Abstract
Balancing the body in upright standing and balancing a stick on the fingertip are two examples of unstable tasks that, in spite of strong motor and sensory differences, appear to share a similar motor control paradigm, namely a state-space intermittent feedback stabilization mechanism. In this study subjects were required to perform the two tasks simultaneously, with the purpose of highlighting both the coordination between the two skills and the underlying interaction between the corresponding controllers. The experimental results reveal, in particular, that upright standing (the less critical task) is modified in an adaptive way, in order to facilitate the more critical task (stick balancing), but keeping the overall spatio-temporal signature well known in regular upright standing. We were then faced with the following question: to which extent the physical/biomechanical interaction between the two independent intermittent controllers is capable to explain the dual task coordination patterns, without the need to introduce an additional, supervisory layer/module? By comparing the experimental data with the output of a simulation study we support the former hypothesis, suggesting that it is made possible by the intrinsic robustness of both state-space intermittent feedback stabilization mechanisms.
Highlights
Balancing the body in upright standing and balancing a stick on the fingertip are two examples of unstable tasks that, in spite of strong motor and sensory differences, appear to share a similar motor control paradigm, namely a state-space intermittent feedback stabilization mechanism
In other balancing paradigms the stiffness mechanism is physically ineffective: for example, in upright standing on a very narrow support basis, like a tight-rope, the activation of ankle muscles will not produce any torque for compensating the medio-lateral oscillations of the body; in the manual stabilization of a stick on the fingertip no stabilizing torque can be produced on the virtual stick-finger joint
The strategy of the central nervous system (CNS) would be to learn the optimal impedance for compensating the destabilizing effect of gravity, by selecting the appropriate groups of antagonist muscles and optimally tuning co-activation levels in a preprogrammed manner[39]
Summary
Balancing the body in upright standing and balancing a stick on the fingertip are two examples of unstable tasks that, in spite of strong motor and sensory differences, appear to share a similar motor control paradigm, namely a state-space intermittent feedback stabilization mechanism. The main challenge, for the family of balancing tasks we are considering, is the strong delay of the feedback information about the ongoing sway, of the order of 0.2 s, and the fact that this delay is comparable to the potential falling time constant of the oscillating body Such feedback is noisy and of very small amplitude, since the involved sensory channels operate near the perceptual thresholds. The event-driven solution implies a threshold that can operate in two different manners: 1) it is applied to an error signal, implementing a simple switch-like controller in which corrective movements are made only when the vertical displacement angle exceeds a certain threshold21; 2) it operates in the state space, taking advantage of the affordance provided by the saddle-type instability that characterizes the dynamics of an inverted pendulum (see Fig. 1)
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