Legs Feel Heavy When Running? Here’s 7 Reasons Why

Why do my legs feel heavy when running?

There’s nothing worse than strapping on your running shoes early in the morning, convincing yourself you’ll feel healthy and empowered once you get started…only to suffer from heavy legs throughout the run. 

At first, you might think it’s because you’re tired or you’ve ‘fallen off the wagon’ with your training plan. But after several days of heavy legs, you’re wondering if something else might be the culprit. 

And it just might be.

In this article, we’ve outlined 7 common causes of heavy legs.

Work your way through the list, and see if any of these might be true for you. 

legs feel heavy when running

7 Reasons Why Your Legs Feel Heavy When Running

1. Poor Running Form

There are many reasons avoiding bad running form is important. It improves your efficiency to help you run faster and prevents you from injuries. 

But you know that. 

The other important reason to keep good running form is to prevent you from fatiguing too quickly. If your body is not using energy efficiently, it not only makes you slower; it also tires you out too fast. 

You can read our guide on proper running form here, but these might be a few indications that your form is fatiguing you:

  • You’re over-striding: While running, your ‘forward’ foot should be directly under your body – don’t pick your leg up and reach out too far. Instead, bend your knee slightly, take a shorter stride, and keep the leg closer to the ground. Focus on getting a higher running cadence.
  • You’re swinging your arms too much: Some people think they need to ‘push’ their bodies forward with the arms, but the real goal is just to keep you stable and balanced.

    Your legs, hips, and the slight forward bend of your torso is enough to propel you. Think about keeping your arms at right angles, and keep them in line with your shoulders. 
  • You’re not running from the hips: Don’t make the mistake of starting the push with your quads (unless it’s a steep hill). Start your leg movement from the hips and use the hips and core to power the leg movement. 

2. Excessive Weight Training

Strength training with weights is an important aspect of running. You need that core and leg strength to keep your body doing its job – especially when running uphill. 

In fact, there’s more to a strong core than meets the eye. Many people think the core is just the abdominal muscles. But the true core is all the muscles that support your spine and hold it upright: the hips, pelvic floor, your diaphragm, and – finally – the rectus abdominus and obliques. 

Keeping these muscles strong is essential to good running form and your health in general…but lower body strength training can be overdone

If you’re strength training as part of your training (which we highly recommend), then our head coach Thomas Watson recommends the following:

  • Do your heavy leg workouts (weighted squats, lunges) the same as an intense speed session – just make sure you do the leg workout after the speed session.
  • Be aware that DOMS is very common after lower leg workouts, and it’s common to feel sorest 2 days after your leg workout. During the 2-3 days following a heavy leg session at the gym, expect your running performance to be impeded.
  • If you combine a speed session (like hill sprints or fartleks) with weight training, then strongly consider having a rest day the following day. You’ve earned it!
  • I recommend doing light recovery runs either 1 or 2 days after your leg workout; it can help loosen things up and shake off those DOMS. Just don’t expect to set any PRs!

legs feel heavy when running

If you’re in the off-season and keeping in shape for training season, that’s when you want to focus on building up those core and leg muscles. 

3. Overtraining

If you love to push your limits and work hard, you’re not alone!

It can be exciting to see yourself achieving new goals and watching your body transform. But overdoing it is detrimental in the long term, and overtraining often causes heavy legs while running. 

Minor running injuries fall under this category as well.

If you feel any strains, sprains, or leg fractures, it’s important to rest and allow them heal – don’t ignore them! Research has shown that quick redressal of leg fractures, particularly in female athletes, helps faster recovery. 

legs feel heavy when running

4. Not Enough Carbs and Iron

These are the two food-fuel deficiencies that often cause heavy legs. 


Complex carbohydrates are especially important during long-distance runs. When running a shorter distance, your body starts by converting fat into energy. 

But the energy stores there quickly run out, and the body then turns to carbohydrates for more endurance. If there are not enough carbs there, the body then isn’t able to keep up the necessary levels of oxygen to convert to energy, and that causes fatigue (and heavy legs). 

Related: Carb Loading for Runners


Iron is another important part of the energy-making process during running. Iron helps your blood pump oxygen into the muscles, which then convert it to energy. 

You’re most likely to lack sufficient iron if you tend to experience anemia due to any pre-existing medical condition, or if you’re a woman on your cycle and are losing more-than-usual amounts of blood. 

Here are some iron-rich foods you can eat to combat the low levels:

  • Red meat – we usually recommend lean meats like chicken, but red meats can get you iron quickly. 
  • Beans – Red meat is high in iron, but is not the best for the environment, so if you want to get extra iron while keeping things easy on the earth, beans are a great source (and have carbohydrates and protein too!). 

5. Dehydration

You know hydration is important during a run, but water is essential during a long run because it replaces all the fluids you lose in the process. When you sweat, you need water to carry important nutrients through your body to help with the energy process. 

Since 50-60% of your body is made up of water, it’s no wonder you might feel fatigued and have trouble with heavy legs when you’re dehydrated. 

legs feel heavy when running

6. Sleep Deprivation

Before you started running long distances, you probably had no trouble functioning on very few hours of sleep. The CDC says that 1 in 3 people don’t get enough sleep

While our bodies do adapt to our current situations (hence, the ‘built-in alarm clock,’ that helps you wake up naturally when you’ve been getting up at the same time for many days), there’s a limit to how much you should force your body to adapt. 

Especially when it comes to running. 

Sufficient sleep enhances your running performance, while lack of sleep causes drowsiness, lack of mental stamina, and (you guessed it) heavy legs. 

legs feel heavy when running

7. Poor Blood Circulation

By now, you’ve probably noticed a consistent theme throughout this article: energy.

When you get heavy legs running, the cause is often tied to a lack of energy production in your body. 

Determining the cause for your heavy legs is ultimately a matter of finding out where your body is missing the fuel it needs to produce energy. 

Drs. Michael Joyner and Darren Casey discuss the effects that high-intensity exercise has on your blood circulation:

As you run, your blood pumps oxygen into the muscles to convert into energy. Over time (during a long-distance run), the heart and lungs have a hard time getting enough blood to those muscles to meet the high demand. When not enough blood circulates into those muscles, there is not enough oxygen to convert into energy. 

Usually, this means you’re over-exerting yourself and are trying to run longer distances than your body can handle. In this case, you need to work on cross-training and strength training, so that your muscles have the capacity to convert the necessary amount of oxygen. 

Bonus: One Additional Reason Your Legs May Feel Heavy When Running

legs feel heavy when running

8. Wearing the Wrong Shoes

When you started running, you might have thought stability was the most important aspect to watch out for. But if you typically run on flat, paved surfaces, try running with a lighter shoe. 

You’ll also want a shoe that offers some mobility, so your feet and legs can move together, offering you a better range of motion. 

If you’ve been wondering, ‘Why do my legs feel heavy when I run?’, and you now suspect your running shoe might be the culprit, read this guide on choosing the right running shoes for you

legs feel heavy when running
Mia Kercher

Mia Kercher

Mia Kercher is a hiker, cyclist, and runner. After finishing her first marathon in 2013, she continued the sport but found a new passion in trail running. She now explores the glorious mountains in Portland, Oregon where she works as co-founder of Evoke.

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