running fatigue

Part of getting better at running is getting used to running while tired. Learn how you can use your fatigue to improve your running and how much is too much.

Many times, when you go out for a run, you feel better than when you left. A good run can leave you feeling energized and ready to tackle just about anything.

But then there are those runs that leave you dragging and bone tired. You’re struggling to keep up with the last few miles of your long run. Or your cool down from a tough track session feels like you are running through a pool of mud.

All you want to do when you get home is snuggle up on the couch for the rest of the day. And by the time you lace up in the morning, the thought of doing it all over again can be daunting.

Does that mean you overdid it? Just how much fatigue should you feel in training and when have you crossed the line?

running fatigue

In this post, I’m going to get into the science of fatigue. You’ll learn:

  • why a certain amount of fatigue is actually important as a runner,
  • how to use fatigue to your advantage, and
  • how to know how much is too much fatigue.

Expect and Accept Some Fatigue

Unless you only do a couple of short, easy runs a week, you are going to feel fatigue at some point as an athlete. Our bodies simply do not immediately repair from the beneficial damage that we do to them in our workouts.

When we exercise, our muscle fibers sustain tiny micro tears that are repaired and rebuilt during the recovery process. Those repaired muscles end up being faster and stronger than they were before, but that process can take days or even weeks depending on what you did to them.

In an attempt to keep you on the couch to repair and avoid more damage, the brain will send pain signals to your muscles and will create a mental sensation of exhaustion or fatigue. Your brain’s main job is to keep you alive and if that means slowing you down mentally and physically, it will do it.

But if we waited the days or weeks necessary to be 100% recovered from a single run, well, we wouldn’t get very far in our training.

The Weight of Fatigue

One way to think about it is that fatigue is kind of like running with a weighted backpack. Each run you do puts another weight in your pack. Each recovery day takes some weight out, but it doesn’t completely empty it. As we train, we get more used to running with a weighted pack, so much so that we might not even notice that it’s there after a while. But it’s always there. The goal is to get so accustomed to the weighted training so that when we finally set the bag down on race day, running feels as light and easy as possible.

During training, we need to find that balance between recovery and fatigue so that our load doesn’t ever get so heavy that we can’t lift it.

Concentrate Your Harder Efforts

The first step is to alternate your hard days and easier days. This is the main reason behind the phrase, “hard days hard, easy days easy.” The goal is to concentrate your most fatigue-producing efforts together so that you are not in a constant state of muscle breakdown and fatigue.

Your slow, easy mileage is the best way to build aerobic endurance which is why endurance runners should spend 80% of their runs at a conversational pace. We even call some of this easy running a “recovery run” when it’s scheduled the day after a speed workout or a long run.

Full Recovery Between Workouts Isn’t the Goal

Are you truly getting recovered from the track on Tuesday by running easy on Wednesday? No. You are not getting 100% recovered. But you do not need to be 100% to run well by Thursday or Friday. In a smart training plan, you should not be trying to run 100% of your potential on any given run. That’s something we want to save for race day, not for an average training run.

This is one of the biggest reasons not to go faster than intended in your workouts. If your run is supposed to be run at, let’s say, your 5k race pace, and you decide that you want to go faster just because you can, not only are you completely changing the energy system that the workout is trying to target, but you are building more fatigue that you will carry into your next few runs.

Your backpack is going to feel extra heavy and even an easy run is going to be a struggle to run with good form. Not only is this run going to feel terrible, but you are likely to reinforce bad running habits if you are too sore and tired to run well.

On the other hand, if you actually are feeling light and fresh as a daisy after a big speed workout, you might be ready to bump up your training intensity or volume if you are trying to reach your potential. A lack of fatigue isn’t a perfect sign of this, as some people are more fatigue resistant than others, but it can be a clue.

How to Use Fatigue to Your Advantage

Now I’d like to go over some ways you can actually use fatigue strategically in your training. Because just like hiking with a weighted backpack will make you a stronger hiker when you go without it, carrying some fatigue into certain workouts can help you maximize some of your runs.

Most of the time, we are trying to recover and relieve ourselves from the fatigue of training. But sometimes, you can actually harness that fatigue to your advantage.

The Steady/Long Run Combo

Let’s start with the marathon.

One thing that separates the marathon from shorter distances is that you don’t want to run the full 26.2 miles or 42 kilometers in training. Research has shown that once your training run goes longer than 3 hours, the aerobic benefits go down, while recovery times and injury risk goes way up. Since the marathon is about 99% aerobic, we want to maximize all the aerobic training we can, while minimizing the risks, and shortening the recovery time between runs.

So unless you can comfortably jog a marathon in less than 3 hours, you are going to want to run less than 26 miles in a single run in training. And in order to make those under-distance long runs more like the experience we will feel on race day, without actually running all the miles at once, we can get creative by using our backpack of accumulated fatigue.

A great way to do this is by running a shorter steady or marathon-paced run the day before the long run. Now, at the beginning of this episode, I told you to alternate your hard days with easy days, but this is when you want to do the opposite for a specific purpose.

Instead of thinking of the steady before the long run as two separate runs, think of them as one run, broken up by a night of rest. So perhaps your steady run is 6 or 8 miles on a Saturday. It’s not an easy run, other than the warm up and cool down miles, but it’s not a super challenging run either. Then on Sunday, you’ll be effectively beginning your long run at mile 6 or 8 instead of mile zero, since you’re still carrying Saturday’s miles in your legs. If your long run is 18 miles on Sunday, you are effectively running 22 or 24 miles, but with far less risk or recovery time.

