Experts are debunking the viral myth that claims running makes your waist bigger.
Fitness influencers and social media trolls alike have taken over online discourse with declarations that running as exercise can have an adverse effect to working out by building thicker muscles in the midsection, making the torso more boxy — which would make it more difficult to achieve an hourglass figure.
However, science proves that running regularly does not typically cause muscle gain in the midsection.
Although running engages your core muscles, including the obliques, the aerobic exercise does not work the muscles in a way that would make them bulky or oversized, according to a medically reviewed article by Healthline.
Building a muscle, which is called hypertrophy, often takes specific and consistent work, meaning that it is unlikely that you would grow abdominal and oblique muscles by accident.
Going into hypertrophy is achieved by working with heavy resistance like weights, and building up intensity.
On the other hand, running is known as a low resistance exercise that focuses on repetitive and endurance-based movements to burn calories, rather than to create muscle.
A 2025 analysis found that internal obliques are active during running, but within a normal range and not intense enough to overload into hypertrophy.
Instead, running can spark weight loss. A 2022 research paper found that regular aerobic exercise typically results in a decreased waist size, and less fat in the total body.
This means that running causes fat loss, not gain, and that it does not work hard enough to cause the stomach muscles to change shape.
Another factor that might have contributed to the runner’s waist myth is the trim appearance of elite sprinters or trained distance runners. However, it is important to keep in mind that professional athletes train very differently than a recreational runner — and they might have specialized training to provide a stronger core on their intense runs. Fat distribution, rib cage width and hip width cannot be controlled, and every body is different.
Anyone who experiences a temporary change in how their waist looks after running might have post-run inflammation, bloating, or water retention.
While you might not bulk up your stomach muscles on the treadmill, running long distances like in a marathon can cause micro-tears to build other muscles throughout the body.
Performance physiologist Francesca Bagshaw told The Independent last month: “Running heavily engages your lower muscles – especially your hamstrings, calves and quadriceps – but then also engages your abdominal muscles to help keep you running in a straight line and your arms to help drive you forward.
“These little micro-tears that we get during training will stimulate inflammation and various other responses around the muscle that adapt so the next time you go out and run, the muscle will be stronger and will be able to cope with the load and the intensity that you put on those muscles.”