Abstract
It has been considered that the brain stabilizes unstable body dynamics by regulating co-activation levels of antagonist muscles. Here we critically reexamined this established theory of impedance control in a postural balancing task using a novel EMG-based human-computer interface, in which subjects were asked to balance a virtual inverted pendulum using visual feedback information on the pendulum's position. The pendulum was actuated by a pair of antagonist joint torques determined in real-time by activations of the corresponding pair of antagonist ankle muscles of subjects standing upright. This motor-task raises a frustrated environment; a large feedback time delay in the sensorimotor loop, as a source of instability, might favor adopting the non-reactive, preprogrammed impedance control, but the ankle muscles are relatively hard to co-activate, which hinders subjects from adopting the impedance control. This study aimed at discovering how experimental subjects resolved this frustrated environment through motor learning. One third of subjects adapted to the balancing task in a way of the impedance-like control. It was remarkable, however, that the majority of subjects did not adopt the impedance control. Instead, they acquired a smart and energetically efficient strategy, in which two muscles were inactivated simultaneously at a sequence of optimal timings, leading to intermittent appearance of periods of time during which the pendulum was not actively actuated. Characterizations of muscle inactivations and the pendulum¡Çs sway showed that the strategy adopted by those subjects was a type of intermittent control that utilizes a stable manifold of saddle-type unstable upright equilibrium that appeared in the state space of the pendulum when the active actuation was turned off.
Highlights
Human motor control requires the generation of a sequence of motor commands for muscles that actuate a mechanical body plant with the correct magnitude at the right time to achieve the goal of a motor task
A reaction time to visually supplied falling motions of the virtual inverted pendulum in this study corresponds to the feedback time delay, and in the later section, we show that it was much larger than 0.2 s, implying that stabilization of the upright equilibrium by the use of continuous feedback control is not easy
If the smoothed integrated electromyograms (iEMGs) was above the slow trend in a time interval, we considered that the muscle activity was high in that interval
Summary
Human motor control requires the generation of a sequence of motor commands for muscles that actuate a mechanical body plant with the correct magnitude at the right time to achieve the goal of a motor task. If a target moves regularly, the brain predicts a subsequent motion, and compensates delay-affected feedback information to better perform the task using an internal inverse and/or forward models [5,6,7,8,9,10]. If this is not the case, for example during inherently unstable body dynamics [11,12] or where there is little knowledge of task-related dynamics [13], control might be compelled to rely on the delayed feedback, leading to an increased risk of delayinduced instability. Stabilization of unstable body dynamics is another important issue to be addressed by the brain
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