(II) Μι-Verbs.—The stem of the second aorist of μι-verbs is the verb-stem without any thematic vowel. In the indicative active the strong form of the stem, which ends in a vowel, is regularly employed. The middle uses the weak stem form.
ἵ-στη-μι (στα-, στη-)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].