Why can't I run a full marathon at half marathon pace?

Damn good question.

We accept that glycogen and lactate thresholds are probably

the reasons but then they are only really subjective responses.

Sure there is a wealth of evidence in favour of that view

but it's not conclusive.

AstoriaRunner wrote:

Fueling is the second reason -- your body can only store about 2 hours of glycogen stores while the body keeps days worth of fat stores. IF you are an elite athlete you can try to power through primarily on glycogen, but for mere mortals going past 2:15 or so on the marathon, that's not feasible.

People talk about "2 hours of glycogen," but that's misleading; storage of glycogen is not time-based, it's energy-based. In other words, you have a certain quantity of CALORIES in the form of stored glycogen. 2 hours is only the time you happen to use it up in at something around marathon pace.

If you're just sitting in a chair, you won't run out of stored glycogen in 2 hours.

Thanks to everyone for the learned and thought provoking responses. What I have read about running training and performance always talked about physiological systems and their limits, but the limits were always at more extreme levels of exertion. Yet, as the distance increases, speed inevitably and invariably must decline seemingly systematically.

Especially intriguing is the notion that your brain subconsciously limits your speed when it knows you have further to go. Unfortunately, my brain has not always performed this function well and it allowed me more than once to try to run a marathon faster than I was able.

I have to say, based strictly on my intuition, the true reason is physiological rather than mental. It seems like an accumulation of H ions in the muscles is a likely culprit. Is this muscle fatigue? What exactly is muscle fatigue?

Thanks to anyone else who cares to respond. I think this is an interesting topic and I too would love to hear from a specialist in this field.

Have a nice day.

Suitcase of Courage wrote:

Thanks to everyone for the learned and thought provoking responses. What I have read about running training and performance always talked about physiological systems and their limits, but the limits were always at more extreme levels of exertion. Yet, as the distance increases, speed inevitably and invariably must decline seemingly systematically.

Especially intriguing is the notion that your brain subconsciously limits your speed when it knows you have further to go. Unfortunately, my brain has not always performed this function well and it allowed me more than once to try to run a marathon faster than I was able.

I have to say, based strictly on my intuition, the true reason is physiological rather than mental. It seems like an accumulation of H ions in the muscles is a likely culprit. Is this muscle fatigue? What exactly is muscle fatigue?

Your brain doesn't subconsciously limit your speed when it knows you have further to go. Your body responds to feedback from neurons, which tells it your overall state. Its not a subconscious thing. You can't trick yourself into thinking you're only running a half marathon and keep going for the full at the same pace.

Fatigue (reduced ability to generate force) may occur due to the nerve, or within the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This causes there to be less calcium available for the muscle cell. In addition an enzyme is proposed to be activated by this released calcium which eats away at muscle fibers.http://en.wikipedia.org/wiki/Muscle_fatigue

You can think of it being similar to why you can only lift a weight so many times. Even at lighter weight you still reach a limit. You also have soreness the next day due to inflammation from the damaged muscles. There's also neurological control. A guy on PCP can flip a car and rip his muscles completely due to the inhibition of feedback.

With all due respect to the respective posters, I think you didn't get very many good answers at all. I would say that lactate threshold is barking up the wrong tree, and it looks like a couple of key causes haven't yet been mentioned.Lactate threshold, or accumulation of H+ ions likely has very little to do with why you slow down at marathon distances.What is fatigue? Simply defined, it's the inability to perform at your normal ability.What causes it? That is the question for PhD's (so I will avoid it here).Timothy Noakes "Lore of Running" addresses a lot of these causes (but reading the book is like running an ultra-marathon). He evaluates what scientists have found on each subject, based on a broad survey of what's been studied, and sometimes drawing conclusions when it's possible.I would say likely causes come from:1) Glycogen depletion. Noakes spends a lot of time distinguishing from muscle glycogen and liver glycogen, and theorizing that the brain causes you to stop using glycogen as a fuel to avoid hypoglycemia.2) Body temperature. Running at marathon pace generates heat faster than you can dissipate it. If your body temperature gets dangerously high, you have two options: slow down to reduce the rate of temperature increase; or pass out, and possibly risk permanent damage or death. Noakes theory is that your brain detects and predicts the high rise in temperature, and slows you down so you don't overheat.3) Muscle damage/loss of elasticity: Running causes damage to muscles. At a certain point, before the end of a marathon, your muscles get damaged to a point where they lose their elasticity. This means to maintain the same speed, you must push harder, producing all of the running force from energy, without the aid of elastic return, causing you to consume energy faster.Your body is a complex machine, and running is a complex mechanism. Factors such as lactate accumulation and glycogen consumption, are simply a couple of factors among many. As such, they may be limiting factors in some cases, but it is a mistake to over-simplify them, and conclude that what stops you after 100m, 800m, 5K, 10K, or marathons are caused by the same thing.

