I am trying to model a system with a series of springs. Each spring is connected to the world frame on one end. The other end of each spring is either mechanically coupled to the world frame (holding it's extension constant) or mechanically coupled to a 1-DOF joint (linear slider or wrapped around a cylinder/moment arm). The coupling would be determined by a binary control input (u = 0 connect to world frame, u = 1 connect to output). Is there a way to model this using tendons and actuators in mujoco?