Natural finish. Ball-bearing rollers encourage rocking motion. 3 in. height adjustment. Non-skid, inset pads. Rubber feet protect flooring. Height Range: 3.75 in. to 6.75 in.. 11.875 in. D x 16 in. W. The FM300, with their ball bearing rollers, encourage gentle rocking of the feet in the same way a rocking chair encourages movement. This rocking motion uses most of the lower leg muscles to increase healthful circulation. In addition, both offer 3 in. height adjustment for custom positioning.