Fit and sizing

Chafing

Also called chafe, friction burn

What is chafing?

Chafing is skin worn raw by repeated friction, usually where wet fabric rubs skin or skin rubs skin over hours. On a run it starts at the inner thigh, and dried sweat leaves abrasive salt behind that makes it considerably worse.

Needs: Movement, moisture and time
Usual site: Inner thigh
Made worse by: Salt left behind by dried sweat
First fix: A longer liner, not a thicker balm

What is actually happening

Chafing needs three things: movement, moisture and time. Skin that is damp has a much lower tolerance for friction than dry skin, and a running stride supplies several thousand repetitions an hour.

The part people miss is salt. As sweat evaporates it leaves crystals behind on skin and in fabric, and those are abrasive, which is why a hot multi-hour run chafes far more than a cool run of the same distance.

Fix the garment before the balm

Most chafing is a cut problem wearing a fabric problem's clothes. The inner thigh is the usual site, and a brief liner simply does not reach it, so a boxer-brief liner or a 2-in-1 removes the cause rather than treating it.

Remove cotton from the system entirely. It holds water, stays wet, and stays heavy. Then look for flat seams rather than raised ones anywhere fabric meets skin, and be suspicious of any seam sitting directly at the inner thigh.

What to do on a long day

Apply an anti-chafe balm before you start rather than once it hurts. Skin that is already raw will not be rescued mid-run, and the run you are on is the one you have to finish.

Rinsing at a creek genuinely helps on a hot day, because it takes the salt off. If one specific spot chafes every single time, treat that as the garment telling you it is the wrong cut for your body rather than as something to keep managing.

What this means when you're choosing

A liner that reaches the inner thigh rather than stopping above it
Flat seams anywhere fabric touches skin
No cotton anywhere in the system
A liner anchored to the shell so it cannot ride up mid-run

The common mistake

Treating chafing as a balm problem

Balm buys time, it does not fix a liner that stops above the part of you that rubs. If the same spot goes every run, change the garment rather than applying more product to it.

Common questions

Does merino chafe less than synthetic?

Often, next to skin. Merino holds less odour and stays comfortable when damp, which helps on multi-day efforts. It is not automatically better: fit and seam placement matter more than fiber, and merino liners wear out faster.

Should I put balm on before or after a run?

Before. Anti-chafe balm works by reducing friction while skin is still intact, so applying it at the trailhead is what prevents the problem. Once skin is raw, balm on top will not restore the run you are already on.

Where this comes up

Guides on Tom's Hiking where chafing changes which product is right for you.

More in fit and sizing

Torso length

Torso length is the distance from the bony bump at the base of your neck down to the top of your hip bones. It's the measurement backpack sizes are built around, and it has almost nothing to do with how tall you are.

Iliac crest

The iliac crest is the curved top edge of your pelvis, the ridge you feel when you put your hands on your hips. It's the shelf a backpack hipbelt sits on, and the lower anchor point for measuring torso length.

Last

A last is the foot-shaped mould a shoe or boot is built around. It sets the width, the volume, the shape of the toe box and the heel cup, which is why two pairs in the same size from different brands can fit completely differently.

Toe box

The toe box is the front of a shoe, from roughly the ball of the foot forward. Its width and height decide whether your toes can splay under load, which is what stops black toenails and blisters on long descents.

Surgeon's knot

A surgeon's knot is a lacing trick where you wrap the laces twice instead of once before pulling tight. The extra wrap holds tension in place, letting you lock your heel down while leaving the toe box loose.

Gusseted crotch

A gusseted crotch is an extra diamond or triangle of fabric sewn into hiking pants so four seams do not meet at a single point. It spreads the load of a high step across several seams, which is why gusseted pants do not bind or split at the crotch.

Short liner

A short liner is the brief, boxer or compression layer sewn inside running shorts and worn instead of underwear. Its cut is what decides whether long runs chafe: a brief covers little, a boxer liner reaches the inner thigh, and a 2-in-1 anchors to the shell so it cannot ride up.

Inseam

Inseam is the length of a garment's inner leg seam, measured from the crotch to the hem, and it is how running shorts are sized. Trail shorts run from about three inches to nine, and five to seven covers most trail running.

Last reviewed August 29, 2026. Back to the glossary