We are wondering about the origin of the term 'prison workout'. It is a series of exercises that you work down to finish a set or a group of sets. For example of a 15–1 prison workout for pushups. Start set of 15 pushups, next set 14, next set 13, and so on to one pushup.
A prison workout refers to a set of exercises that could be done in a small indoor space and without much equipment - like a prison cell. http://www.urbandictionary.com/products.php?term=prison%20workout&defid=4512034