![]() Following perturbations, dynamically stable systems return towards an unchanged equilibrium trajectory. Consequently, although static stability is useful to explain some aspects of morphology and behavior during slow, precise tasks, a consideration of dynamic stability is critical to understanding rapid locomotion ( Full and Koditschek,1999).Īlthough many types of periodic, dynamically stable motions are possible( Guckenheimer, 1982), one simple way of defining dynamic stability is the maintenance of an equilibrium trajectory over time: a defined pattern of positions and velocities that repeats with a characteristic frequency such as the stride frequency( Full et al., 2002). Animals or robots with fewer legs or aerial phases or both must rely on dynamic stability because the conditions necessary for static stability seldom apply ( Raibert et al.,1984). Ting et al.( 1994) showed that, while remaining statically stable over a wide range of speeds, at their highest speeds cockroaches are not statically stable and must rely on dynamic stability by using the momentum of the body to bridge periods of static instability. The stepping patterns used by hexapods at slow and moderate speeds, for example, ensure that the center of mass falls within the polygon of support provided by their legs over the course of the entire stride( Alexander, 1982 Cruse and Schwarze, 1988 Delcomyn, 1985 Jander, 1985). Many-legged animals with sprawled postures can be highly statically stable. ![]() The rapid onset of recovery from lateral perturbations supports the possibility that, during fast locomotion, intrinsic properties of the musculoskeletal system augment neural stabilization by reflexes. Instead, they exhibited viscoelastic behavior in the lateral direction, with spring constants similar to those observed during unperturbed locomotion. Cockroaches did not require step transitions to recover from lateral perturbations. Lateral velocity began to decrease 13±5 ms (mean ± S.D., N=11) following the start of a perturbation, a time comparable with the fastest reflexes measured in cockroaches. Cockroaches were able to recover from these perturbations in 27☑2 ms(mean ± S.D., N=9) when running on a high-friction substratum. The apparatus was mounted onto the thorax of the insect, oriented to propel the projectile laterally and loaded with propellant sufficient to cause a nearly tenfold increase in lateral velocity relative to maxima observed during unperturbed locomotion. The apparatus used chemical propellants to accelerate a small projectile, generating reaction force impulses of less than 10 ms duration. We studied the stabilizing mechanism employed by rapidly running insects by using a novel apparatus to perturb running cockroaches ( Blaberus discoidalis). To stabilize locomotion, animals must generate forces appropriate to overcome the effects of perturbations and to maintain a desired speed or direction of movement.
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