Side Stitches in Runners: What ETAP Is, Why It Happens, and How to Fix It

Runner holding side during run due to exercise-related transient abdominal pain

A side stitch is one of the most common pains runners experience, but it is also one of the most manageable.

The medical term is exercise-related transient abdominal pain, or ETAP. It is usually felt as a sharp, stabbing, cramping, aching, or pulling pain along one side of the abdomen, often near the lower ribs or the side of the mid-abdomen.

The good news: in healthy runners, a typical side stitch is usually short-lived and not dangerous. The better news: most runners can reduce how often it happens by adjusting meal timing, fluid choices, breathing, posture, core control, and pacing. The frustrating part is that science has not identified one single cause for every case. ETAP is real, common, and well-described, but the exact mechanism is still debated.

Quick answer: what stops a side stitch mid-run?

Slow down, take deep controlled breaths, press firmly on the painful spot, exhale fully, walk if needed, then restart gradually. Do not try to “win” the stitch by sprinting through it. That usually turns a small problem into a run-ending problem.

What a Side Stitch Feels Like

A classic side stitch appears during exercise, not hours later. It is usually well localized: a runner can often point to the painful spot with one or two fingers. It may occur on either side, though many runners report it near the right or left lower ribs. Some people also feel pain at the tip of the shoulder, which may be referred pain through the phrenic nerve, a nerve that supplies the diaphragm and nearby structures.

ETAP is different from general stomach upset. Runner’s gut symptoms may include nausea, reflux, bloating, diarrhea, or an urgent need to use the bathroom. A side stitch is more specific: it is a localized exercise-related pain in the abdominal or lower-rib region. A runner can have both at the same time, but they are not the same issue.

Why Side Stitches Happen

Researchers have proposed several explanations, including diaphragm stress, pulling on ligaments that support abdominal organs, gastrointestinal distension, muscle cramping, nerve irritation, and irritation of the abdominal lining. The current leading explanation is irritation of the parietal peritoneum, a sensitive lining attached to the abdominal wall and underside of the diaphragm.

This theory fits many features of ETAP: the pain is sharp, localized, worsens with movement, can occur in different parts of the abdomen, and often resolves quickly when the irritating movement stops.

Older explanations are not completely useless, but some are less convincing than once thought. For example, the diaphragm-ischemia theory suggests a side stitch comes from reduced blood flow to the diaphragm. However, ETAP can occur in activities with relatively low breathing demand, such as horse or motorbike riding, and studies have not consistently shown breathing-function compromise during ETAP. The “muscle cramp” theory is also weaker than it sounds: one study found that local electromyographic activity was not elevated at the pain site during ETAP, which argues against a true skeletal muscle cramp.

A practical way to think about ETAP is this: running repeatedly moves the trunk and abdominal contents while breathing, posture, food, fluid, and core control all influence how much irritation or tension occurs inside the abdomen. That is why the fix is rarely one magic trick. It is usually a combination of small changes.

The Main Triggers Runners Can Control

Food and fluid timing are the biggest controllable variables. Research summarized in the ETAP literature reports that food and fluid intake before exercise consistently provokes symptoms, and larger pre-event volumes of food and drink relative to body weight have been associated with more symptoms.

Fluid type matters too. Hypertonic drinks - fluids with a high concentration of carbohydrate or solutes, such as many sugary drinks, juices, or overly concentrated sports drinks - appear more provocative than hypotonic or isotonic fluids. That does not mean runners should fear carbohydrates. It means the drink has to match the session, the athlete, the gut, and the timing.

Posture and trunk mechanics also matter. ETAP is most common in activities involving repetitive torso movement, especially when the torso is extended. Running fits that pattern: every stride creates vertical movement through the trunk. Studies also suggest that poorer postural alignment, especially increased thoracic kyphosis, may increase susceptibility, while better core stability may reduce symptoms.

HYROX and Hybrid Athlete Note

HYROX athletes should pay extra attention to side stitches because the race does not ask for fresh running only. It asks you to run while breathing is elevated, grip is fatigued, the trunk is bracing from carries or sled work, and the stomach may be reacting to race-day fueling. That combination can make a stitch more likely if the athlete starts too fast, drinks too much at once, overuses concentrated carbohydrates, or loses posture under fatigue.

