This is an excerpt from Foundations of Professional Personal Training-3rd Edition by canfitpro.
The kinetic chain is a classification system that refers to interrelated parts of the body (i.e., joints and muscles) and how they work together to perform movement. The upper-body kinetic chain starting at the spinal column includes the scapulae, shoulders, upper arms, elbows, forearms, wrists, and fingers. The lower-body kinetic chain starting at the spinal column includes the pelvis, hips, thighs, knees, lower legs, ankles, feet, and toes. Thinking about the joints as a chain, if the distal part of the chain is fixed it is classified as a closed kinetic chain, and if the distal part is free to move it is classified as an open kinetic chain.
During the execution of an open kinetic chain exercise, the hand (upper-body exercise) or foot (lower-body exercise) is free to move. An example of an open chain lower-body exercise is a machine seated knee extension, in which the feet are free to move in space. An example of an open chain upper-body exercise is a standing barbell or dumbbell arm curl in which the biceps and brachialis are recruited to flex the elbow joint.
A closed kinetic chain exercise is characterized by the hand or foot being in a position where it cannot freely move. An example of a closed kinetic chain exercise for the lower body is the body-weight or loaded squat, in which the body pushes itself away from the immovable surface of the floor. When applying the principle of closed kinetic chain exercise to the upper body, the body-weight push-up from the floor can be used to recruit the entire shoulder girdle and core.
In terms of program design, note that open kinetic chain exercises usually develop skills specific to sport or performance (e.g., throwing a baseball), whereas closed chain exercises generally have more benefits to functional movements.