Legs Feel Heavy When Running? Here Are 7 Reasons Why + How To Fix It

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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 tired and 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 that feel like lead when you’re running, 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 when running and how to fix them.

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

Runner's legs with a sunset behind. Overlaid text with the article title and 'Marathon Handbook'.

7 Reasons Why Your Legs Feel Heavy When Running

1. Poor Running Form

There are many reasons why avoiding bad running form is important. Good running form improves your running efficiency, helps you to run faster, and reduces the chance of injury.

Take note of the first point here. 

If your body is not using energy efficiently due to poor running form, it not only makes you slower; it also tires you out faster, meaning your legs are more likely to feel heavy. Therefore, maintaining good running form can prevent you from fatiguing too quickly and reduce the chance of tired legs when running, as your body is working more efficiently.

You can read our guide on proper running form here, but here are a few indicators of poor posture to watch out for that could help overcome heavy legs when running:

  • 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 their arms, but the real goal is just to keep you stable and balanced. Your legs, hips, and the slight forward bend of your torso are enough to propel you. Think about keeping your arms at right angles, and keeping 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. 
A lady doing a leg press on a machine.

2. Excessive Weight Training

Strength training with weights is an important aspect of training for many runners. 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, 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, resulting in stiff legs that feel like lead when you run. 

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!

In particular, if you have heavy and tired legs when running but you’re not out of breath, this may be the cause, as the root of the problem isn’t cardio related, but rather due to your muscles being tired and repairing themselves.

A lady stretching her leg on a railing.

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. 

In particular, if you find yourself with tired and heavy legs when running but you’re not out of breath, or you feel that you could run further but your legs are aching too much, overtraining is a very common cause. This is due to the fact that your joints and bones strengthen at a slower rate than your cardio fitness improves.

Overtraining is also more likely to be a cause of tired legs when running if you are a beginner runner or are just starting out.

Cardio-wise, you might be capable of running 10k, but if your legs haven’t had a chance to strengthen, you shouldn’t be running that far regularly.

Minor running injuries fall under the category of overtraining as well. If overtraining, you may find that your legs stiffen up whilst you run, as they are still healing.

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

In order to fix heavy legs from running due to overtraining, the key is to rest and don’t increase the intensity or duration of your runs too quickly. It’s always best to follow a professional training plan.

A lady climbing over a wooden exercise wall

4. Not Enough Carbs and Iron

There are 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, which causes fatigue and heavy legs as the muscles in your legs don’t have enough fuel. 

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 low iron 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 it 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!). 
  • Dark leafy greens such as spinach and kale are also packed full of iron.

5. Dehydration

We know that keeping hydrated is important during a run in order to replace 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. 

In fact, your muscles can actually cramp up and fail to work properly when dehydrated, again resulting in tired legs when running.

In order to prevent and overcome dehydration-induced heavy legs when running, make sure to drink sufficient water and maybe try some electrolyte-based solutions.

a man stretching his leg on grass with a bottle of water in front of him

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. 

a lady stretching her leg against a fence

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 of 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 it into energy. Overtime (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

a lady running across a tiled plaza with trees behind her

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

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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.

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