US Patent: 7,648,450 This patent discloses a hydraulic cylinder for use in exercise machines to deliver a controllable fast acting force. The invention uses a hydraulic cylinder with features that allow high acceleration rates, rapid changes of force level and direction, and positive force limitation.

This patent discloses an apparatus for physical fitness physical strength, agility and endurance training.

Abstract: An apparatus for physical fitness physical strength, agility and endurance training. A controllable pressurized flow of fluid is first received via a hose to a source of the pressurized fluid. In response to this flow of fluid, a force is generated along at least one of a plurality of selected directions using one or more apertures or nozzles. The force is then transferred to a user of the apparatus so as to supply physical resistance training to the user. The apparatus further includes a user engaging mechanism that is connected to the propulsion system, and is configured to transmit exercise forces to an associated user. The apparatus further includes controls for controlling the magnitude and direction of the reaction force vector applied to the user engaging mechanism. Various embodiments of user engaging members, including blocking dummies, handles, gloves, helmets, shoes, straps or balls, may be attached to the propulsion system.