among other things , a structure supports the ischial tubularities of a user of a device to which the structure is attached , while gravity is applying a downward force on the ischial tubularities and the user &# 39 ; s legs are engaged in motion during which forces are being applied that tend to cause the user &# 39 ; s respective ischial tubularities to move relative to a frame of the device . the structure includes two buttock supports having surfaces against which the bottom surfaces of the user &# 39 ; s respective buttocks bear during the motion . a mechanism connects the two buttock supports to the frame of the device to permit simultaneously free motion of the two buttock supports up and down relative to a frame of the device and back and forth relative to the frame of the device and free variation of the angular orientations of the bottom surfaces of the user &# 39 ; s buttocks back and forth , relative to a hypothetical horizontal plane that is fixed relative to the frame of the device .