Chafing: Why It Happens and How Your Kit Causes or Fixes It

Three things have to be present. Remove any one and the problem largely goes away, which is why the fix is usually a garment change rather than a product.

The location tells you which of the three factors is dominant, and therefore which change will fix it.

In any fitted garment a seam is pressed against skin for the whole session. A raised seam in the wrong place will chafe in an otherwise excellent garment.

Turn a garment inside out before buying and run a thumb along the inner thigh and underarm. If you can feel a ridge standing proud, you will feel it at mile eight too.

This is the trade-off nobody states plainly. Shorter shorts reduce fabric on skin and increase skin on skin. Longer shorts do the reverse.

Which length suits your training is covered in our gym shorts length guide. If your thighs touch, the reliable answer is a fitted inner layer rather than a longer outer one — see the tight and compression shorts guide.

Cotton is the clearest mistake. It absorbs sweat and holds it against the skin, which softens the skin and raises friction at the same time.

Fibre behaviour in detail is in our workout t-shirt fabric guide, and why bamboo behaves differently when wet is in the bamboo fabric guide.

Both extra factors are about moisture. More sweat means more friction, and a longer session means more repetitions of it.

• Salt concentrates as you dehydrate and irritates skin further.
• Rain and wet kit produce the same softening as heavy sweat.
• New kit often chafes once and never again, as seams settle.
• Old kit chafes when the fabric has thinned and the seam stands proud.

Training in heat multiplies all of this, which is why our hot-weather guide covers the same problem from the temperature side.

They reduce the friction coefficient at the contact point. That is genuinely useful and it is not a substitute for kit that fits.

Apply before the session, not after the skin is already sore. Applying balm to broken skin traps bacteria against a wound.

Chafing is a friction injury rather than an inconvenience, and treating it as a minor wound is the right approach.

1. Clean gently with water; skip anything that stings.
2. Dry fully before covering.
3. Use a barrier or healing ointment, not a drying one.
4. Wear loose clothing until it has healed.
5. See a clinician if it weeps, spreads or does not improve.

General skin guidance from a non-commercial source is available through the NHS healthy living pages. Persistent chafing in the same spot usually means the garment, not the skin, is the problem.

Two people can run the same route in the same shorts and only one comes back raw. That is not imagination, and it is worth knowing why before you assume the garment is at fault.

The practical consequence is that recommendations are a starting point rather than an answer. If a short that works for everyone else does not work for you, the short is not necessarily wrong.

Build changes which of these apply more than most kit advice admits. Where cut and grading go wrong on broader frames is covered in our guide to training kit for larger and broader builds.

1. Work out whether it is skin on skin or fabric on skin.
2. Check the inner seam before you buy anything.
3. 7 inch is the inseam that avoids both failure modes.
4. Never cotton for anything long or hot.
5. Balm before, never on broken skin.

The same mechanism one layer down causes blisters — see our training socks guide, and our guide to what goes under training kit.