An exoskeleton configured to be coupled to a lower extremity of a user, said exoskeleton comprising: a thigh link configured to couple to a user's thigh; a shank link configured to couple to a user's shank; a knee joint positioned between said thigh link and shank link and configured to allow flexion and extension between said shank link and said thigh link; a torque generator at least one leg sensor capable of producing a leg signal representing the absolute angle of said thigh link with respect to a line selected from the group consisting of a vertical gravitational line and a line substantially parallel with a user's torso; a controller in communication said sensor and with said torque generator, wherein said controller is configured to move said exoskeleton to a resistive state wherein in said resistive state, said torque generator provides selected resistive torques in response to flexion between said thigh link and said shank link; an exoskeleton trunk configurable to be coupled to said user's upper body, said exoskeleton trunk comprising: a torso link capable of being coupled to said user's torso; a trunk thigh link configurable to be rotatably coupled to said torso link; an actuator capable of providing torque between said torso link and said trunk thigh link; and an actuator controller characterized in that the torque generator is configured to create a controllable resistive torque between said thigh link and said shank link along flexion direction by use of friction force between first and second friction surfaces. a thigh link configured to couple to a user's thigh; a shank link configured to couple to a user's shank; a knee joint positioned between said thigh link and shank link and configured to allow flexion and extension between said shank link and said thigh link; a torque generator at least one leg sensor capable of producing a leg signal representing the absolute angle of said thigh link with respect to a line selected from the group consisting of a vertical gravitational line and a line substantially parallel with a user's torso; a controller in communication said sensor and with said torque generator, wherein said controller is configured to move said exoskeleton to a resistive state wherein in said resistive state, said torque generator provides selected resistive torques in response to flexion between said thigh link and said shank link; an exoskeleton trunk configurable to be coupled to said user's upper body, said exoskeleton trunk comprising: a torso link capable of being coupled to said user's torso; a trunk thigh link configurable to be rotatably coupled to said torso link; an actuator capable of providing torque between said torso link and said trunk thigh link; and an actuator controller characterized in that the torque generator is configured to create a controllable resistive torque between said thigh link and said shank link along flexion direction by use of friction force between first and second friction surfaces. a torso link capable of being coupled to said user's torso; a trunk thigh link configurable to be rotatably coupled to said torso link; an actuator capable of providing torque between said torso link and said trunk thigh link; and an actuator controller characterized in that the torque generator is configured to create a controllable resistive torque between said thigh link and said shank link along flexion direction by use of friction force between first and second friction surfaces.