To avoid extra stretching of the topology in the bending body parts you should start with the right topology.
The topology scheme on the image allows saving the topology with no stretching as it is possible.
It made from three parts: central loop, flat knee area which surrounded by a border loop.
The key principle is to increase the polygon count in the area of the knee cap because this part will be susceptible to stretching. While on the opposite the polygons may be fewer because this area will be susceptible to compressing.
The video shows four examples of topology bending.
The first two save the mesh while stretching and the second two are not.
