Frank is correct. Exercise burns fat cells. When fat cells burn, the stored THC metabolizes into THCA and is released back into the bloodstream. The kidneys then filter the THCA from the bloodstream and deposits them into the bladder. This is the main reason why THC carries a longer detection period than most other types of drugs.

For this reason, you want to avoid any fat-burning activities and be as dormant as possible for at least 3 days before the scheduled U/A to prevent a bunch of THCA from being released into the bloodstream, which will end up in the urine to be detected.