Skin Under Pressure: How Your Body Becomes a Course to Manage, Anticipate and Protect

Running is a repetitive sport. A 42-kilometre marathon means between 35,000 and 40,000 strides. That is 35,000 to 40,000 times the same areas of skin being compressed, pulled and rubbed against fabric, a seam or another patch of skin. At that level of repetition, even the toughest skin eventually gives way. There is an entire field of science devoted to these phenomena: tribology, the study of frictional forces between two surfaces in contact. Applied to the human body, it helps explain why lesions appear where they do, and not elsewhere. Dr Stéphanie Leclerc-Mercier distinguishes between two separate mechanisms. The first involves superficial friction: fabric, a seam or a strap abrades the epidermis over the course of the miles. “It’s a superficial abrasion of the epidermis that removes the first layers and exposes the top of the dermis,” she explains. It’s painful, it stings, and this is what English speakers call chafing.” Redness, burning and irritation around the groin or armpits belong to this first category.

The second mechanism is deeper and potentially more disabling. In the feet especially, the skin is trapped between an external surface, such as the sock or insole, and the internal bony structures. When the movement is repeated thousands of times, the different layers of skin no longer move together. The epidermis sticks to the sock while the bones continue along their path. This dissociation creates internal shearing forces that eventually cause separation at the dermal-epidermal junction: a blister. A 2016 study by Lipman and colleagues involving 128 runners found that 50% of blisters form on the toes and 23% on the heel, precisely the areas where shearing forces are highest because of the foot’s anatomy and its repeated contact with the ground.

Sweat, accomplice and accelerator

You might therefore imagine that sweat acts as a lubricant, a layer of water between skin and fabric that makes sliding easier. Wrong. “For things to glide, you need grease. Sweat contains salt, and salt is exactly what skin does not need constantly,” says Dr Leclerc-Mercier, who is active on Instagram under the name @the_outdoor_dermato. In reality, sweat makes friction worse in two ways. First mechanically: damp skin is more adherent to fabric than dry skin, increasing frictional forces rather than reducing them. Then chemically: salt in sweat directly irritates the epidermis, creating an osmotic effect on skin cells that weakens them until they burst. “Salt, sweat and friction make a volatile cocktail,” the dermatologist sums up.

Heat makes the problem worse. High temperatures, humidity and moisture build-up: summer races bring together every aggravating factor. Winter events are not exempt, either. In cold weather, the nipples contract and become mechanically more prominent, leaving them more exposed to rubbing from fabric.

Feet: the first line of defence

This is the most documented and studied area, and paradoxically the one runners often manage with the least discipline. Blisters are one of the leading causes of withdrawal over long distances, a statistic that should be enough to put foot care at the heart of every runner’s routine. The source of the problem is almost always equipment. A pair of shoes with a toe box that is too narrow or too wide, socks with poorly positioned seams or, a classic mistake, a familiar model in a new version that has not yet been broken in. “I run in the Adizero 7 and bought the 8 thinking they were the same. But they don’t have the same foam, and the fit may not be the same either,” says Dr Leclerc-Mercier. The result is almost guaranteed by the finish.

Hardening the skin, a common practice among distance runners, can genuinely help by thickening the epidermis and making it more resistant to shearing forces. But too much has the opposite effect: an overly thick callus rubs against the deeper layers and creates deep, particularly painful blisters. That is why seeing a podiatrist before an important race can be useful, not to get orthotics, but to have areas of hyperkeratosis cleaned up and brought under control.

The classic mistake is the race T-shirt. You pick up your bib and the race T-shirt, then wear it on Sunday without ever having washed or tested it. That’s a recipe for jogger’s nipples.

Nipples: the hallmark of an avoidable injury

The jogger’s nipple or runner’s nipple in the English-language literature has become the classic image of the suffering marathon runner. Studies show that nearly 36% of runners covering more than 65 km per week experience it. At mass-participation races, bloodstained shirts around the chest are not uncommon. Nipples are vulnerable because of their anatomical position and the lack of natural protection in men. Unlike other areas, they come directly into contact with fabric without any cushioning. In cool weather, they become erect and therefore more prominent and exposed. In hot weather, sweat worsens the abrasion.

The dermatologist is categorical about the source of the problem: “The classic mistake is the race T-shirt,” she says. You pick up your bib and the race T-shirt, then wear it on Sunday without ever having washed or tested it. That’s a recipe for jogger’s nipples.” Barrier creams or kinesiology tape applied before the start remain the most effective solutions.

The groin is a case apart

A zone subject to two types of stress, the groin combines mechanical friction with moisture build-up, bringing together two of the three aggravating factors on a permanent basis. The shearing movement between the thighs with every stride creates continuous abrasion, amplified by the heat and humidity trapped in this enclosed area.

