Figure 1
Main concept: dynamic behaviour of balancing by delayed feedback. (a) Inverted pendulum model for human standing balance. (b) Stability diagram for delayed PD feedback \(Q(t)=P \theta (t-\tau ) + D \dot{\theta }(t-\tau )\). Light and dark grey shading indicates oscillatory (spiral type) and nonoscillatory (node type) stable responses, respectively. The two types of responses are separated by the node-spiral separation line indicated by (black-green) dashed line. Contour lines \(\gamma _1={\rm const}\) are associated with different settling time of the response. Black \(\times \) marker shows the parameter point \((p^*,d^*)\) associated with fastest settling time. Green 
marker shows the parameter point \((\hat{p}, \hat{d})\) on the node-spiral separation line that is closest to the experimentally fitted parameter point \((\bar{p}, \bar{d})\). (c–e) Responses for different control gains (p, d). Fastest response is shown in panel (e).
