An exoskeleton leg configured to be coupled to a leg of a person, said exoskeleton leg comprising: a thigh link configured to move in unison with the person's thigh; a shank link coupled to said thigh link and configured to move in unison with the person's shank; a force generator comprising a first end and a second end capable of providing a force between said first end and said second end, wherein the first end is coupled to the shank link; at least one leg sensor (310,405) configured to produce at least one leg signal; a constraining mechanism comprising a constrained operational mode and an unconstrained operational mode; and a signal processor configured to move the constraining mechanism between unconstrained and constrained operational modes based on said at least one leg signal, characterised in that, when the constraining mechanism is in the constrained operational mode, the constraining mechanism is configured to constrain the second end of the force generator to a rotational motion relative to the thigh link and is configured to provide support to a wearer when a knee of the wearer is flexing, where said force generator provides a force in response to flexion of said shank link relative to said thigh link and, wherein, when the constraining mechanism is in the unconstrained operational mode, the constraining mechanism allows the second end of the force generator to be main unconstrained along the line between the first end and the second end allowing unconstrained flexion and extension motion of said thigh link sand shank link relative to each other and providing no support to the wearer when the knee of the wearer is flexing. a thigh link configured to move in unison with the person's thigh; a shank link coupled to said thigh link and configured to move in unison with the person's shank; a force generator comprising a first end and a second end capable of providing a force between said first end and said second end, wherein the first end is coupled to the shank link; at least one leg sensor (310,405) configured to produce at least one leg signal; a constraining mechanism comprising a constrained operational mode and an unconstrained operational mode; and a signal processor configured to move the constraining mechanism between unconstrained and constrained operational modes based on said at least one leg signal, characterised in that, when the constraining mechanism is in the constrained operational mode, the constraining mechanism is configured to constrain the second end of the force generator to a rotational motion relative to the thigh link and is configured to provide support to a wearer when a knee of the wearer is flexing, where said force generator provides a force in response to flexion of said shank link relative to said thigh link and, wherein, when the constraining mechanism is in the unconstrained operational mode, the constraining mechanism allows the second end of the force generator to be main unconstrained along the line between the first end and the second end allowing unconstrained flexion and extension motion of said thigh link sand shank link relative to each other and providing no support to the wearer when the knee of the wearer is flexing.