A model based controller which regulates the braking intensity of a bicycle ergometer based on information on the heart-rate and breathing volume of the bicyclist is developed. The control system enables training at a desired heart-rate by automatically adjusting the load. The mathematical model was determined using linear continuous time system identification techniques and can reproduce the behavior of the human subject under aerobic metabolic conditions sufficiently well. Data for identification is obtained through an easy to perform test sequence. A linear quadratic controller is synthesized based on the identified model. Experimental results of one subject show the effectiveness of the developed system.