Aerobic Threshold

Aerobic threshold is the intensity at which you are going hard enough to recruit all your endurance (slow twitch) muscle fibers, but none of your speed-endurance (fast oxidative glycolytic) or sprint (fast twitch) muscle fibers. This is a VERY easy training intensity.

It generally correlates with maximal fat burning, which is not at rest, but a very light trainig intensity - usually between 50 and 60 percent of maximal oxygen consumption (which usually occurs at about 60-70% maximal heart rate). The percentage of fat burned may be highest at rest, but the total metabolism of fat is far higher during low intensity aerobic exercise.

I use a metabolic analyzer to test aerobic threshold (http://www.fitness-concepts.com/voxmax.htm) and have found that almost every athlete overestimates aerobic threshold. Many beginner triathletes should use very fast walking along with running for their zone 1 and 2 run workouts.

Hope this helps, Ken

Technically speaking there is no such thing as AT (or, as you said, it would be anything from zero to OBLA - but, the term threshold insinuates a point of change.., which further perverts the "idea" of AT). But, that doesn't really mean anything... it's just terminology that is used to define a point where the body begins to build up more lactic acid than can be "removed". It typically means that it's that effort level that is below the LT (LT defined as first 1.0 rise in lactate from baseline) range. And, as far as I'm concern is virtually the same concept as LT. However, (IMO) the AT and LT are simply different applications of concepts that very simular.

The idea of "fat burning" zone (which some say is equally a non-existent concept) is simply another conceptualizing of training at a point where the body is capable of maintaining is ability to perform (simular to the idea of stayng below the point of build up of more lactic acid faster than it can be "removed").

Both ideas are actually simular idea... they simply look at the methods of training at an efficient point where performance improvements are achieved or allows for frequency of training to be consistent enough to provide performance improvement. It's less about terminology and more application of the terms that will make the difference.

I think the important thing is simply find what ever terminology which conceptualize those ideas that are intended to define training zones and stick with them... I will not make a point as to which is "better", because that's simply too controversial. And in the end, the net results are very simular.

FWIW Joe Moya

Anaerobic or lactate threshold are completely different from aerobic threshold. Anaerobic threshold is the intensity at which Respiratory Quotent is equal to 1 (CO2 production and O2 consumption are equal). Lactate threshold is the highest intensity at which acid does not accumulate in the muscle. These terms essentially reflect two ways of measuring the same threshold. This intensity is approximately race pace for an event of 75 minutes in duration for a well trained athlete (of any ability level).

Aerobic threshold is a completely different intensity, the one used for basic endurance training and at which the body is able to oxidize fat most effectively. This is an intensity that can be maintained for hours. Nobody with any education or experience would argue with the existance of a fat-burning threshold, that is simply fact, but some argue that training at this intensity is not useful for bodyfat reduction because an athlete burns more calories at higher intensity.

Ken

I am no expert, but I understood the aerobic threshold to be the level of exertion whereby some 98% of all oxygen consumed is coming from inhalation, in other words a pace that would be maintainable for some length of time by just breathing hard. Doesn't the term anaerobic mean going into oxygen deficit whereby you are using much more than you are taking in, hence a very temporary state? I assume most people find their race pace by reaching a threshold that they can barely but still sustain without going into acute oxygen deficit? I always think of the threshold as the place once I go past it, breakdown in pace will shortly follow.

Ken

"Aerobic threshold is a completely different intensity, the one used for basic endurance training and at which the body is able to oxidize fat most effectively. This is an intensity that can be maintained for hours. Nobody with any education or experience would argue with the existance of a fat-burning threshold, that is simply fact, but some argue that training at this intensity is not useful for bodyfat reduction because an athlete burns more calories at higher intensity. "

....

I'm not exactly sure about this paragraph... It seems that the term "fat burning" is frequently mis-used.

I'm not aware of a training "zone" (i.e., specific intensity level) where that fat is burned (i.e., oxidized) at a rate more efficiently (or faster) than any other calorie type (i.e., fat, protein, carbohydrates).

In fact... correct me if I am wrong but... Anaerobic training raises your respiratory quotient. This could mean the percentage of energy derived from sugar increases and fat burning decreases. In time, this may force more anaerobic metabolism and less aerobic function.

I've not seen any verification of this conclusion that increasing the respiratory quotient decreases the burning of fat... no more than it increases the level of fat being utilized.

