This is a well known medical phenomenon that used to be called march hemoglobinuria because soldiers get it from marching too long. It is also well known among conga and bata players, and may actually be hazardous to our health.
What happens is the repeated impact of each step, or in the case of hand drumming, each hit of the drum, causes a few red blood cells to break within the capillaries. The tiny red cells get crushed by the physical pressure and impact and break open, spilling their contents into the blood plasma that surrounds all the red cells. Blood may look like a liquid but is actually billions of microscopic red and white blood cells suspended in a liquid called plasma. The hemoglobin molecules escape from the broken cell into the blood plasma. Small amounts of hemoglobin in the blood can be eliminated by normal mechanisms, and do not cause a problem. This happens all the time, every day, and does not cause hemoglobin to get into the urine. When there are too many red cells breaking and too many hemoglobin molecules floating around in the blood, free hemoglobin (not inside red cells or bound by other proteins in the blood) gets filtered through the kidneys and gets into the urine. This appears as dark or rust colored urine, not as bright red blood in the urine. Red blood in the urine is due to some other cause, generally bleeding in the lower or upper urinary tract, and should be evaluated by an internist, urologist, or kidney specialist. Free hemoglobin that is outside the red cells and not bound up by the natural proteins in the blood plasma can actually be toxic to the kidneys and can do damage. There are case reports of renal failure in patients with this kind of hemoglobinuria, making me wonder whether conga players with kidney problems have gotten kidney damage from a lifetime of playing congas. Two AfroCuban master conga players have died of or with renal failure in the past 6 months. There were other factors contributing as well, so the link is not completely clear, but that anecdotal evidence is consistent with several published medical studies, two of which were referenced by LightSeeker above.
This is not due to direct force to the kidneys or straining the kidneys, bursting of capillaries, or hemorrhaging, but rather the physical crushing of red blood cells inside the capillaries in the fingers. Given the little bit of published medical evidence that links march hemoglobinuria with renal failure (shutting down of the kidneys, often permanently, that may require dialysis or kidney transplant), I think it would be wise to try to avoid having this happen. I know lots of conga players have had this happen and not developed frank renal failure, but that does not prove it is safe, just that there is variability in people's susceptibility to this. As much as we like to hit hard and play over amplified bands etc, we should remember that John Henry died after beating the mechanical drill. I strongly recommend using microphones when playing with loud instruments.
By the way, this post is based on several studies in the medical literature, and is presented here as medical and public health advice. This is a potentially serious medical problem that can be prevented. I am a practicing physician, board certified in internal medicine and preventive medicine (occupational) and have been playing congas for 37 years, since before I studied medicine. Feel free to ask more questions, I think it is important that everyone here understands this phenomenon well enough to take steps to prevent it. I was actually thinking about this when we were playing rumba at the funeral of one of the great AfroCuban master conga players who passed on a couple of months ago after several years on dialysis. It is coincidental and important that this question came up here on the website.
George Friedman-Jimenez, MD
Director, Occupational & Environmental Medicine Clinic
Bellevue Hospital Center
NYU School of Medicine
New York, NY
george.friedman.jimenez@nyu.edu212 562-4572