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SUMMARY Animals can expend energy to acquire sensory information by emitting signals and/or moving sensory structures. We propose that the energy from locomotion itself could permit control of a sensor, whereby animals use the energy from movement to reconfigure a passive sensor. We investigated high-speed, antenna-mediated tactile navigation in the cockroach Periplaneta americana. We discovered that the passive antennal flagellum can assume two principal mechanical states, such that the tip is either projecting backward or forward. Using a combination of behavioral and robotic experiments, we demonstrate that a switch in the antenna's state is mediated via the passive interactions between the sensor and its environment, and this switch strongly influences wall-tracking control. When the tip of the antenna is projected backward, the animals maintain greater body-to-wall distance with fewer body collisions and less leg–wall contact than when the tip is projecting forward. We hypothesized that distally pointing mechanosensory hairs at the tip of the antenna mediate the switch in state by interlocking with asperities in the wall surface. To test this hypothesis, we performed laser ablation of chemo-mechanosensory hairs and added artificial hairs to a robotic antenna. In both the natural and artificial systems, the presence of hairs categorically increased an antenna's probability of switching state. Antennal hairs, once thought to only play a role in sensing, are sufficient for mechanically reconfiguring the state of the entire antenna when coupled with forward motion. We show that the synergy between antennal mechanics, locomotion and the environment simplifies tactile sensing.

Synopsis Animals that fall upside down typically engage in an aerial righting response so as to reorient dorsoventrally. This behavior can be preparatory to gliding or other controlled aerial behaviors and is ultimately necessary for a successful landing. Aerial righting reflexes have been described historically in various mammals such as cats, guinea pigs, rabbits, rats, and primates. The mechanisms whereby such righting can be accomplished depend on the size of the animal and on anatomical features associated with motion of the limbs and body. Here we apply a comparative approach to the study of aerial righting to explore the diverse strategies used for reorientation in midair. We discuss data for two species of lizards, the gecko Hemidactylus platyurus and the anole Anolis carolinensis, as well as for the first instar of the stick insect Extatosoma tiaratum, to illustrate size-dependence of this phenomenon and its relevance to subsequent aerial performance in parachuting and gliding animals. Geckos can use rotation of their large tails to reorient their bodies via conservation of angular momentum. Lizards with tails well exceeding snout-vent length, and correspondingly large tail inertia to body inertia ratios, are more effective at creating midair reorientation maneuvers. Moreover, experiments with stick insects, weighing an order of magnitude less than the lizards, suggest that aerodynamic torques acting on the limbs and body may play a dominant role in the righting process for small invertebrates. Both inertial and aerodynamic effects, therefore, can play a role in the control of aerial righting. We propose that aerial righting reflexes are widespread among arboreal vertebrates and arthropods and that they represent an important initial adaptation in the evolution of controlled aerial behavior.

The simplest prey-selection model predicts that predators select prey to maximize energetic benefit and minimize handling time. We tested whether or not energetic indices predict stomatopod preference for prey. The energetic indices included: 1) the ratio of energetic return (mass of prey tissue, M) per unit handling time (feeding duration, T); 2) the ratio of energetic benefit (caloric value of prey tissue, E) to energetic cost (caloric value of energy expended, C); and 3) the ratio of net energetic benefit (caloric value of food minus the caloric value of energy expended, NE) to handling time. Stomatopods, characterized by highly specialized raptorial feeding appendages, strike and smash the shells of prey such as snails. The prey-capture strike is one of the fastest known animal movements. Stomatopods, presented with snail shells of three size classes (5–6, 8–9, 11–12 mm in length), preferred intermediate and small snails. Oxygen consumption increased an average 2.5-fold above resting rates during feeding and increased linearly with strike frequency. The energetic cost per strike was 1.6 ± 0.2 μl O2. Stomatopods obtained more snail tissue from smashing larger snail shells. However, longer handling times were required to open shells of greater size. To obtain snail tissue, stomatopods struck larger shells a greater number of times. Strike frequency (0.72 strikes/min) was independent of shell size. The amount of snail tissue obtained per strike (0.044 mg/strike), M/T, NE/T, and E/C were all independent of snail size. The mean energetic benefit per strike (0.23 calories/strike) was 13-fold greater than the energetic cost per strike (0.017 calories/strike). Energetic cost, energetic benefit and handling time appear not to be the sole variables that explain stomatopod prey selection.

Spiders use hemolymph pressure to extend their legs. This mechanism should be challenged when required to rapidly generate forces during jumping, particularly in large spiders. However, effective use of leg muscles could facilitate rapid jumping. To quantify the contributions of different legs and leg joints, we investigated jumping kinematics by high-speed video recording. We observed two different types of jumps following a disturbance: prepared and unprepared jumps. In unprepared jumps, the animals could jump in any direction away from the disturbance. The remarkable directional flexibility was achieved by flexing the legs on the leading side and extending them on the trailing side. This behaviour is only possible for approximately radial-symmetric leg postures, where each leg can fulfil similar functions. In prepared jumps, the spiders showed characteristic leg positioning and the jumps were directed anteriorly. Immediately after a preliminary countermovement in which the centre of mass was moved backwards and downwards, the jump was executed by extending first the fourth and then the second leg pair. This sequence provided effective acceleration to the centre of mass. At least in the fourth legs, the hydraulic and the muscular mechanism seem to interact to generate ground reaction forces.

SUMMARYA musculo-skeletal structure can stabilize rapid locomotion using neural and/or mechanical feedback. Neural feedback results in an altered feedforward activation pattern, whereas mechanical feedback using visco-elastic structures does not require a change in the neural motor code. We selected musculo-skeletal structures in the cockroach (Blaberus discoidalis)because their single motor neuron innervation allows the simplest possible characterization of activation. We ran cockroaches over a track with randomized blocks of heights up to three times the animal's `hip' (1.5 cm),while recording muscle action potentials (MAPs) from a set of putative control musculo-skeletal structures (femoral extensors 178 and 179). Animals experienced significant perturbations in body pitch, roll and yaw, but reduced speed by less than 20%. Surprisingly, we discovered no significant difference in the distribution of the number of MAPs, the interspike interval, burst phase or interburst period between flat and rough terrain trials. During a few very large perturbations or when a single leg failed to make contact throughout stance, neural feedback was detectable as a phase shift of the central rhythm and alteration of MAP number. System level responses of appendages were consistent with a dominant role of mechanical feedback. Duty factors and gait phases did not change for cockroaches running on flat versus rough terrain. Cockroaches did not use a follow-the-leader gait requiring compensatory corrections on a step-by-step basis. Arthropods appear to simplify control on rough terrain by rapid running that uses kinetic energy to bridge gaps between footholds and distributed mechanical feedback to stabilize the body.

