Foot Care, Blisters, and Chafing

What actually causes a blister — shear, not "rubbing"

It's tempting to think of a blister as skin being rubbed raw, like sandpaper. That picture is wrong, and getting it right changes how you prevent them. The modern understanding — laid out in two Journal of Athletic Training papers by podiatrist Doug Richie and physiotherapist Rebecca Rushton, and built on in John Vonhof's "Fixing Your Feet"¹ — is that friction blisters are caused by **shear**: the repeated stretching and tearing of soft tissue *beneath* the skin's surface.

Here's the mechanism. Your skeleton is rigid, but the soft tissue around it can move. As ultrarunning coach Jason Koop puts it, citing Vonhof, "your soft tissue (skin on the feet) moves more than your skeletal system (rigid bone)," and "friction and the underlying shear forces are what ultimately cause the dreaded blister"². When your foot loads on each step, the bone shifts but the skin surface stays gripped against your sock — so the layers in between stretch and distort. Do that thousands of times and the tissue separates, fluid floods the gap, and you have a blister.

This is why the single most useful preventive idea is **encouraging slippage**. Rushton and Richie's paradigm holds that if you reduce the friction force at one of the interfaces — between shoe and sock, or sock and skin — the skin can move *with* the bone instead of staying stuck, so each shear event is smaller. Every prevention tool that follows (socks, lubricant, tape) is really just a different way to let that little bit of slippage happen at the right interface, so your skin doesn't have to absorb the full shear.

Two things make shear far worse, and a long day delivers both in abundance, which is why prevention is never one thing — it attacks friction, heat, and moisture together:

• **Heat.** A warm foot blisters more easily. Koop quantifies it: heat "loosen[s] the bond between the dermis and epidermis," and "an increase in skin temperature of 4°C will accelerate blister formation by 50 percent"². Hours of friction inside a closed shoe is a heat factory.
• **Moisture.** "Moisture weakens the outer layer of skin, making it more prone to injury," and "the more your feet sweat, the more saturated your socks, shoes, and skin become"². Sweat, stream crossings, and rain all soften the skin until it gives way under shear it would otherwise shrug off.

The sock-and-shoe system

Your sock is the foundation of blister prevention — more than any lubricant or powder. Koop is blunt about it: "a well-fitting wicking sock offers the best blister prevention strategy"². Build the rest of the system around getting that sock right.

**Wicking material, never cotton.** Cotton soaks up sweat and holds it against your skin, which is exactly the moisture that softens skin into blistering. Vonhof's advice is simply to avoid cotton socks³; choose merino wool or a technical synthetic that moves moisture away from the foot.

**Toe socks and double-layer socks move the shear into the sock.** This is the slippage principle made physical. Toe socks (Vonhof names Injinji) encase each toe separately, so the friction happens between sock and sock rather than skin against skin — useful if you blister between your toes²⁴. Double-layer socks work the same way over the whole foot: the two layers slide against each other and absorb the shear your skin would otherwise take. One caveat from Vonhof: "If conditions will be sandy, avoid double-layer socks as the sand can build up between sock layers"⁵ — grit trapped between layers becomes its own abrasive.

**Size up for foot swelling — but keep the heel locked.** Over a 100-mile day your feet swell; "it is not uncommon for the swelling to cause a half to full size change"⁶. A shoe that fit at the start can crush your toes by mile 60, jamming them into the toebox on every descent and bruising or blistering the nails. The standard move is a trail shoe about a half size larger than your road shoe, with roughly a half inch (about 1 cm) of space ahead of your longest toe⁴. The trap is going too big: "too much extra room can cause slippage and blisters, so make sure any sizing change still gives you a secure midfoot and heel hold"⁶. The fit you want is roomy in the toebox but "relatively snug" through the midfoot, with your heel sitting "snugly into the heel cup"⁴.

**Lacing is your lock-down lever.** If a slightly roomier shoe lets your heel lift, a heel-lock lacing pattern (the "runner's loop" through the top eyelets) cinches the rearfoot without choking the toebox — letting you have toe room and heel security at once. Vonhof's in-shoe tricks for dialing fit as feet change: add a thin flat foam insole under the regular insole, or switch to thicker socks⁶⁵.

**Carry a larger backup pair.** Many 100-mile runners stage a half-size bigger shoe in a late-race drop bag and swap into it once the swelling sets in. It's one of the cheapest race-savers there is.

**Gaiters keep the trail out of your shoes.** Mesh uppers breathe, but they "let in the sand, grit, and dirt, which gets into socks and cakes on your feet and causes hot spots"³. Even a single small pebble "can rub wear holes in your sock and rub against your skin until a blister has formed"⁵. Lightweight trail gaiters keep debris out and save you the stops to empty your shoes.

