Running over rough terrain reveals limb control for intrinsic stability

Abstract

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, fo...

Structured variability of steady-speed locomotion in rats

“…Indeed, our statistical analyses revealed few parameters that discriminate between footfall orders. Those parameters include relative limb length as well as relative limb protraction and retraction -parameters that play a predominant role in self-stability during perturbed locomotion in bipeds (Geyer et al, 2005;Daley and Biewener, 2006;Seyfarth et al, 2002). We therefore suggest that these parameters play an overall important role during terrestrial locomotion.…”

“…During perturbed locomotion, animals instead rely on passive dynamic mechanisms that include spring-mass mechanics and intrinsic mechanisms (see also Biewener and Daley, 2007). In humans and birds, simple spring-mass mechanics mitigate sudden changes in terrain height (Daley and Biewener, 2006;Geyer et al, 2005;Grimmer et al, 2008;Seyfarth et al, 2002). Limbs act as springs and help the system to return to the locomotor trajectory between steps.…”

Leg muscles that mediate stability: mechanics and control of two distal extensor muscles during obstacle negotiation in the guinea fowl

“…The kinematic differences may have resulted from the intrinsic mechanical interaction of the swing leg with the obstacle, resulting in an early transition from swing to stance. Drop perturbations result in similar but inverse dynamics-delayed ground contact leads to a more extended leg posture and shorter stance duration [21].…”

“…Animals were trained to run on a level motorized treadmill (Woodway, Waukesha, WI, USA) at speeds of 1.7 -2.0 m s 21 . Training sessions were 20-30 min in duration, with breaks for 1-3 min as needed.…”

Climbing, falling, and jamming during ant locomotion in confined environments

“…Such simplifications have allowed discovery of general principles in locomotor modes of walking, running, and climbing (6)(7)(8)(9). Recent studies have generated appreciation for the importance of mechanical interactions with the environment, and through biological experiment (10) and robot modeling (11,12) have demonstrated that stable and robust movement can emerge as a result of appropriately tuned dynamics of limb-ground interaction (13,14). For example, rapid perturbations to locomotion may be corrected by so-called "preflexes" (15) in which mechanical design of the limb and appropriate kinematics enable rapid recovery from perturbation (6,8,10).…”