The active components of the system are the skeletal muscles that produces forces through voluntary contraction. The inertial properties of the segments can be obtained from direct or indirect methods. Human body is best described as the linked-segment system . 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.
Muscles are the only elements that can produce eccentric forces about the joint centers.
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. Although the inverse dynamics approach is fairly useful, it has its own limits. One must clearly understand these limits to prevent misinterpretation of the calculation results.