Zombie Research Society
Establishing a time frame for the zombie lifespan is a central area of focus for the Zombie Research Society (see: Fresh Stage Decay & The Zombie Lifespan). With that in mind, ZRS Researcher, Rick Lowe, suggests that the undead may increase their longevity by literally eating themselves from the inside out.
Lowe argues that many of 
the body’s internal organs – vital to the survival of a living human – are not needed for a zombie to function. Because of this, the newly reanimated corpse has a portable store of food built right into its framework.
“Humans can go weeks without food. But as they eventually reach the later stages of starvation, the body will literally eat itself. In fact, digestive-related organ failure is what usually kills a starved person. Zombies likely have no need for their organs, so they are free to digest the heart, lungs, liver, and intestines without suffering any ill effects, thereby substantially increasing their lifespan.”
Regardless of whether or not zombies actually swallow the human flesh they bite into (see: Do Zombies Really Eat People?), if Lowe’s digestion theory is correct, then zombies can avoid going “hungry” for some time, making thei lifespan a matter of months at very least, and potentiall a great deal longer.
what makes you think zombies dont need their internal organs. if zombies will be able to move and attack that means they need some source of power for their mucles, meaning theywill need their stomach and digestive tracks as well as their lungs and if they need their lungs then they need their heart. im pretty convinced that they will function just like normal humans, because for a virus to usee and control a body through its brain the brain and body must be able to work. that also brings up the fact that headshots probably arent the only way to kill zombies, i mean its great for movies but if they were real it would be different
It should be further noted that unless the Zombie is making active use of it’s mucous membranes (notably within the digestive system) then the Gastric acid in the stomach would eventually dissolve the lower organs and leak out of the anus after dissolving that barrier.
Muscles need oxygen but can continue without oxygen as long as their glycogen reserves are adequate, however once those reserves are depleted then the muscles will no longer contract no matter how much they are simulated.
The muscles need energy to move somehow. This is assuming it’s a scientific zombie and not a demonic zombie.
but how would they digest their organs if they ate their stomache and intestines?
nad since they re dead, would they even need food? their organs aren’t working, so they dont need energy.. just some thoughts..