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Author Notes:

E-mail: e-perreault@northwestern.edu

Conceived and designed the experiments: EJP.

Performed the experiments: RDT MAK BSY.

Analyzed the data: RDT MAK BSY.

Wrote the paper: RDT MAK.

The authors would like to thank Bob Kirsch, who provided constructive suggestions during the early stages of this study. They also would like to thank Timothy Haswell for his technical assistance and Eileen Krepkovich for her assistance with manuscript preparation.

The authors have declared that no competing interests exist.

The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.


Research Funding:

This work was supported by NIH grants K25 HD044720 and NS053813.


  • Science & Technology
  • Multidisciplinary Sciences
  • Stiffness
  • Arms
  • Hands
  • Postural control
  • Robots
  • Limbs (anatomy)
  • Sternum
  • Mechanical properties

Use of Self-Selected Postures to Regulate Multi-Joint Stiffness During Unconstrained Tasks


Journal Title:



Volume 4, Number 5


, Pages e5411-e5411

Type of Work:

Article | Final Publisher PDF


Background: The human motor system is highly redundant, having more kinematic degrees of freedom than necessary to complete a given task. Understanding how kinematic redundancies are utilized in different tasks remains a fundamental question in motor control. One possibility is that they can be used to tune the mechanical properties of a limb to the specific requirements of a task. For example, many tasks such as tool usage compromise arm stability along specific directions. These tasks only can be completed if the nervous system adapts the mechanical properties of the arm such that the arm, coupled to the tool, remains stable. The purpose of this study was to determine if posture selection is a critical component of endpoint stiffness regulation during unconstrained tasks. Methodology/Principal Findings: Three-dimensional (3D) estimates of endpoint stiffness were used to quantify limb mechanics. Most previous studies examining endpoint stiffness adaptation were completed in 2D using constrained postures to maintain a non-redundant mapping between joint angles and hand location. Our hypothesis was that during unconstrained conditions, subjects would select arm postures that matched endpoint stiffness to the functional requirements of the task. The hypothesis was tested during endpoint tracking tasks in which subjects interacted with unstable haptic environments, simulated using a 3D robotic manipulator. We found that arm posture had a significant effect on endpoint tracking accuracy and that subjects selected postures that improved tracking performance. For environments in which arm posture had a large effect on tracking accuracy, the self-selected postures oriented the direction of maximal endpoint stiffness towards the direction of the unstable haptic environment. Conclusions/Significance: These results demonstrate how changes in arm posture can have a dramatic effect on task performance and suggest that postural selection is a fundamental mechanism by which kinematic redundancies can be exploited to regulate arm stiffness in unconstrained tasks.

Copyright information:

© 2009 Trumbower et al.

This is an Open Access work distributed under the terms of the Creative Commons Attribution 3.0 Unported License (http://creativecommons.org/licenses/by/3.0/).

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