An assembly for stabilizing an individual while exercising or performing various sports or physical activities in a seated position, such as a wheelchair or other support structure. A harness assembly comprises one or more harness structures each removably secured to a predetermined part of the individual's body in a manner which safely stabilizes the individual while the individual is exercising or performing sporting or other physical activities. A restraint assembly comprises a plurality of restraining members interconnected to each of the plurality of the harness structures being used, and to an anchor assembly disposed in underlying supporting relation to the individual and the support structure. The restraint assembly is cooperatively structured with the anchor assembly to maintain any of the harness structures being utilized and the corresponding body portion of the individual in a stable, operative position best suited to effectively and safely perform the intended exercise.