Exercise devices for exercising the abdominal muscles and other muscle groups are disclosed. Each of the exercise devices has a housing from which opposing interconnected ends of an axle extend and a support ball that is captured within and partially extends from the housing. The housing has a cavity defining a partial sphere for receiving the support ball. Each of the exercise devices also comprises a plurality of strips which are attached to the surface of the cavity. Each of the strips includes a plurality of bearings each of which is captured within a bearing race. The strips are positioned upon the cavity surface so as to engage the support ball surface and to provide free-rolling support of the housing on the support ball in any direction when the exercise device is in use. Methods of using the exercise devices are also disclosed.