An exoskeleton (100) adapted to be coupled to a lower extremity of a person includes a thigh link (102), a shank link (104) and a knee joint (106) allowing flexion and extension between the thigh and shank links (102, 104). A torque generator (156) connected to the knee joint (106) includes a wrap spring (110) having a first end (112) coupled to the thigh link (102), and a second end (118) coupled to an electric actuator (116) capable of selectively positioning the second end (118) of the wrap spring (110). A controller (120) causes the electric actuator (116) to position the wrap spring (110) to provide a selective torque between the thigh and shank links (102, 104) based on a signal (212, 214, 216) produced by a sensor (164, 166, 168).