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Yes, Poser will allow you to save an animated pose for the legs only. Proceed to save a pose, in the first dialogue box that pops up, click the "Select Subset" button. In the "Hierarchy Selection" window, clear the check mark in the "UNIVERSE" box, this will deselect all actors. Scroll down and place a check mark in each leg part. Click "OK". Give the pose a name, and proceed to save as an animated pose.