Meat has two major components: muscle fibers which contract and relax, and connective tissue, which basically support the muscle fibers. Muscle fibers usually shorten and stiffen right after slaughter and at the start of rigor mortis. This usually lasts for six to 12 hours in beef cattle. Cooling the carcass shortly after slaughter will allow the rigor to go only so far. Also as the carcass ages, is hung after slaughter, and the temperatures in the cooler are set properly, there are enzymes within the muscle that are released, causing a further breakdown of connective tissue that will increase the tenderness of the meat.
How long should a carcass be hung to allow the enzymes to increase tenderness? Data would suggest 10 to 14 days. There is not much difference in tenderness in carcasses aged 10 to 14 days compared to longer. Many times, small slaughter facilities don't have the cooler space to hang carcasses very long. If the meat is tough to start with, aging will help. If the meat is tender to start with, aging will, in theory, make it more tender.