Practical niggles
Evidence: limited
These are the small, common problems that rarely reach a clinic and rarely reach a journal either. Some prevention advice has trial support (sock fibre and moisture for blisters; the associations behind the side stitch); much of the rest is sensible practical consensus with little formal study. Where that is the case, this page says so rather than dressing lore up as science.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
None of these will end a season, but they can ruin a race or a run. They get little research attention because they are minor, so prevention rests partly on a few studies and partly on long-standing practical consensus.
Blisters
A friction blister forms when repeated shear rubs the skin layers against each other; heat and especially moisture make it worse, because even mild skin hydration raises friction (Rushton & Richie 2024).
Prevention follows from the cause: cut friction and manage moisture.
- Shoe fit. Shoes that fit and do not slide are the foundation; a heel that lifts or a foot that slips creates the shear that blisters need. See running shoes.
- Socks. Densely padded acrylic or synthetic socks reduced blister number and size compared with cotton in trial data, with the benefit tied to sock construction and lost in thin socks (Rushton & Richie 2024). Cotton holds moisture and dries slowly, so it is the worst choice.
- Lubricant and taping. Smearing a barrier balm over hot spots, or taping them before a long effort, is standard practice. The evidence for blister-prevention lubricants is actually uncertain, partly because reapplication mid-run is impractical (Rushton & Richie 2024); taping is practical consensus rather than well-tested. Both are cheap, so a personal trial is reasonable.
- Managing moisture. Moisture-wicking kit and, for some, foot powder are widely used; antiperspirants on the feet do not prevent blisters (Rushton & Richie 2024).
If a blister forms, a small intact one is usually best left covered and protected. Practical consensus is that a large, painful blister can be drained with a sterilised needle at the edge while leaving the roof of skin in place as a dressing, then covered; broken or infected blisters need cleaning and watching. This is standard first-aid lore, not trial-tested.
Chafing
Chafing is friction at skin folds and seams: inner thighs, underarms, groin, sports-bra lines and, for many runners, the nipples. Sweat and salt make it sting. The fixes are practical consensus rather than studied: apply an anti-chafe balm or barrier cream to the spots that rub, wear fitted seamless kit that does not flap, and tape the nipples for long runs. Showering promptly afterwards limits the salt-and-sweat sting on already-raw skin.
Black toenails
A black toenail is usually a subungual haematoma: bleeding under the nail from repeated impact of the toe against the front of the shoe, classically on long downhills where the foot slides forward. It is the same friction-and-shear story as a blister, one layer deeper.
Prevention is fit: enough length in front of the longest toe (often a thumb’s width), a shoe that holds the heel so the foot does not slide, snug lacing, and keeping the toenails trimmed straight and short. Most black toenails are painless and resolve on their own as the nail grows out, which can take months; the nail may lift and shed. A toenail that is acutely and severely painful from pressure, or any sign of infection, warrants medical attention.
Side stitch (ETAP)
The ‘side stitch’, or exercise-related transient abdominal pain (ETAP), is sharp or stabbing when severe and cramping or aching when milder, usually felt at the side of the mid-abdomen. It is very common: around 70% of runners report it in a given year (Morton & Callister 2015).
The mechanism is genuinely unsettled. The older idea of diaphragm cramp or ischaemia has largely given way; the review by Morton and Callister judges that irritation of the parietal peritoneum (the membrane lining the abdominal wall) best fits the features, while stating plainly that the cause is not settled (Morton & Callister 2015). Reported associations include eating or drinking shortly before exercise, especially concentrated or hypertonic drinks, and being less trained (Morton & Callister 2015). See pre-race fuelling and hydration and electrolytes.
Management is practical and largely untested by trials: slow down, deepen and steady the breathing, and apply pressure to the painful spot or bend forward briefly. Leaving more time after meals and avoiding very concentrated drinks before running follows from the associations. The reassurance is real: it is transient and harmless, and eases as fitness builds.
The common thread
Three of these four are friction-and-shear problems (blisters, chafing and black toenails), so the same levers fix them: good shoe and kit fit, less rubbing, and managing moisture. The side stitch is the odd one out, a poorly-understood internal pain with sensible but largely unproven self-management. For the load-related injuries that do reach clinics, see common overuse injuries, and for getting the foundations right, the basics.