A recent idea in exercise and rehabilitation divides exercises into closed chain and open chain.
Chains are links of body parts, such as foot, ankle, knee, and hip during walking.
In a closed chain the end of the chain farthest from the body is fixed, such as a squat where your feet are fixed and the rest of the leg chain moves. In open chains the end is free, such as in a seated leg extension. Closed and open chain exercises provide somewhat different benefits. Closed chain exercises tend to emphasize compression of joints, which helps stabilize the joint, such as your knee during the upright stance phase of squats. Closed chains tend to involve more muscles and joints than open chains and lead to better coordination around each structure, which improves overall stability.