A header for an agricultural vehicle. The header includes a header frame and an elongate sickle knife mounted along a leading edge of the header. The header further includes an actuator for reciprocally moving the elongate sickle knife in its longitudinal direction relative to the header frame between a first position and a second position over an intermediate position. At least one accumulator extends between the header frame and the elongate sickle knife to accumulate energy during a first movement of the elongate sickle knife from the intermediate position towards at least one of the first position and the second position, and to release accumulated energy during a second movement of the elongate sickle knife opposite to the first movement.