Jointed exoskeletons permit rapid appendage-driven locomotion but retain the soft-bodied, shape-changing ability to explore confined environments. We challenged cockroaches with horizontal crevices smaller than a quarter of their standing body height. Cockroaches rapidly traversed crevices in 300-800 ms by compressing their body 40-60%. High-speed videography revealed crevice negotiation to be a complex, discontinuous maneuver. After traversing horizontal crevices to enter a vertically confined space, cockroaches crawled at velocities approaching 60 cm⋅s(-1), despite body compression and postural changes. Running velocity, stride length, and stride period only decreased at the smallest crevice height (4 mm), whereas slipping and the probability of zigzag paths increased. To explain confined-space running performance limits, we altered ceiling and ground friction. Increased ceiling friction decreased velocity by decreasing stride length and increasing slipping. Increased ground friction resulted in velocity and stride length attaining a maximum at intermediate friction levels. These data support a model of an unexplored mode of locomotion--"body-friction legged crawling" with body drag, friction-dominated leg thrust, but no media flow as in air, water, or sand. To define the limits of body compression in confined spaces, we conducted dynamic compressive cycle tests on living animals. Exoskeletal strength allowed cockroaches to withstand forces 300 times body weight when traversing the smallest crevices and up to nearly 900 times body weight without injury. Cockroach exoskeletons provided biological inspiration for the manufacture of an origami-style, soft, legged robot that can locomote rapidly in both open and confined spaces.

Fibrillar, or “hairy,” adhesives have evolved multiple times independently within arthropods and reptiles. These adhesives exhibit highly desirable properties for dynamic attachment, including orientation dependence, wear resistance, and self-cleaning. Our understanding of how these properties are related to their fibrillar structure is limited, although theoretical models from the literature have generated useful hypotheses. We survey the morphology of 81 species with fibrillar adhesives to test the hypothesis that packing density of contact elements should increase with body size, whereas the size of the contact elements should decrease. We test this hypothesis in a phylogenetic context to avoid treating historically related species as statistically independent data points. We find that fiber morphology is better predicted by evolutionary history and adhesive mechanism than by body size. As we attempt to identify which morphological parameters are most responsible for the performance of fibrillar adhesives, it will be important to take advantage of the natural variation in morphology and the potentially suboptimal outcomes it encompasses, rather than assuming evolution to be an inherently optimizing process.

SUMMARYCockroaches recover rapidly from perturbations during high-speed running that allows them to cross unstructured terrains with no change in gait. Characterization of the exoskeletal material properties of the legs suggests that passive mechanical feedback could contribute to the self-stabilizing behavior. We imposed large, dorsal-ventrally directed impulsive perturbations to isolated hind legs having both a fixed and free body–coxa joint and measured their recovery. We tested a frequency-independent hysteretic damping model that effectively predicted the behavior of sinusoidal oscillations of isolated legs. Leg position reached its peak amplitude within 4–6 ms following an impulse. Position was 99% recovered within 16±3.3 ms for the stiffest possible leg configuration and within 46±6.6 ms for the most compliant leg configuration. The rapid recovery supports the hypothesis that passive musculo-skeletal properties play an important role in simplifying the control of high-speed locomotion.

Insects run with far greater speed and agility for their size than even the most advanced legged robots produced to date. The single-leg ground reaction forces of running insects such as the cockroach B. discoidalis provide valuable insight into the biomechanical basis for this rapid robust locomotion. To better study the running kinematics and biomechanics of these insects, a multiaxis silicon micromachined force sensor has been fabricated. The sensor consists of a 5.3-mm square plate that is supported at its corners by thin springlike beam elements. Each flexure beam is instrumented with two piezoresistive strain gauges, allowing the determination of both normal and in- plane bending force components. Typical unamplifled normal and in-plane flexure force sensitivities of 55 and 12 V/N, respectively, have been demonstrated for a sensor with 18-mum-thick flexures and a mechanical bandwidth of 1.3 kHz. Nominal normal force resolution is 2.2 nN/Hz1/2 at 1 kHz. This paper details the design, fabrication, calibration, performance, and analytical modeling of the first-generation micromachined ground reaction force sensor. Preliminary data obtained from running cockroaches show that this sensor represents a marked improvement in performance over the techniques previously available for studying small-animal biomechanics.

Terrestrial arthropods negotiate demanding terrain more effectively than any search-and-rescue robot. Slow, precise stepping using distributed neural feedback is one strategy for dealing with challenging terrain. Alternatively, arthropods could simplify control on demanding surfaces by rapid running that uses kinetic energy to bridge gaps between footholds. We demonstrate that this is achieved using distributed mechanical feedback, resulting from passive contacts along legs positioned by pre-programmed trajectories favorable to their attachment mechanisms. We used wire-mesh experimental surfaces to determine how a decrease in foothold probability affects speed and stability. Spiders and insects attained high running speeds on simulated terrain with 90% of the surface contact area removed. Cockroaches maintained high speeds even with their tarsi ablated, by generating horizontally oriented leg trajectories. Spiders with more vertically directed leg placement used leg spines, which resulted in more effective distributed contact by interlocking with asperities during leg extension, but collapsing during flexion, preventing entanglement. Ghost crabs, which naturally lack leg spines, showed increased mobility on wire mesh after the addition of artificial, collapsible spines. A bioinspired robot, RHex, was redesigned to maximize effective distributed leg contact, by changing leg orientation and adding directional spines. These changes improved RHex's agility on challenging surfaces without adding sensors or changing the control system.

© 2015 IOP Publishing Ltd. Natural substrates like sand, soil, leaf litter and snow vary widely in penetration resistance. To search for principles of appendage design in robots and animals that permit high performance on such flowable ground, we developed a ground control technique by which the penetration resistance of a dry granular substrate could be widely and rapidly varied. The approach was embodied in a device consisting of an air fluidized bed trackway in which a gentle upward flow of air through the granular material resulted in a decreased penetration resistance. As the volumetric air flow, Q, increased to the fluidization transition, the penetration resistance decreased to zero. Using a bio-inspired hexapedal robot as a physical model, we systematically studied how locomotor performance (average forward speed, vx) varied with ground penetration resistance and robot leg frequency. Average robot speed decreased with increasing Q, and decreased more rapidly for increasing leg frequency, ?. A universal scaling model revealed that the leg penetration ratio (foot pressure relative to penetration force per unit area per depth and leg length) determined vxfor all ground penetration resistances and robot leg frequencies. To extend our result to include continuous variation of locomotor foot pressure, we used a resistive force theory based terradynamic approach to perform numerical simulations. The terradynamic model successfully predicted locomotor performance for low resistance granular states. Despite variation in morphology and gait, the performance of running lizards, geckos and crabs on flowable ground was also influenced by the leg penetration ratio. In summary, appendage designs which reduce foot pressure can passively maintain minimal leg penetration ratio as the ground weakens, and consequently permits maintenance of effective locomotion over a range of terradynamically challenging surfaces.

This article provides a summary of the ideas discussed at the 2015 Annual Meeting of the Society for Integrative and Comparative Biology society-wide symposium on Leading Students and Faculty to Quantitative Biology through Active Learning. It also includes a brief review of the recent advancements in incorporating active learning approaches into quantitative biology classrooms. We begin with an overview of recent literature that shows that active learning can improve students' outcomes in Science, Technology, Engineering and Math Education disciplines. We then discuss how this approach can be particularly useful when teaching topics in quantitative biology. Next, we describe some of the recent initiatives to develop hands-on activities in quantitative biology at both the graduate and the undergraduate levels. Throughout the article we provide resources for educators who wish to integrate active learning and technology into their classrooms.

