An energy-storing prosthetic knee includes an upper leg structure and a lower leg structure hingedly connected to the upper leg structure. The lower and upper leg structures move pivotally relative to one another. A fluid actuator is operatively connected to the upper and lower leg structures such that relative pivotal movement of the upper and lower leg structures causes a fluid to flow in a predetermined manner within the fluid actuator. At least two controllable variable fluid flow resisting devices, an accumulator, and a control system are also provided. The control system is actuable to cause storage of the fluid energy for a predetermined length of time and release of the fluid energy at a predetermined time during the gait cycle. Both the storage and release of the fluid energy are variable by action of the control system without physical modification of other components of the prosthetic knee.