Why isn't script detecting npc movement?

I put this script into each one of my 150 npcs to animate them, but it seems to not detect when the velocity of the base part has changed or something. The print does not fire. Here is my code:

local idle = Instance.new("Animation")
idle.AnimationId = "http://www.roblox.com/asset/?id=507766388"
local run = Instance.new("Animation")
run.AnimationId = "http://www.roblox.com/asset/?id=507767714"

local idleTrack = script.Parent.Humanoid:LoadAnimation(idle)
local runTrack = script.Parent.Humanoid:LoadAnimation(run)

script.Parent.UpperTorso:GetPropertyChangedSignal("Velocity"):Connect(function()
	print("Fired")
	local speed = script.Parent.Humanoid.MoveDirection.Magnitude
	if speed > 0 then
		runTrack:Play()
	elseif speed == 0 then
		idleTrack:Play()
	end
end)

I tried detecting the movement of the npcs by making the code execute on the Humanoid.Running event, and then using the speed parameter, but that just caused all the npcs to play the running animation at all times, even when standing still.

What am I doing wrong? Is there a better way to detect when something moves?