The marathon slump: causes and how to avoid it

A sudden, sustained drop in pace, legs that stop responding and a growing urge to walk describe one of the hardest moments in a marathon. This collapse in performance is often associated with insufficient carbohydrate availability, but starting pace, muscle fatigue, heat and digestive problems can also play a part.

There is no exact mile at which it appears and no formula that guarantees you can avoid it. Consistent preparation, a realistic pace and a well-rehearsed fuelling strategy reduce the risk. If the slump arrives, the priority is to slow down and distinguish fatigue from symptoms that require medical attention, such as confusion or loss of coordination.

What happens when your pace collapses

Finishing a marathon of 42.195 kilometres demands effort even when everything goes to plan. It is normal for perceived exertion to rise and for your legs to feel heavy towards the end. A collapse, by contrast, involves a substantial and sustained loss of speed: a pace that once felt manageable becomes impossible to maintain despite continued effort.

There is no single clinical definition. One study of more than four million marathon records used a specific criterion: a speed reduction of at least 25% lasting for at least 5 kilometres after the 20-kilometre mark. That measure is useful for studying results, but it cannot establish what caused each slowdown or diagnose a runner from watch data.

The slump is often associated with the section after 30 or 32 kilometres, although it can happen earlier, later or not at all. Race duration, preparation and conditions all affect when it might occur. A hill, an aid-station stop or one slower kilometre is not enough on its own to establish that a sustained collapse has occurred.

The role of carbohydrates and glycogen

During a race, the body gets energy from a combination of fat and carbohydrates. Some carbohydrate is stored as glycogen in the muscles and liver, while glucose circulates in the blood. As relative intensity rises, carbohydrates generally provide a greater share of the energy needed to sustain the effort.

Glycogen stores are limited and vary from person to person. If carbohydrate availability falls substantially, holding a demanding pace may become harder, even though the body still has fat available. This is not an instant switch from one fuel to another: both are used at the same time, in proportions that change with intensity and other circumstances.

Distance covered cannot reveal how much glycogen remains, and a watch cannot measure it directly either. A drop in pace does not by itself prove complete depletion of those stores. Eating during a race helps maintain carbohydrate availability, but it does not automatically make up for an unsuitable pace, excessive heat or an injury.

Why a slump rarely has a single cause

Starting too fast raises the cost of the race from the outset. The apparent ease of the opening kilometres can conceal an effort beyond what a runner can sustain for hours. That excess speeds up carbohydrate use and increases muscular and thermal strain, even if the consequences do not become apparent until much later.

Preparation also influences the ability to maintain form and pace under fatigue. A short session at race speed does not prove that the same speed is sustainable over the full distance. Marathon-specific endurance is built through months of training that allows for recovery, not through a few exceptionally long runs.

Heat, humidity, wind, hills and repeated changes of pace add to the challenge. Nausea, a pre-existing illness or injury pain can also contribute. Mood and attention influence decisions, but mental strength cannot replace fuel or rule out a medical problem. Several factors often accumulate before the pace finally drops.

Training and pacing: prevention starts early

Solid preparation combines consistency, runs suited to a person’s experience and enough rest. Long runs help runners become accustomed to spending more time on their feet and observe how their legs and digestive system respond. Their length should reflect training history: there is no universal number of 30-kilometre runs that guarantees a good race.

Adding kilometres without recovering from the workload can have the opposite effect to the one intended. Persistent pain, fatigue that does not ease or repeated sessions ending at the limit are signs that training needs review. Reducing volume in the days before the race helps limit accumulated fatigue; one final exhausting long run cannot make up for lost weeks.

Target pace should be based on recent results, race-specific sessions and the training actually completed, not just a desired finishing time. In the opening kilometres, avoid accelerations to regain seconds or keep up with a group that is moving too fast. A controlled, relatively even effort leaves room to respond to the course and conditions on the day.

How much carbohydrate to take during the race

For prolonged exercise, a common guideline is approximately 30 to 60 grams of carbohydrate per hour. In events lasting more than about two and a half hours, some trained athletes tolerate up to around 90 grams per hour from different carbohydrate sources. The upper figure is neither compulsory nor suitable for everyone and requires prior practice.

The actual amount depends partly on expected duration and digestive tolerance. Gels, sports drinks and some foods can provide carbohydrates, but their contents vary. For example, two gels containing 25 grams each provide 50 grams; if a sports drink also contains carbohydrate, its contribution must be added to the total rather than treated as plain water.

Spreading intake from the early part of the race avoids relying on a large dose once weakness has already appeared. Timing and quantity should be rehearsed on long training runs with familiar products and access to the planned fluids. A gel taken when the slump arrives does not work instantly and cannot by itself reverse muscle fatigue or heat stress.

