Cybernetics of Balance Control

Fighting against gravity is a common challenge for all terrestrial animals, including most mammals. It means, in particular, avoiding falls to the ground while performing daily tasks, such as standing up, locomotion, or foraging for food. This means that balance control in humans involves a wide variety of contexts and balance paradigms, such as upright standing, hand standing, tightrope walking, ice-skater spinning, bicycling, whole-body gesturing, and fingertip stick balancing, among others. From the cybernetic point of view, the underlying control problem is to keep the CoP (Center of Pressure) and the CoM (Center of Mass) aligned dynamically on the common vertical axis, and this means that the variety of balance strategies can be reduced to two basic paradigms: the CoP strategy (the CoP is the control variable and the CoM is the controlled variable) and the CoM strategy (the CoM is simultaneously the control and the controlled variable). It is suggested that the two balance strategies are implemented by combining four basic control paradigms, as a function of the task and environmental conditions: • Opportunistic control: exploitation of a physical phenomenon as the gyroscopic effect. • Stiffness control: exploitation of the elastic properties of skeletal muscles. • Feedback control: measuring an incipient fall index and closing the loop in real time. In particular, it is shown that a phase-space-based formulation of intermittent feedback control can compensate for the destabilization effect of conventional continuous control due to the large feedback delay. • Feedforward control: exploitation of an internal body model to generate stable whole-body synergies in an anticipatory manner. Such control paradigms are illustrated by summarizing the results of experimental and simulated data.