Muscle location as related to tenderness
Generally beef tenderness is related to two factors: 1) function of muscles, and 2) frequency of use in the live animal. Muscles used for locomotion (walking, lifting of head, etc.) tend to be less tender than muscles used for supporting the skeleton.

Locomotion Muscles:
Found in the front shoulder (chuck) and hind leg (hip), these muscles were used for working. Therefore, they are stronger then the support muscles of the back (loin) and rib. Locomotion muscles have higher percentages of connective tissue or gristle. Connective tissue contains collagen. Both heat and moisture (water) are required to dissolve collagen during cooking.

Support Muscles:
Found in the rib and the back (loin), these muscles were used to support the midsection of the live animal's skeleton as the animal moved. Since they did less work, support muscles have less connective tissue than locomotion muscles and are finer and smoother in texture. Less exposure to work results in a more tender cut of beef.