Vertebrate Locomotion Research Articles

Paleoinspired robotics as an experimental approach to the history of life.

Paleontologists must confront the challenge of studying the forms and functions of extinct species for which data from preserved fossils are extremely limited, yielding only a fragmented picture of life in deep time. In response to this hurdle, we describe the nascent field of paleoinspired robotics, an innovative method that builds upon established techniques in bioinspired robotics, enabling the exploration of the biology of ancient organisms and their evolutionary trajectories. This Review presents ways in which robotic platforms can fill gaps in existing research using the exemplars of notable transitions in vertebrate locomotion. We examine recent case studies in experimental paleontology, highlighting substantial contributions made by engineering and robotics techniques, and further assess how the efficient application of robotic technologies in close collaboration with paleontologists and biologists can offer additional insights into the study of evolution that were previously unattainable.

The impact of measurement technique and sampling on estimates of skeletal muscle fibre architecture.

Skeletal muscle fibre architecture provides important insights into performance of vertebrate locomotor and feeding behaviours. Chemical digestion and in situ sectioning of muscle bellies along their lengths to expose fibres, fibre orientation and intramuscular tendon, are two classical methods for estimating architectural variables such as fibre length (Lf) and physiological cross-sectional area (PCSA). It has recently been proposed that Lf estimates are systematically shorter and hence less accurate using in situ sectioning. Here we addressed this hypothesis by comparing Lf estimates between the two methods for the superficial masseter and temporalis muscles in a sample of strepsirrhine and platyrrhine primates. Means or single-specimen Lf estimates using chemical digestion were greater in 17/32 comparisons (53.13%), indicating the probability of achieving longer fibres using chemical digestion is no greater than chance in these taxonomic samples. We further explored the impact of sampling on scaling of Lf and PCSA in platyrrhines applying a bootstrapping approach. We found that sampling-both numbers of individuals within species and representation of species across the clade significantly influence scaling results of Lf and PCSA in platyrrhines. We show that intraspecific and clade sampling strategies can account for differences between previously published platyrrhine scaling studies. We suggest that differences in these two methodological approaches to assessing muscle architecture are relatively less consequential when estimating Lf and PCSA for comparative studies, whereas achieving more reliable estimates within species through larger samples and representation of the full clade space are important considerations in comparative studies of fibre architecture and scaling.

Coordinating upper limbs for octave playing on the piano via neuro-musculoskeletal modeling

Understanding the coordination of multiple biomechanical degrees of freedom in biological organisms is crucial for unraveling the neurophysiological control of sophisticated motor tasks. This study focuses on the cooperative behavior of upper-limb motor movements in the context of octave playing on the piano. While the vertebrate locomotor system has been extensively investigated, the coherence and precision timing of rhythmic movements in the upper-limb system remain incompletely understood. Inspired by the spinal cord neuronal circuits (central pattern generator, CPG), a computational neuro-musculoskeletal model is proposed to explore the coordination of upper-limb motor movements during octave playing across varying tempos and volumes. The proposed model incorporates a CPG-based nervous system, a physiologically-informed mechanical body, and a piano environment to mimic human joint coordination and expressiveness. The model integrates neural rhythm generation, spinal reflex circuits, and biomechanical muscle dynamics while considering piano playing quality and energy expenditure. Based on real-world human subject experiments, the model has been refined to study tempo transitions and volume control during piano playing. This computational approach offers insights into the neurophysiological basis of upper-limb motor coordination in piano playing and its relation to expressive features.

The robotic approach to the passive interlocking mechanism in the hindlimb musculoskeletal system of Crocodylus porosus

Efficient locomotion in terrestrial vertebrates relies on intra-limb coordination that emerges through interaction with the environment and passive interlocking of the musculoskeletal system. These structures have been well investigated in some mammals and birds, but the presence of similar structures in other terrestrial vertebrates has been under appreciated. This study addresses the functionality of the passive interlocking system in reptilian hindlimb locomotion by dissecting Crocodylus porosus and constructing a robot. Based on the dissection, we hypothesized that the passive interlocking mechanism consisted of multiple muscles to maintain the semi-erect limb posture and support its weight. The mechanism is provided by the caudifemoralis longus muscle, along with its tendon and the gastrocnemius externus, acting as a passive element that interacts with the ground reaction force. Accordingly, the feasibility of the interlocking function was demonstrated by a robot implementing the hindlimb and pelvic musculoskeletal system of crocodilians. This study provides new insights into the locomotion mechanism of crocodilians and emphasizes the importance of passive interlocking in efficient locomotion in terrestrial vertebrates. The findings could have implications for developing biomimetic robots and understanding the evolution of terrestrial locomotion in vertebrates.