Many animals, modern aircraft, and underwater vehicles use fusiform, streamlined body shapes that reduce fluid dynamic drag to achieve fast and effective locomotion in air and water. Similarly, numerous small terrestrial animals move through cluttered terrain where three-dimensional, multi-component obstacles like grass, shrubs, vines, and leaf litter also resist motion, but it is unknown whether their body shape plays a major role in traversal. Few ground vehicles or terrestrial robots have used body shape to more effectively traverse environments such as cluttered terrain. Here, we challenged forest-floor-dwelling discoid cockroaches (Blaberus discoidalis) possessing a thin, rounded body to traverse tall, narrowly spaced, vertical, grass-like compliant beams. Animals displayed high traversal performance (79 ± 12% probability and 3.4 ± 0.7 s time). Although we observed diverse obstacle traversal strategies, cockroaches primarily (48 ± 9% probability) used a novel roll maneuver, a form of natural parkour, allowing them to rapidly traverse obstacle gaps narrower than half their body width (2.0 ± 0.5 s traversal time). Reduction of body roundness by addition of artificial shells nearly inhibited roll maneuvers and decreased traversal performance. Inspired by this discovery, we added a thin, rounded exoskeletal shell to a legged robot with a nearly cuboidal body, common to many existing terrestrial robots. Without adding sensory feedback or changing the open-loop control, the rounded shell enabled the robot to traverse beam obstacles with gaps narrower than shell width via body roll. Such terradynamically 'streamlined' shapes can reduce terrain resistance and enhance traversability by assisting effective body reorientation via distributed mechanical feedback. Our findings highlight the need to consider body shape to improve robot mobility in real-world terrain often filled with clutter, and to develop better locomotor-ground contact models to understand interaction with 3D, multi-component terrain.

SUMMARYThe mechanical behavior of muscle during locomotion is often predicted by its anatomy, kinematics, activation pattern and contractile properties. The neuromuscular design of the cockroach leg provides a model system to examine these assumptions, because a single motor neuron innervates two extensor muscles operating at a single joint. Comparisons of the in situmeasurements under in vivo running conditions of muscle 178 to a previously examined muscle (179) demonstrate that the same inputs (e.g. neural signal and kinematics) can result in different mechanical outputs. The same neural signal and kinematics, as determined during running, can result in different mechanical functions, even when the two anatomically similar muscles possess the same contraction kinetics, force-velocity properties and tetanic force-length properties. Although active shortening greatly depressed force under in vivo-like strain and stimulation conditions, force depression was similarly proportional to strain, similarly inversely proportional to stimulation level, and similarly independent of initial length and shortening velocity between the two muscles. Lastly, passive pre-stretch enhanced force similarly between the two muscles. The forces generated by the two muscles when stimulated with their in vivo pattern at lengths equal to or shorter than rest length differed, however. Overall, differences between the two muscles in their submaximal force-length relationships can account for up to 75% of the difference between the two muscles in peak force generated at short lengths observed during oscillatory contractions. Despite the fact that these muscles act at the same joint, are stimulated by the same motor neuron with an identical pattern, and possess many of the same in vitro mechanical properties, the mechanical outputs of two leg extensor muscles can be vastly different.

SUMMARYLegs of sprawled-posture, quadrupedal trotting geckos (Hemidactylus garnotii) each functioned differently during constant average-speed locomotion. The center of mass decelerated in the first half of a step and accelerated in the second half, as if geckos were bouncing in fore–aft and side-to-side directions. Forelegs decelerated the center of mass only in the fore–aft direction. Hindlegs provided all the acceleration in the latter half of the step. Lateral ground reaction forces were always directed toward the midline and exceeded the magnitude of fore–aft forces. The differential leg function of sprawled-posture geckos resembled sprawled-posture hexapods more than upright-posture quadrupeds. The pattern of leg ground reaction forces observed may provide passive, dynamic stability while minimizing joint moments, yet allow high maneuverability. Integrating limb dynamics with whole body dynamics is required to resolve the trade-offs,if any, that result from stable sprawled-posture running with differential leg function.

Cheetahs and beetles run, dolphins and salmon swim, and bees and birds fly with grace and economy surpassing our technology. Evolution has shaped the breathtaking abilities of animals, leaving us the challenge of reconstructing their targets of control and mechanisms of dexterity. In this review we explore a corner of this fascinating world. We describe mathematical models for legged animal locomotion, focusing on rapidly running insects and highlighting past achievements and challenges that remain. Newtonian body--limb dynamics are most naturally formulated as piecewise-holonomic rigid body mechanical systems, whose constraints change as legs touch down or lift off. Central pattern generators and proprioceptive sensing require models of spiking neurons and simplified phase oscillator descriptions of ensembles of them. A full neuromechanical model of a running animal requires integration of these elements, along with proprioceptive feedback and models of goal-oriented sensing, planning, and learning. We outline relevant background material from biomechanics and neurobiology, explain key properties of the hybrid dynamical systems that underlie legged locomotion models, and provide numerous examples of such models, from the simplest, completely soluble "peg-leg walker" to complex neuromuscular subsystems that are yet to be assembled into models of behaving animals. This final integration in a tractable and illuminating model is an outstanding challenge.

SYNOPSIS. Many animals that locomote by legs possess adhesive pads. Such organs are rapidly releasable and adhesive forces can be controlled during walking and running. This capacity results from the interaction of adhesive with complex mechanical systems. Here we present an integrative study of the mechanics and adhesion of smooth attachment pads (arolia) in Asian Weaver ants (Oecophylla smaragdina). Arolia can be unfolded and folded back with each step. They are extended either actively by contraction of the claw flexor muscle or passively when legs are pulled toward the body. Regulation of arolium use and surface attachment includes purely mechanical control inherent in the arrangement of the claw flexor system. Predictions derived from a ‘wet’ adhesion mechanism were tested by measuring attachment forces on a smooth surface using a centrifuge technique. Consistent with the behavior of a viscid secretion, frictional forces per unit contact area linearly increased with sliding velocity and the increment strongly decreased with temperature. We studied the nature and dimensions of the adhesive liquid film using Interference Reflection Microscopy (IRM). Analysis of ‘footprint’ droplets showed that they are hydrophobic and form low contact angles. In vivo IRM of insect pads in contact with glass, however, revealed that the adhesive liquid film not only consists of a hydrophobic fluid, but also of a volatile, hydrophilic phase. IRM allows estimation of the height of the liquid film and its viscosity. Preliminary data indicate that the adhesive secretion alone is insufficient to explain the observed friction and that rubbery deformation of the pad cuticle is involved.