The solution is not to avoid fueling or avoid hard training. The solution is to practice fueling, warm-up, breathing, and compromised running in a controlled way before race day.

Training for HYROX and tired of guessing? Download the free HYROX First-Timer Guide + 4-Week Starter Plan from Warrior Performance Lab. You will learn the race flow, readiness basics, race-day essentials, and how to start building smarter HYROX fitness. Download here

What To Do When a Stitch Starts Mid-Run

When the pain hits, the goal is to reduce abdominal movement, reduce irritation, and restore comfortable breathing. Use this sequence:

1. Slow down immediately. You do not always have to stop, but you should reduce the pace enough that your breathing becomes controllable. If the pain continues to build, walk for 30-90 seconds.
2. Use deep, controlled breathing. Try inhaling through the nose or mouth while expanding the lower ribs, then exhale slowly and forcefully through pursed lips.
3. Apply firm pressure with your fingers or palm directly over the painful spot. Many runners find that pressing into the stitch while exhaling helps the pain fade.
4. Experiment with a slight forward bend or gentle side stretch. Keep it gentle. Avoid aggressive twisting or hard stretching while running fast.
5. Restart gradually. The mistake is going right back to the same pace that triggered the stitch. Ease back in over the next few minutes.

How To Prevent Side Stitches Before Your Run

Avoid large meals for at least 2 hours before running. Runners who are especially stitch-prone may need 3-4 hours after a large meal. That does not mean running on empty is always best; it means the pre-run meal should be timed and portioned so the stomach is not heavy when the run begins.

Avoid large amounts of fluid immediately before the run. Hydration matters, but chugging water or sports drink right before the first mile can make ETAP more likely. Use small, regular sips instead of large boluses. During longer runs, small and consistent fluid intake is often better tolerated than drinking a lot at once.

Be careful with hypertonic drinks before and during running. Concentrated carbohydrate drinks, fruit juice, soda, or heavy sports mixes may raise stitch risk in susceptible runners. For most easy runs under an hour, plain water is usually enough. For longer or hotter runs where carbohydrate and electrolytes are needed, use a properly mixed sports drink rather than an overly concentrated one.

Warm up gradually. Although the evidence linking poor warm-up to ETAP is not as strong as the evidence around pre-run food and fluid, a progressive warm-up is still sensible because it reduces abrupt changes in breathing, trunk motion, and intensity. Start easy for 5-10 minutes before faster running.

Core and Posture: The Long-Term Fix

If side stitches happen repeatedly, especially at similar paces or distances, look beyond food and fluids. A runner with poor trunk control may bounce, rotate, arch, or collapse excessively with each stride, increasing stress through the abdomen.

A 2-3 day per week core routine can help. Focus less on high-rep crunches and more on anti-rotation, anti-extension, and trunk-control exercises:

The goal is not to brace hard while running. The goal is to build enough control that the torso stays quiet and efficient at normal running speeds. Posture work can also help. If you run with a collapsed chest, exaggerated arch, or excessive rotation, use drills that promote a tall but relaxed torso: short strides, quiet shoulders, ribs stacked over pelvis, and arms swinging forward-back rather than across the body.

A Practical 2-Week Side-Stitch Reset

For the next two weeks, treat ETAP like a pattern you can troubleshoot. For every run, record four things: what you ate, what you drank, when you ate or drank, and when the stitch appeared. This turns guesswork into data.

When It Might Not Be a Side Stitch

A typical side stitch appears during exercise, improves when you slow or stop, and does not leave you feeling sick afterward. Get medical advice if the pain is new, severe, worsening, happening at rest, or repeatedly preventing training despite basic changes.

Seek urgent medical help if abdominal pain is sudden and sharp, associated with chest, neck, or shoulder pain, vomiting blood, blood in stool, a hard or tender abdomen, difficulty breathing, faintness, fever, persistent vomiting, unexplained weight loss, or possible pregnancy. Most stitches are benign, but persistent or unusual pain deserves evaluation.

