Used by NASA astronauts to maintain their upper body conditioning in space, this is the gyroscopic dumbbell that strengthens and rehabilitates muscles in the hands, wrists, arms, and shoulders. Holding the tennis ball-sized device and rotating your wrist causes the internal gyroscope to spin up to 13,000 rpm and provide up to 38 lbs. of torque resistance to strengthen the wrists, biceps, triceps, and deltoids. A study performed by Southern Utah University proved the gyroscopic dumbbell increased maximum handgrip strength by an average of 11%, and this low-impact exercise can also increase the range of motion in your hands and help rehabilitate carpal tunnel and tennis elbow injuries. A docking station automatically initiates the spinning, and an integrated tachometer measures and displays the current rpm, highest rpm achieved, and total number of revolutions, allowing you to easily track progress. Docking station requires two AA batteries. 3 1/2" H x 3 3/4" W x 4 1/4" L. (1 lb.)