Scaling the tail beat frequency and swimming speed in underwater undulatory swimming

Undulatory swimming is the predominant form of locomotion in aquatic vertebrates. A myriad of animals of different species and sizes oscillate their bodies to propel themselves in aquatic environments with swimming speed scaling as the product of the animal length by the oscillation frequency. Although frequency tuning is the primary means by which a swimmer selects its speed, there is no consensus on the mechanisms involved. In this article, we propose scaling laws for undulatory swimmers that relate oscillation frequency to length by taking into account both the biological characteristics of the muscles and the interaction of the moving swimmer with its environment. Results are supported by an extensive literature review including approximately 1200 individuals of different species, sizes and swimming environments. We highlight a crossover in size around 0.5–1 m. Below this value, the frequency can be tuned between 2–20 Hz due to biological constraints and the interplay between slow and fast muscles. Above this value, the fluid-swimmer interaction must be taken into account and the frequency is inversely proportional to the length of the animal. This approach predicts a maximum swimming speed around 5–10 m.s−1 for large swimmers, consistent with the threshold to prevent bubble cavitation.

Integration of feedforward and feedback control in the neuromechanics of vertebrate locomotion: a review of experimental, simulation and robotic studies.

Animal locomotion is the result of complex and multi-layered interactions between the nervous system, the musculo-skeletal system and the environment. Decoding the underlying mechanisms requires an integrative approach. Comparative experimental biology has allowed researchers to study the underlying components and some of their interactions across diverse animals. These studies have shown that locomotor neural circuits are distributed in the spinal cord, the midbrain and higher brain regions in vertebrates. The spinal cord plays a key role in locomotor control because it contains central pattern generators (CPGs) - systems of coupled neuronal oscillators that provide coordinated rhythmic control of muscle activation that can be viewed as feedforward controllers - and multiple reflex loops that provide feedback mechanisms. These circuits are activated and modulated by descending pathways from the brain. The relative contributions of CPGs, feedback loops and descending modulation, and how these vary between species and locomotor conditions, remain poorly understood. Robots and neuromechanical simulations can complement experimental approaches by testing specific hypotheses and performing what-if scenarios. This Review will give an overview of key knowledge gained from comparative vertebrate experiments, and insights obtained from neuromechanical simulations and robotic approaches. We suggest that the roles of CPGs, feedback loops and descending modulation vary among animals depending on body size, intrinsic mechanical stability, time required to reach locomotor maturity and speed effects. We also hypothesize that distal joints rely more on feedback control compared with proximal joints. Finally, we highlight important opportunities to address fundamental biological questions through continued collaboration between experimentalists and engineers.

Locomotor pattern generation and descending control: a historical perspective.

The ability to generate and control locomotor movements depends on complex interactions between many areas of the nervous system, the musculoskeletal system, and the environment. How the nervous system manages to accomplish this task has been the subject of investigation for more than a century. In vertebrates, locomotion is generated by neural networks located in the spinal cord referred to as central pattern generators. Descending inputs from the brain stem initiate, maintain, and stop locomotion as well as control speed and direction. Sensory inputs adapt locomotor programs to the environmental conditions. This review presents a comparative and historical overview of some of the neural mechanisms underlying the control of locomotion in vertebrates. We have put an emphasis on spinal mechanisms and descending control.

Hindlimb muscle spindles inform preparatory forelimb coordination prior to landing in toads.

