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Trumbower - 2009 - Use of self-selected postures to regulate multi-joint stiffness during unconstrained tasks


Trumbower RD, Krutky MA, Yang BS, Perreault EJ. Use of self-selected postures to regulate multi-joint stiffness during unconstrained tasks. PLoS One. 2009;4(5):e5411. PUBMED - FULL TEXT

10 Word Summary

Postural configuration can be exploited to modulate arm stiffness.


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.


  • Subjects were asked to point to targets in the presence of a destabilizing negative stiffness.
  • Subjects were asked to maintain a constant endpoint force.  Sudden displacements were applied to measure stiffness.
  • Both tracking accuracy and endpoint stiffness were effected by arm configuration.
  • Self-selected configurations had the least amount of tracking error.
    • Self-selected configurations changed with disturbance
  • Maximal stiffness aligned closely with direction of instability, but just a little off.
  • Rotating the fore-arm did not affect the stiffness estimates.