Figure 2. Neural control organization.

A, the control is organized in spinal and supraspinal layers. The spinal layer consists of 10 reflex modules, M₁ to M₁₀, for each leg which are active in stance or swing. The supraspinal layer adjusts the desired foot placements (α_tgt) and the desired minimum swing leg length (l_clr), and selects which leg should transition into swing control during double support. B, desired foot placement is calculated as target leg angles and for sagittal (s) and frontal (f) plane motions based on the velocity v^{s, f} of the COM and its distance to the stance leg ankle, d^{s, f}. C, in double support, swing control is initiated for the leg whose angle α is farther from the target.