Shock absorber for an at least partially muscle-powered bicycle having a damper device having a first and a second damper chamber assigned thereto which are hydraulically coupled with one another through a damping valve. In the damping valve a flow duct with a field-sensitive, rheological medium is provided. The damping valve has a field generating device assigned to it which serves for generating and controlling a field intensity in the flow duct of the damping valve. The flow duct comprises in the damping valve a collection chamber which is hydraulically connected with the first damper chamber through a plurality of flow apertures, wherein at least one flow aperture is configured as a through hole and at least one flow aperture, as a closable valve opening provided with a one-way valve. The collection chamber is hydraulically connected hydraulically with the second damper chamber via a plurality of fan-type damping ducts each separated from one another by a fan wall.