Sitting down and standing up require a coordinated effort from muscles surrounding your hip, knee and ankle joints. Joints, or articulations, are junctions between two bones that permit a degree of movement. When you sit down, your hip joints flex, which means that the joints bend so that your thighs move forward, your knees flex and your ankles dorsiflex -- where the top of your feet move up toward your shins. When you stand up, your hip joints extend or straighten so that your thighs move backward while your knees extend and your ankles plantar flex, which means that your feet point and move away from your shins.
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.