the other interventions in the phys flex Cert. Number one is gonna be temperature. This is both cold and hot. Number two is pH changes, which we didn't talk about today.
But this is involved a lot with high intensity exercise and different types of breathing techniques. Number three is expanded fuels, so lactate and ketones, right? So if we're exercising in extreme environments, we're more likely to have higher levels of lactate. If we're really pushing high intensity intervals, we're gonna produce [00:21:00] lactate as a byproduct.
Ketones are on the other end more from fasting and running a ton of fat through the system. And it turns out both of these expanded fuels have a lot of really cool molecular effects. And with ketones, you have the advantage of consuming an exogenous source of ketones. You don't necessarily have to do a ketogenic diet to get there.
And so this opens up a lot of cool stuff you can do of, consuming exogenous ketones. My bias is Teton ketone EERs. Full disclosure, I do some work for them and I am an ambassador for them. But we've seen data from them and other companies that consuming these ketone nesters on an empty stomach within 20 minutes you can hit, 2, 3, 4 millimolar blood levels of ketones.
So that's pretty fun to play around with that. And there are some benefits to that especially with high level exercise, in cognitive function and potentially even in some [00:22:00] pathologies such as TBI concussion. I did a whole educational series for the care Institute. I do a lot of functional or clinical neurology, so I'm an associate professor with them.
I did a whole program for them for their docs on potential benefits of ketones there. Last component is gonna be, like I said, oxygen and CO2 regulation, and where the most cross adaptation literature right now is between temperature and hypoxia. If I were a betting person, I am going to bet that there are other cross adaptation effects within those four pillars I just talked about.
Although I can't point to a lot of data yet because no one has really looked at it, but all of these things overlap. They cross over each other and I think improvements in each one of those areas. From what I've seen over the past, man, I've probably been doing work in this area for. Almost seven, eight years [00:23:00] now.
It just seems like people are more resilient to just other stressors overall, even if it's not a heat stressor, even if it's not a cold water stressor. And I do think that is because of these cross adaptations running in the background. Now, again, we fully elicited all of those. No, we've only the surface there.
But if I'm, again, if I'm a betting person, my bet is we're gonna find a lot more of these cross adaptations working in the background. And then the last part is most people are not super adapted to these four areas, so they don't need a ton of work to get better in those areas. Again, if you haven't done a lot of cold water emergent or cold water, even just taking a cold shower is a great start.
And again, you probably don't need a lot of exposure because you're not adapted to it. Just like if you're new to exercise, you don't need a super fancy program, you just need some basic stuff to provide overload to your system. And you're good to go. If you haven't done a lot of super high [00:24:00] intensity intervals work again, in an intelligent manner, you don't need a ton of it.
Even when you are more experienced, like you still don't need a ton of it. So I think these are areas you can see a lot of bang for your buck. Again, once you're getting a B on basic exercise, nutrition, and sleep without investing a ton of time. Now again, there are some adaptations you may have to invest a little bit more time in.
But most of 'em are not super time intensive if exactly you know what to do. So there we go. There's all the information on human dynamic range and the cross adaptation effect. Both of those I find absolutely fascinating and I think you'll be hearing a lot more about them in the future. If you like those concepts, you'll love the physiologic flexibility certification.
It is closing tonight. However, as of this recording, which is a Monday, October 20th, 2025 closes at midnight Pacific Standard Time, we will put a link to the [00:25:00] newsletter there. Hit me up on the newsletter with any questions. We'll have a couple more information going out there. If you wanna purchase the link, you can go to the Instagram account, which will link, it's in my bio there also anything I can do to help you out.
Last part is when you sign up and enroll in the search. You get personal access to me, like you literally get my private email, which is me. It's not an AI bot or my assistant or anyone. They go directly to me and you can ask any questions about the certification because I do want to actually help you.
I'm not making it difficult for you to get in touch with me. I know some other, we'll say educational products, like you don't have any interaction with the instructor. But I do want to physically help you with whatever I can. Whether that's protocol development, whether it's something in the coursework you didn't understand all of it is a hundred percent online, so you can go through it at your own pace.
There's no rush to complete it. You get the big picture, how everything works together in terms of [00:26:00] physiologic flexibility, and then you get a really deep dive into the research of each area. It is presented in a way that it is more, I'd say intermediate to some advanced material. But I do my best to break it down into terms you can actually understand.
And then each area has five explicit action items. So you're not left holding the bag going, oh man, I have no idea what the hell to do with any of this stuff. Like I will literally explicitly tell you what to do next, how to determine what action item is for yourself or for your clients. And in some of the areas, I even have more advanced protocols.
So for example, like in cold water immersion, I have a whole PDF protocol breakdown of. If you happen to have exposure to say a cold plunge or you make one out of a freezer like I did what protocol do you want to use? There's different ways you can do different things. So there is a method where you can get a massive amount of parasympathetic upregulation [00:27:00] within literally minutes.
So if you really want to jam on the brakes as hard as you can and push the body into a state of recovery. That is definitely a way you can do that. Now, again, that has a cost that you probably don't wanna do that the night before, really big performance because the timeline on that can extend into that performance.
If you want something to upregulate your body and you've only got a couple minutes, there's a way you can do that by using called water immersion again. So again, there's multiple ways you can use these interventions for sometimes. Paradoxically different things depending upon how you're doing it.
So those advanced protocols we all have in there, and as I mentioned, I've got two bonus items that are not in the public domain that I sent out all the information via the private newsletter. So there you go. Any questions hit me up. Thank you so much for listening. I really appreciate it. If you miss this round hop on to the newsletter.
The Fiz Flexer will open again in March of 2026. If you [00:28:00] miss line, the deadline of tonight, October 20th of midnight, 2025 Pacific Standard Time. We've got another podcast coming up very soon. Thank you so much for listening to this. We really appreciate it. I hope to see you in the cert. If not, hope to see you on the newsletter or talk to all of you again here very soon.
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