Fig. 8. Postural influences on intrinsic limb dynamics following a perturbation. Intrinsic mechanical changes can simplify control and stabilization of running dynamics, because these mechanisms rely on the natural dynamics of the body and limb interacting with the environment, without the need for altered muscle activation through central or reflex neural pathways. (A) Following an unexpected change in substrate properties during hopping in place, humans exhibit an intrinsic decrease in limb stiffness (k_leg) due to increased flexion of the knee in response to rapid limb loading (Moritz and Farley, 2004). (B) Upon encountering an unexpected drop in terrain height, guinea fowl exhibit an intrinsic increase in limb contact angle (_o) due to the normal backward motion of the limb during late swing phase just prior to ground contact (to match foot and ground speed). If the limb acts as a simple compression spring, limb loading (and ground reaction force) depends on the angle between the limb and the bird's velocity (_o). The backward motion of the limb in the drop step results in altered geometry as shown, which is associated with an intrinsic decrease in the ground reaction force impulse over stance (the time integral of force, F_g), due to decreased peak force and shorter contact duration. These dynamics are consistent with the spring-mass model (Daley and Biewener, 2006).