The principal muscle in the thigh is the great muscle running down the front of the thigh, and fastening to the upper border of the _patella_, or knee cap.

This muscle, when it shortens, straightens or extends the limb, or lifts the foot from the ground and swings it forward as in walking, or raises the knee up toward the body when we are sitting or lying down.

You can easily tell how much it is used in walking by remembering how stiff and sore it gets when you have taken an unusually long tramp, particularly if there has been much hill-climbing in it.

On the back of the thigh, runs another great group of muscles, which bend or flex the limb when they shorten.

When the knee is bent, you can feel their tendons, or sinews, stand out as hard cords beneath the knee; hence, this group is called the _ham-string_ muscles.[23] How the Muscles are Fed.