Accumulated Fatigue Workouts for Other Distances

But it’s not just in marathon training that you can use this technique. In 5k training, you can combine hill sprints in the same week as a track session. Or you might try a tempo run followed by some quick 400 meter repeats for a half marathoner. By starting a workout fatigued, you automatically intensify the workout, allowing you to run less with the same effect.

Of course, any time you intensify a workout, you need to be smart about it. You can’t just keep adding weights to your backpack and expect to get stronger and stronger. Specific accumulated fatigue workouts are very challenging and should only be done every other week during the race-specific portion of your training schedule. This ensures that you don’t overdo it and find yourself on the wrong side of fatigue.

Chronic Running Fatigue and Overtraining

Chronic fatigue is one of the first signs of overtraining syndrome which can be a tough hole to climb out of. Overtraining is not always about simply training too much and it includes everything else you’ve got going on in your life that contributes to your overall stress.

If you are not quite feeling recovered over a few days of rest, that should start to get your attention and you might consider keeping things easy for a while before adding more stress from speed or long runs. But, if you’re experiencing fatigue after taking two weeks of relative rest, it’s time to investigate further.

There’s no blood test for overtraining. Some signs and symptoms of overtraining are a suppressed immune system, which might lead to more frequent colds or illness. You might also feel psychological effects like low level depression, chronic down moods, or changes in your sleep. Your performance in training and races will likely begin to suffer as you feel mentally and physically exhausted.

This is not the kind of fatigue that is a normal, healthy part of training. This is when you’ll need to take action.

You might still be able to run some when you are suffering from over training, but drop your volume in half and stop your speed work and racing. For some runners, you might need to stop running completely for a few weeks. Walking is a great way to stay lightly active as you recover.

Low Iron Can Cause Fatigue

If you are feeling overly fatigued, but you don’t fit the description of overtraining, there might be another issue going on.

A very common culprit of running fatigue is low iron levels. Slight fatigue and shortness of breath happen to everyone after a hard workout, so low iron levels can be missed without a blood test.

I recommend that all runners get a simple blood test to check their iron levels because you don’t want to supplement with iron if you are not deficient. Supplementing with iron can be toxic for some people, so make sure you need it before you take it.

Ask your doctor to test for hemoglobin, hematocrit, iron, total iron binding capacity and ferritin. If you can’t remember all of that, explain to your doctor that you are a runner and you want a full anemia panel with ferritin, because it’s not always included in a standard blood panel.

Anemia means your hemoglobin and hematocrit, which measures of your red blood cell count, are low. Iron deficiency is when your ferritin, which is a measure of your stored iron, is low, and your total iron binding capacity is high, indicating there is extra room to bind more iron. For most runners in training, you’ll want to make sure that ferritin number is over 30 nanograms/milliliter for women and over 40ng/ml for men.

If you test low in iron, your doctor will recommend supplementation and getting more iron in your diet. Common sources of iron are meat and egg yolks and plant-based runners can get their iron from dark leafy greens, legumes and molasses.

There are many other medical reasons for excessive fatigue, such as illness or other deficiencies in your diet, so check with your doctor if you are not feeling better in a couple weeks.

Fatigue During Your Runs

And finally, I’d like to talk about the kind of fatigue that you feel while you are actually running. We’ve talked about chronic fatigue from your overall training, but now let’s get into why you’re getting tired during an individual run.

In long runs and races, you’ll eventually start to fatigue if you go long enough. If you are running longer than 90 minutes and you haven’t fueled, your brain will start to slow your legs down. This is the most common reason for the marathon bonk. Your glycogen stores are getting depleted and your brain sends out the mental and physical fatigue signals. This can be largely prevented, or at least pushed back, by properly fueling during the run.

The next reason you will feel fatigued is if your legs aren’t up to the task that you are trying to do. Try running two or three times as long as you’ve ever run before on untrained legs and guess what? You’ll feel tired, both mentally and physically.

And it’s not just running longer that will trigger the fatigue signals. If you are attempting a speed workout that is beyond what you’ve trained for, your brain immediately imagines that you are being chased by a saber-toothed tiger. Stress hormones like adrenaline and cortisol shoot through your system and it won’t take long for you to be bent over with your hands on your knees, completely exhausted.

Heat, hills, cold, and dehydration all exacerbate these effects, making you feel even worse even faster.

Smart Training Builds Running Fatigue Resistance

The good news is that your body has an incredible ability to adapt to fatigue-producing stimuli. With gradual, patient, and smart training, you can teach your brain and your body to run faster and longer while increasing your running fatigue resistance. And then, you can use your fatigue in clever ways to maximize your workouts making them more effective and easier to recover from.

And on race day, you’ll finally set down that heavy pack of fatigue, so you can be faster than ever. But don’t get too comfortable. Your fatigue will be waiting for you at the finish line.

About Claire

Coach Claire has helped hundreds of runners chase their dreams and conquer big goals. Her coaching philosophy combines science-based training, plant-based nutrition, and mindset techniques to unlock every runner's true potential. She's an ASFA certified running coach, sports nutrition specialist, a 2:58 marathoner, mom, and borderline obsessive plant lover.

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