However, I am aware of the concept that energy consumed by the body on a hiarchial bases. Where the primary source of energy begins with carbohydrates... then the protein... then the fats. This is in essence a shift in energy sources. The primary difference between the amount of time required to achieve this shift to predominately "fat burning" is a determined by genetics. Some people burn more fat as a source more quickly and for an extended period of time than others. While training can significantly change the bodies ability to access fats for energy, it doesn't not change the fundamental fact that the body uses all sources of energy.

It's not just intensity level that makes the body shift in energy sources, it is also length of activity. Not to mention the energy available by diet. However, what doesn't happen automatically is that there is "Fat Burning Zone" defined in only one term - intensity. This is what I origniallly meant when I said, "...the term "fat burning" is frequently mis-used."

FWIW Joe Moya


Edited by Joe M 2005-10-09 7:35 PM

What he was suggesting was that while the percentage of fat as a total of all energy sources is higher down in the aerobic zone, the total 'quantity' of fat burned may still be higher overall up in the near-LT zones. So if you burn 400 cal/hr and 50% is fat, you burn 200cal of fat per hr. But if you go hard and burn 1000 cal/hr and fat is only 30% you still burn 300cal/hr which is better if you are trying to reduce fat.

This fat burning zone is a term created by the fitness industry to try to encourage people to exercise in a low-intensity mode and makes sense. You want to create positive rewards for the athlete so you try to reduce injury and overall pain, while still suggesting a weight-bearing exercise. Talk about a 'fat-burning' zone and it's easier to keep clients from feeling like they need to work harder than they're ready to. PLUS working out for longer rather than more intense is better for the heart and probably other body systems including those that produce endorphins.

Dr. Maffetone is a big supporter of low-intensity workouts and it works for his people, ie Mark Allen.

Hi Joe,

Yes, the term "fat buning" is misused all the time, frequently to try to sell something.

The slow twitch muscle fibers are the only ones that have the enzymes to burn fat as a significant energy source. They burn a combination of fat and carbohydrate, based on the availability of oxygen (which is based on work intensity). At aerobic threshold, the intensity at which we are going hard enough to use all of the slow twitch musce fibers, but not hard enough to recruit any of the more powerful fibers (FOG or FT), most athletes burn between 40 and 60 percent fat with carbohydrate making up almost 100% of the difference. We use a byproduct of carbohydrate metabolism (acetly coenzyme A) to burn fat, so we can never burn fat without carbohydrate breakdown occurring.

It requires more oxygen to burn fat than carbohydrate. As we increase intensity, we begin to recruit our speed-endurance (FOG) fibers, which have just slightly more than zero ability to burn fat. Recruitment of these fibers makes less oxygen available to the ST fibers, so they shift to burning more sugar and less fat. A 3-5% increase in running speed or cycling wattage above aerobic threshold reduces fat oxidation by 30% or more. This delays workout recovery by burning up glycogen (which takes several days to replace) and using speed endurance (FOG) fibers, which require longer to recover between workouts than ST fibers.

Aerobic threshold generally occurs in mid zone 2 using Joe Friel's Training Bible method of developing heart rate training zones.

Anaerobic training does increase RQ and almost no fat burning occurs in zone 4 or higher. The terms aerobic threshold and anaerobic threshold sound similar, but refer to very different intensities.

RQ increasing is a direct result of fat oxidation decreasing. Beta oxidation of one calorie of fat requires more oxygen and produces more CO2 than glycolysis of one calorie of sugar.

The genetic component is primarily a result of inhereted muscle fiber type. An athlete with a high percentage of ST fibers will burn more fat at any sub-LT intentisty and will have an aerobic threshold that occurs at a higher percentage of LT and a higher percentage of max. But differences in the time to start fat burning is a very small aspect ... the amount of fat burned is the greatest issue, both for workout recovery and for long course racing.

Training at the correct AeT intensity is critical as well. Athletes who train too hard for basic endurance workouts do not burn fat effectively, regardless of genetics. Road cyclists are the worst at this because most of their training is group rides and ego, not planning, dictate training intensity. Just a couple of months of keeping the intensity down when appropriate can dramatically increase fat oxidation.

Ken

thanks ken, this last explanation made the most sense to me.

