Abstract: A supernumerary robotic limb (SRL) system can include a plurality of rigid links, a joint that connects one rigid link to another rigid link in the plurality of rigid arms, and two variable stiffness actuators (VSAs) configured to drive the plurality of rigid links in order to complete at least one task. The VSAs can exhibit infinite rotation and infinite stiffness. The VSAs can include an output link. Additionally, the VSAs can include a set of elastic elements mounted on the output link. The VSAs can include an input link configured to provide kinetic energy for the output link. The VSAs can include a dynamic chassis configured to connect with the input link. Additionally, the VSAs can include a stiffness adjustor included in the dynamic chassis and configured to adjust an elastic transmission between at least one elastic element of the set of elastic elements and the output link.