Should you wear underwear under running shorts?

In the rain, running shorts cling to your skin, and the question comes up again: should you wear underwear underneath, or nothing at all? Underwear feels reassuring, but it can rub with every stride and ruin your run. This article explains when underwear is really useful, when it gets in the way, and how to choose running shorts you can wear on their own.

No, running shorts are worn directly against the skin

No, running shorts are worn directly against the skin. Their liner already acts as underwear. On the trails of Finistère, in persistent drizzle, well-fitting running shorts stay in place without needing an extra layer. Additional underwear can add a layer that shifts and rubs, especially over long distances.

For comfort, wear the running shorts directly against your skin if their cut and support suit you. Shorts with an integrated liner can replace underwear, but not all running shorts have one: check the label before deciding. For a pair with built-in boxers, the AEROK 6 Ultra Trail is a solid option. Wearing an extra layer is still your choice, so try it out on a run: without a liner, fitted technical underwear is better than cotton; with a liner, wear nothing underneath.

Why underwear mostly adds friction

Seams and folds in the groin

On a muddy trail, halfway up a climb, your underwear is the first to let you know. Chafing caused by friction between skin and clothing often occurs in the groin, a particularly sensitive area. A seam or fold in your underwear rubs against your skin with every stride, and repeated throughout a run, this friction eventually causes irritation. Check your skin after running: a red mark in the groin says it all.

The underwear's crease and elastic affect the skin in three ways, and the effects add up with every stride:

• The skin rubs against itself for a long time in the groin crease with every step.
• The L's elastic presses against a strip of skin and rubs it with every leg movement.
• Sweat trapped in the fold causes moisture and heat, and repeated movement damages the skin's outer layer.

Cotton and trapped sweat

On a rainy trail in Brittany, you feel the cotton from the very first hill. Soaked with sweat, it stays damp against the skin and chafes with every stride. A well-fitting technical fabric wicks away sweat and dries quickly, even in the rain. For quick-drying shorts, men's trail shorts and women's shorts and skirts by Kinetik are made from technical fabrics that leave cotton behind.

When sports underwear is appropriate

Very thin or light-colored shorts

There is one case where an extra layer makes sense: thin or light-colored shorts. In the rain, they get wet, cling to the skin, and become see-through. If you want to wear a layer underneath, choose a close-fitting style made from technical fabric that wicks away sweat. Polyester or nylon helps draw sweat away from the skin, whereas cotton traps moisture. Before heading out, try this test: take a few strides in front of a bright light, then bend forward.

If in doubt, switch to 2-in-1 shorts: it's less hassle. Their built-in liner replaces underwear, reduces chafing, and lets your skin breathe. The L'AEROK 6 Ultra Trail is designed exactly for this: ultra-light SKARTEX fabric on the outside, irritation-resistant SILKMILK liner on the inside.

The support some men look for

For some men, support comes first. They wear a jockstrap, a sports brief with a rear strap and waistband, under their running tights. It’s a personal comfort choice, not a requirement. If you wear one, choose a synthetic fabric rather than cotton, as cotton holds sweat and causes more rubbing. NOnly add a jockstrap if you really need extra support: if your compression shorts stay in place and nothing shifts when you run, there’s no need to wear one.

What to check before buying compression shorts

On a Breton trail at dawn, slightly loose compression shorts can rub your skin on the very first descent. It’s frustrating, especially when you only notice halfway through the run. If they’re too loose, they rub; if they’re too tight, they dig into your skin. Don’t keep compression shorts that leave marks on your skin.

Before buying, check:

• Seams: none should run along the inner thighs.
• Gusset: the crotch lining should lie flat. For support, see our trail running shorts guide.
• Fabric: stretchy fabric that doesn’t mark the skin.
• Length: try them on while running, making sure there’s no seam in a chafing area.
• Compression: it covers the skin and limits contact between the thighs. If the inseam causes irritation, see our guide to the best trail running shorts.

How to prevent chafing before a long run

Before a long run, spend twenty minutes wearing the outfit you plan to run in. You’ll quickly feel any seam or waistband that rubs, often within the first 10 to 20 minutes. On a coastal trail, a poorly placed seam can leave a mark on your skin in twenty minutes: if you see a red mark under the seam, don’t push through it - try a different style. For a 2-in-1, see our guide to women’s running shorts that prevent chafing.

Apply balm or petroleum jelly to areas that rub before heading out. A layer creates a barrier between your skin and clothing, reducing friction. No need to wait: you can run right after applying it. After an hour of running, check these areas. Redness that remains under a seam means the rubbing hasn’t been addressed: apply more balm or switch to a different style. Apply it before every long run, even when nothing is rubbing yet.

In most cases, running shorts are enough on their own. Their lining replaces underwear, and an extra layer mainly adds another source of friction. Check the label on your shorts: if they have a built-in liner, don’t wear anything underneath. If they don’t have a liner, or become see-through in the rain, a 2-in-1 pair of shorts like the AEROK 6 Ultra Trail settles the question once and for all. Available with 4 interest-free payments on orders of €70 or more and free delivery on orders of €100 or more.

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