Dr Leclerc-Mercier systematically recommends compression shorts for all long-distance efforts, even if they are worn under shorts or a running skirt. “With compression shorts, there’s much less friction,” says the founder of Dans la Peau des Sportifs (@danslapeau_dessportifs), an association dedicated to preventing, educating about and researching athletes’ skin problems. With short shorts and built-in briefs, there’s much more rubbing.” A barrier cream applied before the start provides a second line of defence, provided it is reapplied regularly, since its effectiveness generally lasts no more than one to two hours.

Armpits and lower back

Less often discussed but regularly seen in consultations, the armpits are a high-friction area. The pendulum-like movement of the arms is to running what the stride is to the legs. A garment with thick or poorly positioned seams can be enough to turn a long run into an ordeal.

When I see someone whose T-shirt is riding up during a run, I always warn them.

The lower back deserves particular attention from runners wearing a running belt or backpack. When a T-shirt rides up beneath the equipment, it exposes a strip of thin skin that is not used to repeated contact, leaving it vulnerable over the miles. “When I see someone whose T-shirt is riding up during a run, I always warn them,” says Dr Leclerc-Mercier. The same applies to runners wearing wired headphones around the neck: with sweat, the cable can become an unsuspected source of irritation.

Why some people suffer more than others

The question comes up often, and the answer is more nuanced than you might think. Contrary to popular belief, body shape is only a secondary factor. “Larger thighs will rub a little more, yes. But overall, the two most decisive factors remain sweat and the number of repetitions: the duration, distance and intensity of the effort,” the dermatologist explains.

There are, however, genetically determined skin vulnerabilities. Some people naturally have a weaker dermal-epidermal junction, with attachment systems that are structurally less resistant to shearing forces. These runners develop blisters disproportionately often compared with people around them under the same conditions, and it is sometimes only during a first long race that they discover this reality.

What really works, and what doesn’t

So how do you find something that works? The principle behind barrier creams is simple: apply a greasy film to the surface of the skin to prevent direct contact between salty sweat and the epidermis. “I compare them to a small shield,” says Dr Leclerc-Mercier. But they have a limited lifespan over long distances, meaning you need to carry some or leave it in your drop bag so you can reapply on the way. On ultras, some runners carry small individual sachets to renew the application.

I was stunned to see elite trail athletes finish races with their feet in an appalling state. Seeing a podiatrist beforehand is non-negotiable for me.

The effectiveness of tape on the feet is debatable. While runners use it widely on high-risk areas, the scientific literature does not recognise any proven benefit in preventing blisters. Worse, poorly applied tape that starts to peel during a race can create new friction points where it curls up. On nipples, however, it remains one of the most reliable solutions when applied correctly and suited to the effort. The once-popular double-sock technique has not demonstrated any effectiveness in the available studies. The trend has passed, probably because the results did not follow. A well-chosen technical sock, with as few seams as possible and a thickness suited to the shape of the foot, remains the best option.

Few runners think about this during their preparation. Yet a consultation a few weeks before a race is important, not just for orthotics. Cleaning up areas of hyperkeratosis, identifying potential friction points and preparing the skin for the effort can radically change the experience of a race. “I was stunned to see elite trail athletes finish races with their feet in an appalling state. Seeing a podiatrist beforehand is non-negotiable for me,” says Dr Leclerc-Mercier.

During the race, what can you do when it’s already too late?

In a marathon or half marathon, the honest answer is often to grit your teeth and keep going. Few runners stop to treat irritation when they have a time goal in mind. But over longer distances, and even more so in trail races, the right response to a developing blister is to act at the first sign rather than wait until it makes finishing impossible. The recommended survival kit for a trail pack: a small pair of round-ended scissors, a disinfectant wipe and hydrocolloid dressings. These create a protective cushion over the injured area without sticking to the wound, unlike standard plasters, which can tear away skin when removed. Secured with a light adhesive strip to stop them rolling under the strain, they have saved many a race finish.

For irritation on the chest or in the groin, the window for action is narrower. Going shirtless remains an option for men at events where it is allowed. Otherwise, having access to spare barrier cream and a second pair of dry socks can make all the difference, including at long-distance events where you may be tempted never to stop.

The golden rule: test everything beforehand

Dr Leclerc-Mercier returns to this point every time: “There is no magic formula. The best way to stop it happening is to stop it happening.” Prevention ultimately rests on a disarmingly simple principle: never wear something unfamiliar on race day. No new shoes, no new T-shirt, no new socks. If possible, test every item in your kit on long runs before approving it for competition.

For marathon runners, she sums it up in three pieces of advice: test the full outfit on long training runs until you find the combination that causes no problems; apply a barrier or anti-chafing cream to known sensitive areas before the start; and see a podiatrist a few weeks before race day. No mystery, no miracle gadget. Just preparation, method and the instinct never to underestimate what thousands of strides can do to three millimetres of skin.

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