Individuals with quadriceps muscle weakness often have difficulty generating the knee-extension moments required for common mobility tasks. A new device that provides a knee-extension moment was designed to help individuals perform sit-to-stand and stand-to-sit. The knee-extension-assist (KEA) was designed as a modular component to be incorporated into existing knee-ankle-foot-orthoses (KAFO). The KEA loads a set of springs as the knee flexes under bodyweight and returns the stored energy as an extension moment during knee extension. The springs can be locked in place at the end of flexion to prevent unwanted knee extension while seated. When the affected leg is unloaded, the device disengages, allowing free joint motion. A prototype KEA underwent mechanical testing and biomechanical evaluation on an able-bodied individual during sit-to-stand and stand-to-sit.