The Blend file of a fully rigged model, if altered and saved, reverts to original

I have a fully rigged human model. The original is the usual standing pose, arms outstretched.
I now lower the hips to bend the knees and lower the arms sidewise, bend spine etc.
I then save the Blend file without any key frames being inserted.
I open another file and then return to the first file.
I notice the model has reverted to its original (rest) pose.

Why have the changes made been lost?
Help requested please

Did you move the object in “Pose Mode”. Are there other keyframes that are set in the file?

Yes, the bones were moved in pose mode and there is a key frame at Frame 0 with all bones selected.

I deleted this keyframe - leaving nothing in the Dope Sheet.
But I noticed that when the tineline is scrubbed, a previous animation tends to be reproduced patchily even though the Dope Sheet is empty.
It is as if the file remembered remnant of the old animation.

check on the action editor, maybe you forget to set the action to a fake user (clicking the F next to the action name)

Thanks, Belofilms, for the suggestion. I don’t see an F in the Action Editor header.
However, if I click the browse action button, a number of F names turn up (Action, armatureactyion, feet, arms etc). Some have a 0 (instead of F). These were from a previous animation, using the same model (mesh).
How do you delete these F entries? They seem stuck there permanently.
I’d much appreciate suggestions on this matter.

read this to understand what you need to do:
is a tutorial explaining what the action editor does, and how to use it

the action selected is the one you view on the 3d view, the f saves the action even if it is not in use, you delete them by pressing the x, or by not clicking the f, save and reopen the .blend


Thank you again, Belofilms, for taking the trouble to comment again.
I confess I don’t really understand the Action Editor, so I’ll read the tutorial suggested carefully.
Much appreciated.