[quote]rekrunner wrote:

With all due respect to the respective posters, I think you didn't get very many good answers at all. I would say that lactate threshold is barking up the wrong tree, and it looks like a couple of key causes haven't yet been mentioned.

Lactate threshold, or accumulation of H+ ions likely has very little to do with why you slow down at marathon distances.

What is fatigue? Simply defined, it's the inability to perform at your normal ability.

What causes it? That is the question for PhD's (so I will avoid it here).

Timothy Noakes "Lore of Running" addresses a lot of these causes (but reading the book is like running an ultra-marathon). He evaluates what scientists have found on each subject, based on a broad survey of what's been studied, and sometimes drawing conclusions when it's possible.

I would say likely causes come from:

1) Glycogen depletion. Noakes spends a lot of time distinguishing from muscle glycogen and liver glycogen, and theorizing that the brain causes you to stop using glycogen as a fuel to avoid hypoglycemia.

2) Body temperature. Running at marathon pace generates heat faster than you can dissipate it. If your body temperature gets dangerously high, you have two options: slow down to reduce the rate of temperature increase; or pass out, and possibly risk permanent damage or death. Noakes theory is that your brain detects and predicts the high rise in temperature, and slows you down so you don't overheat.

3) Muscle damage/loss of elasticity: Running causes damage to muscles. At a certain point, before the end of a marathon, your muscles get damaged to a point where they lose their elasticity. This means to maintain the same speed, you must push harder, producing all of the running force from energy, without the aid of elastic return, causing you to consume energy faster.

Your body is a complex machine, and running is a complex mechanism. Factors such as lactate accumulation and glycogen consumption, are simply a couple of factors among many. As such, they may be limiting factors in some cases, but it is a mistake to over-simplify them, and conclude that what stops you after 100m, 800m, 5K, 10K, or marathons are caused by the same thing.

[quote]Suitcase of Courage wrote:

WOW! I am VERY surprised to see some scientific debate/knowledge displayed on this running board...This might be the first time since a few people with basic physics tried to explain the airplane/treadmill issue.

To the above poster saying the TS did not get very good replies you are wrong...this is a great question and there have been some great answers...Some even take me back to my metabolism class during my Ph.D.

So I'll chip in. 1st off I've heard the name Noakes but I really don't know who he is or his educational level. But if he believes/argues the brain "shuts off" glycogen usage during longer distances he needs to review the physiological/metabolic or even the anatomical/structural literature. The brain simply does not tell the muscle what fuel to use. You may be quoting him wrong though.

2) Distance running is much easier on the heat generation than sprinting. Think about homeostasis...Although it's technically improper to use the term to refer to the time spent running...its relevent. The body has a much easier time attaining/maintaining "homeostasis" during distance running than during sprinting.

3) Tissue elasticity...I don't know where to start with this one but I don't think a credible scientist would have made the relationship you say Noakes made!?

This topic is certainly a good one...I might be willing to sponsor/suport the first dissertation ever written entirely on a message board...Any aspiring Ph.D.s out there? :)

1st off I've heard the name Noakes but I really don't know who he is or his educational level. But if he believes/argues the brain "shuts off" glycogen usage during longer distances he needs to review the physiological/metabolic or even the anatomical/structural literature. The brain simply does not tell the muscle what fuel to use. You may be quoting him wrong though.

This isn't exactly what Noakes is saying (more like the shutoff is neural and not fuel specific).

