Side Stitch Root Cause Research

A side stitch (or “stitch in one’s side”) is an intense stabbing abdominal pain under the lower edge of the ribcage that occurs during exercise.



I experience sharp pains in my side(s) when running, particularly after riding in TT at hard effort. Each breath brings a stabbing sensation, so I neither can inhale completely, not can I run at the full capacity. The pain could come in either on the upper side of the abdomen, under the bottom ribs, or it could arise on different sides of the abdomen. During the last race (70.3 Marbella) it came up right under my lower ribs and it seemed like a spasm of the diaphragm.

I can swim, bike, and run individually, at all heart rates, different levels of effort for varying times with no spasms (or cramps). But for the past three IM 70.3 races, I would get a killer spasm during the run portion of triathlon, even though there was no indication of one during my bike ride. The pain only seems to appear after transitioning from the bike to the run, never just on the run.

Exercise-related transient abdominal pain (ETAP), colloquially referred to as ‘stitch’ and ‘side ache’, is a common condition observed in many sporting activities.


Exercise-related transient abdominal pain (ETAP), colloquially referred to as ‘stitch’ and ‘side ache’, is a common condition observed in many sporting activities.

  • Diaphragmatic Ischemia

  • Mechanical Stress on the Visceral Ligaments

  • Gastrointestinal Disturbances

  • Muscular Cramp

  • Median Arcuate Ligament Syndrome

  • Neurogenic Pain

  • Irritation of the Parietal Peritoneum

Screenshot 2023-06-04 at 07.32.51

Diaphragmatic Ischemia


Several theories have been presented to explain the mechanism responsible for the pain, including ischemia of the diaphragm; stress on the supportive visceral ligaments that attach the abdominal organs to the diaphragm. I found a strong argument that among potential factors contributing to side stitches and abdominal cramps could be Diaphragmatic Ischemia. During intense exercise, blood flow is diverted away from the diaphragm, potentially leading to reduced oxygen supply and ischemia (lack of blood flow) in this muscle. This theory was proposed in 1941, by Capps [].

Proposed to be caused by hypoxia of the diaphragm:

  • Pain-relieving techniques: bending forward, tightening belt around abdomen, breathing through pursed lips
  • Right side pain twice as common as left side pain
  • Runners who consumed a large pre-event meal 1–2 h before the event were more likely to experience ETAP
  • ETAP unrelated to the nutritional profile of the pre-event meal
  • Related to shoulder tip pain

Shoulder tip region—extending from the lateral third of the trapezius border to the acromion process—is the referred site for pain arising from tissue innervated by the phrenic nerve, which includes the diaphragm and neighbouring structures.


The phrenic nerve controls your diaphragm (the large dome-shaped muscle between your abdominal and chest cavities). It’s essential to breathing. Your nerve sends signals that cause your diaphragm to contract (become thicker and flatter). This movement gives your lungs room to expand and take in air (inhalation). The phrenic nerves provide motor innervation to the diaphragm and work in conjunction with secondary respiratory muscles (trapezius, pectoralis major, pectoralis minor, sternocleidomastoid, and intercostals) to allow respiration.

Screenshot 2023-06-04 at 08.25.04
  • While ETAP is commonly considered a gastrointestinal complaint, elements of the pain are not consistent with a gastrointestinal origin.
  • This is NOT a muscular cramp. EMG activity was not elevated at the site of ETAP during an episode of the pain, which convincingly discredited the muscular cramp theory.
  • Experience of ETAP appears to be affected by poor posture, especially in the thoracic region (thoracic spine is the middle section of your spine. It starts at the base of your neck and ends at the bottom of your ribs)

Morton, D. P. & Aune, T. (2004). Runner’s stitch and the thoracic spine. British Journal of Sports Medicine

… an elite runner who, after a thoracic spine trauma, developed severe and recurrent episodes of ETAP which were relieved by localised treatment. In this study, researchers made observations on 17 other runners who often experience episodes of ETAP and found that palpation of specific facet joints could reproduce symptoms of ETAP shortly after an episode of the pain had been relieved. “…torso hyperextension coupled with the jolting movement of downhill running provoked the pain. In addition, tight erector spinae after a surfing session seemed to increase the likelihood of experiencing ETAP. Palpation of the left T8–T9 facet joint resulted in reproduction of subcostal pain consistent with the patient’s exercise induced experience of ETAP. Further, the site of the pain reproduction was consistent with the dermatome distribution arising from the T8–T9 level. Palpation of the joints above and below the joint caused no pain referral. Localised treatment aimed at mobilising the T7–T8 joint was administered in the clinic, and the patient was given mobilising exercises to perform. After about four weeks of treatment, he reported a reduction in the symptoms of ETAP. He has since resumed competitive running.”

In 1941 Capps anecdotally observed that people with an increased kypholordotic postural alignment (see “B” below) seemed most susceptible to the pain. To investigate this further, we recently confirmed in a study involving a comprehensive postural assessment of over 150 subjects that increased kyphosis is provocative of ETAP.4 This observation is interesting given that intercostal nerves arising from the thoracic region supply the anterior abdomen.


Gastrointestinal Disturbances

One hypothesis suggests that increased intestinal permeability could lead to the translocation of substances, such as bacteria or toxins, from the intestines into the bloodstream. These substances may trigger an immune response and cause localized inflammation or irritation in the diaphragm or surrounding tissues, resulting in side stitches or abdominal cramps. However, this theory requires further investigation to establish a definitive connection.


Capps RB. Causes of the so-called side ache in normal persons. Arch Intern Med. 1941;68:94–101. doi: 10.1001/archinte.1941.00200070104006.  

Morton, D., & Callister, R. (2015). Exercise-related transient abdominal pain (ETAP). Sports medicine (Auckland, N.Z.)45(1), 23–35. 

Morton, D. P. & Aune, T. (2004). Runner’s stitch and the thoracic spine. British Journal of Sports Medicine, 38(2), 240. doi:10.1136/bjsm.2003.009308