Matisse began Back (II) from a plaster cast of Back (I), which he reduced by carving and built up with more plaster as he worked, creating a more angular and massive form. He removed details from the head, hair, neck, and shoulders and reformulated the curving, S-shaped spine and soft flesh to produce a solid, stocky figure with a more geometric build.
Gallery label from Matisse: Radical Invention, 1913-1917, July 18–October 11, 2010.