Might I also suggest you try google before you assert the superiority of your expertise over someone you don't know. It seems unlikely you are both more qualified than Noakes is to make statements about running physiology *and* ignorant about who he is. Certainly he has peers who disagree with his theory. But they do know who he is.

No Way wrote:

Your half pace should become intolerable long before 14 miles.

Well said. My half marathon pace becomes "intolerable" around mile 11 or 12. I only get through the last 1-2 miles through sheer will.

..and in the interest of actually contributing something useful to the discussion...

1. I'd agree that heat is not the major cause of slow down from 1/2M to M distances. I've run marathons in cold conditions where that clearly wasn't an issue and they didn't become magically easier. I've also run ones where heat was terrible, I was unconditioned for the heat and the heat was responsible for my race falling apart. The sensation was different from the usual fatigue of running fast.

2. I think glycogen is a bit of a red herring. Certainly there are folks who bonk in the marathon due to running out/low on glycogen (I've done that myself). And for some folks that may be an issue. But in many ways (at least perceptually) not being able to hold 1/2M pace for a full marathon is much like not being able to hold 10k pace for a 1/2M. There's a sense of fatigue that builds up over the course of a race that makes it hard to maintain a given pace any longer. Clearly in a 10k that fatigue isn't due depletion of all the ones glycogen. But the sensation is still there. Which points to glycogen depletion not being the key element that's limiting performance. At least in events that are shorter than ones glycogen stores (which may or may not include the marathon depending on your physiology and training).

3. Noakes central governor theory doesn't really give an answer. It just ads a layer of complexity over the top. The 'standard' model is that this fatigue is due to peripheral physiological (presumably biochemical) processes. Could be H+ levels. Could be Ca. It's pretty clear we don't actually know. In any case Noakes' theory is that the sensation of fatigue we get in a race is not a *direct* result of this peripheral breakdown. But instead the brain is integrating information about these peripheral conditions and then a 'Central Governor' is providing a generalized signal to the body about its level of fatigue. If necessary it will also shut down muscles to prevent damage. Hence the sensation of fatigue we get isn't a direct signal from the tissue in distress, but a decision by the brain that gives us global sensation that we simply can't continue moving as fast (coupled with increase perceived effort levels due to the brain simply refusing to fire as many muscle groups).

I personally find this theory appealing in that it agrees with my perception (and that of every other runner I've talked to) that the fatigue at the end of a race is something that is a whole body fatigue and at some level a mental barrier. If this was all purely peripheral physiology wouldn't runners simply complain about localized pain? "I couldn't continue at 1/2M pace anymore because of a burning in my upper quad". We don't see most runners slowing because of some complete localized failure of whatever was the weakest element in their chain of running muscles. The brain (the Central Governor) stops us before we get there.

But that all begs the question of what physiological variables the Central Governor is monitoring. It's these that effectively constitute the root cause of fatigue (even if what we perceived is actually some sort of integrated response from the CG). So is it H+ levels (in muscle? in blood?). Certainly the fatigue at mile 5 of a 10k feels different than that burning in the legs at the end of a 600 (which seems more likely to be H+ levels somewhere). Or maybe Ca levels, or tissue damage? Or maybe it really is a wide range of things depending on the distance and the CG just serves to gloss over the details leaving the 'hardness' of 5k to feel not unlike the hardness of a 15k even though the underlying source of fatigue is radically different.

There are two basic arguments for why you slow down the further you run, and both are based on fatigue. The first is the peripheral fatigue model and the second - the central fatigue model (this _mostly_ is Noakes' theory).

The peripheral fatigue model involves some of the following:

1) Disrupted ion concentration gradients could _possibly_ mean that "fatigued" muscle cell membranes do not depolarize with the same frequency of "unfatigued" muscle cell membranes despite the firing frequency of the motoneurons. This means that if a unfatigued muscle cell membrane can depolarize at say 100 hz, a fatigued may only be able to depolarize at 75 hz.

