Here's my layman's understanding. As you tire and get dehydrated, your heart rate starts to climb just to hold the same power and pace. I.e., they are no longer linked. Eventually, the heart rate will spike on the most minimal exertion and you are done.

Happens to me all the time; especially in hot and humid, summer weather down here in Sauna Land (Tampa, FL). Yesterday was a great example. I ran 7 miles in the morning (solid run, but nearly evaporated by the end), and went out for another 7 in the evening. After 5 miles into the second run, my heart rate was going up fast and I started to fall apart. I made it to a water fountain at 6 miles and took a good 5 minutes to try and recover. Started back and my HR went through the roof by 6.2 miles ... totally decoupled. Walked home.

Technically this isn't really decoupling in my book, you are simply cooked, which is really easy to do in Florida in the summer time. To me true decoupling is just your heart not beating as efficiently, this can be measured by ejection fraction. We see this for instance with Ironman athletes post Ironman when a relatively healthy EF in the 60% is relegated into the 40s. The heart isn't pumping as efficiently in the 40s, and while the BPM may stay the same, lower EF means lower cardiac output. To do the same amount of work it takes more heart beats to make up for the lack of EF.

Taking your own example, where you overheated (IMHO), I could do the same sort of workout,and reach the same sort of melting point just because I can't shed the heat, but I could also keep going, say past 7:30pm, as the sun falls, temp fall and continue on to a point where I am NOW maintaining the same pace with less heart beats not more. If I was truly de-coupled this wouldn't happen I would keep seeing a decline in aerobic function as the duration increases. That HR decoupling that you experience is a result of overheating but that can be corrected.

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Cooked is exactly right, but before I am cooked, I am simmering; and the HR is rising while at steady pace. Then I finally move into the boiling stage and my goose is cooked. If coupled is roughly the concept that HR is linked to effort (low effort = low heart rate and high effort = high heart rate), then I start these runs coupled and eventually the HR goes off the rails.

When I was younger I could take the heat a lot better, but not anymore. Even though I sweat buckets and drink tons during the run, I just can't hack it anymore. I have no idea what my EF is, but apart from the heat, I cannot really detect any decoupling. BTW, the temperatures really don't fall much when the sun goes down here. Lows running about 78-82 this time of year.

Sure I am down here too. I run all the time at 1:00pm in the afternoon. Radiant energy is what is really the nail in the coffin and why running at 7:30 is so much more sustainable than 1:00pm. But what I was trying to say is your are not decoupling in the true sense of the definition in triathlon. If you think about it ,basically your heart rate is never staying flat. It is rising rising rising with each additional step. When looking at decoupling in the aerobic sense you are looking for say a stabilization of HR at a pace/effort, then over duration, with constant conditions, your hr starts to drift upward (ie cardiac drift).

Basically the metric is useless IMHO for anyone training in a place like Florida because the metric doesn't take into account changing conditions, and also doesn't take into account UNSUSTAINABLE conditions which is basically what all of us train in Florida in summer.

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As has been outlined.
There are multiple causes as to why your heart rate will continue to increase with a constant effort.
Ultimately the notion of "decoupling" is the human need to simplify complex topics into understandable chunks. It never is 100% conceptually correct, but as with horseshoes and hand-grenades can often be "close enough".

It could be that dehydration is lowering your blood volume which lowers your heart's stroke volume (how much is pumped per beat) which causes an increase in Heart Rate. (Less pumped with each beat = more pumps to achieve the same output).

Typically, however, decoupling in someone with reduced aerobic fitness is due to Muscle Fiber utilization.

Slow Twitch fibers are rather efficient. More so than their Fast-twitch counterparts.

Therefore, you begin exercise at a low to moderate intensity and slow twitch fibers are the primary contributors to moving you down the road. However, in a low aerobic fitness individual, these fibers will soon begin to fatigue. To compensate, additional fibers begin helping out... fast twitch fibers. Because they're less efficient, increased cardiac output is required to maintain the same workload. This causes "decoupling". In more-fit individuals this is less likely to happen as the slow-twitch fibers are better adapted and presumably more "fatigue" resistant. (Fatigue is a completely different topic)

Now, in certain instances, glycogen depletion due to 2 - a -day workouts etc may cause individuals may be more prone to decoupling.

