A passive exercise apparatus for use in an erect, non-recumbent seating position that provides an unstable platform capable of encouraging an omni-directional involuntary rocking motion that will result in the involuntary contractions of muscles involved in maintaining muscle tone and promoting proper posture (viz., the gluteus maximus, gluteus medius, and gluteus minimus muscles, external oblique muscles, and other muscles in the abdominal, lumbar/sacral, and pelvic regions) in order to counterbalance the rocking forces. The purpose of the device is to provide continuing involuntary contractions of these muscle groups (a recognized form of EXERSITTING, which is the combined execution of concentric and isometric muscle exertion and contractions performed while sitting on the apparatus of the present invention in order to help maintain muscle tone and encourage good seated posture.