page_title: Neural control of limb stiffness
section_title: Neuroprosthetics and exoskeletons
hierarchical_section_title: Neural control of limb stiffness / Applications in robotics / Neuroprosthetics and exoskeletons
caption_attribution_description: Robot leg, with Air Muscles. Designed and built by David Buckley for The Shadow Robot company Ltd.
context_page_description: As humans move through their environment, they must change the stiffness of their joints in order to effectively interact with their surroundings. Stiffness is the degree to a which an object resists deformation when subjected to a known force. This idea is also referred to as impedance, however, sometimes the idea of deformation under a given load is discussed under the term "compliance" which is the opposite of stiffness.
  In order to effectively interact with their environment, humans must adjust the stiffness of their limbs. This is accomplished via the co-contraction of antagonistic muscle groups.
  Humans use neural control along with the mechanical constraints of the body to adjust this stiffness as the body performs various tasks. It has been shown that humans change the stiffness of their limbs as they perform tasks such as hopping, performing accurate reaching tasks, or running on different surfaces.
  While the exact method by which this neural-modulation of limb stiffness occurs is unknown, many different hypotheses have been proposed.
context_section_description: Knowledge of human stiffness variation and stiffness selection has influenced robotic designs as researchers attempt to design robots that act more like biological systems. In order for robots to act more like biological systems, work is being done to attempt to implement stiffness modulation in robots so that they may interact more effectively with their environment.
  	State of the art neuroprosthetics have attempted to implements stiffness control in their robotic devices. The goal of these devices is to replace the limbs of amputees and allow the new limbs to adjust their stiffness in order to effectively interact with the environment.
  	Additionally, robotic exoskeletons have attempted to implement similar adjustable stiffness in their devices. These robots implement stiffness control for multiple reasons. The robots must be able to interact efficiently with the external environment but they must also be able to interact safely with their human user. Stiffness modulation and impedance control can be leveraged to accomplish both of these goals.
  	These devices achieve variable stiffness in various ways. Some devices use controllers and rigid servomotors to simulate variable stiffness. Other devices utilize specific flexible actuators to achieve a various levels of limb stiffness.
caption: A robotic leg powered by air muscle actuators
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