2) When we talk about muscle contractions at the muscle cell level we are talking about the interaction of two proteins - myosin and actin. We could say that a muscle contraction is _stronger_ when myosin and actin have a higher probability of interacting, conversely a contraction is weaker when they have a lower probability of interacting. Basically when your muscle nerve fibres "fire" they send an electrical impulse to the muscle cell which starts a cascade of events leading to the interaction of myosin and actin... thus a contraction happens... A theory of peripheral fatigue surrounds the notion that as we exercise at a submaximal level the probability of myosin/actin interaction decrease.... thus for the same _effort_ (i.e. firing frequency of our nerves) we get a lower contraction. With repeated muscle contractions certain things happen: slight acidosis, calcium concentrations may change (calcium acts as a messenger in the muscles "telling" actin and myosin to interact) and damage occurs to the myosin/actin proteins... All of these (and more!) contribute to a lower probability that myosin and actin will interact, and thus a lower power contraction is produced.

3) Microtrauma to tendons, ligaments leading to decreased energy return, meaning you have to work harder for the same return.

4) Depletion of energy sources. You can't do as much work per unit of time using fat as you can using glycogen as you can using phosphocreatine. Sooooo if you run out of glycogen and start using fat you have to slow down because your body can't utilize fat nearly as easily as it can glycogen. If glycogen can provide X kcal/minute you can only do ~X kcal/minute exercise... If fat can provide X-1 kcal/min you can no longer do ~X kcal/min exercise... (to the poster that said you still have muscle/liver glycogen post marathon - yes you do, your body would not level these levels deplete to zero, ALSO your body is replenishing its glycogen stores (slowly) even while you exercise)

The central model is a theory of fatigue that is based in the brain... Basically your brain keeps you from exercising at a level that will lead to severe bodily damage. When you are running (or moving at all for that matter), you think "move my right leg", this translates into messages in your motor-cortex which causes patterned nerve firing and consequentially a patterned muscle contraction of "move my right leg"... Okay, this all works good and fine for most people, especially so if your "unfatigued". Now one thing to note here is that your body has tons of "free nerve endings" that essentially act as little "monitors" in your body. These free nerve endings can measure the chemical composition of parts of your body (chemoreceptors) (i.e. lactate levels, free hydrogen levels, electrolyte levels), heat (thermoreceptors), pain (nocioceptors) and some others that I'm sure I've forgotten. Basically these free nerve endings let your brain know whats going on in your body, and allow your body to react. SO.... Noakes model is based on the premise that your mind knows tolerable levels of certain chemicals, tolerable pain limits (although nocioceptors feed more into your conscious mind than sub-conscious) and even tolerable temperature limits. So his model kind of works like this: as you fatigue you're still saying to your brain->"move my right leg!" But somewhere in between your thinking that and the message being translated to your motor-cortex, your mind is like "f*** that, if we keep on moving our right leg at that speed, we'll die", so when you're asking your mind motor-cortex to fire at say 75% of your legs capacity, your mind decides to fire at only 70%.... And voilà that's fatigue.... The cool thing about this model is that it fits nicely with the idea of neural-adaptation to exercise... This is the theory that with repeated exercise (especially weight training), your ability to produce greater muscle contractions comes not from stronger muscles, but from an allowance for high levels of muscle activation from your brain.

Now... Noakes model is no good on its own and the peripheral model is also no good on its own... Why? Because as humans we have both! Each model fails to make note of the other, and since our muscles don't exist in petri dishes, nor do our minds control homeostasis neither theory paints a full picture. So... It's much more likely that both play off each other and both lead to fatigue. This is the generally accepted theory of fatigue.

PLEASE NOTE that there are still tons of unanswered questions related to fatigue, and tons and tons of research that could be done on the topic... I've just (poorly) presented two main theories out there and some of their central ideas... This is not meant at all to be the end-all-be-all of fatigue.