SUMMARYTo stabilize locomotion, animals must generate forces appropriate to overcome the effects of perturbations and to maintain a desired speed or direction of movement. We studied the stabilizing mechanism employed by rapidly running insects by using a novel apparatus to perturb running cockroaches (Blaberus discoidalis). The apparatus used chemical propellants to accelerate a small projectile, generating reaction force impulses of less than 10 ms duration. 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. Cockroaches were able to recover from these perturbations in 27±12 ms(mean ± S.D., N=9) when running on a high-friction substratum. 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 did not require step transitions to recover from lateral perturbations. Instead, they exhibited viscoelastic behavior in the lateral direction, with spring constants similar to those observed during unperturbed locomotion. 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.

Geckos have evolved one of the most versatile and effective adhesives known. The mechanism of dry adhesion in the millions of setae on the toes of geckos has been the focus of scientific study for over a century. We provide the first direct experimental evidence for dry adhesion of gecko setae by van der Waals forces, and reject the use of mechanisms relying on high surface polarity, including capillary adhesion. The toes of live Tokay geckos were highly hydrophobic, and adhered equally well to strongly hydrophobic and strongly hydrophilic, polarizable surfaces. Adhesion of a single isolated gecko seta was equally effective on the hydrophobic and hydrophilic surfaces of a microelectro-mechanical systems force sensor. A van der Waals mechanism implies that the remarkable adhesive properties of gecko setae are merely a result of the size and shape of the tips, and are not strongly affected by surface chemistry. Theory predicts greater adhesive forces simply from subdividing setae to increase surface density, and suggests a possible design principle underlying the repeated, convergent evolution of dry adhesive microstructures in gecko, anoles, skinks, and insects. Estimates using a standard adhesion model and our measured forces come remarkably close to predicting the tip size of Tokay gecko seta. We verified the dependence on size and not surface type by using physical models of setal tips nanofabricated from two different materials. Both artificial setal tips stuck as predicted and provide a path to manufacturing the first dry, adhesive microstructures.

The integration of information from dynamic sensory structures operating on a moving body is a challenge for locomoting animals and engineers seeking to design agile robots. As a tactile sensor is a physical linkage mediating mechanical interactions between body and environment, mechanical tuning of the sensor is critical for effective control. We determined the open-loop dynamics of a tactile sensor, specifically the antenna of the American cockroach, Periplaneta americana, an animal that escapes predators by using its antennae during rapid closed-loop tactilely mediated course control. Geometrical measurements and static bending experiments revealed an exponentially decreasing flexural stiffness (EI) from base to tip. Quasi-static experiments with a physical model support the hypothesis that a proximodistally decreasing EI can simplify control by increasing preview distance and allowing effective mapping to a putative control variable - body-to-wall distance - compared to an antenna with constant EI. We measured the free response at the tip of the antenna following step deflections and determined that the antenna rapidly damps large deflections: over 90% of the perturbation is rejected within the first cycle, corresponding to almost one stride period during high-speed running (~50 ms). An impulse-like perturbation near the tip revealed dynamics that were characteristic of an inelastic collision, keeping the antenna in contact with an object after impact. We contend that proximodistally decreasing stiffness, high damping, and inelasticity simplify control during high-speed tactile tasks by increasing preview distance, providing a one-dimensional map between antennal bending and body-to-wall distance, and increasing the reliability of tactile information.

RHex is an untethered, compliant leg hexapod robot that travels at better than one body length per second over terrain few other robots can negotiate at all. Inspired by biomechanics insights into arthropod locomotion, RHex uses a clock excited alternating tripod gait to walk and run in a highly maneuverable and robust manner. We present empirical data establishing that RHex exhibits a dynamical (“bouncing”) gait—its mass center moves in a manner well approximated by trajectories from a Spring Loaded Inverted Pendulum (SLIP)—characteristic of a large and diverse group of running animals, when its central clock, body mass, and leg stiffnesses are appropriately tuned. The SLIP template can function as a useful control guide in developing more complex autonomous locomotion behaviors such as registration via visual servoing, local exploration via visual odometry, obstacle avoidance, and, eventually, global mapping and localization.

Nocturnal geckos can actively forage at low temperatures. A low minimum cost of locomotion allows greater sustainable speeds by partially offsetting the decrease in maximal oxygen consumption (VO2max) associated with low nocturnal temperatures. The nocturnality hypothesis (Autumn et al. 1997) proposes that the reduced cost of continuous locomotion is a shared, derived characteristic that increases the capacity to sustain locomotion at low temperatures. Yet many lizards move intermittently at speeds exceeding those that elicit VO2max. We exercised the frog-eyed gecko, Teratoscincus przewalskii, continuously and intermittently on a treadmill. At an exercise speed of 0.90 km h-1 (270% maximum aerobic speed), lizards alternating a 15-s exercise period with a 30-s pause period exhibited a 1.7-fold increase in distance capacity (total distance traveled before fatigue) compared with lizards exercised continuously at the same average speed (0.30 km h-1). The average aerobic cost of intermittent exercise was not significantly different from VO2max. Locomoting intermittently could augment the increase in endurance resulting from the low minimum cost of continuous locomotion in nocturnal geckos. Intermittent behavior could increase the endurance of lizard movement in general.

To explore the role of the mechanical system in control, we designed a two dimensional, feed–forward, dynamic model of a hexapedal runner (death–head cockroach,Blaberus discoidalis). We chose to model many–legged, sprawled posture animals because of their remarkable stability. Since sprawled posture animals operate more in the horizontal plane than animals with upright postures, we decoupled the vertical and horizontal plane and only modelled the horizontal plane. The model was feed–forward with no equivalent of neural feedback among any of the components. The model was stable and its forward, lateral and rotational velocities were similar to that measured in the animal at its preferred velocity. It also self–stabilized to velocity perturbations. The rate of recovery depended on the type of perturbation. Recovery from rotational velocity perturbations occurred within one step, whereas recovery from lateral perturbations took multiple strides. Recovery from fore–aft velocity perturbations was the slowest. Perturbations were dynamically coupled—alterations in one velocity component necessarily perturbed the others. Perturbations altered the translation and/or rotation of the body which consequently provided ‘mechanical feedback’ by altering leg moment arms. Self–stabilization by the mechanical system can assist in making the neural contribution of control

This study tested the hypothesis that there has been an evolutionary increase in locomotor performance capacity at low temperature in nocturnal lizards. Nocturnal lizards are often active at low and suboptimal body temperatures. An evolutionary decrease in the minimum cost of locomotion could increase endurance capacity at low temperature, partially offsetting the thermal handicap of nocturnality. In support of the nocturnality hypothesis, we discovered that minimum cost of locomotion of a nocturnal gecko, Coleonyx variegatus (4.2 g), was only 58% of the minimum cost of locomotion of Phrynosoma douglassii, a diurnal lizard (4.5 g). As a result, maximum aerobic speed was 2.3 times as great in the nocturnal lizard compared to the diurnal lizard. By using the method of phylogenetically independent contrasts at the species level, we showed that the relationship between mass and minimum cost of locomotion in diurnal lizards was similar to that of the ahistorical standard allometry and that low minimum cost of locomotion in geckos represents a significant evolutionary change from the ancestral diurnal pattern. The decrease in the minimum cost of locomotion concordant with the evolution of nocturnality suggests that geckos evolved a greater capacity for sustained locomotion at low temperature.

