Why Horace Pippin had a great physical difficulty painting the John Brown going to his hanging?

Pippin was descended from slaves. Born into a family of laborers and domestic servants in Pennsylvania, he worked as a laborer himself before entering the Army in World War I. There he was a proud member of the famed all-black 369th Infantry in France until a sniper's bullet took him out of action, leaving his right arm partly paralyzed. He claimed that he started painting as a form of physical therapy, supporting one hand with the other when using a brush. http://query.nytimes.com/gst/fullpage.html?res=990CE6DC173BF930A35751C0A963958260