Animals move across a wide range of surface conditions in real-world environments to acquire resources and avoid predation. To effectively navigate a variety of surfaces, animals rely on several mechanisms including intrinsic mechanical responses, spinal-level central pattern generators, and neural commands that require sensory feedback. Muscle spindle Ia afferents play a critical role in providing sensory feedback and informing motor control strategies across legged vertebrate locomotion, which is apparent in cases where this sensory input is compromised. Here, we tested the hypothesis that spindle Ia afferents from hindlimb muscles are important for coordinating forelimb landing behavior in the cane toad. We performed bilateral sciatic nerve reinnervations to ablate the stretch reflex from distal hindlimb muscles while allowing for motor neuron recovery. We found that toads significantly delayed the onset and reduced the activation duration of their elbow extensor muscle following spindle Ia afferent ablation in the hindlimbs. However, reinnervated toads achieved similar elbow extension at touchdown to that of their pre-surgery state. Our results suggest that while toads likely tuned the activation timing of forelimb muscles in response to losing Ia afferent sensation from the hindlimbs they were likely able to employ compensatory strategies that allowed them to continue landing effectively with reduced sensory information during take-off. These findings indicate muscle spindle Ia afferents may contribute to tuning complex movements involving multiple limbs.

TO THE HISTORY OF THE LOCOMOTIVE APPARATUS STUDY BY THE SCIENTISTS KYIV SCHOOL OF COMPARATIVE MORPHOLOGISTS

The issue has been considered of studying the apparatus of animal movement by representatives of the Kyiv School of Comparative Morphologists. In the 20s of the 20th century, scientific works of representatives of the Kyiv School of Comparative Morphologists under the leadership of B.O. Dombrovsky began studying the structure of the animal movement apparatus, especially in the comparative aspect. In the course of his scientific activity, B. O. Dombrovskyi introduced a new direction in comparative anatomy to scientific research - the method of establishing and taking into account correlation as a method of synthesizing structures and analyzing processes. V. G. Kasyanenko's scientific research was devoted to the study of the structure and function of the joints of mammals, namely the structure of the horse's hock joint in the light of its biological evolution and domestication. Research on the biomechanics of statolocomotion organs was also new - the distribution of the limit of strength and the modulus of elasticity of parts of the bones of the limbs and the biomechanics of joints (Manziy S.F.). In addition, new methods were introduced in morphological studies, in particular electrophysiological ones, which were carried out by Kyiv morphologists for the first time in the Soviet Union. Along with this, non-structural laboratories were formed at the institute: functional analysis of limb joints and the role of synovium (O.G. Berezkin), zoological biomechanics (K.P. Melnyk, V.I. Klykov), research of limb muscles (V.F. Moroz), morphology of birds (V.F. Sych), morphology of bats (M.F. Kovtun), innervation of joints (M.M. Ilyenko), group for the study of limb vessels (P.M. Mazhuga). In the laboratory of limb muscle research organized by V.F. Moroz., registration of biopotentials from muscles in the state of statics and dynamics was carried out. An important device was a treadmill with track speed control and simultaneously working biopotential recorders. This made it possible for the first time to give a reliable picture of the coordinated functioning of each muscle and muscle group. The direction of scientific research of O.P. Melnyk: Biomorphology of vertebrate locomotion organs in a comparative aspect using a large number of taxonomic groups of animals that occupy different biological niches in nature.

An early midbrain sensorimotor pathway is involved in the timely initiation and direction of swimming in the hatchling Xenopus laevis tadpole.

Vertebrate locomotion is heavily dependent on descending control originating in the midbrain and subsequently influencing central pattern generators in the spinal cord. However, the midbrain neuronal circuitry and its connections with other brainstem and spinal motor circuits has not been fully elucidated. Vertebrates with very simple nervous system, like the hatchling Xenopus laevis tadpole, have been instrumental in unravelling fundamental principles of locomotion and its suspraspinal control. Here, we use behavioral and electrophysiological approaches in combination with lesions of the midbrain to investigate its contribution to the initiation and control of the tadpole swimming in response to trunk skin stimulation. None of the midbrain lesions studied here blocked the tadpole's sustained swim behavior following trunk skin stimulation. However, we identified that distinct midbrain lesions led to significant changes in the latency and trajectory of swimming. These changes could partly be explained by the increase in synchronous muscle contractions on the opposite sides of the tadpole's body and permanent deflection of the tail from its normal position, respectively. We conclude that the tadpole's embryonic trunk skin sensorimotor pathway involves the midbrain, which harbors essential neuronal circuitry to significantly contribute to the appropriate, timely and coordinated selection and execution of locomotion, imperative to the animal's survival.

