An exoskeleton leg apparatus configured to be coupled to a lower extremity of a person, the exoskeleton leg apparatus comprising: a first link; a second link; a knee joint, coupled to the first link and the second link and configured to allow flexion and extension motion between the first link and the second link; a force generator, comprising a first end and a second end, wherein the first end of the force generator is rotatably coupled to the first link; a constraining mechanism, coupled to the second link and comprising an indentation in the second link and an indentation filler capable of moving relative to the second link, and a triggering mechanism; the constraining mechanism having at least a first operational position and a second operational position and being arranged to provide assistance during squatting of the person when in said first operational position, and to allow walking of the person when in said second operational position, wherein the triggering mechanism further includes an actuator, operable to move the constraining mechanism between the first operational position and the second operational position, said actuator being connected or coupled to the constraining mechanism such that the actuator is capable of moving the indentation filler in and out of the indentation; wherein when the first link flexes relative to the second link in a first direction getting close to said second link and the actuator moves the indentation filler of the constraining mechanism in the first operational position out of the indentation, the second end of the force generator engages the indentation of the constraining mechanism, thus causing the force generator to create a force resisting motion in said first direction and providing assistance during squatting; when the first link flexes relative to the second link in said first direction and the actuator moves the indentation filler of the constraining mechanism into the indentation in the second operational position, the second end of the force generator does not engage the constraining mechanism, and the first link and the second link are free to flex and extend relative to each other, thus allowing walking of the person; wherein the triggering mechanism further includes a stance sensor that is capable of generating a stance signal when a leg of person is in the stance phase. a first link; a second link; a knee joint, coupled to the first link and the second link and configured to allow flexion and extension motion between the first link and the second link; a force generator, comprising a first end and a second end, wherein the first end of the force generator is rotatably coupled to the first link; a constraining mechanism, coupled to the second link and comprising an indentation in the second link and an indentation filler capable of moving relative to the second link, and a triggering mechanism; the constraining mechanism having at least a first operational position and a second operational position and being arranged to provide assistance during squatting of the person when in said first operational position, and to allow walking of the person when in said second operational position, wherein the triggering mechanism further includes an actuator, operable to move the constraining mechanism between the first operational position and the second operational position, said actuator being connected or coupled to the constraining mechanism such that the actuator is capable of moving the indentation filler in and out of the indentation; wherein when the first link flexes relative to the second link in a first direction getting close to said second link and the actuator moves the indentation filler of the constraining mechanism in the first operational position out of the indentation, the second end of the force generator engages the indentation of the constraining mechanism, thus causing the force generator to create a force resisting motion in said first direction and providing assistance during squatting; when the first link flexes relative to the second link in said first direction and the actuator moves the indentation filler of the constraining mechanism into the indentation in the second operational position, the second end of the force generator does not engage the constraining mechanism, and the first link and the second link are free to flex and extend relative to each other, thus allowing walking of the person; wherein the triggering mechanism further includes a stance sensor that is capable of generating a stance signal when a leg of person is in the stance phase.