• Articulation, when it refers to anatomy, is simply the movement of two bones at a joint. However the muscles and tendons, which are anchored to the bones, also pull and tug so that the bones move as the brain directs them to. When you sit down or stand up, there are actually several different articulations going on at the same time.
  
  

Knees

• When you sit down, unless you're sitting on a very high chair, your knees are moving. Your knees are diarthrosis joints, which means they're free-moving. As you sit down or stand up, the muscles and tendons in your legs and back flex, and the pull created causes your knees to bend at the joint. When the knees bend, the cartilage between the bones is used as padding and the bones are allowed to move, held in check by the muscles and sinews that act as the cables to the bones' frame in the body.
  
  

Ankles

• As your weight shifts when you walk or change position, your ankles articulate. Like the knees, the ankles are also considered to be diarthrosis, but the ankles are even more flexible. As you sit or stand, your weight is shifting on your feet, and as your weight shifts, your ankles shift to support your weight at that new angle. The ankles move forward and backward, but they also move side to side and in a circular motion, which means that they can support the body in a number of different positions, including the full range of motion from sitting to standing.
  
  

Hips

• Your hips are two large joints that articulate as you stand and sit. Your hips, similar to your shoulders, are ball-and-socket joints. This gives your hips the ability to not only rotate as you go from standing to sitting (or vice versa) but to rotate outward if you want to straddle the back of a chair. As with all joints that articulate in this motion, though, age and mistreatment can make it more painful for them to work, so it's important that you stretch and exercise your joints often.