In this work, we propose a control scheme for attitude control of a falling, two link active tailed robot with only two degrees of freedom of actuation. We derive a simplified expression for the robot's angular momentum and invert this expression to solve for the shape velocities that drive the body's angular momentum to a desired value. By choosing a body angular velocity vector parallel to the axis of error rotation, the controller steers the robot towards its desired orientation. The proposed scheme is accomplished through feedback laws as opposed to feedforward trajectory generation, is fairly robust to model uncertainties, and is simple enough to implement on a miniature microcontroller. We verify our approach by implementing the controller on a small (175 g) robot platform, enabling rapid maneuvers approaching the spectacular capability of animals.

Summary The role in adhesion of the toes and lamellae - intermediate sized structures - found on the gecko foot remains unclear. Insight into the function of these structures can lead to a more general understanding of the hierarchical nature of the gecko adhesive system, but in particular how environmental topology may relate to gecko foot morphology. We sought to discern the mechanics of the toes and lamellae by examining gecko adhesion on controlled, macroscopically rough surfaces. We used live Tokay geckos, Gekko gecko, to observe the maximum shear force a gecko foot can attain on an engineered substrate constructed with sinusoidal patterns of varying amplitudes and wavelengths in sizes similar to the dimensions of the toes and lamellae structures (0.5 to 6 mm). We found shear adhesion was significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. We discovered that the toes are capable of adhering to surfaces with amplitudes much larger than their dimensions even without engaging claws, maintaining 60% of shear adhesion on surfaces with amplitudes of 3 mm. Gecko adhesion can be predicted by the ratio of the lamella dimensions to surface feature dimensions. In addition to setae, remarkable macroscopic-scale features of gecko toes and lamellae that include compliance and passive conformation are necessary to maintain contact, and consequently, generate shear adhesion on macroscopically rough surfaces. Findings on the larger scale structures in the hierarchy of gecko foot function could provide the biological inspiration to drive the design of more effective and versatile synthetic fibrillar adhesives.

As a first step towards the integration of information on neural control, biomechanics and isolated muscle function, we constructed a three-dimensional musculo-skeletal model of the hind leg of the death-head cockroach Blaberus discoidalis. We tested the model by measuring the maximum force generated in vivo by the hind leg of the cockroach, the coxa–femur joint angle and the position of this leg during a behavior, wedging, that was likely to require maximum torque or moment production. The product of the maximum force of the leg and its moment arm yielded a measured coxa–femur joint moment for wedging behavior. The maximum musculo-apodeme moment predicted by summing all extensor muscle moments in the model was adequate to explain the magnitude of the coxa–femur joint moment produced in vivo by the cockroach and occurred at the same joint angle measured during wedging. Active isometric muscle forces predicted from our model varied by 3.5-fold among muscles and by as much as 70 % with joint angle. Sums of active and passive forces varied by less than 3.5 % over the entire range of possible joint angles (0–125 °). Maximum musculo-apodeme moment arms varied nearly twofold among muscles. Moment arm lengths decreased to zero and switched to the opposite side of the center of rotation at joint angles within the normal range of motion. At large joint angles (>100 °), extensors acted as flexors. The effective mechanical advantage (musculo-apodeme moment arm/leg moment arm = 0.10) resulted in the six femoral extensor muscles of the model developing a summed force (1.4 N) equal to over 50 times the body weight. The model’s three major force-producing extensor muscles attained 95 % of their maximum force, moment arm and moment at the joint angle used by the animal during wedging.

Jumping ability varies by two-fold within an instar during the moult cycle in the grasshopper, Schistocerca americana (Acrididae: Cyrtacanthacridinae). Changes in jump distance could result from deviations in jump angle away from the optimum during development, a change in jump energy and/or a change in body mass. Body mass has already been shown to vary by over two- fold within an instar (Queathem, 1991). In the present study, jump angle remained near the optimum of 43° during the time course of maximal jumps throughout the instar. Jump energy was correlated with ground reaction force production because energy lost to backward rotation and drag was small. Ground reaction force production varied by nearly four-fold over the period of the instar. Within instar six, force production and body mass accounted for 85% of the variation in jump distance. Their patterns of change relative to one another explain the four functional stages we define for within instar performance. Jump distance increased early within instar six (Stage I, days 0–2) because force production increased. In Stage II (days 3–8), jump distance remained at its peak because an increase in body mass was offset by an equal increase in force production. Jump distance decreased in Stage III (days 8–11) because body mass continued to increase while force production levelled off. Force production decreased to a greater extent than body mass during Stage IV (days 11–13), resulting in a further decline in jump distance during the three days preceding the moult to adulthood. Our results suggest that further examination of the musculo-skeletal system could provide a causal explanation for this change in jumping ability within an instar. The present study illustrates the remarkable physiological and mechanical changes that affect locomotion within a single instar, and highlights developmental differences between arthropods and vertebrates. Arthropod development is by its very nature a discontinuous process separated by periods of continuous, parabolic changes, and this pattern of growth is reflected in locomotor performance through ontogeny.

Escaping from predators often demands that animals rapidly negotiate complex environments. The smallest animals attain relatively fast speeds with high frequency leg cycling, wing flapping or body undulations, but absolute speeds are slow compared to larger animals. Instead, small animals benefit from the advantages of enhanced maneuverability in part due to scaling. Here, we report a novel behavior in small, legged runners that may facilitate their escape by disappearance from predators. We video recorded cockroaches and geckos rapidly running up an incline toward a ledge, digitized their motion and created a simple model to generalize the behavior. Both species ran rapidly at 12–15 body lengths-per-second toward the ledge without braking, dove off the ledge, attached their feet by claws like a grappling hook, and used a pendulum-like motion that can exceed one meter-per-second to swing around to an inverted position under the ledge, out of sight. We discovered geckos in Southeast Asia can execute this escape behavior in the field. Quantification of these acrobatic behaviors provides biological inspiration toward the design of small, highly mobile search-and-rescue robots that can assist us during natural and human-made disasters. We report the first steps toward this new capability in a small, hexapedal robot.

Stability is fundamental to the performance of terrestrial locomotion. Running cockroaches met the criteria for static stability over a wide range of speeds, yet several locomotor variables changed in a way that revealed an increase in the importance of dynamic stability as speed increased. Duty factors (the fraction of time that a leg spends on the ground relative to the stride period) decreased to 0.5 and below with an increase in speed. The duration of double support (i.e. when both tripods, or all six legs, were on the ground) decreased significantly with an increase in speed. All legs had similar touch-down phases in the tripod, but the shortest leg, the front one, lifted off before the middle and the rear leg, so that only two legs of the tripod were in contact with the ground at the highest speeds. Per cent stability margin (the shortest distance from the center of gravity to the boundaries of support, normalized to the maximum possible stability margin) decreased with increasing speed from 60% at 10 cm s−1 to values less than zero at speeds faster than 50 cm s−1, indicating instances of static instability at fast speeds. The center of mass moved rearward or posteriorly with respect to the base of support as speed increased. Moments about the center of mass, as shown by the center of pressure (the equivalent of a single ‘effective’ leg), were variable, but were balanced by opposing moments over a stride. Thus, hexapods can exploit the advantages of both static and dynamic stability. Static or quasi-static assumptions alone were insufficient to explain straight-ahead, constant-speed locomotion and may hinder discovery of behaviors that are dynamic, where kinetic energy and momentum can act as a bridge from one step to the next.

