A treadmill has an endless belt and a housing supporting movement of the endless belt along the housing. The housing has a top surface, a bottom surface and a plurality vertical channels extending between the top surface and the bottom surface. Posts are vertically movable in the vertical channels. The posts are secured in a selected position in the vertical channels, with an upper portion of a selected length extending above the top surface and, if desired, a lower portion of a selected length extending below the bottom surface.