We present an interactive system for physical exercise of older people and provide results of a usability study with target user group. The system motivates an elderly person to do regular physical activity based on an easy exercise in a monitored environment without a direct supervision from care-givers. Our system employs multi-modal interface including speech synthesis and speech recognition, as well as distance measurement using an ultrasound range finder. The system coaches the user through a sequence of body movements in the exercise utilizing an underlying human activity model. For evaluation of the performance of the user we present a statistical human activity model to estimate physical load of the user. The system tracks user load by monitoring heart rate and by scanning movement patterns using statistical estimators. At well-defined moments and when the scanning suggests there is a problem with the user, the user is asked to verify his ability to continue with the exercise. The system was tested on a set of elderly users to gather usability data and to estimate the acceptance of the system. While simplicity of the setup proved to work well for the users, suggestions for further extensions of the system were gathered. Usefulness of the concept was verified with a physiotherapist.