Using Avimator

What settings work well for a fixed or ridged animation with no head or body movements.

Priority
In%
Out%
Ease in in sec
Ease out in sec

Ridged?

The proper settings to use depend on your animation, it's not like there is some blanket setting we could recommend for any arm animation (I'm assuming arms, since you say neither the head nor body is animated.) Also, note that In/Out are only for animations where you want to play through it all once and then loop part; for most animations, just leave them at 0/100.

So do you mean rigid? As in not moving (no breathing or head turning, IE, like a statue?)

If so, make keys for your head and body on frame two different than frame one, and no other keys for them.

The things listed in the OP don't directly do that...

Priority depends on use and should be as low as possible to properly play (search or see other threads on priorities), or other animations won't work properly because of yours.

Except for special circumstances, Ease In and Out should be set far longer than defaults IMO, like one full second. (Of course it depends how different the start of your animation is from the expected starting pose.)

Do you mean like a pose? Or do you not want certain body parts to move at all during a regular animation of other parts?

Summarizing it here won't give you all the answers (check the sticky topics for more) but generally you take your first frame and treat it as a reference one. You never see that frame in any SL animations, it's used to calibrate the avatar. In the second frame, if any body part is moved even slightly, SL will then "animate" that part, but leave everything else alone. (this means they will not be used and can be animated by other animations with lower priority - like the default animations).

So for your case, if I understand correctly, you want the head and body to not move during this animation. Avimator starts your avatar out in the classic "T-pose" so leave frame one as it is and go to frame two. Move the head/neck and whatever else you want to stay ridged a little bit and then leave it alone - and you're done!

If this is a pose, you can reduce the number of total frames to just two and have the upload set to loop in SL. You would also have to move EVERY body part into whatever stance you want so that no pieces are ignored.

As for priority, level three overrides most of the default upper body animations, but I've found that level four is needed to overcome the default walking animation. If this is just a typical standing animation, I think level two would be fine - but don't quote me on that.

Ease in/out is how fast your avatar twines out of and into the animation. This is something you'll need to test out in the preview window to get a good idea.

% in/out is a bit buggy but it's supposed to tell SL what part of the animation to use in the loop (if it is set to loop). For poses, you can set both to 100, or leave it at the default.