Energy storing and releasing for use in a prosthesis or orthosis, and including a unidirectional locking system enabling storage of energy in one direction. The system includes a first body, a second body have a device for storing mechanical energy, which includes a support, a carriage connected to at least one elastic element for storing energy, a unidirectional locking system, a slider slidably inserted into the carriage, a priming system configured to switch between a first state of the device and a second state of the device and vice-versa. The system further includes an articulated joint between the first body and the support of the second body, the articulated joint allowing the rotation of the first body and the support of the second body with respect to one another around a joint rotation axis, and a transmission element between the articulated joint and the slider of the device.