Your flank is the area of your side between the bottom of your ribs and the top of your hip. Your kidneys are found in this region, so pain originating in your flank is often the result of an injury to, or disorder in, the urological system. If you experience flank pain while running, it may be caused by something related to running or it may be unrelated to running but brought to the fore by the stress of exercising.
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Nonrenal Flank Pain
Flank pain often results from a kidney problem, but it can also be caused by degenerative disk disease of the lower back, arthritis, muscle spasms and spinal fractures. If you have any of these, they are likely to be made worse by running. A side stitch also causes pain in the flank, and you can usually identify a stitch from its sudden onset and equally sudden disappearance. Rest and physical therapy can help you overcome muscle spasms, while anti-inflammatory drugs are useful to combat arthritis.
Infection of the kidney, or pyelonephritis, is a type of urinary tract infection normally caused by bacteria gaining access to the genitourinary system through the urethra. If, in addition to flank pain on the run, you have to stop often to urinate and experience a burning sensation when you do, you may have a kidney infection. Blood in the urine is another sign of pyelonephritis. If this happens to you, stop running and seek medical attention immediately, as this kind of infection can spread to the rest of the body and become life-threatening if you are not given antibiotics.
Kidney stones, or renal calculi, are a common and extremely painful urological disorder. They consist chiefly of calcium, with other elements present to varying degrees. Men over 40 are the most common victims, and having kidney stones once predisposes you to recurrence. Their causes remains largely unknown. If you have sudden onset of severe flank pain, perhaps with nausea and vomiting, and pass pink or bloody urine, you may have a stone. Stop running and contact your doctor, who may recommend waiting for the stone to pass or may elect to perform extracorporeal shock wave lithotripsy, a noninvasive dissolving of the stone using sonic energy. Other options, such as laser lithotripsy, require minimally invasive surgery.
Ultramarathons and Kidney Failure
The niche world of ultramarathons, in which competitors cover distances up to and even in excess of 100 miles, is fraught with a range of physiologic hazards that rarely if ever appear in races lasting less than a few hours. Kidney failure is a well-documented consequence of rhabdomyolysis, a widespread breakdown of skeletal muscle. Because of the extremely high demands of running very long distances, rhabdomyolysis is common in ultramarathon runners. If you have been running for many hours, are experiencing flank pain, and are passing sparse reddish-brown or otherwise dark urine despite remaining well hydrated, stop and seek medical help. Hot weather and the use of anti-inflammatories increase your chances of harming your kidneys under these conditions.