Cybernetics of Balance Control

Fighting against gravity is a common challenge for all terrestrial animals, including most mammals. It means, in particular, avoiding falls on 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 stick balancing on a finger,tip 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, 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). The two balance strategies are implemented by combining different control paradigms: • Opportunistic control: exploitation of a physical phenomenon as the gyroscopic effect. • Stiffness control: exploiting the elastic properties of skeletal muscles. • Feedback control (continuous or intermittent): measuring an incipient fall index and closing the loop in real-time. • Feedforward control: exploiting an internal body model for generating stable whole-body synergies in an anticipatory manner. Such control paradigms are illustrated with experimental and simulated experiments.