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Human body is a system of rigid segments inter-linked at the joints. One of the 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.

It is possible to assess the interaction between the segments through the joint & muscle and the work done by the muscles through a procedure called inverse dynamics. 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.

Human body is best described as the linked-segment system. The basic assumptions here are:

 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.

 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.

The following pages in this section explain the inverse dynamics procedure, and the joint energetics:

 Joint Torque Joint Torque Revisited Data Interpretation Joint Energetics

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.