Neural control of limb stiffness

As humans move through their environment, they must change the stiffness of their joints in order to effectively interact with their surroundings.

[5] While the exact method by which this neural-modulation of limb stiffness occurs is unknown, many different hypotheses have been proposed.

A thorough understanding of how and why the brain controls limb stiffness could lead to improvements in many robotic technologies that attempt to mimic human movement.

The human body is able to modulate its limb stiffnesses through various mechanisms with the goal of more effectively interacting with its environment.

The body varies the stiffness of its limbs by three primary mechanisms: muscle cocontraction,[1][8][9] posture selection,[6] and through stretch reflexes.

One study showed that adults had more feedforward neural control, muscle reflexes, and higher relative leg stiffness than their juvenile counterparts when performing a hopping task.

[3] The nervous system also controls limb stiffness to modulate the degree of accuracy that is required for a given task.

For example, the accuracy required to grab a cup off of a table is very different from that of a surgeon performing a precise task with a scalpel.

To accomplish these tasks with varying degrees of required accuracy, the nervous system adjusts limb stiffness.

Because of this, the central nervous system increases limb stiffness to allow the user to accurately maneuver the tool and perform a task.

Studies postulate the humans do this in order to stabilize unstable dynamics of the environment and also to maximize the energy efficiency of a given movement.

[1] Impedance control has served as the basis for much of the work done in the area of determining how humans interact with their environment.

State of the art neuroprosthetics have attempted to implements stiffness control in their robotic devices.

[17] Additionally, robotic exoskeletons have attempted to implement similar adjustable stiffness in their devices.

Other devices utilize specific flexible actuators to achieve a various levels of limb stiffness.

By utilizing these variable stiffness actuation technologies, new robots have been able to more accurately replicate the motions of biological organisms and mimic their energetic efficiencies.