Running on Tired Legs: Cumulative Fatigue Explained

Cumulative fatigue in running (or accumulated fatigue) is tiredness that slowly builds throughout a long distance running training cycle. The goal of cumulative fatigue prepares a runner’s body and mind for running a marathon, or long distance running event. With cumulative fatigue, you can still perform workouts and daily tasks. Learn how to use cumulative fatigue in your training to boost performance.

cumulative fatigue
Cumulative fatigue occurs naturally in marathon training and has mental and physical benefits.

As I write this, I am in the last few weeks of my marathon training cycle for the California International Marathon. And I must tell you, I have been running on tired legs as a result of cumulative fatigue.

My easy run pace has slowed by almost a minute. In these past few weeks, my legs have not felt “fresh or springy.” They have felt like they’ve already run many miles before I even begin my run. I call these dead legs.

If this were my first marathon cycle, I might be freaking out about how much slower my easy pace is and how tired and heavy my legs feel when I run. But since this isn’t my first rodeo, I know that running on tired legs during a marathon cycle is common-and actually a good thing. 

How on earth can running on tired legs which makes you run slower be a good thing? Because it is the result of something called cumulative fatigue or accumulated fatigue and it prepares your body and mind for running 26.2 miles.

In this article, I’m going to explain what cumulative fatigue is and why it is good to run on tired legs.

So, let’s go!

What causes heavy legs when running?

Before we dive into cumulative fatigue, let’s look into other reasons why your legs may feel heavy when running. 

  1. Iron deficiency. Low iron, which is common in female runners, can lead to low energy and make running feel hard. This is because iron is needed for red blood cells which carry oxygen in blood flow to your working muscles. I experienced this first hand when I realized I had very low ferritin. Ensure you are getting enough iron-rich foods in your diet and consider taking an iron supplement if directed by your doctor.
  2. Strength training. If you are lifting heavy weights, especially on easy days when you should be recovering, your leg muscles fatigued and legs may feel heavy when running. Aim to do your weight training on harder run days to concentrate stress and optimize rest. 
  3. Sleep deprivation. If you aren’t getting enough sleep each night, you are hurting your recovery. Because your recovery (and fitness gains!) happen when you are sleeping not when you are hitting the pavement or track. Aim to get 7-9 hours of sleep a night!
  4. Not enough rest or recovery. If there is not enough time between hard workouts, you may not be recovering enough and need to space out your workouts. If you have only one easy day,  recovery day, or rest day in between hard sessions, consider adding another easier day in between. 
  5. Diet. If you aren’t getting enough nutrients, you may have a feeling of heavy legs when running. This is especially true for those who eat a low-carb diet. Carbs are your body’s main source of energy (especially for runners!) so make sure you are eating enough carbs! Also ensure you are getting enough protein especially after running for more than an hour. Aim for about 20 grams within an hour of running.
  6. Poor running form. If you are running with poor running form such as a slow turnover or hunched upper body, your legs may have a heavy feeling when running. Practice running tall with quick and light feet which are key for good form. If you are a beginner runner, it may be worth getting your running gait checked by a physical therapist or running coach.
  7. Wrong running shoes. It may be as easy as changing your running shoes to a lighter shoe to rid that heavy leg sensation. For example, I love how cushioned the Hoka Bondi shoes are, but they make my legs feels so heavy!
  8. Other medical conditions. If you didn’t check any of these boxes and have feeling of heaviness in your legs, it may be worth getting your blood levels checked and medical advice to ensure you don’t have any other medical conditions. 

What is cumulative fatigue in running?

Cumulative fatigue (or accumulated fatigue) is tiredness that slowly builds throughout a marathon cycle. It is a term associated with the Hansons Marathon Method which defines it as “the accumulation of fatigue over days, weeks, and even months of consistent training.”

With cumulative fatigue, you are still able to perform your workouts, hit your paces and go the distance, but on the easy runs and recovery runs, your legs will feel heavy and your pace will likely be slower.

Cumulative fatigue is an intentional part of marathon training to teach your mind and body to keep running on tired legs—because no matter how experienced a marathoner you are, your legs will be tired that last 10k!

Related: The Benefits of an Easy Running Pace

Pin this information on cumulative fatigue in running for later!

What causes cumulative fatigue in running?

Cumulative fatigue is caused by days, weeks, and months of marathon training. When you are training for a marathon, you don’t do one big run and then rest for several days. Instead, you continue to run, sometimes even the day after a big long run.

You continue to run even though it can take days (or even weeks) to fully recover from a long run or hard workout.

Cutback weeks or down weeks where mileage is reduced by 20-30 percent every few weeks buffers a runner against injury. But overtime, this lack of full recovery and existence of fatigue builds causing “cumulative fatigue” which feels like running on tired legs.

Related: The Science of the Long Run

Why do my legs feel heavy and tired when I run?

If you are training for a long-distance event like a marathon or running high mileage, your legs likely feel heavy and tired because of the accumulated fatigue that happens with long distance running.

After long runs and workouts with fast paces, your body needs several days to recover. However, we cannot take days off training, as running is a sport built on consistency. So, over time, layers of fatigue build in your legs and body, leaving your legs feeling heavy and tired.

Should you run on tired legs?

Yes, you should run on tired legs if it’s not painful potentially risking injury and it’s not a persistent feeling on every run.

In a marathon training cycle, your harder workouts and longer runs should be spread out with easy days and rest days in between. These easy run days are when you will likely be running on tired legs. Embrace running slow on these tired legs as a way to get fitter and train your mind for running a full marathon.

