Research into anatomical drawings.
He uses lots of hatching and cross hatching.
Anatomical drawings by Degas:
Although the figures in Degas drawings here are clothed, they show the angles and joints of the body to a good advantage.
It is also useful to see how Degas builds light and shade using chalk and charcoal.
Anatomical drawings by Vincent van Gogh
In these drawings there is a lot of hatching and blending, and a really good use of tonal scale. The drawings look three dimensional because of the shading.
He uses curved lines to hatch the shade in, describing the shape of the hands and arms, the gentle curves and the more angular joints.
Picasso anatomy drawing