"Knee" is a noun referring to a body part. Verbs related to the knee usually describe actions involving or affecting the knee:
* Bend: to move the knee joint so that the angle between the thigh and lower leg decreases.
* Straighten: to move the knee joint so that the angle between the thigh and lower leg increases.
* Kneel: to rest on one or both knees.
* Flex: to bend the knee joint.
* Extend: to straighten the knee joint.
* Sprain: to injure the ligaments around the knee.
* Fracture: to break the bone in the knee.
* Amputate: to surgically remove the knee.
To understand which verb fits best, you need more context. For example, "What did she do to her knee?" requires a different verb than "What did she do with her knee?"