Role of locomotor efference copy in vertebrate gaze stabilization.

Vertebrate locomotion presents a major challenge for maintaining visual acuity due to head movements resulting from the intimate biomechanical coupling with the propulsive musculoskeletal system. Retinal image stabilization has been traditionally ascribed to the transformation of motion-related sensory feedback into counteracting ocular motor commands. However, extensive exploration of spontaneously active semi-intact and isolated brain/spinal cord preparations of the amphibian Xenopus laevis, have revealed that efference copies (ECs) of the spinal motor program that generates axial- or limb-based propulsion directly drive compensatory eye movements. During fictive locomotion in larvae, ascending ECs from rostral spinal central pattern generating (CPG) circuitry are relayed through a defined ascending pathway to the mid- and hindbrain ocular motor nuclei to produce conjugate eye rotations during tail-based undulatory swimming in the intact animal. In post-metamorphic adult frogs, this spinal rhythmic command switches to a bilaterally-synchronous burst pattern that is appropriate for generating convergent eye movements required for maintaining image stability during limb kick-based rectilinear forward propulsion. The transition between these two fundamentally different coupling patterns is underpinned by the emergence of altered trajectories in spino-ocular motor coupling pathways that occur gradually during metamorphosis, providing a goal-specific, morpho-functional plasticity that ensures retinal image stability irrespective of locomotor mode. Although the functional impact of predictive ECs produced by the locomotory CPG matches the spatio-temporal specificity of reactive sensory-motor responses, rather than contributing additively to image stabilization, horizontal vestibulo-ocular reflexes (VORs) are selectively suppressed during intense locomotor CPG activity. This is achieved at least in part by an EC-mediated attenuation of mechano-electrical encoding at the vestibular sensory periphery. Thus, locomotor ECs and their potential suppressive impact on vestibular sensory-motor processing, both of which have now been reported in other vertebrates including humans, appear to play an important role in the maintenance of stable vision during active body displacements.

Intersegmental coordination of the central pattern generator via interleaved electrical and chemical synapses in zebrafish spinal cord.

A significant component of the repetitive dynamics during locomotion in vertebrates is generated within the spinal cord. The legged locomotion of mammals is most likely controled by a hierarchical, multi-layer spinal network structure, while the axial circuitry generating the undulatory swimming motion of animals like lamprey is thought to have only a single layer in each segment. Recent experiments have suggested a hybrid network structure in zebrafish larvae in which two types of excitatory interneurons (V2a-I and V2a-II) both make first-order connections to the brain and last-order connections to the motor pool. These neurons are connected by electrical and chemical synapses across segments. Through computational modeling and an asymptotic perturbation approach we show that this interleaved interaction between the two neuron populations allows the spinal network to quickly establish the correct activation sequence of the segments when starting from random initial conditions, as needed for a swimming spurt, and to reduce the dependence of the intersegmental phase difference (ISPD) of the oscillations on the swimming frequency. The latter reduces the frequency dependence of the waveform of the swimming motion. In the model the reduced frequency dependence is largely due to the different impact of chemical and electrical synapses on the ISPD and to the significant spike-frequency adaptation that has been observed experimentally in V2a-II neurons, but not in V2a-I neurons. Our model makes experimentally testable predictions and points to a benefit of the hybrid structure for undulatory locomotion that may not be relevant for legged locomotion.

The light-independent locomotion response to a static magnetic field in Xenopus tadpoles

Increasing numbers of behavioral observations have shown that many organisms can sense magnetic fields in the environment. However, the underlying mechanisms of magnetosensation remain elusive, and a major problem is the lack of an animal model convenient for detecting magnetic responses, especially in vertebrates. We have previously reported that the physical performance of mice decreased after 1 month of exposure to a hypomagnetic field, which probably includes a light-independent mechanism. In this study, we investigated the effects of a static magnetic field on the locomotion of Xenopus tadpoles (stage 48) using a real-time recording system. Because it is speculated that the photosensing system is related to magnetosensation, we triggered the tadpoles to swim by a 30 s light pulse after a 5 min adaptation in the dark and measured the swimming activities of the tadpoles under static magnetic field exposure in both the dark and bright conditions. Compared to the sham control treatment, the presence of a magnet inhibited the movement of the tadpoles under both bright and dark conditions, as shown by reductions in swimming distance, speed, and counts of path adjustment. Moreover, the directional preference for path adaptation was altered when swimming in the dark. These results suggest that tadpoles in the dark can exert a rapid locomotion response to changes in the environmental magnetic field, providing evidence for an intrinsic, light-independent rapid magnetoresponse pathway in tadpoles. In addition, this rapid vertebrate locomotion assay paradigm will be a practical tool to facilitate further investigation of the mechanisms of biomagnetic effects.

