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Fig. 7. Body CoM dynamics following perturbations in relation to spring-mass model dynamics. (A) Computer simulation of the spring-mass trajectory following a change from a soft to hard surface, with no change in leg stiffness (k_leg), resulting in an asymmetrical CoM path and a steep trajectory during take-off at the end of support. (B) Actual CoM vertical displacement of a human runner encountering an abrupt but expected change from a soft to hard substrate. The runner maintains similar (symmetrical) CoM motion by anticipating the change and adjusting k_leg within the first step (Ferris et al., 1999). (C) Computer simulation of the spring-mass trajectory following a sudden, unexpected drop in substrate height (40% of leg length). In the perturbed step, the only model value that differs from the level step is the limb contact angle (_o), which is steeper due to the backward motion of the limb before it contacts the ground (see also Fig. 8B). The angle used in the simulation was that measured experimentally from guinea fowl. (D) Actual CoM trajectory and energy changes of running guinea fowl following an unexpected drop in substrate height equal to 40% of leg length (PE_g, solid blue line; KE_v and KE_h, solid green and broken purple lines; E_com, total center of mass energy, solid black line). The broken vertical gray line indicates when the foot contacted the tissue paper `false floor', and the gray box indicates the duration of stance. The bar graph (right) shows the net energy changes during the step (between the start and end of the traces on the left). Two response modes were observed: in most cases the body dynamics of the bird match the conservative spring-mass model (as shown in C), converting lost PE_g to forward KE (D, top graphs). In some cases, however, the limb muscles absorb net energy, decreasing the total body mechanical energy (E_com; D, bottom graphs). The different response modes are associated with different limb postures when the foot contacts the ground (Daley and Biewener, 2006; Daley et al., 2006).

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