Why Do Running Shorts Have Built in Underwear? (Benefits of Liners)
The liner replaces underwear. It reduces chafing, wicks sweat, and adds light support, so you usually do not wear underwear under lined running shorts. The liner is built from light, breathable fabric that absorbs sweat before it builds up on your legs.
This guide covers why the liner is there, whether you still need underwear, and what to wear instead if you skip the lining.
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Why running shorts have a built-in liner
Most running shorts use a liner instead of plain underwear because it keeps you cooler. The fabric is thin and breathable, so air moves through it easily and sweat does not pool on your skin.
- Keeps you cool: breathable fabric lets air reach your legs instead of trapping heat.
- Wicks sweat: moisture gets pulled away from your skin instead of sitting on it.
- Prevents chafing: dry skin rubs less, so you can run farther without irritation.
- Stops ride-up: only the thin liner can ride, not a bulky pair of underwear.
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Do running shorts need to be lined?
No, but it helps. Most running shorts come with a liner already built in, and there is a clear benefit to choosing one that does. The combination of cooling, sweat control, and chafe prevention is hard to match with separate underwear.
Do you still need underwear with lined shorts?
No. The liner already does the job, so extra underwear is not necessary. Wearing some anyway will not hurt, but it adds a layer that can trap heat the liner is designed to release.
Lined shorts worn without underwear still feel comfortable, since sweat gets wicked away as you go. You can compare lined running shorts on Amazon if your current pair is missing a liner or has worn thin.
What to wear instead of a built-in liner
If built-in liners are not for you, two options work reasonably well.
Wear your own underwear
Regular underwear under unlined shorts can help prevent chafing and lets you run longer in comfort. The catch is that most underwear fabric absorbs sweat rather than wicking it away.
Over a long run that sweat builds up, and damp fabric can eventually cause chafing on its own. If you are running anything close to marathon distance in regular underwear, expect some irritation by the finish.
A better fix is running underwear that prevents chafing, which matters most for men.
Try compression shorts
Compression shorts hug the leg closely, which keeps air moving across the skin and helps prevent chafing in the same problem spots. The snugness also presses lightly on the leg muscles as you run.
Some runners find that mild compression helps muscles feel less fatigued late in a run. Skip wearing underwear underneath, though. It shows through the fabric and blocks the cooling effect that makes compression shorts work.
You can see running compression shorts on Amazon if you want to try this route instead of a liner.
The bottom line
Most running shorts have a built-in liner, and it replaces underwear for good reason. The lightweight fabric keeps you cool, wicks sweat, and cuts down on chafing better than plain underwear can.
If you would rather skip the liner, chafe-resistant underwear or compression shorts are solid substitutes. Either way, you will also want to sort out where to carry your phone on a run once the shorts are settled.