We tested a general model predicting the effect of body temperature $(T_{b})$ on the aerobic capacity, metabolic cost, and endurance of sustained, terrestrial locomotion. In the ghost crab, Ocypode quadrata, $T_{b}$ was a function of ambient temperature $(T_{a})$, relative humidity (RH), and the duration of acute exposure. At 15°C and 24°C, $T_{b}$ was similar to $T_{a}$. At high $T_{a}$ (30°C to 35°C) and low RH (40% to 50%), $T_{b}'s$ were 6°C below $T_{a}$. When the RH was 99%-100% at a $T_{a}$ of 30°C, $T_{b}$ stabilized at 29.7°C. The depression in $T_{b}$ resulted from evaporative water loss. The maximal rate of oxygen consumption $(\dot{V}O_{2max})$, determined during treadmill exercise, decreased by nearly 75% as $T_{b}$ was decreased from 24°C to 15°C. The minimum cost of locomotion ($C_{min}$, the slope of the steady state oxygen consumption vs. speed function) did not change at low $T_{b}$ (15°C). As $T_{b}$ was increased from 24°C to 30°C, $\dot{V}O_{2max}$ decreased to half of its original va...

The effect of dehydration on the aerobic metabolism and endurance of sustained, terrestrial locomotion was determined for the ghost crab, Ocypode quadrata. The rate of evaporative water loss, measured as the percentage of decrease in body mass per hour, was influenced by ambient temperature $(T_{a})$. Increasing $T_{a}$ from 24°C to 30°C (40%-50% relative humidity) increased the rate of water loss from 2.3% h⁻¹ ± 0.2% h⁻¹ to 3.6% h⁻¹ ± 0.6% h⁻¹. Crabs were divided into three treatment groups to determine the effect of dehydration on aerobic metabolism: hydrated control crabs, slowly dehydrated crabs, and rapidly dehydrated crabs. Hydrated control crabs lost only 1.2% of their initial body mass. Slowly dehydrated crabs were dehydrated by 3.6% of their initial body mass at a rate of 2.3% h⁻¹. Finally, rapidly dehydrated crabs were dehydrated by 3.6% of their initial body mass at a rate of 3.6% h⁻¹. The maximal rate of oxygen consumption $\dot{V}o_{max}$ determined during treadmill exercise was decreased by ...

Thermal optima for physiological processes are generally high (30°-40° C) in lizards. Performance decreases substantially at low temperatures, yet some lizards are nocturnal and are active with body temperatures below 15° C. We corroborated three hypotheses about the ecophysiological consequences of the evolution of nocturnality in lizards: (1) nocturnality requires activity at low temperature; (2) activity at low temperature imposes a thermal handicap that constrains performance capacity; (3) nocturnal species have higher performance capacity at low temperature than do comparable diurnal species. Field body temperatures during activity averaged 15.3°C in Teratoscincus przewalskii, a nocturnal, terrestrial gecko from northwestern China. Individuals of T. przewalskii sustained exercise at 15° C on a treadmill for more than 60 min at 0.18 km · h⁻¹. However, 15° C was suboptimal for sustained locomotion. Resting and maximum oxygen consumption at 15° and 25° C were similar to predicted values for diurnal liza...

In 1969, a palaeontologist proposed that theropod dinosaurs used their tails as dynamic stabilizers during rapid or irregular movements, contributing to their depiction as active and agile predators. Since then the inertia of swinging appendages has been implicated in stabilizing human walking, aiding acrobatic manoeuvres by primates and rodents, and enabling cats to balance on branches. Recent studies on geckos suggest that active tail stabilization occurs during climbing, righting and gliding. By contrast, studies on the effect of lizard tail loss show evidence of a decrease, an increase or no change in performance. Application of a control-theoretic framework could advance our general understanding of inertial appendage use in locomotion. Here we report that lizards control the swing of their tails in a measured manner to redirect angular momentum from their bodies to their tails, stabilizing body attitude in the sagittal plane. We video-recorded Red-Headed Agama lizards (Agama agama) leaping towards a vertical surface by first vaulting onto an obstacle with variable traction to induce a range of perturbations in body angular momentum. To examine a known controlled tail response, we built a lizard-sized robot with an active tail that used sensory feedback to stabilize pitch as it drove off a ramp. Our dynamics model revealed that a body swinging its tail experienced less rotation than a body with a rigid tail, a passively compliant tail or no tail. To compare a range of tails, we calculated tail effectiveness as the amount of tailless body rotation a tail could stabilize. A model Velociraptor mongoliensis supported the initial tail stabilization hypothesis, showing as it did a greater tail effectiveness than the Agama lizards. Leaping lizards show that inertial control of body attitude can advance our understanding of appendage evolution and provide biological inspiration for the next generation of manoeuvrable search-and-rescue robots.

Animals can swerve, dodge, dive, climb, turn and stop abruptly. Their stability and maneuverability are remarkable, but a challenge to quantify. Formal stability analysis can allow for quantitative comparisons within and among species. Stability analysis used in concert with a template (a simple, general model that serves as a guide for control) can lead to testable hypotheses of function. Neural control models postulated without knowledge of the animal's mechanical (musculo-skeletal) system can be counterproductive and even destabilizing. Perturbations actively corrected by reflex feedback in one direction can result in perturbations in other directions because the system is coupled dynamically. The passive rate of recovery from a perturbation in one direction differs from rates in other directions. We hypothesize that animals might exert less neural control in directions that rapidly recover via passive dynamics (e.g., in body orientation and rotation). By contrast, animals are likely to exert more neural control in directions that recover slowly or not at all via passive dynamics (e.g., forward velocity and heading). Neural control best enhances stability when it works with the natural, passive dynamics of the mechanical system. Measuring maneuverability is more challenging and new, general metrics are needed. Templates reveal that simple analyses of summed forces and quantification of the center of pressure can lead to valuable hypotheses, whereas kinematic descriptions may be inadequate. The study of stability and maneuverability has direct relevance to the behavior and ecology of animals, but is also critical if animal design is to be understood. Animals appear to be grossly over-built for steady-state, straight-ahead locomotion, as they appear to possess too many neurons, muscles, joints and even too many appendages. The next step in animal locomotion is to subject animals to perturbations and reveal the function of all their parts.

