Ezekiel 1:7

וְרַגְלֵיהֶם רֶגֶל יְשָׁרָה וְכַף רַגְלֵיהֶם כְּכַף רֶגֶל עֵגֶל וְנֹצְצִים כְּעֵין נְחֹשֶׁת קָלָל

Their legs were straight legs and the soles of their feet were like the sole of a calf's foot, and they sparkled like the color of burnished copper.

Rashi explains the “straight leg” to mean without knee joints, as angels (who are described in this verse) neither sit nor recline. It is this “straight leg” we emulate when reciting Kedusha, which is a form of praise recited by the angels (see Isaiah 6:3 and Ezekiel 3:12).