importing legacy animation fbx with mask

Hi there

I'm making animation with the legacy animation (not humanoid) in Unity5.4 .

Now, I have an animation fbx file of the whole module , but what I want is only the anim clip of upper body.

I tried the mask option of tag in animation

1. select Definition: Create from this model

2. uncheck the useless parts of bones from Transform

3. save and build up prefab from this anim But nothing happened, the animation still affected the whole body of the model

Is there a way to mask the useless parts off in building up the prefab from an fbx, so I can pick up only the part I wanna use to make the correct blend result and reduce the anim cost(mem cost and cpu cost in calcing blend of useless parts).

BTW, PLZ PLZ PLZ just talk something about legacy animation. THANK U ALL THE SAME if you wanna post your plan of animator(mecanim), but I recommand not to waste your time coz animator leads to lot of performance and updating/hot-fixing issues and is NOT ACCEPTABLE in my case.