Types of Articulations
Your ankle, knee and hip joints are all considered synovial, or freely movable articulations. Your knees and ankles are hinge-type articulations, meaning that they allow movement in only one plane, similar to a door hinge. Your hips are considered ball-and-socket joints in which a rounded head of one bone fits into the cavity of the other and allows for both rotational movement as well as movement in any plane, or direction.
Movement at the hips is primarily handled by the gluteus maximus muscle. Your quadriceps muscles at the front of your thighs are responsible for movement at your knees, and the soleus muscles in your calves control movement in your ankles. When you stand up, these muscles contract concentrically, or shorten as they contract. When you sit down, your muscles contract eccentrically, or lengthen as they contract, to help you control your body and resist the pull of gravity as you lower yourself.