Skeleton Builder skeletons are relatively simple, direct and fast calculations of segments (bones) that are defined and calculated from one marker center to another. The markers can be real or virtual (calculated) and are typically from one virtual joint center to a second virtual joint center. A 3D local coordinate system is defined with three markers:origin, bone (Y) axis, and XY plane. The advantage of Skeleton Builder skeletons is that they compute very quickly and they represent the method of how most biomechanical models have been computed for many years.