What did Valdek's father do to avoid being drafted?

Vladek tells his son that when he was twenty-one, his own father purposefully starved him and kept him deprived of sleep so that he would fail his army physical and not have to join along with the rest of the boys his age. The plan worked, but the army told him to work out for a year and then return. Vladek begged his father not to starve him again, and the next year he joined the army. Basic training was eighteen months, and he returned every four years for another month of training.