Ageing and neurological disorders can alter the knee functions during activities of daily living. Though active knee exoskeletons have been widely developed in the last decades, only a few are able to assist during non-stationary walking and sit-to-stand/stand-to-sit (STS) transitions. First, this work introduces the mechanical, electronic and control design of a novel single-motor Active Knee Orthosis (AKO) with a dual-reduction actuator that selects between two different trains of gears (high torque/low velocity and low torque/high velocity). Other novelties include (i) a symmetrical design, (ii) a torsional spring, (iii) a servo-controlled clutch system and (iv) a non-linear reduction ratio. Then, we present a proof-of-feasibility test with a healthy volunteer performing treadmill walking and STS. The AKO fulfils the requirements of walking and STS tasks in terms of range of motion, torque delivery and angular speed and is able to deliver torque during the application scenario with the two complementary gears. However, an upgraded version should include a simpler clutch design that could lead to a lighter and more reliable exoskeleton with a more adequate weight distribution. The new version will allow for further investigation of the potential of the device and for intense subject testing with healthy volunteers and gait-impaired individuals.