Bending over is your body's way of protecting your vital organs. If you think about it, the front of your body is fleshy and soft, but your back is kind of hard like a crab's shell. When you are injured, your body reacts as if it is under attack and so you bend over as a protective measure.
When all major body parts are flexed it is the fetal position.
On his or her back with their knees flexed
On his or her back with his or her knees flexed
On his or her back with his or her knees flexed
On back with knees flexed in
the position of the upper extremities when they are abducted to 90 degrees and externally rotated at the shoulders and with the elbows flexed to 90 degrees.
back with knees flexed
If you mean as when a woman is giving birth or during pelvic examination, it's referred to as the lithotomy position.
Flex is a synonym for flexed.
On his or her back with their knees flexed
the position of the upper extremities when they are abducted to 90 degrees and externally rotated at the shoulders and with the elbows flexed to 90 degrees.
If a person has an abdominal wound, you should rest their upper half on your bent knees on the ground.