where n is the number of muscles in the model; am is the activation level of muscle at a discrete time step; $//$ is its maximum isometric force; lm is its length; vis its shortening velocity; $//$ is its force-length-velocity surface; rm,j is its moment arm about the jthjoint axis; $//$ is the generalized force acting about the jth joint axis; and is a user defined constant. Note that for static optimization$//$ computes the active fiber force along the tendon assuming an inextensible tendon and does not include contribution from muscles' parallel elastic element.