Therefore, you cannot necessarily hang our hat on any one reason. You need to look at the trend of the situation.
If you always decouple, it's probably an aerobic fitness situation
If you rarely decouple significantly, but do so on hot days where you didn't drink enough, it's most likely a dehydration / blood volume issue
If you decouple after a fast or period of low-carb eating, it could be glycogen depletion.
etc.
etc.

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That's it exactly. I guess I don't really understand cardiac drift/decoupling then because I've always thought that's what I was experiencing. Meanwhile, WTF are you running at 1pm for? You are Clermont now, right? Heck, I think it is worse there than on the coasts.

It's like a divorce, but different ... in a Deepak Chopra kinda way

http://goop.com/conscious-uncoupling-2/

Oh, wait, that's something else

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The reason for the 1pm run is aerobic training, I am actually getting in a heck of aerobic workout with bread-and-butter heart rate zones without beating my legs up by running at the very hottest part of the day. My heart is a good 15-30 beats higher. I'll pull the plug and take a break and do some mobility work for a minute or two in the shade if my heart rate goes past 160. I definitely don't recommend this to anyone unless they know what they are specifically doing and why. In addition I am just churning thru things that I am cognizant to replace, like magnesium.

Anecdotally the other day I saw this gal going the other direction (1pm run again), turns out I knew who she was, but it took me a bit to put it together because she was flying and in my mind I was like "what pro female is still around training." On the way back I passed her again. Her pace had fallen by a good 2-3 minutes per mile in the matter of 2 miles. I talked to her a bit for about 1/2 mile so got the full scoop on the workout etc. Was just an implosion. When I passed her the first time I knew she was cashing paycheck that her body couldn't handle. It amazes me that people don't understand simple things like you aren't going to be shedding heat at the temp, sun, humidity we were running at even with reasonable pace, which means the only way you can continue is to slow down. Better just to start slow and stay consistent than do the death march home.

This is my first summer here. Another pro, who has a place in both Clermont and coast of Florida says it is a good 10 degrees warmer here. If you get that coastal breeze I wouldn't doubt it. The air started standing still sometime last night, like no air flow just crazy how poorly the body can dissipate heat. This is no different than your ac working. The hotter it is outside, the less efficient your AC system is as it has a tougher time actually cooling the Freon. It is why they recommend putting your ac in at least the shade and get all the twigs and bushes away from it so it could suck air.

Again, YOU ARE experiencing drift but it is expected given the temp, humidity, dew point etc. If you want to learn about growing the aerobic engine you have to take in account the changes that conditions bring as that is not an apple-to-apples comparison. If you start your run at 78 degrees (78 heat index0 and the heat index is 102 when you finish, of course you heart rate is going to be higher for the same pace, anyone's HR would be. In a super simple example, assume the conditions don't change at all, which obviously never happens but just go with it.

Ride 1:
Hour 1-2 avg power 200 = avg heart of say 130
Hour 2-4 avg power 200 = avg heart of say 145

then you experienced drift. If on the same sort of day (conditions the same, fatigue level the same etc) you ride the following week

Hour 1-2 avg power 200 = avg heart of say 130
Hour 2-4 avg power 200 = avg heart of say 130

You have grown the aerobic system. It can handle the additional 2 hours without increased HR. The more variables that change, level of fatigue in the body, changes in temp, different roads etc etc the harder it is to tease out whether your fitness is changing by looking at drift.

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Now I admit I've never paid much attention to this decoupling metric but conceptually it seems to make little sense to me. You don't have to live in Florida to have changing or even unsustainable conditions. If I'm supposed to pay attention to long term trends with decoupling, how do I separate those trends from the fact in the winter I might be running in 10 degrees F, in the spring I might be running in 50 degrees, in the summer 90 degrees and humid, and in the fall 50 degrees again? Not to mention the changes from day to day in relative humidity, or unseasonable temperatures, rainy day vs. sunny, etc. all of which can change your heartrate relative to a given pace dramatically.

Seems there are so many caveats and conflating circumstances as to make this metric useless.

Right you don't have to live Florida and wherever you are there are going to be changes the occur. Ultimately the metric isn't useful because people don't use it right and account for the changes. For instance the majority of people who train for Ironman do their long ride on Saturday morning. They start 7-8am and guess what constantly creeps up between 7 and 1pm??? The heat. So if at 1pm you heart rate is 15bpm than when you start you don't really know how much is from heat and how much is from just the heart rate actually decoupling. Does that make sense?

