Great question — answering as the UphillTech team:
There's several major reasons why meat and poultry can be tough: Collagen: muscles exist to impart a force between bones, when they contract there must be a connection between that muscle and the bone or the muscle would not be able to do work. Collagen is a strong protein that is distributed throughout a muscle and connects to a tendon, allowing the muscle to distribute its' force to the skeletal structure of the animal. The more load a muscle takes, the more collagen is needed to distribute the force, and the tougher the meat is. The animal was stressed before slaughter: if the animal was stressed before slaughter it will have lactic acid buildup resulting in tougher meat Poor treatment...
Full answer by GdD: https://cooking.stackexchange.com/a/28028 (CC BY-SA 3.0).
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