The blister you got at kilometre 28 of your last marathon was almost certainly not bad luck. The published literature on friction blisters, while sparser than the literature on most running injuries, identifies a small number of training and equipment patterns that account for the majority of cases. The same applies to subungual haematomas, the medical term for the black toenail. Both injuries are predictable, and both follow from mistakes that are identifiable in retrospect and avoidable in advance.
This article maps the most common running mistakes that produce blisters and black toenails, particularly in the Indian distance-running context. The aim is to give you a checklist to audit your own training and race-day choices against.
Mistake 1: testing new shoes on race day
The 2014 review by Knapik in the Journal of Special Operations Medicine on blister prevention is unambiguous on this point. Unfamiliar footwear is the single most consistent risk factor for blister development in distance events. New shoes have new pressure points, new flex patterns, and a different relationship between the foot and the upper material.
The mistake compounds in Indian running because race-day excitement, combined with the social signalling of new gear at major events like the Tata Mumbai Marathon or the Airtel Delhi Half, produces a predictable annual cluster of first-marathon blisters. The runner has trained 16 weeks in trainer A, decided three weeks before the race to buy trainer B, used it for two short runs, and worn it for 42 kilometres on race day.
The acclimatisation principle
The practical rule supported by the literature is at least 50 to 80 kilometres of running in any new shoe before using it for a race longer than 10 kilometres. The skin needs time to adapt to the new pressure pattern, the foot intrinsics need time to learn the new flex profile, and the runner needs time to identify any hotspots in lower-stakes conditions. For a marathon, the figure is closer to 100 to 150 kilometres.
Mistake 2: cotton socks
The published comparisons on sock material and blister rates are limited but consistent. The 2017 work by Polliack and colleagues on hosiery and friction characterised the moisture-friction relationship of cotton versus synthetic materials. Cotton retains moisture; synthetic blends, particularly merino-synthetic, transport it away from the skin.
The 2010 work by Brennan in Foot and Ankle Specialist established that moderately moist skin blisters more readily than dry or saturated skin. Cotton socks in a humid Indian morning produce exactly the moisture range in which blisters form most rapidly. The mistake of training in technical socks and then racing in cotton, or of choosing a cotton sock for the perceived comfort of breathability, is common.
The toe sock case
Inter-toe blisters, where two toes rub against each other through a single fabric sleeve, are particularly common in long humid races. The toe sock, with individual sleeves for each toe, addresses this specifically. The evidence base is modest but the mechanism is logical, and runners with a history of inter-toe blisters frequently report meaningful improvement after the switch.
Mistake 3: wrong-size shoes, particularly in the toe box
The 2015 work by Hagen on running shoe fit established that approximately one centimetre of space between the longest toe and the front of the shoe at the end of a long run is the appropriate practical fit threshold. Less than this is associated with elevated rates of subungual haematoma. More than this is associated with increased forefoot blistering due to excessive foot movement within the shoe.
The Indian retail context has a specific risk pattern. Many running shoe purchases happen at multi-brand stores where the fitting time is brief, the runner tries the shoe in a fresh-foot morning state, and the longer-run foot swelling is not accounted for. The foot at kilometre 25 is meaningfully larger than the foot at the start of the run.
The end-of-run fit test
The practical sequence supported by the literature is to fit running shoes in the late afternoon or evening, after the foot has had a full day of activity and is closer to its maximum daily volume. Always with the socks you intend to run in, never with thin retail socks. Always with both feet measured, because foot length asymmetry of up to 6 millimetres is common and the larger foot should determine the size.
Mistake 4: ignoring the early hot spot
The published prodromal pattern for friction blisters is consistent. A localised warm or burning sensation precedes the visible blister by anywhere from 10 to 30 minutes. The 2008 work by Knapik described the staging clearly, and the practical implication is that the runner who stops at the first hot spot, applies tape or a hydrocolloid dressing, and continues, almost always finishes the run without the blister forming.
The runner who notices the sensation and decides to push through is almost certainly creating a blister that will be visible in 15 to 30 minutes. The decision to stop is rarely costly in time terms; a 90-second pause to apply tape adds nothing meaningful to a four-hour marathon, and saves what is often a multi-week disruption to subsequent training.
The race vest hotspot kit
A small kit consisting of two pieces of pre-cut zinc oxide tape, a hydrocolloid blister plaster, and a small sachet of antiseptic wipe takes up minimal space in a race vest and is the difference between finishing comfortably and finishing in distress. The cost is under one hundred rupees from any chemist. The investment-to-return ratio is exceptional.
Mistake 5: dehydration and the swollen foot
The 2012 work by Sawka in Medicine and Science in Sports and Exercise on hydration and exercise reviewed the physiological responses to dehydration, including the soft tissue volume changes that occur during prolonged exercise. The relevant pattern for blisters is that the foot tends to expand during long runs, primarily due to capillary fluid shifts, with the magnitude depending on environmental conditions and individual factors.
This matters for race-day shoe selection. A shoe that fits perfectly at the start may become tight at kilometre 30, particularly in hot or humid Indian race conditions. Runners with chronic forefoot blisters on long races should consider sizing up by half a size for marathon-distance events specifically, with the understanding that the trade-off is slightly more forefoot movement at lower distances.
The carbon-plate consideration
Carbon-plated racing shoes, increasingly common since 2017, often have unforgiving uppers that prioritise lockdown over accommodation of foot expansion. The 2022 narrative review by Burns and Tam noted the pattern. For first-time carbon-plate users in marathon racing, particularly in humid conditions, the risk of forefoot blistering and subungual haematoma is elevated compared to traditional racing shoes. Acclimatise to the shoe over substantial training mileage before racing in it.
Mistake 6: downhill running without training for it
Subungual haematomas, the black toenails, are produced primarily by repeated forefoot impact against the shoe upper during deceleration phases. Downhill running magnifies this loading. Trail races with significant net descent, common in the Western Ghats and the Himalayan running calendar in India, produce predictable black toenail clusters.
The mistake is to train primarily on flat terrain and race on courses with substantial descent. The forefoot is not conditioned for the loading pattern, and the toe-box space that was adequate on training runs becomes inadequate when the foot is repeatedly driven forward in the shoe by downhill deceleration.
The training-specificity principle
The 2017 work by Vernillo and colleagues in Sports Medicine on downhill running adaptations established that the eccentric loading of downhill running produces specific neuromuscular adaptations that transfer poorly from flat training. The practical implication is that runners targeting downhill-heavy courses should include downhill-specific training in the final eight weeks of preparation, both for performance reasons and for soft-tissue conditioning.
Putting the corrections in place
The pattern from the literature is consistent. Most blister and subungual haematoma cases are produced by a small number of identifiable mistakes, and most cases are preventable. Wear race shoes you have trained in extensively. Use synthetic technical socks, not cotton. Fit shoes with foot volume accounted for. Stop at the first hot spot, every time. Hydrate adequately. Train for the terrain you will race on.
The treatment side of the equation, for cases that occur despite prevention, sits in our companion article on blister and black toenail treatment in the injuries hub. The exercise and conditioning work that supports skin and nail durability is part of the broader programme in our exercises library. For the recovery framework that fits around the running, see our recovery guides.
For the planning side, if recurrent skin or nail injuries have disrupted previous race cycles, the STRIDD plan generator can build a load-managed plan that includes the specific training adaptations needed for your target event. The rest of the Running Lab covers race-specific guides for events across the Indian calendar. The data on what causes blisters and black toenails is reasonably clear. The mistakes that produce them are, in most cases, avoidable with planning and discipline.