Cause
The repetitive nature of these types of activities cause mechanical trauma to the red blood cells causing hemolysis. Free haemoglobin released from lysed red blood cells is filtered into the urine. Defects in red blood cell membrane proteins have been identified in some of these patients. The dead RBC's are filtered by the glomerulus and are removed from the body. Due to destruction of a large number of RBC's, a large number of damaged blood cells are filtered out of the body at the same time, making the urine have noticeable amounts of hemoglobin in solution.
Read more about this topic: March Hemoglobinuria