Bottom Line

A side stitch is not a sign that you are “bad at running.” It is usually a fixable mismatch between running intensity, trunk movement, breathing, stomach contents, and core control. The most useful interventions are simple: avoid large meals and large fluid volumes close to running, be cautious with concentrated sugary drinks, warm up gradually, slow down when symptoms start, breathe deeply, press the painful area, and build better trunk stability over time.

ETAP is common, annoying, and sometimes race-ruining - but for most runners, it is manageable.

Want your running, fueling, and race-day strategy to feel less random? Download the free HYROX First-Timer Guide + 4-Week Starter Plan, or work with Warrior Performance Lab for personalized HYROX coaching, nutrition, and performance support. Download here

Frequently Asked Questions

What is a side stitch called medically?

The medical term is exercise-related transient abdominal pain, or ETAP. It describes temporary abdominal pain that occurs during exercise, especially activities with repetitive trunk movement like running.

Should I stop running when I get a side stitch?

Not always. First, slow down. If the pain keeps building, walk for 30-90 seconds, breathe deeply, press the painful area, and restart gradually. If the pain is severe, unusual, or does not improve after stopping, do not ignore it.

Is a side stitch caused by the diaphragm?

The diaphragm may be involved in some cases, but current research does not support one single cause for every side stitch. The leading explanation is irritation of the parietal peritoneum, but food/fluid timing, posture, breathing, and trunk control all seem to influence symptoms.

Can drinking water cause a side stitch?

Water itself is not the enemy, but drinking a large volume right before running can increase stomach movement and may provoke symptoms in some runners. Small, regular sips are usually better tolerated than chugging.

Are sports drinks bad for side stitches?

Not necessarily. The problem is usually concentration and timing. Very concentrated sugary or hypertonic drinks may be more provocative in stitch-prone runners. For longer sessions, use a properly mixed sports drink rather than an overly strong mix.

Can core training help side stitches?

It may help, especially for runners who repeatedly get stitches at similar paces or distances. Focus on trunk-control work like dead bugs, side planks, Pallof presses, bird dogs, and carries rather than only high-rep crunches.

Can HYROX athletes get side stitches during compromised running?

Yes. HYROX combines running with stations that challenge breathing, posture, bracing, grip, and trunk control. If fueling, pacing, breathing, or warm-up is off, side stitches can show up during the run sections or after stations.

When should I worry that it is not just a side stitch?

Seek medical help if the pain is sudden, severe, happening at rest, associated with chest/neck/shoulder pain, fever, vomiting, blood in stool or vomit, difficulty breathing, faintness, a rigid abdomen, possible pregnancy, or if it repeatedly disrupts training despite basic changes.

References

Morton, D. P., & Callister, R. (2015). Exercise-related transient abdominal pain (ETAP). Sports Medicine, 45, 23-35. https://doi.org/10.1007/s40279-014-0245-z

Morton, D. P., & Callister, R. (2000). Characteristics and etiology of exercise-related transient abdominal pain. Medicine & Science in Sports & Exercise, 32(2), 432-438.

Morton, D. P., & Callister, R. (2002). Factors influencing exercise-related transient abdominal pain. Medicine & Science in Sports & Exercise, 34(5), 745-749.

Morton, D. P., Aragón-Vargas, L. F., & Callister, R. (2004). Effect of ingested fluid composition on exercise-related transient abdominal pain. International Journal of Sport Nutrition and Exercise Metabolism, 14(2), 197-208. https://doi.org/10.1123/ijsnem.14.2.197

Morton, D. P., & Callister, R. (2006). Spirometry measurements during an episode of exercise-related transient abdominal pain. International Journal of Sports Physiology and Performance, 1(4), 336-346.

Mole, J. L., Bird, M.-L., & Fell, J. W. (2014). The effect of transversus abdominis activation on exercise-related transient abdominal pain. Journal of Science and Medicine in Sport, 17(3), 261-265.

Viola, T. A. (2010). Evaluation of the athlete with exertional abdominal pain. Current Sports Medicine Reports, 9(2), 106-110.

MedlinePlus. (2026). Abdominal pain. U.S. National Library of Medicine. https://medlineplus.gov/ency/article/003120.htm

Note:

This article is for general education and coaching support. It should not be used to diagnose, treat, or replace care from a licensed medical professional. Runners with severe, unusual, persistent, or worsening abdominal pain should seek medical evaluation.