I'm always learning something new. Yesterday a friend told me about Woody Breast Syndrome. This might explain some of the tough chicken breasts I've encountered in the past few years.
http://www.poultrytimes.com/poultry_today/article_cd6d0284-5ab9-11e7-a8e4-c399c839e7e8.html
http://www.poultrytimes.com/poultry_today/article_cd6d0284-5ab9-11e7-a8e4-c399c839e7e8.html