Given names often turned into family names when people were identified by their father's name. For example, the first name Ahrend developed into the family name Ahrends by adding a genitive s-ending, as in Ahrend's son. Examples: Ahrends, Burkhard, Wulff, Friedrich, Benz.
OK the definition you have used for crouching associates it with stooping (which on Wikipedia redirects to bowing. Squatting essentially involves bending at the knees and hips. If there is any bending at the waist (and maybe lowering of the head) then that is considered stooping.
A person in the Trendelenberg position is lying supine with their head slightly lower than their feet. Abduction is movement away from the midline, or to abduct. Adduction is movement toward the midline, or to add. Flexion is to bend at a joint, or to reduce the angle.