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First published online on September 14, 2006.
Copyright © 2006 by The Physiological Society
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Received August 7, 2006
Revised September 11, 2006
Accepted after revision September 12, 2006

The frequency of human, manual adjustments in balancing an inverted pendulum is constrained by intrinsic physiological factors

Ian David Loram1*, Peter Gawthrop2, and Martin Lakie3

1 Manchester Metropolitan University
2 University of Glasgow
3 University of Birmingham

* To whom correspondence should be addressed. E-mail: i.loram{at}mmu.ac.uk.

While standing naturally and when manually or pedally balancing an equivalent inverted pendulum, the load sways slowly (characteristic unidirectional duration ~1s) and the controller, calf muscles or hand, makes more frequent adjustments (characteristic unidirectional duration 400ms). Here we test the hypothesis that these durations reflect load properties rather than some intrinsic property of the human neuromuscular system. Using a specialised setup mechanically analogous to real standing, subjects manually balanced inverted pendulums with different moments of inertia through a compliant spring representing the Achilles tendon. The spring bias was controlled by a sensitive joystick via a servo motor and accurate visual feedback was provided on an oscilloscope. As moment of inertia decreased inverted pendulum sway size increased and it became difficult to sustain successful balance. The mean duration of unidirectional balance adjustments did not change. Moreover, the mean duration of unidirectional inverted pendulum sway reduced only slightly remaining around 1 s. The simplest explanation is that balance was maintained by a process of manual adjustments intrinsically limited to a mean frequency of 2 to 3 unidirectional adjustments per second corresponding to intermittent control observed in manual tracking experiments. Consequently the inverted pendulum sway duration, mechanically related to the bias duration, reflects an intrinsic constraint of the neuromuscular control system. Given the similar durations of sway and muscle adjustments observed in real standing, we postulate that the characteristic duration of unidirectional standing sway reflects intrinsic intermittent control rather than the inertial properties of the body.

Key words: Motor control • Postural control • Sensorimotor control

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Copyright © 2006 The Physiological Society.