An inverted pendulum with a springy control as a model of human standing
  The normal and the inverted pendulum continue to be one of the main physical models and metaphors in science. The inverted pendulum is also a classic study case in control theory. In this paper we consider a special demonstration version of the inverted pendulum which is controlled via a spring. If the spring constant is below a critical level the springy control will be unstable and the pendulum will be kept from falling only by exercising a dynamically varying control. This situation resembles the case of human bipedal quiet standing with the Achilles tendon serving as the spring. The paper is written with physics students in mind. 
Frank Borg