Legged animals routinely negotiate rough, unpredictable terrain with agility and stability that outmatches any human-built machine. Yet, we know surprisingly little about how animals accomplish this. Current knowledge is largely limited to studies of steady movement. These studies have revealed fundamental mechanisms used by terrestrial animals for steady locomotion. However, it is unclear whether these models provide an appropriate framework for the neuromuscular and mechanical strategies used to achieve dynamic stability over rough terrain. Perturbation experiments shed light on this issue, revealing the interplay between mechanics and neuromuscular control. We measured limb mechanics of helmeted guinea fowl (Numida meleagris) running over an unexpected drop in terrain, comparing their response to predictions of the mass-spring running model. Adjustment of limb contact angle explains 80% of the variation in stance-phase limb loading following the perturbation. Surprisingly, although limb stiffness varies dramatically, it does not influence the response. This result agrees with a mass-spring model, although it differs from previous findings on humans running over surfaces of varying compliance. However, guinea fowl sometimes deviate from mass-spring dynamics through posture-dependent work performance of the limb, leading to substantial energy absorption following the perturbation. This posture-dependent actuation allows the animal to absorb energy and maintain desired velocity on a sudden substrate drop. Thus, posture-dependent work performance of the limb provides inherent velocity control over rough terrain. These findings highlight how simple mechanical models extend to unsteady conditions, providing fundamental insights into neuromuscular control of movement and the design of dynamically stable legged robots and prosthetic devices. biomechanics ͉ locomotion ͉ motor control ͉ mass-spring model A ll legged terrestrial animals use similar basic mechanisms for steady locomotion (1-7). In the stance phase of bouncing gaits, such as hopping and running, kinetic energy (KE) and gravitational potential energy of the body cycle in phase, decreasing during the first half of stance and increasing during the second half (1). Elastic recoil allows this energy to be stored and returned by the elastic structures of the limb (8-11). Consequently, a simple mass-spring model is often used to describe the stance phase dynamics of these gaits. The model consists of a point mass and a linear compression spring (2-4). Despite the simplicity of this model, it appears that humans and animals maintain mass-spring dynamics over a broad range of locomotor conditions by adjusting model parameters: limb contact angle ( o ), effective limb length (L o ), and leg stiffness (k leg ) (4, 5, 12, 13). Nonetheless, the mass-spring model is a conservative system, meaning that the total mechanical energy of the body (E com ) does not change over a stride. If energy must be absorbed or produced to change E com (in acceleration or deceleration, for example), the animal must deviate from the simplest spring-like mechanics. Thus, although this model describes well the body mechanics of steady running, it is not yet clear whether it applies to animals running in more complex conditions, including rough or unpredictable terrain. We address this issue by using a simple perturbation experiment to investigate limb dynamics following a sudden, unexpected drop in terrain height (⌬H) (Fig. 1A) . We test the hypothesis that helmeted guinea fowl (Numida meleagris) use conservative mass-spring limb mechanics to maintain running stability following this perturbation. The ⌬H perturbation results in a 26-ms delay in limb loading relative to that anticipated by the bird (14). In response, the animal can (i) rapidly adjust limb position and stiffness to prevent changes in mechanical energy,
REF identified that stabilization of the first step into the pothole was led by feed-forward control, and dynamics of the mass-spring system.
Running over rough terrain reveals limb control for intrinsic stability
Proceedings of the National Academy of Sciences of the United States of America
Proceedings of the National Academy of Sciences of the United States of America
Computer Science Medicine