Does Running Build Muscle Or Does It Burn It?

Does Running Build Muscle Or Does It Burn It?

Fitness seems straight-forward on the surface. But the more attention a person pays to their fitness, the deeper the topic becomes. Many athletes tend to turn to their fellow gym-goers and online fitness forums to answer their questions. This seems like a good idea but it can lead to some aggressive runs of misinformation. One of the best examples of this is the rumor that running – or any low-impact cardio – burns muscle.

The myth goes something like this: When an athlete engages in low or slow-burn cardio such as distance running, their body burns muscle as fuel. People who believe this myth will point to the stereotypical frame of a distance runner as their proof. This stereotype is, of course, that distance runners are all whip-thin with little muscle mass and even less body fat.

But it is just that: a myth. Unless an athlete is starving themselves or wildly undercutting their protein intake, there is no reason for the human body to consume its own muscle. In both cases, it isn’t the distance running that causes the body to turn on its own muscle stores. It’s malnutrition. Low-impact cardio can actually help athletes build muscle, according to studies on the subject.

When DOES the Body Burn Muscle?

Our bodies want to burn fat before they burn protein. It breaks down easier which means it releases energy faster. If an athlete’s body is consuming its own muscle stores, something abnormal is happening.

The most common reason for a body to burn its own muscle for fuel is that it isn’t receiving enough new protein. This means that the person isn’t eating enough or isn’t eating at all. Athletes may fall into this trap without realizing it as their protein needs change. Other symptoms of a protein deficiency can include exhaustion, brittle hair, water retention, and consistent hunger. If an athlete is losing muscle while experiencing these symptoms, the solution is not to cut low-impact cardio. It’s to increase their protein consumption.

There are also a few medical conditions that can cause a body to burn its muscle stores. They are uncommon enough, however, that athletes would be better served by adjusting their protein consumption first.

Does Low-Impact Cardio Have ANY Effect?

Proponents of the “muscle burn” myth will be astounded to find that, according to science, low-impact cardio can actually help an athlete build muscle as well as avoid some post-workout soreness, improve their fitness readiness, and recover faster. It might seem like too much to ask from a long-distance jog, but the science doesn’t lie.

Can Reduce Soreness

Low-impact cardio increases blood flow to the muscles it puts the most strain on. This increased blood flow helps alleviate some post-workout soreness while bringing fresh oxygen  – meaning fresh energy – to the muscles during the workout.

Increase Fitness Readiness

Readiness in fitness is the word for how well the human body can recover from different types of exercise. If an athlete only trains for one kind of exercise  – or overtrains for that one kind of exercise – they have poor readiness. This can and will lead to injury, fatigue, and a limitation on their fitness gains. Improved readiness allows athletes to adapt to whatever training they need to do, keep their muscle gains well-rounded, and will help them avoid injury.

Improve Recovery Times

The central nervous system (CNS) governs how fast the body recovers from intense use. It orchestrates all the things necessary to get an athlete back on their feet and high-impact exercise can lead to CNS fatigue. Low-impact cardio, on the other hand, more or less presses the reset button. It “re-energizes” the CNS so that athletes can start to recover faster, from the cardio and from the next few workouts.

Assist Muscle Building

This is where the myth of low-impact cardio’s “muscle burn” reputation really falls apart.

Fast-Twitch Muscle Cells

It turns out that cardio increases the aerobic capacity of the body’s fast-twitch muscle cells. These cells produce the raw strength and power that allows for explosive motion, such as intense jumps and weight lifting sessions. A cell’s aerobic capacity is the amount of oxygen a cell consumes. Oxygen translates into energy. So the more oxygen a cell consumes, the more energy it can produce. Scientists are still trying to nail down the exact cardio intensity and duration to best exploit this benefit, but endurance cardio is absolutely guaranteed to increase the cells’ aerobic capacity.

Growth Hormone Spikes

There is a second myth that runs alongside the “muscle burn” myth. This second myth claims that intense weight training is the only way to cause natural spikes in a person’s growth hormone (GH) level. GH is a key component in muscle building so, the myth reasons, anything other than weight training is a waste of time for people seeking to bulk up.

The US Army’s research institute took this myth down themselves. They found that GH does spike after an intense weight training session, but the level falls again almost immediately after the workout ends. Steady endurance-style cardio causes a similar spike in GH levels, but the spike lasts for more than an hour after the workout ends, which can lead to more muscle over time if exploited properly.

Why Are Runners Slender, Then?

Despite the muscle-promoting benefits of low-intensity cardio, runners still tend to be a slender bunch. This is not because running is burning their muscles, however. Most runners are skinny because they prefer cardio over weights. No matter how helpful running is for muscle gains, an athlete has to actively engage in both weight training and cardio to see results. Most runners tend to focus on legs when they hit the weight room at all. This is so prevalent an issue among runners that a movement has started among training coaches to increase their push toward the weight room.

The Importance of Intensity

If an athlete is going to incorporate running into their workouts, science says they should be more worried about the intensity of their workouts than muscle loss. Most people who aim for either end of the intensity spectrum usually miss the mark. They fall somewhere in the middle. Athletes seeking high-intensity workouts don’t realize how long they’re taking on their runs, which reduces the intensity. And those seeking low-intensity workouts may up their pace because they aren’t “feeling the burn”.

There are a few ways to combat this middle ground muddle. Athletes can employ a trainer or workout buddy to keep an outside perspective on the intensity of their workouts. They can also keep the intensity of their workouts low by breathing only through their noses. Switching up cardio styles is another good way to keep the intensity low, since the athlete’s body will not be used to the muscular and cardio demands of the new workout style.

Still Worried?

On the off-chance that an athlete is still worried about cardio affecting their muscle gains, there are safeguards they can put in place. Branch chain amino acids (BCAA) are excellent additions to any pre-workout protein shake. BCAAs prevent the body from breaking down stored protein and reroute the body’s consumption to any stored fat that may be available. Athletes can take a serving of BCAAs in their protein shakes 15-30 minutes before their workout and rest easy knowing that their muscles won’t be affected.


  1. Mens Health, How Long, Slow Runs Help You Build Muscle
  2. Mens Health, How to Build Muscle With Running
  3. Runners World, Can I Run Long Distance and Still Build Muscle?
  4. Muscle and Fitness, 7 Ways to Burn Fat Without Losing Muscle