The present invention relates to an exercise device that allows the user to strengthen the muscles that close the hand, against the resistance of a flexion member, as well as to strengthen the muscles that open the hand and spread the digits, against resistance of an extension member, all through full natural planes of motion, without lulls of resistance during the exercise. An external part and an internal part may be arranged concentrically to a rotation axis and may be adjustable rotationally relative to each other about the rotation axis. Constituent software products of the bundle are identified.