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's legs are engaged in motion during which forces are being applied that tend to cause the user'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's respective buttocks bear during the motion. A mechanism connects the two buttock supports to the frame of the device to permit simultaneously (1) free motion of the two buttock supports (a) up and down relative to a frame of the device and (b) back and forth relative to the frame of the device and (2) free variation of the angular orientations of the bottom surfaces of the user's buttocks back and forth, relative to a hypothetical horizontal plane that is fixed relative to the frame of the device.