Abstract: Hand-held, portable exercise device which enables the user to strengthen muscles and joints by performing various exercises, the device having a base structure defining a receptacle and supporting at least three omnidirectional rotational elements. A grip element, which may be spherical, has a lower surface positionable on the receptacle and has an upper gripping surface that is convex in at least two dimensions. A hand retaining strap is positionable over the gripping surface of the grip element such that a hand of a human user, the hand having a palm surface and a back surface, can be inserted between the retaining strap and the gripping surface, the palm surface of the user's hand contacting the gripping surface while the retaining strap contacts the back surface of the user's hand to secure the exercise device to the hand.