Abstract: A mobile and adaptable fitness system allows a fitness device to be used to control an application running on a host device. The system includes one or more controllers, a motion sensing device, and a host device. The hand controllers may be mounted on the handlebars of the fitness device or held in a user's hands and include buttons that the user can press. The motion sensing device may be mounted on the user or on a moving part of the fitness device and contains sensors that capture motion performed by the user. The host device receives input data from the hand controllers and the motion sensing device and generates an activity vector representing the intensity at which the user performs a fitness activity. Based on the activity vector, the host device controls an object in an application, such as a character, based on the input data.