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This Article Full Text Full Text (PDF) All Versions of this Article: 586/5/1265    most recent jphysiol.2007.146910v1 Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Google Scholar Articles by Chew, J. Z. Z. Articles by Fitzpatrick, R. C. PubMed PubMed Citation Articles by Chew, J. Z. Z. Articles by Fitzpatrick, R. C. Related Collections Neuroscience

¹ Prince of Wales Medical Research Institute and University of New South Wales, Sydney, NSW 2031 Australia

In our movements and posture, we always act against a physical load. A key property of any load is its elastic stiffness (K), which describes how the force required to hold it must change with position. Here we examine how load stiffness affects the ability to maintain a stable posture at the wrist. Loads having positive (like a spring) and negative stiffness (like an inverted pendulum) were created by varying the position of weights on multiarm rigid pendulum. Subjects (n = 9) held 15 loads (K = ± 0.04, ± 0.01 and 0 N m deg^–1 at mean torques of 0.2, 0.4 and 0.6 N m) still for 60 s. Residual wrist movement (sway) increased with mean torque and increased as stiffness became more negative. Large effects of load stiffness were seen at low frequencies (< 1.5 Hz) but not at higher frequencies that reflect load resonance and reflex activity. Subjects accurately perceived their postural sway while holding the loads but measured psychophysical thresholds showed that load stiffness was not perceived. We conclude that load stiffness, independent of force levels, affects the ability to control a load and that the postural control process relies on perception and volitional tracking rather than more automatic reflex pathways. Despite an awareness of their postural errors, we see no evidence for adaptation of postural control processes to compensate for changes in load properties. This is unlike the adaptation of feedforward control processes that produce targeted volitional movements when load properties are altered. We propose that postural control and movement control are fundamentally different neural processes.

(Received 17 October 2007; accepted after revision 9 January 2008; first published online 10 January 2008)
Corresponding author R. C. Fitzpatrick: Prince of Wales Medical Research Institute, Easy Street, Randwick, NSW 2031, Australia. Email: r.fitzpatrick{at}unsw.edu.au

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