A familiar scene: you are running along fine, your breathing is even, and suddenly something stabs under the right rib hard enough that you have to slow down. Folk wisdom usually blames the liver, supposedly engorged with blood. In sports medicine the phenomenon has a name and an abbreviation: ETAP, exercise-related transient abdominal pain.

Let us look at what is known about it, because a surprising amount is.

How common it is

A review in Sports Medicine by Morton and Callister collected the prevalence figures:

around 70% of runners had experienced ETAP at least once in the previous year
in any given race the pain hits roughly one participant in five (27% in one study, 17% in another, 21% over 10K)

Broken down by sport over a year, the picture looks like this: swimming 75%, running 69%, horse riding 62%, aerobics 52%, basketball 47%, cycling 32%.

Note the order. Swimming is ahead of running, and cycling trails far behind. That is an important clue about the mechanism: this is not about impact loading through the foot.

Seven hypotheses and one survivor

The review works through the explanations proposed over the decades:

Ischaemia of the diaphragm
Stress on the ligaments suspending the abdominal organs from the diaphragm
Ischaemia or distension of the gastrointestinal tract
Cramping of the abdominal wall musculature
Ischaemic pain from compression of the coeliac trunk by the median arcuate ligament
Irritation of the spinal nerves
Irritation of the parietal peritoneum — the membrane lining the abdominal cavity from the inside

The last one fits the features of ETAP best. The arguments:

The character of the pain matches. Irritation of the parietal peritoneum produces sharp, sharply localised pain — which is exactly how sufferers describe it.
It explains shoulder tip pain. Some sufferers report referred pain at the tip of the shoulder, the classic distribution of the phrenic nerve.
It explains the link with food. A full stomach increases friction between the layers of the peritoneum.
It explains the scatter of locations. ETAP most often appears along the sides of the abdomen near the costal margin, but it can turn up anywhere in it.

At the same time the authors state honestly that the aetiology remains hypothetical and needs further research. This is not an established fact but the best of the available accounts.

While we are here, the folk explanations can be closed out. The liver has nothing to do with it: ETAP occurs equally often on the left and on the right, and it happens more often in swimmers than in runners.

Who is at risk

Age is the strongest factor, and the relationship is inverse. Among people under 20, 77% experience the pain; among those over 40, 40% do. Which is to say the problem literally goes away with the years.

Food and drink. Large volumes 1–2 hours before exercise. Hypertonic drinks are especially provocative — concentrated juices, sports drinks mixed too strong, anything sweet and thick.

Posture. People with kyphosis — an exaggerated thoracic curve — are more prone to ETAP.

The spine. Palpation of the thoracic vertebrae T8–T12 reproduced the symptoms in some subjects.

Sex and body build showed no meaningful association. Fitness does not fully protect either — well-trained athletes are not immune, although their incidence is lower.

What sufferers actually do about it

A survey of sufferers produced this ranking of manoeuvres that bring relief:

deep breathing — 40%
pressing on the painful spot — 31%
stretching the affected area — 22%
bending forward — 18%

The review authors note that the evidence on how well these manoeuvres work is contradictory, and that the most reliable way to end the pain is to stop exercising. Which, as they also observe, is not always practical and not always desirable.

How to apply this

Do not eat a big meal in the 2 hours before the start. This is the one recommendation that holds up across every source.
Isotonic rather than hypertonic on the course. Concentrated juice or a mix made up too thick is a direct provocation. If you make your own drink, do not make it stronger for good measure.
Practise drinking on the run rather than working it out for the first time in a race.
Work on the thoracic spine. Opening up the chest and minding your posture is not esoterica but one of the few confirmed risk factors.
Core and a wide belt. The review mentions both strengthening the core musculature and a wide supportive belt as ways to reduce the load on the suspensory structures.
During an attack: a deep breath with forceful exhalation, pressure from the palm on the painful point, a forward bend. It does not work for everyone, but it is what genuinely helps people.
If the pain does not settle at rest, radiates elsewhere or returns every single time, it is no longer ETAP and it is time to see a doctor.

Key points

ETAP is transient abdominal pain during exercise; 70% of runners had it within the past year, and one in five suffers it in any given race.
By sport: swimming 75%, running 69%, cycling 32% — impact loading is not the culprit.
Of the seven hypotheses, the one that fits the facts best is irritation of the parietal peritoneum; officially the aetiology remains unproven.
The liver and the spleen have nothing to do with it: the pain is equally common on the left and on the right.
Age is the main factor: 77% under 20 versus 40% over 40.
Provocations: large volumes of food and drink 1–2 hours beforehand, hypertonic drinks, kyphosis; tenderness at the T8–T12 level.
Relief manoeuvres: deep breathing (40%), pressure on the spot (31%), stretching (22%), bending forward (18%).

Source: Morton D., Callister R. «Exercise-Related Transient Abdominal Pain (ETAP)», Sports Medicine, 2015. https://doi.org/10.1007/s40279-014-0245-z