What makes chicken tough and chewy?

Online Answer
The poultry industry has a fowl problem: an emerging phenomenon called "woody breast." While it's not harmful to humans, the condition causes chicken breasts to be tougher because of hard or woody fibers that lace the meat.
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The condition seems to be associated with vascular problems in the birds. Broiler chickens have been bred to strengthen the genetic markers for large breast meat, and this is thought to be a factor in the development of woody breast in recent years.