Joint Kinetics

Kinetics can be defined as being the scientific study of the turnover, or rate of change, of a specific factor in the body, commonly expressed as units of amount per unit time. The Human body is a system of rigid segments inter-linked at the joints. One of the most important aspects of this type of system is the interaction between the inter-linked segments at the joints. The joints (ligaments, joint capsule, etc.) themselves are passive structures that only apply constraining forces, when they are stretched, on to the bony structures involved in the joints. The active components of the system are the skeletal muscles that produces forces through voluntary contraction. Through a process called inverse dynamics one can asses the interaction between the segments through the joint & muscle and the work done by the muscles. Inverse dynamics literally means that one tries to figure out the cause of motion based on the effects (motion) and the inertial properties of the object in motion. One can quantify the human (or animal) motion through motion analysis. The inertial properties of the segments can be obtained from direct or indirect methods. The human body is most accurately described as being the "linked segment system," in which the natural assumption is that the joints are all pin joints and there is no friction at the joint. Thus, the forces produced by the elements of the joint such as ligaments and joint capsules are all concentric about the joint centers. In other words, all these forces pass through the joint centers. That muscles are the only elements that can produce eccentric forces about the joint centers. And that all muscles are uni-articular and there is no intervening structure that can act as pulley. In other words, muscles are straight and directly attach to the segments. Such a procedure will be later describe through the use of Joint Energetics.