Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, PC74
Manual and isometric pedal control of an inverted pendulum: intentional control in both cases?
C. Spiga1, D. Marple-Horvat1, M. Lakie2, I. Loram1
1. IRM, Manchester Metropolitan University, Manchester, Cheshire, United Kingdom. 2. School of Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.
Previously, visuo-manual control of an inverted pendulum has been studied as an analogy of quiet standing (1,2). The high feedback variability and feedback time delays of the order of 200 ms, were consistent with intentional rather than automatic control, where intentional control indicates flexibility over the direction as well as the magnitude of response to a stimulus. Manual control is well adapted for fine, voluntary, visually guided movement with direct cortical innervation, a short motor transmission nerve pathway, low innervation ratios, and many degrees of freedom using multiple muscles. Conversely, in pedal control, tibialis anterior is normally passive, leaving only soleus and gastrocnemius. These muscles have more indirect innervations via the spinal chord, a high innervation ratio, a longer transmission pathway, slower muscles, and more peripheral control via proprioceptive and vestibular reflexes. This experiment compares isometric pedal with manual control. We ask: how effective is the pedal v. manual control? What are the feedback time delays? Is isometric pedal control intentional? Participants controlled a virtual 1st order and an unstable 2nd order (inverted pendulum) load using either a unidirectional contactless manual joystick or alternatively isometric modulation of left ankle moment onto a fixed footplate. In both cases identical visual information from a dot in a screen represented load movement. Six trained participants were instructed to minimize deviation of the spot from the centred visual target, while an unpredictable motor disturbance, containing power from 0.1 to 10 Hz, was applied to the load. As previously feedback time delays were calculated using cross correlation between the disturbance and the control signal (1). Values quoted are mean ± s.d.. Load movement is greater during pedal control. Manual control feedback time delays for 1st order are 136±19 ms, and for the 2nd 211±11 ms (stable) and 206±22 ms (unstable). Correspondingly, for the pedal tasks, they are 178±23, 238±12, 246±12 ms, respectively. Manual control is more effective than pedal control in rejecting the disturbance and reducing load movement. Longer nerve transmission at most accounts for 10 of the 40 ms difference between pedal and manual control. The remaining 30 ms is accounted for by speed muscle contraction and possible longer central processing. Eye-hand reactions may be inherently faster. As shown previously for manual control (1), the time delay for isometric pedal control supports a conclusion of intentional rather than automatic control.
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