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Are there any tables out there that show how long a person can exert themselves at a given heart rate or %LT (assuming adequate fueling)?

events lasting up to(LT = 185): 

3 hours: 170+ 
4 hours: 165 
6 hours: 150 
??? 
IM: 130? 
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I'm quite sure it depends on your physical condition, both training and genetics. Keep in mind that folks differ considerably in their heart rate range, and likely they differ in lactate clearance as well. –  Daniel R Hicks Jul 29 '13 at 21:05
    
some insight: forum.slowtwitch.com/gforum.cgi?post=3671017; –  jdl Jul 29 '13 at 21:19
    
zones: 3-fitness.com/tarticles/zones.htm –  jdl Jul 29 '13 at 21:29
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I read a paper on this (for marathon running) I'll see if I can find it. Basically exercise is fuelled by a mix of fat and glycogen, the harder you exert yourself the higher the % of glycogen you use. You've got lots of fat but limited glycogen. –  Tom77 Jul 29 '13 at 21:37
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And it's important to note that glycogen is stored not just in the liver but also in the muscles. And repeated exercise of sufficient duration/intensity to draw down blood glucose and substantially dip into glycogen stores causes the body to further build glycogen stores, resulting in increases stamina. –  Daniel R Hicks Jul 30 '13 at 2:56

2 Answers 2

up vote 3 down vote accepted

Source - Metabolic Factors Limiting Performance in Marathon Runners

(While the above paper is aimed at marathon runners, I'm not aware of any reason it can't be applied to other endurance sports such as long-distance cycling.)

Exercise is fuelled by a combination of fat (plasma free fatty acids plus muscle triglycerides) and carbohydrate (plasma glucose plus muscle glycogen). The proportions used vary depending on the intensity of the exercise.

intensity

Exercise at a particular intensity can be maintained until you run out of carbohydrate - "hitting the wall" or "bonking". Intensity is expressed as a % of VO2max, which can be estimated from heart rate.

In order to work out how much carbohydrate you have available, you need to estimate how much glycogen you can store in your leg muscles, how much glycogen you can store in your liver and how much carbohydrate you can take on as food (gels, sports drinks, etc) during exercise. The methods section of the paper goes into this in detail.

In order to apply this practically to cycling you should take measurements of the power you can produce at various intensities (heart rates). If you don't have the equipment to measure power, then measure speed on a flat course and estimate the power from that.

Having done all that you should be able to calculate the total energy required for a particular distance at a particular speed and what proportion of that energy will come from carbohydrate. You also have an estimate of how much carbohydrate you have available and that will tell you if you're likely to hit the wall / bonk.

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Very insightful paper! Opens quite a few doors to explore... thx –  jdl Aug 2 '13 at 6:08

The short answer is "no", there are no charts or tables that can tell you this directly. There is a lot of variability that determines how hard a person can work out for a specific amount of time - a few of the factors include:

  1. Genetics Amount of training
  2. Type of training (if you train only for long periods, your ability to work hard at a high intensity will be low, for example).
  3. Recent training history (if you've trained a lot, you will have accumulated fatigue)
  4. Recent stress history
  5. Recent health or sickness
  6. Current mental state

For a given person at a given point in time, it is possible to determine what you are asking experimentally; for example, a runner can, over time, figure out their speed for different time periods.

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