A fitness device is provided, the fitness device comprising: an elastomeric member having an interior cavity, the elastomeric member comprising a cylindrical portion extending between a first hemispherical endcap and a second hemispherical endcap, wherein a surface of the elastomeric member defines an aperture positioned between the interior cavity and an exterior of the elastomeric member; and a structured surface positioned on the surface of the elastomeric member over the aperture, the structured surface extending into the interior cavity of the elastomeric member.