Unless you are a truly dedicated couch potato, your calves get a workout as you walk through your daily life. With every step you take, you push off the ball of your foot, and your calf muscles lift your entire body weight. Since they receive so much work, the calves are some of the strongest muscles in your body.
Any muscle, whether big or small, tends to shorten as it gets worked and strengthened. Without regular stretching, a powerful muscle group like that in the calf can become short and tight, seriously limiting the range of motion at the ankle, and this can get in your way in yoga poses.
Adho Mukha Svanasana (Downward-Facing Dog) is one of the best poses for stretching the calves. The next time you come into Down Dog, spend a few more minutes gently releasing your heels toward the floor to give your calves a much-needed stretch.