A muscle exercise device can include a crank having an axis of rotation. A first arm can be connected to the crank so that a downward external force applied to the distal end of the first arm causes rotation of the crank. The devices of the present invention can further include a second arm to which a counterweight is connected. Optionally, an assist mechanism other than a counterweight can be used to provide upward movement of the first arm with minimal application of force by a subject. A retrofit kit can be provided for modifying an existing device such as an ergometer or road bicycle. The retrofit kit can include a counterweight and a fastener for attaching to the device to the existing device. The devices of the present invention allow for increased exercise capacity, isolation of muscle groups, and biomechanical forces which closely approximate comparable paired limb motion.