I'm not afraid to have some tell me I'm wrong. In fact, I always think I might learn something if it's true. I'm more mathematician and software developer than physiologist.Maybe there were great answers, but in my opinion, lactate threshold and H+ ions aren't among them, and two other important ones (again IMO) were not mentioned at all. But I can be wrong on all 3 counts -- I don't mind.Noakes is not a new name, and certainly well-known if nothing more because his new model is controversially received, and his ideas attempt to challenge our existing ideas about physiology, by showing that research doesn't support it the way it was thought. Might be in your interest to do a little research on him, but briefly, he is a long-time ultra-runner, M.D., and leads an exercise physiology research team in South Africa. He is most noted for his "Central Governor Theory" which states that the brain incorporates a bunch of self-defense mechanisms to prolong homeostasis and prevent physiological dangers long before they become dangerous (my summary). (Someone above mentioned the debate between central fatigue versus peripheral fatigue. With the Central Governor, the brain controls everything centrally, rather than fatigue being caused by peripheral issues in organs or muscles). He is secondly most noted for the sheer weight and volume of his book, which embodies the sum of his decades worth of personally accumulated knowledge about running, and reflecting his experiences, and his broad survey of all the running research he has come across.I surely have mistated him, if not for my own lack of knowledge, simply due to the length of what he writes, and the limited space here, and the fact that I do this from memory from a very large book I read last year. But I will try to clarify what I mean further.1) The brain doesn't shutoff glycogen usage directly, but rather indirectly, by causing you to use less muscle fibers. The brain receives lots of bio-feedback, which includes levels of glycogen in the liver and blood. It also directly activates the muscle fibers. One learned response from accumulated running experience, is that, by recruiting fewer muscles, you use less glycogen. So when levels are low, the brain reacts by recruiting fewer muscles, thereby prolonging or preserving homeostasis. He does point to research where they measured brainwave activity to confirm this.2) While distance running may generate less heat than sprinting, the original question asked about maintaining half-marathon pace. Dr. Noakes covers heat generation quite extensively, explaining how it is affected by bodyweight, air temperature, humidity, and speed. He gives graphs with all the variables about when heat/weight/humidity/speed combination becomes a limiting factor for a race. Based on my sketchy memory of these graphs, running at half-marathon paces may create a heat issue for all, but the very lightest or slowest of athletes and/or the coolest and driest of conditions. Similarly as for blood-sugar management, with experience that comes with training, the brain predicts with remarkable accuracy, the required pace for a certain temperature and humidity. This has been confirmed by quick calculations of elite marathon winners in the heat. They settle into the right pace (as calculated by a heat/weight model) in the first few kilometers, long before any body temperature increase can be noticed peripherally.3) I may be on shaky ground here, but I'm sure I recall him saying that somewhere around 25-30K during a marathon, that muscle elasticity simply degrades. I think he said specifically in the quads, during pre-activation of the muscles, before the footstrike (although I may be confusing this with downhill running). Noakes does give very detailed explanations of the muscles, expanding into fiber bundles, expanding into a single fiber, expanding into certain smaller components, including a little "springy thing" in his diagrams that elastically absorb energy, and recoil it back, but as I'm not too familiar with internal biology, or too interested, I may have missed some relevant details. I will refer you to his book to decide the credibility of what he said, but will add that he is widely considered a credible scientist. I also recall, similarly, that this muscle elasticity is also one of the things that fail as we get older.I'm not looking for any sponsorship though, unless you want to sponsor an unlikely running career. :-)

because you get all tired and junk.

AnEngineer wrote:If you're just sitting in a chair, you won't run out of stored glycogen in 2 hours.

Sitting in a chair watching porn is another story.

Excellent theory.

wouldacoulda wrote:

because you get all tired and junk.

Meant to quote this.

AstoriaRunner wrote:

IF an elite were to try to run at LT pace for the full, they would blow up somewhere around an hour as their body would simply not be able to handle the LT buildup..

False, LT buildup is a good thing. Still living in the dark ages i see.

rekrunner wrote:

What is fatigue? Simply defined, it's the inability to perform at your normal ability.

What causes it? That is the question for PhD's (so I will avoid it here).

[/quote]

I'm so happy someone said this. Ultimately a lot of the running stuff we know is just guesswork. People on this site post like they know, but most of the central questions are unanswered.

i'm a fan wrote:

I'm so happy someone said this. Ultimately a lot of the running stuff we know is just guesswork. People on this site post like they know, but most of the central questions are unanswered.

Although there have been a few mistakes as to why fatigue occurs in this thread, "most of the central questions" surrounding fatigue have in fact been answered. The "running stuff we know" is not just guesswork. Take some time and read the literature available.

Excellent response, I thoroughly enjoyed reading it, as I am about 70 pages into Lore and am really excited to learn more on this topic.

Are you by chance going to the ACSM conference next week?