The woman in the painting is Anna Christina Olson (May 3, 1893 – January 27, 1968). Anna had a degenerative muscular disorder, possibly polio or Charcot-Marie-Tooth disorder, which left her unable to walk.[1] She was firmly against using a wheelchair, so she would crawl everywhere. Wyeth was inspired to create the painting when he saw her crawling across a field while he was watching from a window in the house. 

:-(

Man, I'm glad that we figured out the polio vaccine.