I suppose it also depends on which type of zombie you subscribe to: The classic, lumbering Romero-esque zombie that shuffles along, or the rather more unbelievable fleet-footed zombie seen in the Dawn of the Dead remake and World War Z.
Being a purist, I naturally buy into the former, although the power output of Romero's undead would be lower, which is a bit of a bummer come the apocalypse.
In your "calculations" you fail to consider the reaction force on the Zombie.
Lets get this into sensible units:
4kW of output, 54 inches/second on the treadmill belt = 1.4m/s
the force required to move the belt will be 4000/1.4 = 2857 N
assume the Zombie weighs 75kg, then you'll need a coefficient of friction between the Zombie's foot and the treadmill of 3.8 - which is not credible without teeth on the belt which would be cruel.
Worse, in all the films the Zombies are about human equivalent in power and strength and humans can't manage more than 100W or so for long periods - if Zombies can do 40x better they'll have us on the treadmills.
AND - you haven't allowed for losses in the treadmill, generator etc.
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