Pre-race food and digestive tolerance

Food in the preceding days helps runners start with adequate stores. Giving carbohydrate-rich foods such as rice, pasta, potatoes, bread or fruit a prominent place can be part of that preparation, alongside a reduction in training load. There is no need to turn the previous evening’s meal into an excessive or unfamiliar feast, particularly if it causes discomfort.

Breakfast is also worth rehearsing. A familiar meal containing carbohydrates, eaten with enough time to digest, is generally more predictable than changing routine at the start line. Tolerance of fibre, fat and caffeine varies, and there is no menu that works for everyone. Diabetes, digestive disease and other medical circumstances call for individual professional advice.

Practising food intake while running tests not only a product’s taste, but also its quantity, timing and combination with fluids. Digestive adaptation may improve tolerance, although it cannot guarantee an absence of symptoms. Severe abdominal pain, persistent vomiting or blood in the stool requires medical assessment.

Heat and hydration: two variables that change the plan

A target time set on a cool day may be too ambitious in heat or humidity. The body must dissipate heat generated by exercise, and the same speed can impose a greater load. Adjusting pace early, using available shade and making use of the race’s cooling resources are more prudent responses than waiting until you feel unwell.

Fluid needs vary with temperature, sweat rate, duration and individual characteristics. Thirst and experience from training in similar conditions are useful guides, alongside the location of aid stations. No fixed hourly amount suits everyone; forcing yourself to drink at every station does not guarantee better hydration either.

Drinking more fluid than the body needs can contribute to exercise-associated hyponatraemia, a dangerous fall in blood sodium concentration. Taking salt or sports drinks does not remove that risk if fluid intake is excessive. A severe headache, confusion, vomiting or worsening general condition during the race calls for medical attention, not an improvised decision about how much to drink.

What to do if your pace drops sharply

The first change is to stop chasing the planned finishing time. Slowing down or walking reduces the demand and gives you a chance to assess the situation; before doing so, move aside without endangering other runners. A persistent fall in performance will not be fixed by trying to regain all the lost seconds at once.

If the runner is alert, coordinated, able to swallow safely and has no warning signs, a small serving of familiar carbohydrate may be appropriate, along with fluid according to thirst and a rehearsed plan. Avoid taking several gels at once or forcing large quantities of water. Absorption takes time, and an excessive dose can worsen digestive discomfort.

What happens next depends on how the person feels. If walking is comfortable and the discomfort eases, an easy jog with a revised goal may be possible. If weakness increases, localised pain appears or normal control of movement does not return, the right decision is to stop and seek medical assistance. Finishing must not take priority over safety.

Signs that should not be dismissed as normal fatigue

Confusion, disorientation, fainting, collapse or an inability to walk normally are warning signs. So are chest pain or pressure, severe or unusual difficulty breathing, and seizures. If any of these occur, stop exercising and seek immediate medical attention.

Localised pain that changes the way a runner lands, inability to bear weight on a leg, repeated vomiting or a severe and worsening headache also require assessment. These symptoms can have different causes and it is unsafe to assume they are due to a lack of energy. Do not give food or drink to a confused person, someone with reduced consciousness or anyone unable to swallow safely.

If medical staff are not available, in Spain you can call 112 and give your location and symptoms. Follow the emergency service’s instructions while help is on the way. If heat stroke is suspected, moving the person somewhere cool and starting to cool them, if possible, must not delay the call for assistance.

Recovery and lessons after the race

After finishing or stopping, the priority is to make sure symptoms resolve. Eating and drinking gradually as tolerated helps replenish what was used; a meal containing carbohydrates and protein is a common option. Avoid taking anti-inflammatory medication automatically and allow the body to recover before returning to demanding sessions.

Muscle soreness can be expected, but very dark or scant urine, substantial muscle swelling, fainting, persistent vomiting or chest pain warrant medical attention. Localised pain that does not improve should also be assessed. Returning to running makes more sense when everyday activities are comfortable and general symptoms have gone; a serious episode requires individual clinical guidance.

Once some time has passed, reviewing split times alongside training, weather, food and fluid intake, and symptoms offers a more useful picture than blaming a single gel. Watch data can help put events in order, but cannot identify the cause on its own. A difficult marathon does not prove a lack of willpower: it shows that performance depends on how preparation, pacing, fuelling and real race conditions fit together.

Este artículo contiene enlaces de afiliado: si compras a través de ellos, se genera una pequeña comisión sin coste adicional para ti. Más información.