How can muscle damage lead to myoglobin in the urine?

The presence of myoglobin in the urine, a condition sometimes associated with McArdle disease and other severe muscle pathologies, occurs when muscle tissue undergoes significant destruction or degeneration.12

Myoglobin is an oxygen-binding protein normally stored within muscle cells to facilitate oxidative metabolism. When muscle fibers are damaged—whether through traumatic injury, excessive exertion, or specific metabolic disorders—the integrity of the muscle cell membrane is compromised. This damage allows the contents of the muscle cells, including myoglobin, to be released into the bloodstream. Once in the circulation, if the concentration of myoglobin is sufficiently high, it is filtered by the kidneys and excreted in the urine. This process is a hallmark of rhabdomyolysis, a condition that can lead to serious complications such as acute renal failure.312