Definition
Blisters in runners are one of the most common problems in running, trail running and marathons. They can cause significant discomfort and may even force you to stop during a race.
A blister (phlyctena) is a fluid-filled pocket that forms under the skin as a result of repeated friction, often combined with heat and moisture. Fortunately, there are effective ways to prevent them, notably by tanning the feet before a race.
Anatomy
A blister forms in the superficial layers of the skin: under repeated friction, these layers separate from one another and fluid builds up in the space created, forming the characteristic pocket of a phlyctena. Three main factors explain why it appears: friction, heat and moisture.
Epidemiology
Blisters are among the most common problems encountered in running, trail running and marathons, where prolonged friction, heat and moisture build up over long periods of exertion.
Causes & risk factors
Unsuitable shoes
This is the number one cause of blisters. The most common risks are shoes that are too small, too big, brand new, or that hold the foot poorly: an ill-fitting shoe greatly increases friction.
Unsuitable socks
Some socks encourage blisters to form: standard cotton, thick seams or poor sweat wicking should be avoided in favour of breathable technical socks made from synthetic fibres, possibly double-layer models.
Symptoms
A blister appears as a fluid-filled pocket under the skin, at the spot exposed to repeated friction. It can cause significant discomfort during exercise and may even force you to stop during a race.
Treatment
Choosing the right running shoes
A well-suited shoe greatly reduces the risk of blisters: go half a size up, leave room in front of the toes, test your shoes before a competition and avoid any new shoes on race day.
Choosing the right running socks
Socks play a major role in prevention: choose seamless, breathable models suited to long-distance running that allow moisture to wick away quickly. Double-layer socks are particularly effective against friction.
Tanning the feet before a race
Foot tanning involves hardening the skin to increase its resistance to friction; it is used by many runners, particularly in trail running and ultra-endurance events. It should be started 3 weeks before the race and consists of twice-daily foot baths (plantar arch only), morning and evening, made with lemon juice and water, for 5 to 10 minutes, combined with a daily evening application of a tanning cream such as NOK, to be started 10 days before the race.
Prevention
Preventing blisters relies on reducing the three factors that cause them — friction, heat and moisture — above all through the choice of shoes and socks. Foot tanning, started several weeks before a long race, effectively complements these measures by hardening the skin in areas exposed to friction.
Frequently asked questions
What causes a blister when running?
How do you choose the right shoes to avoid blisters?
What is foot tanning and when should you start it?
Key points — Institut Basque du Coureur
A blister appears when friction, heat and moisture combine on the foot. Choosing well-suited shoes and socks remains the first preventive measure, and foot tanning, started several weeks before a long race, helps strengthen the skin's resistance to friction.