Neuromechanics seeks to understand how muscles, sense organs, motor pattern generators, and brain interact to produce coordinated movement, not only in complex terrain but also when confronted with unexpected perturbations. Applications of neuromechanics include ameliorating human health problems (including prosthesis design and restoration of movement following brain or spinal cord injury), as well as the design, actuation and control of mobile robots. In animals, coordinated movement emerges from the interplay among descending output from the central nervous system, sensory input from body and environment, muscle dynamics, and the emergent dynamics of the whole animal. The inevitable coupling between neural information processing and the emergent mechanical behavior of animals is a central theme of neuromechanics. Fundamentally, motor control involves a series of transformations of information, from brain and spinal cord to muscles to body, and back to brain. The control problem revolves around the specific transfer functions that describe each transformation. The transfer functions depend on the rules of organization and operation that determine the dynamic behavior of each subsystem (i.e., central processing, force generation, emergent dynamics, and sensory processing). In this review, we (1) consider the contributions of muscles, (2) sensory processing, and (3) central networks to motor control, (4) provide examples to illustrate the interplay among brain, muscles, sense organs and the environment in the control of movement, and (5) describe advances in both robotics and neuromechanics that have emerged from application of biological principles in robotic design. Taken together, these studies demonstrate that (1) intrinsic properties of muscle contribute to dynamic stability and control of movement, particularly immediately after perturbations; (2) proprioceptive feedback reinforces these intrinsic self-stabilizing properties of muscle; (3) control systems must contend with inevitable time delays that can simplify or complicate control; and (4) like most animals under a variety of circumstances, some robots use a trial and error process to tune central feedforward control to emergent body dynamics.

A renaissance in organismal biology has been sparked by recent conceptual, theoretical, methodological, and computational advances in the life sciences, along with an unprecedented interdisciplinary integration with Mathematics, Engineering, and the physical sciences. Despite a decades-long trend toward reductionist approaches to biological problems, it is increasingly recognized that whole organisms play a central role in organizing and interpreting information from across the biological spectrum. Organisms represent the nexus where sub- and supra-organismal processes meet, and it is the performance of organisms within the environment that provides the material for natural selection. Here, we identify five "grand challenges" for future research in organismal biology. It is intended that these challenges will spark further discussion in the broader community and identify future research priorities, opportunities, and directions, which will ultimately help to guide the allocation of support for and training in organismal biology.

Muscles are multi-functional structures that interface neural and mechanical systems. Muscle work depends on a large multi-dimensional space of stimulus (neural) and strain (mechanical) parameters. In our companion paper, we rewrote activation to individual muscles in intact, behaving cockroaches (Blaberus discoidalisL.), revealing a specific muscle's potential to control body dynamics in different behaviours. Here, we use those results to provide the biologically relevant parameters forin situwork measurements. We test four hypotheses about how muscle function changes to provide mechanisms for the observed control responses. Under isometric conditions, a graded increase in muscle stress underlies its linear actuation during standing behaviours. Despite typically absorbing energy, this muscle can recruit two separate periods of positive work when controlling running. This functional change arises from mechanical feedback filtering a linear increase in neural activation into nonlinear work output. Changing activation phase again led to positive work recruitment, but at different times, consistent with the muscle's ability to also produce a turn. Changes in muscle work required considering the natural sequence of strides and separating swing and stance contributions of work. Bothin vivocontrol potentials andin situwork loops were necessary to discover the neuromechanical coupling enabling control.

We investigated the effects of orientation angle on the adhesion of single gecko setae using dual-axis microelectromechanical systems force sensors to simultaneously detect normal and shear force components. Adhesion was highly sensitive to the pitch angle between the substrate and the seta's stalk. Maximum lateral adhesive force was observed with the stalk parallel to the substrate, and adhesion decreased smoothly with increasing pitch. The roll orientation angle only needed to be roughly correct with the spatular tuft of the seta oriented grossly towards the substrate for high adhesion. Also, detailed measurements were made to control for the effect of normal preload forces. Higher normal preload forces caused modest enhancement of the observed lateral adhesive force, provided that adequate contact was made between the seta and the substrate. These results should be useful in the design and manufacture of gecko-inspired synthetic adhesives with anisotropic properties, an area of substantial recent research efforts.

Equivalent gaits may be present in pedestrians that differ greatly in leg number, leg design and skeletal type. Previous studies on ghost crabs found that the transition from a slow to a fast run may resemble the change from a trot to a gallop in quadrupedal mammals. One indication of the trot–gallop gait change in quadrupedal mammals is a distinct alteration in bone strain. To test the hypothesis that ghost crabs (Ocypode quadrata) change from a trot to a gallop, we measured in vivo strains of the meropodite of the second trailing leg with miniature strain gauges. Exoskeletal strains changed significantly (increased fivefold) during treadmill locomotion at the proposed trot–gallop transition. Maximum strains attained during galloping and jumping (1000×10−6–3000×10−6) were similar to the values reported for mammals. Comparison of the maximum load possible on the leg segment (caused by muscular tension) with the strength of the segment under axial loading revealed a safety factor of 2.7, which is similar to values measured for jumping and running mammals. Equivalent gaits may result from similarities in the operation of pedestrian locomotory systems.

We examined the effects of aerodynamic forces on the mechanical power output of running insects for which kinematic data were available. Drag and lift on the cockroaches Periplaneta americana (a small, rapidly running species) and Blaberus discoidalis (a larger, more slowly moving species) were measured in a wind tunnel. Although lift would be expected to affect power output by altering functional body weight, the magnitude of the lift on these cockroaches was less than 2% of their weight. Drag, which increases the horizontal force that must be exerted to run at a given speed, accounted for 20–30% of the power output of P. americana running at speeds of 1.0–1.5 ms−1, but had a much smaller effect on B. discoidalis. Aerodynamic drag on the body (parasite drag) can significantly increase the mechanical power output necessary for small, rapidly running insects in contrast to larger running animals and to flying insects.

SUMMARY In nature, cockroaches run rapidly over complex terrain such as leaf litter. These substrates are rarely rigid, and are frequently very compliant. Whether and how compliant surfaces change the dynamics of rapid insect locomotion has not been investigated to date largely due to experimental limitations. We tested the hypothesis that a running insect can maintain average forward speed over an extremely soft elastic surface (10 N m−1) equal to 2/3 of its virtual leg stiffness (15 N m−1). Cockroaches Blaberus discoidalis were able to maintain forward speed (mean ± s.e.m., 37.2±0.6 cm s−1 rigid surface versus 38.0±0.7 cm s−1 elastic surface; repeated-measures ANOVA, P=0.45). Step frequency was unchanged (24.5±0.6 steps s−1 rigid surface versus 24.7±0.4 steps s−1 elastic surface; P=0.54). To uncover the mechanism, we measured the animal's centre of mass (COM) dynamics using a novel accelerometer backpack, attached very near the COM. Vertical acceleration of the COM on the elastic surface had a smaller peak-to-peak amplitude (11.50±0.33 m s−2, rigid versus 7.7±0.14 m s−2, elastic; P=0.04). The observed change in COM acceleration over an elastic surface required no change in effective stiffness when duty factor and ground stiffness were taken into account. Lowering of the COM towards the elastic surface caused the swing legs to land earlier, increasing the period of double support. A feedforward control model was consistent with the experimental results and provided one plausible, simple explanation of the mechanism.

