Limiting factors in the marathon

I attempted to do my 20 miler yesterday… failed miserably thanks to sun, heat, and humidity – also my mistake for running at 1pm. There was absolutely no shade. I’ll go into more detail later – I did make it 11 miles though. In the mean time, I found this article on by Jonathan Dugas, and he explains common issues during the Marathon, and how you can avoid them. It’s pretty lengthy, but very helpful.

These are issues that can suddenly appear after 18 miles of feeling great, or after 22 miles of staying on pace for a PR. (Which is exactly where I always hit the wall – between 19 and 22)


There’s solid evidence that the optimal way to pace yourself over any distance longer than 800m is to run an even or negative split, i.e., running the second half of the distance at the same speed, or perhaps even slightly faster than the first half. It’s challenging, yes, but mostly because it means we have to exercise a little (or a lot) of patience from the start, and run what feels too slow for the first one-third or more to ensure that we don’t slow too much towards the end.

To achieve your best race, you’ll still need to avoid the common problems below, any of which will derail your most determined effort. The oversimplified answer is that matching expectations to training level and executing the correct pacing will go a long way to avoiding these obstacles. A common thread running through all these issues, therefore, is adopting an appropriate pacing strategy. It should always be your starting point. Although the speed each runner runs will be different, their pacing strategy should be similar. But even when you get the pacing right, you’ll still need to take action before and during the race to give yourself the best chance of running to your potential.


Late in a marathon, you find the work of maintaining pace getting much harder. Inevitably you slow and miss your goal. You blame it on dehydration, and, in truth, the loss of fluids likely had a small role. Let me say, however, that fluid balance plays a much smaller role than you probably think. Being “dehydrated” probably doesn’t mean what you think it means. The effect and the importance of “hydration” are overstated.

Perhaps the best way to try to understand the role of fluid during your racing and training is that restricting fluid will lead to a less than optimal performance. As long as you’re not fluid restricted, however, you can rule out hydration as an obstacle. “Restricted” is a relative term, of course, and the amount each of us needs is different according to our individual characteristics as well as the environmental conditions. Following the dictates of your thirst is adequate, however, to ensure that you get sufficient fluid in any given racing situation. Full disclosure: It’s probably a lot less than you think it is.

If for some reason you’re not able to meet your thirst, you’ll in all likelihood go slower, especially if it’s hotter outside. By how much? Probably only around 3–4 percent. That’s a lot if you’re aiming for a best time, when even only a 1–2 percent difference is meaningful, easily the difference between making the goal and not making it. But 3–4 percent is small when you see some of the claims thrown around by product manufacturers and even some sports scientists.

It’s important to note that drinking more fluid than you would when following your thirst will not magically make you go faster. Our normal physiological response to exercise and sweating is to lose some of our body weight in the form of fluid. Typically when we drink to thirst we lose about 2–5 percent of our body mass because we don’t replace 100 percent of our sweat losses. That’s normal, and doesn’t represent dehydration. To put it another way, drinking to thirst prevents dehydration. So drinking enough fluid to prevent any weight losses won’t make you go faster than if you drink to thirst, and in fact represents over-hydration.

Restricting fluids, however, will affect your performance. So when you’re aiming for your best time, and you’re not able to drink to thirst because of not enough aid stations on the course or you’re unable to take in enough on the run, your performance will likely suffer. However, given that most marathons now have aid stations nearly every mile, this is an unlikely scenario. Learn to tune in to your thirst sensations, and to drink comfortably at race pace, and then don’t psych yourself out. Trust your physiological thirst mechanism and know you’ll perform well even if you don’t “hydrate” right from the gun. You might be surprised how it changes your running if you can unchain yourself from the hydration issue.


Putting in all the miles and attempting to race 26.2 miles produce a situation rife for muscle cramps. A cramp will immediately dash any hope of a best performance, while simultaneously ensuring a miserable remainder of whatever distance you’re attempting. This is certainly bad news all around.

First, muscle cramps are not related to fluid balance. Data from the Two Oceans Ultra Marathon (35 miles) examined crampers and non-crampers who finished at the same time. On paper, you can’t tell the difference between them, as all the electrolytes measured look exactly the same between the two groups. That indicates the runners were all hydrated/dehydrated to a similar extent. So fluid doesn’t seem to play a role. In addition, everyone who finishes the marathon has lost some amount of fluid–since 99.99 percent of all finishers lose around 2–3 percent of their starting body mass–so if “dehydration” (defined in this case as losing even 2–3 percent of your body mass) were really to blame for the cramps, then that would mean we should be seeing many more crampers at the finish line. But as it turns out, only around 1 percent of any race’s entrants are seen in the medical tent, and for numerous conditions ranging from abrasions to sprains to collapse.

Fatigue, specifically local fatigue of a particular muscle or muscle group, seems to be a more important factor. The issue at hand is neuromuscular in nature, and we think is caused by a particular reflex between your muscle and your spinal cord that experiences a bit of a malfunction. We aren’t sure entirely what causes this to happen, but local muscle fatigue is a common thread. Therefore, preventing that local fatigue, or any fatigue, is your best path to preventing cramps. This can be done by adequate preparation and training, but that speaks to only part of the issue. The other equally important part of adequate training is “appropriate execution.” By that I mean you should pace according to your training level, and ensure your performance expectations match your current training status. So if you’re trained to run a 3:40, don’t complete the first 10 miles at 3:10 pace. It requires honesty, discipline and patience in particular, because we all know that in the first 10 or so miles we feel amazing, partly due to the atmosphere and crowds early in the race and partly because since we’ve trained to complete 26.2 miles, running 10 miles at a slightly faster pace still seems easy to us.