Lubricant, powder, and antiperspirant — the honest tradeoffs

These are the three "apply something to your feet" strategies, and it's worth being honest up front: the evidence that any of them reliably prevent blisters is thin. Reviewing the Rushton/Richie assessment of prevention strategies, Koop notes "there is little scientific evidence that any of the aforementioned strategies work," and warns that "some preventive measures actually exacerbate the problem by adding moisture and increasing the skin's coefficient of friction." His rule of thumb is worth taping to the inside of your drop bag: **"This is an area where less is more"**².

With that framing, here's the division of labor — pick the lightest intervention that works for *your* feet, prove it over long training runs, and resist the urge to slather on more:

• **Lubricant attacks friction.** Balms like Squirrel's Nut Butter, Body Glide, or older standbys reduce the friction force so the skin can slip earlier. The catch over a day-long effort: lube wears off, and a greasy film can trap moisture against the skin — the very thing you're trying to avoid. If you lube, plan to reapply at aid stations, and don't drown your feet in it.
• **Powder attacks moisture.** As iRunFar frames the split, "foot powders are meant to help absorb your foot sweat, and lubes are meant to take on friction so that the skin of your feet doesn't have to"⁴. Powder keeps the skin drier and firmer. Vonhof's own advice is to experiment in both directions — "try different lubricants like Hydropel or BodyGlide. Or try skipping lubricants as much as possible and switch to powders"⁵ — because feet differ, and the only way to know which yours prefer is to test it.
• **Antiperspirant** has the strongest single study behind it — and a real downside. In a peer-reviewed trial of 667 West Point cadets, applying 20% aluminum chloride hexahydrate for five nights before a long hike cut blister incidence from 48% to 21%. But it caused irritant dermatitis in 57% of users, versus about 6% on placebo⁷. So the honest verdict is "the evidence exists, but the irritation makes it a gamble" — not a blanket recommendation. If you try it, you absolutely must test it in training first, because for more than half of people the cure is worse than the blister.

Proactive taping — let the tape eat the shear

If you have a spot that reliably blisters — the ball of the foot, a heel, the side of a big toe — the best prevention is to tape it *before the race starts*, when you have time to do it right. The logic is the slippage principle again: as iRunFar puts it, "pretaping the feet at their friction points allows the tape, not the skin, to bear the brunt of any friction, thereby preventing blistering"⁴. A patch of well-applied tape also spreads the shear load over a wider area, so the peak stress on any one point drops.

Vonhof is emphatic that this is a before-the-start job, not an aid-station scramble: "if you typically blister on the balls of your feet, consider taping before the run when you have the time to do it right rather than at an aid station," and "it is usually easier to tape the night before an event than wait until the morning when time is rushed"⁸. These are the steps that make tape actually stay on for 100 miles:

• **Prep the skin with tincture of benzoin.** This is the step most people skip and then wonder why their tape peeled off at mile 30. Vonhof: tincture of benzoin "clean[s] the feet of their natural oils, dust, and dirt. This is vital to getting a good stick." Apply it to all the skin the tape will cover and let it dry fully before taping⁸⁹.
• **Use a tape that conforms and sticks.** Vonhof's picks: Leukotape P "works well on the bottom of the feet and heels" and is very sticky; Kinesio-Tex is his current choice for toes because it has "a much smoother texture and is far easier to apply... very sticky and has a nice longitudinal stretch"⁸. NOLS wilderness-medicine instructor and longtime ultrarunner Liza Howard reaches for KT tape or Hypafix because they're "thin and stretchy and conform to the shape of your foot"⁹.
• **Apply with the skin stretched.** Vonhof's detail: as you tape the soles or heels, "grasp the toes of the foot and pull back to stretch the skin to its fullest"⁸. Tape applied to slack skin wrinkles the moment your foot loads — and a wrinkle is a new hot spot.
• **Prep nails and skin in the days before, too.** Vonhof recommends trimming toenails down well ahead of the race — long nails jam the toebox on descents and cause black, blistered toes — and he's wary of thick calluses, because blisters can form *under* a callus where they're painful to reach and treat³.

On the course — manage hot spots before they become blisters

The single most important mid-race habit is to act the instant something feels off. Liza Howard's cardinal rule: deal with a hot spot "as soon as you feel discomfort. Don't wait to get to an aid station to make it"⁹. The economics are lopsided — a hot spot covered with a strip of tape costs you two minutes; a deroofed, raw blister at mile 70 can cost you the race. When in doubt, stop and look.

If a blister has formed and genuinely needs draining, do it cleanly and preserve the roof. The points below are the field protocol; the order matters, and most of the damage people do to themselves is in skipping the "leave the roof on" rule or reaching for a flame-heated needle.

