Aerobic vs Anaerobic in marathon runners

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  • #4377
    mikecochran
    Participant

    This question comes up for me, because I notice that if I am running my general fast pace (~6:30 minute miles), I start to slow down significantly after about 13 miles or 1.25-1.5 hours. I presume this is because of lactate acid build-up in my muscles.

    So when it comes to marathon runners who run sub 6 minute mile paces, are their anaerobic systems so strong that they don’t get lactic acidosis in their muscles by the time they finish their marathon or are their aerobic systems so strong that they are able to run that pace for theoretically many more miles (i.e., their cardiovascular system is so strong that their heart rate is never high enough for their anaerobic system to come into play)? I presume it is the later.

    This, in part, also brings up a further question I have about the division of the aerobic and anaerobic systems. So does your body generally rely on one or the other? In other words, if your heart rate is low enough, is your body using just the aerobic system or is it always using a combination of both systems until you reach lactic acidosis after which your body relies solely on the aerobic system?

    Thanks!
    Mike

Posted In: Mountain Running

  • Keymaster
    Scott Johnston on #4394

    Mike:

    Great set of questions. Here goes so hold on:

    1) You slow down because you are exceeding your aerobic capacity during much if not all of these fast runs. If the slowing occurs in the 1-1.5hr range then you are slowing because of glycogen depletion and not so much from the accumulation of lactate or increased acidosis. Your 6:30 mile pace is probably right at your endurance limit, often called your lactate threshold (LT) (used to be commonly called anaerobic threshold). This LT pace requires the maximum sustained output of both of your metabolic pathways: Aerobic (using fat or glycogen for fuel) and the Anaerobic or Glycolytic (which can only use glycogen/sugar). At this pace both systems are cranking at their max sustainable output so you are burning through your glycogen reserves fast. When those reserves run low you are forced to reduce your pace. Your brain does this automatically and against your wishes. It is trying to preserve the organism from death or damage so it needs to husband those precious glycogen reserves (the brain can only on glycogen so you can imagine what would happen if completely drained that fuel tank).

    Because the glycolytic metabolic system is operating at near max capacity you can be sure that lactate production is high. But at that pace/effort the lactate production is just equalled by the lactate removal.

    2) The marathon is an event that is competed at every individual’s aerobic threshold. The aerobic threshold represents the metabolic limit of energy production where fat dominates as the fuel. The world class marathoner has an aerobic threshold pace of about 4:45/mile. The weekend runner also competes at his aerobic threshold but it happens to be at a 10:00/mile pace. But from a metabolic standpoint they are both at the same limit. The world class runner just can crank out so much more ATP/second with his fat metabolism that he can run twice as fast.

    As you have discovered in your fast runs you begin to slow at about 60 minutes. Events that take about an hour, like a world class half marathon or a recreational 10km all are competed at that individual’s Lactate Threshold.

    As to your last question: Both the aerobic and anaerobic metabolic pathways are each contributing to your overall energy demands at all times. How much is coming from fat and how much is coming from carbs (glycogen) is based largely on your dietary choices and on your training history. I coach top endurance athletes that are barely sipping from their glycogen fuel tank all the way up to 80+% of their max HR. I also deal with many folks who come from high intensity backgrounds how are using 80% carbs at just about resting intensity, like a walking pace. These folks have severe Aerobic Deficiency Syndrome. (ADS).

    If lactate production does exceed lactate removal, as happens in short, very high intensity efforts sustainable for only a few minutes, then yes, the anaerobic/glycolytic system will slow and finally shut down as lactate reaches very high levels. When that happens you will be forced to slow WAY down until the lactate clears and the acidity is restored to normal levels. During this slowing process you will mainly be relying on your aerobic metabolism to power your forward progress.

    More info in Training for the New Alpinism Pages 101-113.

    And this article here: https://uphillathlete.com/what-is-it-that-enables-endurance/

    I hope this helps,
    Scott

    Participant
    mikecochran on #4424

    Thank you for your detailed reply, Scott. I always appreciate understanding the science behind the training, and is what I really enjoyed about your’s and Steve’s book.

    Having reread that section of the book and the article you linked, I have a couple of follow up questions if you do not mind.

    1. If I were to replenish my glycogen stores (e.g., eat an energy gel) when I start to slow down after 1-1.5 hours of running at my LT pace, would I theoretically be able to continue running at that pace?

    2(a). From your description of the marathon runner, I presume they do not do anything to replenish their glycogen stores while running their marathon. Is that correct?

    2(b). I often hear about ultra marathon runners having to consume energy gels or other sorts of food when they compete. If their bodies are so trained to produce energy from fat and are always running at their aerobic threshold, then why is there such a need for them to eat? Is it done to fuel their muscles or just to fuel their other body systems (e.g., the nervous system)?

    3. When one is purposefully trying to train their aerobic system with long duration exercise, should one refrain from replenishing their glycogen stores so that they force their body to rely off of their fat stores for fuel instead?

    Thank you again šŸ™‚

    Participant
    mikecochran on #4425

    Forgot my last question.

    4. When you say “I coach top endurance athletes that are barely sipping from their glycogen fuel tank all the way up to 80+% of their max HR.” I presume you are able to know this by simply measuring the athletes lactate levels at various heart rates? Just curious, as I am considering investing in a blood lactate meter at some point.

    Keymaster
    Scott Johnston on #4445

    Mike:

    1. If I were to replenish my glycogen stores (e.g., eat an energy gel) when I start to slow down after 1-1.5 hours of running at my LT pace, would I theoretically be able to continue running at that pace?
    Theoretically and in most cases, yes, you will be able to keep running at your previous pace. This is what happens when you bonk, eat and feel great again in 10 minutes.

    2(a). From your description of the marathon runner, I presume they do not do anything to replenish their glycogen stores while running their marathon. Is that correct?
    Unlikely. At your aerobic threshold you are consuming about 50% of calories from carbs. You will deplete in 2 hours and want to consume easy to break down calories like carbs in the run or risk bonking and slowing dramatically. Happens all the time in marathons.

    2(b). I often hear about ultra marathon runners having to consume energy gels or other sorts of food when they compete. If their bodies are so trained to produce energy from fat and are always running at their aerobic threshold, then why is there such a need for them to eat? Is it done to fuel their muscles or just to fuel their other body systems (e.g., the nervous system)?
    See above. You are ALWAYS using both carbs and fat. AT AeT you are plowing through carbs and fat at the same rate. Glycogen stores are small and need replenishment if the hope to last a long event.

    3. When one is purposefully trying to train their aerobic system with long duration exercise, should one refrain from replenishing their glycogen stores so that they force their body to rely off of their fat stores for fuel instead?
    Yes: There is a ton of information about this stuff right here on the UA website. Just do some reading. Like this: https://uphillathlete.com/burn-fat-to-go-fast/

    4. When you say ā€œI coach top endurance athletes that are barely sipping from their glycogen fuel tank all the way up to 80+% of their max HR.ā€ I presume you are able to know this by simply measuring the athletes lactate levels at various heart rates? Just curious, as I am considering investing in a blood lactate meter at some point.
    Best was is with a Gas Exchange Test in a lab. Next week there will be an article about this posted on UA so keep tuned in. Lactates can tell you much of this but measuring an athletes expired air is the only way to see this for sure. By ‘siping’ I mean their AeT is clear up near their AnT and Max HR. Also discusses on the forum under General Training.

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