Great explanation, Ken, thanks! Don Fink (in his book Be Ironfit) talks about how athletes train too much in the in-between zone in terms of intensity, not explicitly training either the aerobic or anaerobic systems. He advocates being religious with the HRM, not worry about distance, just time spent in the appropriate HR zone. That seems to be consistent with what you are saying?

Ken

A couple of finer points that are really only important to physiologists:

"The slow twitch muscle fibers are the only ones that have the enzymes to burn fat as a significant energy source. They burn a combination of fat and carbohydrate, based on the availability of oxygen (which is based on work intensity). "

Not true. It has been shown in several published studies that muscles do not go "anaerobic." Availability of oxygen has little or nothing to do with whether a muscle metabolizes substrates aerobically or anaerobically. Fast twitch - glycolytic fibers produce lactate because they have a high level of lactate dehydrogenase. They get less oxygen because they have a lower capillary density, but they need less oxygen. However, they never go "anaerobic" because of lack of oxygen. Muscles burn what is delivered. Fast-twitch fibers burn carbohydrates because they are full of glycogen. Slow twitch fibers are full of intramuscular triglycerides.

"Recruitment of these fibers makes less oxygen available to the ST fibers, so they shift to burning more sugar and less fat. "

Not exactly correct. Slow twitch fibers don't ever get hypoxic either. The shift towards burning more carbohydrates is due to recruiting more fast twitch fibers as exercise intensity increases and due to hormones released during exercise.

"Anaerobic training does increase RQ and almost no fat burning occurs in zone 4 or higher. "

There is some fat burning going on, but your body is relying more on carbohydrates. Again, this is due to the recruitment of fast twitch muscle fibers. They are packed with muscle glycogen. The slow twitch fibers that are recruited are burning fat.

"Training at the correct AeT intensity is critical as well. Athletes who train too hard for basic endurance workouts do not burn fat effectively, regardless of genetics. Road cyclists are the worst at this because most of their training is group rides and ego, not planning, dictate training intensity. Just a couple of months of keeping the intensity down when appropriate can dramatically increase fat oxidation. "

For long course athletes I think you are exactly right here. Improving your ability to metabolize fat (and do it at a higher intensity) is key to race performance; especially late in the run.

Mike

So is it safe to think of fast twitch and slow twtich muscle fibers in terms of geography and proximetry to available capillaries? In other words, if viewing the capillary bed as a watershed, the fast twitch muscle fibers receiving less oxygen are on the ridges of the watershed, like the continental divide, and the slow twich muscle fibers are in the valleys recieving plenty of O2?

If so, how do you account for the predominence of different fuel types in the two muscle fiber types? It's like the sherpas carrying glycogen up the steep mountainsides delivering carbohydrate substrate to the distant fast twitch muscles?

Sorry for the analogy, but I'm a very visual learner, plus, I majored in geography in college the first time around.

"So is it safe to think of fast twitch and slow twtich muscle fibers in terms of geography and proximetry to available capillaries? In other words, if viewing the capillary bed as a watershed, the fast twitch muscle fibers receiving less oxygen are on the ridges of the watershed, like the continental divide, and the slow twich muscle fibers are in the valleys recieving plenty of O2?

If so, how do you account for the predominence of different fuel types in the two muscle fiber types? It's like the sherpas carrying glycogen up the steep mountainsides delivering carbohydrate substrate to the distant fast twitch muscles? "

.........................................................................

It is not really like that. There are really many, many different muscle fiber types (depending on how you define muscle fiber type). It is a contiuum from very fast twitch to very slow twitch. You wouldn't likely have a very fast twitch fiber next to a very slow twitch fiber. They would more likely have similar characteristics. Fast twitch fibers have plenty of blood supply for what they do.

Mike

Mike

OK ... lets see if what I thought all along is correct...

Anaerobic training raises your respiratory quotient. This results int the percentage of energy derived from sugar increases because of an engagement of FT muscles and hormone releases.

In time, this may force more anaerobic metabolism and less aerobic function. But, it doesn't result in a reduction in the "fat burning" aspects even at higher RQ.

So, when I said... "I've not seen any verification of (the) conclusion that increasing the respiratory quotient decreases the burning of fat... no more than it increases the level of fat being utilized." is basically true?

Instead, there is simply a shifting between fats and carbohydrates to supply the muscles with what they need to work.

