Among other things, a compliant element is configured to attach a seat of a device on which a user pedals to a frame of the device. The compliant element has a stiffness that is small enough to permit substantially unimpeded rocking of the seat relative to the frame in a plane in which the spine and pelvic bone of the user lie when the user is on the seat, and is large enough to maintain the seat in a substantially horizontal plane when the user is not on the seat.