• **Decide whether to drain at all.** Rushton's guidance: drain only when the blister is "large, painful and likely to tear anyway." A small, intact, painless blister is often best left completely alone and simply taped over. Her bottom line if you're unsure: "if in doubt, do not lance it"¹⁰.
• **Drain cleanly, with wide holes.** Howard's NOLS field method: clean the blister and a needle or safety pin with an alcohol prep pad, then "use the pin or needle to make four holes in it. Move the pin or needle back and forth to make the holes nice and wide. Use the gauze pad to press the fluid out," then "tape the blister flat"⁹. Multiple wide holes won't seal shut and re-fill the moment you start moving.
• **Keep it sterile — never a flame-heated needle.** Rushton: use implements that are "sterile for starters," a fresh scalpel blade or hypodermic needle, *not* one heated in a yellow flame (which leaves carbon and doesn't reliably sterilize). The blister base is "a few cells width between safety and infection"¹⁰.
• **Leave the roof on.** This is the rule that separates a manageable blister from a miserable one. Koop: "torn or punctured blisters should be cleaned and bandaged, but you can leave the partial blister roof in place," and "don't open it so much that the blister roof becomes detached"². That intact skin is a sterile, built-in dressing over raw, exquisitely tender dermis — peel it off and you've turned a blister into an open wound for the rest of the day.
• **Reset your feet at aid stations.** On a 100-miler, plan to change into fresh, dry socks — and into your larger backup shoes once the swelling sets in. Clean your feet every time you change: dirt and sand particles are abrasives, so brushing them off before fresh socks go on prevents the next round of hot spots¹¹.
• **Manage wet feet and maceration.** Maceration is the pale, wrinkled, waterlogged skin you get from hours of wetness, and it blisters and tears easily. Howard is candid that "there's not much to be done once you have painfully waterlogged and macerated feet," so this one is almost entirely prevention: change socks often, and for a course you know will be wet, "put a very thin layer of Desitin or a similar barrier on them before you start... it's hard to wipe off, it's inexpensive, it's easy to find"⁹. Vonhof's recovery principle — "if the skin macerates, it must be dried and allowed to regain its former shape"⁵ — means at an aid station, get the wet socks and shoes off, dry and air the feet (a little powder helps), and put on fresh dry socks before continuing.

Chafing — same physics, different skin

Chafing is the blister's cousin: the same recipe of friction, moisture, and repetition, except the damage is to the *top* layer of skin rather than the deeper separation of a blister. As Koop notes, "with chafing, the top layer of skin is what is damaged"². And like blisters, it scales with duration — Princeton sports medicine notes chafing happens "in areas where there is friction, moisture, and heat," and "longer runs mean more friction and higher chances of chafing"¹². A day-long effort is the worst case.

Know your hot spots. The usual suspects are "the inner thighs, underarms, nipples, and around the sports bra or waistband"¹². For trail and ultra runners, add the trail-specific ones: the back of the neck under a pack collar, the collarbones and sternum where a hydration vest sits, and anywhere the vest makes contact and rocks over many hours¹³. Prevention is a three-part stack:

• **Lubricate where you can.** Apply an anti-chafe balm, cream, or petroleum jelly to the friction points before you start, and carry a stick to reapply¹². Koop: "chafing can be prevented or treated with the application of a lubricant, like Bodyglide, or taping"².
• **Tape or cover where you can't.** Some spots don't hold lube well — nipples are the classic. Cover them instead: "use adhesive bandages or nipple guards to protect sensitive areas"¹². The same logic applies to a vest-contact line that lube keeps sweating off of — a strip of tape outlasts the balm.
• **Dress to not chafe.** Wear moisture-wicking technical fabric and avoid cotton; choose "clothing with flat seams or seamless designs to minimize friction"; and get the fit "snug but not too tight" so nothing bunches or flaps¹².
• **Dial in your vest fit.** For trail efforts the hydration vest is the biggest chafe variable. A vest that bounces saws at your collarbones and back all day; cinch it snug enough that it moves with you, seat the soft flasks so they don't dig in, and pre-lube or tape the lines where the vest contacts skin. Rehearse it loaded, on a long run — not on race day.

Test everything in training — nothing new on race day

Every choice above — the socks, the shoe size, the lube-or-powder question, the tape and where it goes, the vest fit — has to be proven in training before you trust it for a hundred miles. This isn't a platitude; it's the difference between a foot system that works and a drop bag full of untested guesses. Koop is direct: "your shoe/sock combination should be tested in training, and shoe trials should be completed early in the season"².

Better still, rehearse the *failure* cases on purpose. Koop deliberately stress-tests the wet-foot scenario: "I routinely soaked my laced-up feet with the garden hose before setting out on a run"². Your long runs are the lab — that's where you find out that your favorite socks blister you when wet, or that your vest rubs raw after four hours, while there's still time to fix it.

There's also a quieter benefit that's easy to miss: your skin itself adapts. Koop, in "Training Essentials for Ultrarunning," frames training as the first line of blister defense — "your skin adapts to stress just like any other organ," and gradual, repeated exposure to the frictional forces of long runs "decreases the skin's susceptibility to blisters"¹⁴. So the long run does double duty: it rehearses the whole foot-and-skin system *and* toughens the skin that system protects. Show up on race day with both, and your feet become a non-event — which, over a day on the trail, is exactly what you want them to be.