However, if you are consistently still not recovered by the time your next hard workout or long run arrives, then you need to revisit your training.

You may need to adjust areas such as your training schedule including strength training regimen, nutrition, and sleep to avoid negative effects of cumulative fatigue.

Related: Your Complete Strength Training for Runners Guide

When should you not run on tired legs?

You should not run on tired legs if you are feeling overall exhausted, cannot perform daily tasks, are not recovering in between runs and workouts, and are not able to hit workout paces or complete distances. These are signs of overtraining.

cumulative fatigue in running post
Cumulative fatigue in running in normal for marathoners but shouldn’t hurt performance or impact productivity.

What are the benefits of cumulative fatigue?

The goal of cumulative fatigue prepares a runner’s body and mind for running the marathon.

Your legs and your brain are learning to complete a run even when its tired and doesn’t want to keep going. This callouses both the mind and body.

Related: What is Supercompensation in Running?

In sum, benefits of cumulative fatigue include:

  • Increasing fatigue resistance in legs so they can keep running even when they are tired
  • Strengthening mind to overcome doubts and negative self-talk that arises with pushing oneself in a race
  • Allows you to continue to train and accrue physiological adaptations
  • Strengthens the bones, soft tissues, and muscles to prevent injuries
  • Can spur supercompensation during taper when the body rebuilds and legs feel springy for race day

Related: How to Overcome Race Anxiety

What are the risks of cumulative fatigue?

Cumulative fatigue can easily cross over into overtraining which is BAD NEWS.

With marathon training, you cannot expect your legs to feel fresh all the time. However, the main aim of a marathon training plan should not be to spur excessive fatigue. This is counterproductive.

“Training will cause some accumulated fatigue, but it should never lead to burnout, declined performance in key workouts, and continued breakdown without adequate recovery,” says running coach Laura Norris of Laura Norris Running. 

What is the difference between cumulative fatigue and overtraining syndrome?

This is a very important question as developing cumulative fatigue in a training plan can very easily and dangerously cross over into overtraining or overreaching which can lead to injury, illness, burnout, and poor performance.

The biggest difference between accumulated fatigue and overtraining syndrome is that with accumulated fatigue you don’t feel tired all the time—mostly just on your easy runs. You are still able to perform well during your long runs and workouts, and in your day-to-day life.

Norris explains, “there is a difference between legs with lots of mileage in them and low energy. If you have acute fatigue or fatigue that bleeds into other areas or causes multiple runs to feel poor, something is wrong. The athlete may be undereating and/or overtraining/overreaching.” 

Norris reminds us that although running higher mileage has many indirect effects such as mental toughness, mitochondrial density, angiogenesis, better biomechanical efficiency, and (when in the context of energy availability) musculoskeletal adaptations, it can come at a metabolic cost.

If you are feeling tired most of the time and your workouts are suffering (as well as your productivity as a person), you need to revisit your training!

Related: Signs of RED/S

tired legs pin
Pin this information on cumulative fatigue in running for later!

 Here are the symptoms of overtraining syndrome:

  • increased effort when running
  • feeling tired all the time, even when not running
  • dreading running
  • difficulty sleeping
  • higher resting heart rate
  • increased aches, pains, and/or injuries
  • getting sick more often
  • cranky or irritable.

If you are experiencing some or all of these symptoms, then you may be experiencing overtraining syndrome.

What do I do if I am overtrained?

If you are overtraining or overreaching:

  • Consider getting bloodwork done to ensure there are no deficiencies or imbalances
  • scale back your running in volume and intensity (consider doing 1-2 weeks of just easy volume until things regulate)
  • focus on eating and sleeping
  • get a massage
  • take relaxing walks
  • do what you like to do to relieve stress
  • and enjoy extra time with your family.

When you start to feel better on your runs and in your daily life, you can resume training as programmed.

How do I implement cumulative fatigue into my training?

Cumulative fatigue is a phenomenon that will occur naturally in a marathon training cycle. Therefore, it is not something that should be a sole focus of a training plan design, warns Norris.

Marathon training can be exhausting but you should feel recovered by the time your next hard run or workout arrives.

A marathon training plan that safely introduces cumulative fatigue training will include many of the following training qualities:

  • A gradual increase of weekly mileage by about 10 percent
  • A cutback week in mileage every 3-4 weeks with about a 20 reduction in mileage
  • Hard workout days surrounded by light days, active recovery, or rest days. A running plan should NEVER have consecutive hard days.
  • A shorter run day preceded or followed by a long run day or speed workout day
  • A double day in which you run twice in one day (mostly for elite and sub-elite runners)
  • Unless you are an elite runner, one total rest day per week

Key Takeaways on Cumulative Fatigue in Running:

  • Cumulative fatigue is the “accumulation of fatigue over days, weeks, and even months of consistent training.”
  • It trains your body and mind to run on tired legs for the marathon.
  • It’s most commonly felt in the later portion of a marathon cycle.
  • Your legs may feel heavy and tired on easy runs. Your easy pace may slow.
  • But you shouldn’t feel tired all the time or have it hurt workouts.

Remember, I am not a doctor so if you have health concerns, see your primary healthcare provider. 

If you want guidance with your running goals or marathon training, check out my run coaching services. Also, be sure to check out my free training plans:


Leave a Comment


Download my FREE FULL MONTH of strength workouts for runners!

Looking for a free running plan? Email me at [email protected].

* indicates required

You have Successfully Subscribed!