Muscles can be trained and grown. Stretching can prevent injury to muscles, joints, and connective tissue. But can connective tissues (tendons, ligaments) be trained and strengthened like muscles ...
Let me describe myself: I have been weightlifting since I graduated (2010). Before graduation I weighed close to 120lbs, now I weigh (on a good day) 163lbs (I'm 5' 11"). I have plenty of definition ...