But even if the heat creeps up chances are the conditions are not as oppressive. Not sure where you live, but Florida in the summer is a different problem in that it is simply unsustainable. No matter where you are in the country you can usually shed some heat thru evaporative cooling. Today is the coolest forecasted day in the last 30 days and it is 85 at 10:39am, south wind at 5 mph, DEW POINT 76, humidity 76%. Heat Index 95. You just don't have any leeway to shed heat so it keeps building building building. How long can you last, well if you are running you are NOT going to last long. You are generating way too much heat and there isn't enough cooling to make it meaningful so you either drastically reduce your pace or just melt and get nuked very quickly.

The metric can be use to gain some insight to aerobic function, but it takes a careful eye and understanding how the body work to get precision. I think the point is that the pattern you might see for the typical Ironman triathlete might be quite a bit different if they started their ride at 2pm and finished at 8pm.

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Thanks for this and your subsequent replies ... very informative. You don't have to worry about me trying to run at 1pm over here in Tampa. Although I love the winters here, at 60, I no longer shed heat as well as I did years ago. In fact, I dream about moving to the mountains in North Carolina where at least the summer mornings dip into the 60s. My heat management strategy now is to take several trips across the summer to someplace cooler.

I've been to a Clermont a lot for camps and races...super location but only for half the year. :)

I think there is a pretty good article on it here:


https://arxiv.org/pdf/1205.0639.pdf

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I find it's a lot easier to see aerobic gains on the trainer (or possibly treadmill) as the temperature & effort is controlled.

Over a couple of months a course simulation session moved from 200W to 220W for the same avg heart rate (bumping the target up 5W when matching the prevoius). But even with a controlled environment I'll see a decent amount of variance due to sleep, drinking, etc..

Tracking running is really tricky, takes 2 or 3 miles for the Texas heat to really soak in & the heart rate to settle, then the slow (dehydraton) drift kicks in.

The trainer is the perfect place to use this metric. I can see it being very useful but I think the majority are NOT going to be doing 4 hour trainer rides.

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Thank you Thomas and others for replying.

In my case, the more I read your comments and other articles, it seems that my issues arise from a combination of lack of fitness and heat.
I'm in central NC and the heat and humidity here don't sound much better than in Florida. You just get used to it.
Personally, I run/ride at 1p on the weekends because that's when I can. The kids are down for nap and I have a 2-3 hour window if I'm lucky. This past weekend was 95 degrees with 4000% humidity.
This next weekend they are calling for close to 100 degrees. Maybe I do this weekends' workouts on a trainer/treadmill and see if that helps?
Now lack of fitness is a different issue which I will ignore and continue to find a scapegoat for...

This really went off the rails on heat and dehydration. Like the one guy said, that's not exactly decoupling. Let's take heat out of the equation for a moment...

Imagine you're on a treadmill in a 60 degree room with a fan and all the water and fuel you could ever want. Now run your "easy" pace. Let's say it's a 9 minute mile. Will you be able to run that forever? No. At some point, keeping your HR the same, your speed will fade a tiny bit, like 2%. That's still not decoupling. That's normal. But farther down the line, you'll have a falloff of power (pace) of more than 5% (I think it's 5%, can't remember) while still putting out the same effort (HR). If you're fueling, hydration, and whatever else was all good, that noticable drop in output compared to effort is "the edge of your fitness." THAT is where your body simply can't do endurance anymore. If you train a little, that's at 1 hour. If you train a lot, that's at 20 hours, or whatever.

And you can flip the above and run at a constant speed on the treadmill instead, waiting to see when your HR jumps 5% over the previous period. Either way is pretty much the same thing.

Some people say one way to improve your endurance is to do long sessions until you see that decoupling. Then next week you'll be able to go a bit further, since you pushed the limit last time. But actually, that's only useful in training up until a point. A 10 hour training day isn't really a training day anymore. Once you get up to 4 to 6 hours, then just start increasing speed instead.

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Sure, the fitter you are the more you are capable to some extend of handing heat. Going out to ride with 95 degrees and humidity is no easy task. What I would suggest is that if you are struggling, recognize that, and use some objective tool, power/hr to start even slower/easier the next time.

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Thank you.
Plain English description.

Are you saying you run in the heat because you have an elevated heart rate? Why not just blow angel dust and walk around the house in the cool AC?

I don't do drugs for one, second although some might say the heart doesn't know the difference, the reality is that as a pacemaker rep I saw a lot of drug users totally mess up their hearts. Not saying that isn't possible with athletics as well but I didn't see athlete in my days.

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