Normally urine passes from the the kidneys to the bladder via ureters. There is a valve like mechanism at the junction of ureter with the bladder, that prevents urine coming in the reverse direction. However when this mechanism fails the urine refluxes back towards the kidneys and damages it.
Joint and bone pains can be there due to renal failure
Usually a person can return to almost al normal activities after transplant, including sitting / running