Natural muscle is a spectacular actuator. Why? After millions of years, nature has evolved actuators that allow breathtaking performances. Cheetahs can run, dolphins can swim, and flies can fly like no artificial technology can. It is often argued that if human technology could mimic muscle, then biological-like performance would follow. Unfortunately, the blind copying or mimicking of a part of nature [Ritzmann et al., 2000] does not often lead to the best design, for a host of reasons [Vogel, 1998]. Evolution works on the "€œjust good enough"€ principle. Optimal designs are not the necessary end product of evolution. Multiple satisfactory solutions can result in similar performances. Animals do bring to our attention amazing designs, but these designs carry with them the baggage of their history. Why should these historical vestiges be incorporated into an artificial technology? Moreover, muscle design is constrained by factors that may have no relationship to human-engineered designs. Muscles must be able to grow over time, but still function along the way. Muscles remain plastic in adulthood and can self-repair. Muscles are intimately tied to pressure in the fluid system that supports them. Muscles are involved in metabolic regulation and can even serve as a source of fuel in starvation. Finally, muscles are obviously not the only part of an animal that makes spectacular performances possible. We must understand what muscle uniquely contributes to an integrated, tuned system that includes multiple muscles, joints and sensors, a transport system for fuel delivery, and a complex control system, all of which functions through skeletal scaffolding. To design an artificial muscle is a worthy endeavor. However, we strongly urge that nature's technologies provide biological inspiration for artificial technologies. Biological inspiration should involve the transfer of principles or lessons discovered in a diversity of animals. Our knowledge of biological muscle should be able to assist us in the construction of an actuator with desired performance capacities only observed in animals. However, the performance of biological actuators should not be and has not been the single design by which we measure our success. We have and will continue to design human-made actuators that exceed natural muscle in performance in particular metrics and for specific tasks. If we are to call a human-made actuator an artificial muscle, we must detail precisely the tasks that uniquely define what muscles do. Metrics can best be compared under common conditions. To develop these appropriate tests is an ongoing challenge, because we are still discovering how muscles work in animals. Moreover, engineers have a multitude of metrics that have made relevant, direct comparisons nearly impossible. The design of an artificial muscle will require novel interdisciplinary collaborations between muscle biologists and engineers. Biologists can provide inspiration and detail about what is known at present, but engineers can reciprocate with quantitative hypotheses and novel instrumentation that will lead to new tests and discoveries of muscle function.

In 1979 Bliss predicted that, “land crabs are and will undoubtedly continue to be promising objects of scientific research.” Studies of rapid running ghost crabs support her contention and have resulted in several general findings relating to locomotion and activity. 1) Energy exchange mechanisms during walking are general and not restricted to quadrupedal and bipedal morphologies. 2) “Equivalent gaits,” such as trots and gallops, may exist in 4-, 6- and 8-legged animals that differ greatly in leg and skeletal ( i.e. , exo- vs . endoskeletal) design. These findings support the hypothesis that terrestrial locomotion in many species can modeled by an inverted pendulum or spring-mass system. 3) An open circulatory system and chitin-covered gills do not necessarily limit the rate at which oxygen consumption can be increased or the factorial increase oxygen consumption over resting rates. 4) Interspecific and intraspecific i.e. , ontogenetic) scaling of sub-maximal oxygen consumption and maximal aerobic speed can differ significantly. 5) Locomotion at speeds above the maximal aerobic speed requiring non-aerobic contributions may be far more costly than can be predicted from aerobic costs alone. The cost transport may attain a minimum at less than maximum speed. 6) The speed which elicits maximal oxygen consumption during continuous exercise is attained at moderate walking speeds in crabs and probably other ectotherms. Speeds 15- to 20-fold faster are possible, but cannot be sustained. 7) The low endurance associated with the low maximal oxygen consumption and maximal aerobic speed of ectotherms moving continuously can be increased or decreased by altering locomotor behavior and moving intermittently. Ectotherms can locomote at high speeds and travel for considerable distances or remain active for long periods by including rest pauses. Alternatively, intense activity with extended exercise periods with short pause periods may actually reduce behavioral capacity or work accomplished relative to continuous activity during which the behavior is carried out at a lower intensity level without pauses.

We envision situational awareness developed through warfighters deployment of a system of diverse mobile, communicating platforms that cooperate to provide full coverage of interior and exterior spaces. The goal of the ARL-MAST Center on Microsystem Mechanics is to perform the fundamental research that will enable flying and ambulating platforms to achieve the required mobility for the proposed missions and environments. In this paper the fundamental issues and challenges associated with achieving this goal will be discussed.

Our objective is to provide experimentalists with neuromechanical control hypotheses that can be tested with kinematic data sets. To illustrate the approach, we select legged animals responding to perturbations during running. In the following sections, we briefly outline our dynamical systems approach, state our over-arching hypotheses, define four neuromechanical control architectures (NCAs) and conclude by proposing a series of perturbation experiments that can begin to identify the simplest architecture that best represents an animal's controller. Disciplines Engineering Comments @INBOOK{Revzen-TestArch07, chapter = {Towards Testable Neuromechanical Control Architectures for Running}, pages = {25-56}, title = {Progress in Motor Control A Multidisciplinary Perspective}, publisher = {Springer Science+Business Media, LLC NY}, year = {2008}, editor = {D Sternad}, author = {S Revzen and D E Koditschek and R J Full}, doi = {10.1007/978-0-387-77064-2} This working paper is available at ScholarlyCommons: http://repository.upenn.edu/ese_papers/507 Towards Testable Neuromechanical Control Architectures for

Most animals cannot sustain speeds above that at which the rate of oxygen consumption reaches a maximum (V O O2 max ). Fowler9s toad ( Bufo woodhousii fowleri ), by contrast, has a maximum aerobic speed (MAS, the speed at V O O2 max ) of 0.27 km h −1 but can sustain speeds as high as 0.45 km h −1 without increasing the V O O2 max above the V O O2 max . The present study investigates the discrepancy between MAS and the maximum sustainable speed (MSS). Toads switched from walking to hopping as their speed increased. The cost of a hop (4.1×10 −4 O 2 g −1 hop −1 ) was greater than the cost of a walking stride (2.5 × 10 −4 ml O 2 g −1 stride −1 ) and was independent of speed for both hopping and walking. However, individual hops were much longer than walking strides, which more than offset the greater cost of a hop. The calculated cost to traverse a given distance was approximately 1.9 times as much for walking as for hopping. During natural locomotion animals used combined walking and hopping. Individual toads that favored walking had higher locomotor costs than those that favored hopping. The estimated cost of exclusive hopping was less than the cost of natural locomotion at all but the highest speeds. This discrepancy may reflect the fact that the natural gait is a combination of both the less economical walking gait and the more economical hopping gait. To achieve speeds above the MAS toads walked less and used the more economical hopping gait more, and thus did not increase energy cost above that of V O O2 max . The speed at which the estimated cost of exclusive hopping exceeded the cost of a natural gait and approached the V O O2 max was close to the MSS. Creatine phosphate and lactate concentrations in the muscles of the thigh and calf did not change from resting levels at sustainable speeds greater than the MAS. Note: To whom reprint requests should be addressed