Figure 1.

Model diagram with example endpoint ellipse (A). Three-link model (heavy black lines) with generalized coordinate system (θ⃑ = [θ₁, θ₂, θ₃]^T). The limb axis, v⃑_LA, is directed from the endpoint to the proximal joint. The endpoint stiffness (k_{R_END}) and viscosity (k_{B_END}) ellipses are characterized by the angle ϕ between the limb axis and direction of maximum stiffness, α⃑_MAX. The direction of minimum stiffness is α⃑_MIN. Model responses (B) (top: JU/JS, middle: ES, bottom: EU) are shown for a perturbation applied in the x direction (ψ = 0°). The time of maximum endpoint (dashed gray line) and maximum total joint (solid gray line) displacements are indicated.