So, would you say it's safe to say that the idea of aerobic threshold is simply a range of effort that is below the OBLA? And, that aerobic threshold is more Lactic Threshold dependent to determine the OBLA benchmark (i.e., LT=OBLA in practical terms). And, it's this LT benchmark that defines the optimal aeroebic level? The reason I ask these questions is because I see a application of the idea of AT and LT as if they are mutually independent concepts. While they may not mean the same thing... you can't determine AT without knowing the LT.

I like to think in simplistic terms where the AT as the JFT (ala Gordo) approach. While LT is the benchmark that helps maintain the proper level of JFT/AT. And, training at/near lactate steady state/"anaerobic" threshold/OBLA is probably the most efficient use of an endurance athlete's time, assuming that this intensity is compensated by reduced volume and increased recovery time (ala McCormick). It seems to me that both approaches have value at different times in relation to different situations. In essense, the difference between muscular endurance and aerobic endurance are mutually inclusive concepts.

FWIW Joe Moya

Edited by Joe M 2005-10-10 10:42 PM

I think I'll stick with my sherpa analogy even if it's wrong.

Mike,

I'm going to have to disagree with you on all points. I am an exercise physiologist and I have conducted or supervised over 6,700 tests of aerobic threshold, lactate threshold, and VO2 max. I stand by everything I said in my previous posts. I'm not sure why you would think that changes in RQ would not result from changes in the proportion of glycolysis to beta oxidation or why you would think that fat burning still occurs with an RQ of over 1.0.

I have tested hundreds of athletes repeatedly and observed changes in fat metablism as a result of different training methods. More about the testing I conduct and my training methods can be found on my website www.Fitness-Concepts.com or in my book The Triathlete's Guide to Run Training.

Ken

Ken

I agree with you that changes in RQ result from changes in the proportion of glycolysis to beta oxidation. However, fat burning still occurs at an RQ over 1. It is just dramatically overshadowed by carbohydrate burning. It is a very small percentage at that point. Anyway, it is not important as far as training triathletes is concerned.

I am a PhD exercise physiologist with a specialty in muscle physiology. I have done some testing as well, but not nearly as much as you have. Also, I would not consider myself a competent tri coach by any means.

Mike

Interesting qualifications on both fronts and I have learned a lot from this thread and I know everything (lol).
As I have followed this conversation I've tried to relate the info to my problem. In my training, especially running, I find that at times I'm spending a considerable amount of time hovering around the 90% max HR. If not then my runs become very slow jogs and even at 90% I'm not fast, 9 min/mile. At my age (51) and with my previous health history (smoking, diabetes..all gone now) should I accept this or should I spend the winter working on base aerobic threshold workouts (OK I made that up) which would be runs, rides, and spins that are very slow but that keep my HR down around 70-75%. Would this help in fat loss and in creating speed? Goal is HIM next May. Could do it with current planning regimen but the wagon full of gel and Heed I would have to pull would be awkward.

Yes, this type of training is exactly what is appropriate during the offseason for athletes who are racing any distance next year. The slow twitch muscle fibers will do most of the work in your half ironman, and most of your training should be done at what seems like a ridiculously slow pace. You will find, if you are disciplined about keeping the intensity down, that you will get much faster at this intensity. That is not to say that workouts at a higher intensity are not valuable and important. Spend most of your time between 65 and 70% of HRmax, and some steady efforts at 85-90%, and stay away from too much time in the middle until you add race pace work a few weeks before the race.

Ken

I'm bumping this because I'm interested in the answer to Kathy's question.

I'm really interested in the answer to this as well.  I have a LOT of base-building work to do, particularly on my run.  Am I better off sticking to the 65-70% range for most of the winter or is a periodic speed workout (85-90%) beneficial?  If it is, how often?

In Don Fink's book "Be IRON fit", he pegs the percentage of ironman training that should be done north of zone 2 at only 10%. He argues that most people work out - ineffectvely - in zone 3, neither predominantly working either their aerobic or anaerobic systems. The percentage may well be different for shorter distance racing, I don't know.

Could we get a quick review of "zones" and their corresponding %MHR? Please.

220 - your age = Max HR

Z4= 90-95%
Z3= 86-89%
Z2= 75-85%
Z1= 65-74%

There are variations to be found...some people use 5 zones like my Garmin Forerunner, it will actually display my zone as 4.4 while exercising....Z4 tends to be uncomfortable because you are working so damn hard, and Z1-2 tends to feel very slow. Most people feel most comfortable working out in Z3.