Knee-ankle-foot orthoses (KAFOs) are full leg braces for individuals with knee extensor weakness, designed to support the person during weight bearing activities by preventing knee flexion. KAFOs typically result in an unnatural gait pattern and are primarily used for level ground walking. A novel variable resistance orthotic knee joint was designed and evaluated to address these limitations. This low profile design fits beneath normal clothing. Mechanical and biomechanical testing demonstrated that the design resisted knee motion during stance phase, released the knee joint without restricting the knee’s range of movement, and provided flexion resistance during stair descent. Design modifications and related testing procedures were developed to further improve joint performance and to validate the design prior to testing on individuals with knee extensor weakness.