An exerciser includes a hollow base having two openings. Each opening includes at least one first circular groove. Two bearing members are received in the two openinigs, and each has a frame with which a plurality of balls are rotatably engaged. Two disks are rotatably mounted on the two bearing members respectively, and each disk includes at least one second circular groove and teeth defined in an underside thereof. The balls are rotatably engaged between the at least one first circular groove and the at least one second circular groove. A driving gear is located in the hollow base and engaged with the teeth of both of the two disks.