One object moves other moves opposite way

[tongeert]

I'm animating a banana with arms and legs right now, but when the banana bend his knees, his knees are going trough the bottom of his body.
So I want that when he bends his knees that that part goes up.
Now, I thought that it would be possible that when the big bone (that moves his body) goes down, that a smaller bone (the one that moves the bottom part of his body/the small red one) goes up.
But I don't know how I can do that. Does anyone know how that is possible?