To straighten out s.t. (as one's arms, legs, a rope, etc.); to stretch s.t. (as one's arms, legs, a rope, etc.); to have one's legs extended out [pa-].
Straighten out your leg (lit. foot) so that your veins won't be crumpled.
Don't have your feet extended out in front of people.
If I don't often stretch my knee, it will become more painful.