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Individuals with quadriceps muscle weakness often have difficulty generating the knee-extension moments required to complete common mobility tasks. A new device that provides knee-extension moments through a range of knee angles was designed to help individuals perform stand-to-sit and sit-to-stand tasks. The novel knee-extension assist (KEA) was designed as a modular component to be incorporated into existing knee-ankle-foot orthoses or used in a knee orthosis. During stand-to-sit, a set of springs is loaded as the knee flexes under bodyweight and the KEA thus provides a knee-extension moment that aids in achieving a smoothly controlled knee flexion. The springs can be locked in place at the end of knee flexion to prevent unwanted knee extension while the user is seated. The entire knee extension assist can be disengaged to allow free joint motion anytime the affected leg is unloaded. During sit-to-stand, the KEA assists knee extension by returning the energy stored in the springs as an extension moment. In mechanical testing of a prototype of the new KEA, a mean maximum extension moment of 42.9 ± 0.46 Nm was provided by the device during flexion and 28.4 ± 0.28 Nm during extension. A biomechanical evaluation with two able-bodied individuals demonstrated the effectiveness of the KEA in successfully assisting stand-to-sit and sit-to-stand tasks. During stand-to-sit, the KEA provided 82% and 75% of the total (muscle and KEA) knee-extension moment required by the braced leg for the task for the two subjects, respectively; and during sit-to-stand, the KEA provided 56% and 50% of the total knee-extension moment for the two subjects, respectively. This KEA performance exceeded 50% knee-extension moment assistance for a 70 kg person.