Fig. 1.

Forward simulations with ramp endpoint displacements were performed in 16 directions spaced evenly in the horizontal plane. The endpoint ramp perturbations had a peak acceleration (a_max) of 3.2 m s⁻², peak velocity (v_max) of 18 cm s⁻¹ and displacement (p_max) of 1.4 cm at 150 ms. Endpoint force responses are evaluated at t=0.03, 0.07 and 0.12 s. Three idealized models represent potential endpoint force responses to the perturbations. In the uniform model (U) the change in endpoint force is opposite the imposed endpoint displacement and the magnitude of the response is equal in all directions. In the ellipsoid model (E) there is an axis of maximum response and orthogonal to it an axis of minimum response. The direction of the response is equal to the perturbation direction when the limb is perturbed along these axes. In the force constraint model (F) the response direction is constant for all perturbation directions and the magnitude varies as the dot product between perturbation direction and response direction.