My experience from the many races I’ve worked indicates that it’s the athletes who attempt to outrun their training who are most susceptible to cramps, because they’re the ones who are inducing the local fatigue by asking their muscles to do more than they’re ready for.

The take-home message here is first to ensure you put in the training. But equally as important is matching your performance expectations with your current ability. Doing those two things reduces the chance that you experience a debilitating cramp along the way to the finish. In case you do cramp up, however, the best remedy is to pause and stretch the affected muscle, if possible. I know the last thing you want to do is stop and stretch for 30–60 seconds, but at that point it just might help make those last few miles bearable. In this case you must accept the fact that all performance bets are off, and the goal has now shifted to finishing in a respectable time.


A common pitfall of novice and experienced runners is that they think of running as something only their legs do. Accordingly, they run. And run, and run, and run. And that’s certainly an important part of any marathon training program, because the distance training is absolutely crucial to success, however you define it. But running isn’t just about our legs, and all the other parts of your body have to work so that your legs can continue to do their job. In this case I’m referring to your core and trunk muscles, whose role is to stabilize the body while you take off and land as part of each stride.


Endurance athletes the world over are familiar with this concept, having experienced it on at least one occasion–the narrowing field of vision, exponentially increased effort and deep fatigue, such that you feel like lying down in the ditch and taking a nap. It’s the layman’s term for hypoglycemia, or a low blood sugar. Like cramps, it will dash any hope you had of doing your best, effectively ending the race for you. But unlike cramps, it’s much easier to predict and prevent, mostly because it’s relatively easy for us to study exercise metabolism in the lab. Although the field of carbohydrate metabolism continues to advance, we’ve unlocked most of the important parts that apply to everyday runners.

The first thing you must do to prevent hypoglycemia is to be sure you start the race with at least normal glycogen levels in your muscles and liver, which is the organ responsible for maintaining the blood glucose concentration. There’s much debate now about whether traditional “carbo-loading” is necessary; without going into that whole argument, the minimal requirement is that you toe the line with at least normal concentrations of glycogen in your muscles and liver. Increased levels may or may not add value, but less than normal is detrimental.

Part of your training program should be making sure you ingest sufficient calories to meet the increased expenditure from all the training you’re doing, and that means not only eating regular meals, but depending on how much training you’re doing, eating supplemental food (not necessarily “supplements,” but any food) to ensure you meet your energy demands. If you’re doing that, you’re in all likelihood starting off with normal levels, especially since a normal diet for most runners tends to be high in carbohydrates.

So starting off normal is what you must do before the race, but once the gun goes off you then need to ingest anywhere between 30–60 grams of carbohydrates per hour to prevent your liver glycogen levels from getting too low. The problem is that, as we carry on exercising for many hours, we tend to rely less on the carbohydrates stored in the muscle and more on the carbohydrates from the liver. We do also tend to use more fat, but even as we increase our fat usage we still use more blood glucose over time, and that’s why the liver needs a little help in the form of what we can ingest to make sure it doesn’t get too low. Once the brain senses that it might be getting too low, it has its ways of making you slow, and if you choose to ignore those signals then you’ll most likely bonk.

I realize we don’t think in terms of grams of carbohydrates, so let me put that number of “30–60 grams per hour” in context. A typical gel product has between 20–30 grams of carbohydrates, while sports drinks contain about 10–12 grams per cup that you might receive on the course (around 200 mL or 6–7 oz.). Therefore, one gel and two cups of sports drink per hour will put you in the right range, and most runners’ thirst drive will cause them to ingest anywhere between 400–800 mL (14–28 oz.) per hour depending on the conditions. Of course you’re not limited to gels and sports drinks, but they’ll be the most readily available and easiest to consume.

Trying to go the distance in the absence of ingesting any carbohydrates is risky. Some can do it, but it will depend entirely on their genetic predisposition to burn fat during high-intensity exercise, as well as their training status. Chances are you’re not one of those people, however, and taking in some carbohydrates along the way will therefore be beneficial, if not essential. It might be less as opposed to more, and that’s where experience starts to pay dividends, because it might be a few marathons before you fully understand what’s going to work best for you and in what amounts.


The final point I’d like to share with you here is that, even with our best-laid plans, even with the most accurate and scientific information available to us, getting everything to go exactly right on the day still requires a little (or a lot of) luck. For most of us, racing a marathon really does stretch us to our limit, and predicting what will happen during such a complex situation is terribly hard and often imprecise. So prepare well, pace accordingly, and most importantly, enjoy the scenery.


To set a realistic time goal we really need to understand our potential and what’s possible for us with our current level of training. We tend to overestimate our abilities, and in so doing create a situation that will likely produce only failure. So how fast can you go? The best predictor of your next marathon, even if you’ve run many before, is your most recent time over any shorter distance. If your half marathon time two months before the marathon is 1:28, you’re not going to break 3 hours. If you’re very well-trained, you can expect to multiply your half marathon time by at least 2.1, but you’ll need to add more than that if you’re not as prepared and likely to slow down more during the race. Similarly, to break 3 hours you probably need to be able to run a 10K in less than 40 minutes. You’ll find numerous race prediction charts online, including our Pace Tools here. Regardless of what you set as your goal when you started training, at some point you need to adjust that goal to your ability as revealed in tune-up races leading to the marathon.


2 responses

  1. Reblogged this on Get Going, Get Running! and commented:
    I’ve got a marathon coming up in October so I’m focussing on these sorts of things at the moment. A lot!
    I’ve ‘bonked’ at the 19 mile mark before at its not fun, like ‘the worst half hour running of my life’ not fun, while waiting for a couple of gels and some sports drink to kick in. I learnt a valuable lesson about fuelling that day…

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