A Step Forward: Functional Diversity and Emerging Themes of Slow-Speed Locomotion in Vertebrates.

Walking can be defined broadly as a slow-speed movement produced when appendages interact with the ground to generate forward propulsion. Until recently, most studies of walking have focused on humans and a handful of domesticated vertebrates moving at a steady rate over highly simplified, static surfaces, which may bias our understanding of the unifying principles that underlie vertebrate locomotion. In the last few decades, studies have expanded to include a range of environmental contexts (e.g., uneven terrain, perturbations, deformable substrates) and greater phylogenetic breadth (e.g., non-domesticated species, small and/or ectothermic tetrapods and fishes); these studies have revealed that even a gait as superficially simple as walking is far more complex than previously thought. In addition, technological advances and accessibility of imaging systems and computational power have recently expanded our capabilities to test hypotheses about the locomotor movements of extant and extinct organisms in silico. In this symposium, scientists showcased diverse taxa (from extant fishes to extinct dinosaurs) moving through a range of variable conditions (speed perturbations, inclines, and deformable substrates) to address the causes and consequences of functional diversity in locomotor systems and discuss nascent research areas and techniques. From the symposium contributions, several themes emerged: (1) slow-speed, appendage-based movements in fishes are best described as walking-like movements rather than true walking gaits, (2) environmental variation (e.g., deformable substrates) and dynamic stimuli (e.g., perturbations) trigger kinematic and neuromuscular changes in animals that make defining a single gait or the transition between gaits more complicated than originally thought, and (3) computational advances have increased the ability to process large data sets, emulate the 3D motions of extant and extinct taxa, and even model species interactions in ancient ecosystems. Although this symposium allowed us to make great strides forward in our understanding of vertebrate walking, much ground remains to be covered. First, there is a much greater range of vertebrate appendage-based locomotor behaviors than has been previously recognized and existing terminology fails to accurately capture and describe this diversity. Second, despite recent efforts, the mechanisms that vertebrates use modify locomotor behaviors in response to predictable and unpredictable locomotor challenges are still poorly understood. Third, while computer-based models and simulations facilitate a greater understanding of the kinetics and kinematics of movement in both extant and extinct animals, a universal, one-size-fits-all, predictive model of appendage-based movement in vertebrates remains elusive.

The trunk-tail junctional region in Ciona larvae autonomously expresses tail-beating bursts at ∼20second intervals.

Swimming locomotion in aquatic vertebrates, such as fish and tadpoles, is expressed through neuron networks in the spinal cord. These networks are arranged in parallel, ubiquitously distributed and mutually coupled along the spinal cord to express undulation patterns accommodated to various inputs into the networks. While these systems have been widely studied in vertebrate swimmers, their evolutionary origin along the chordate phylogeny remains unclear. Ascidians, representing a sister group of vertebrates, give rise to tadpole larvae that swim freely in seawater. In the present study, we examined the locomotor ability of the anterior and posterior body fragments of larvae of the ascidian Ciona that had been cut at an arbitrary position. Examination of more than 200 fragments revealed a necessary and sufficient body region that spanned only ∼10% of the body length and included the trunk-tail junction. 'Mid-piece' body fragments, which included the trunk-tail junctional region, but excluded most of the anterior trunk and posterior tail, autonomously expressed periodic tail-beating bursts at ∼20s intervals. We compared the durations and intervals of tail-beating bursts expressed by mid-piece fragments, and also by whole larvae under different sensory conditions. The results suggest that body parts outside the mid-piece effect shortening of swimming intervals, particularly in the dark, and vary the burst duration. We propose that Ciona larvae express swimming behaviors by modifying autonomous and periodic locomotor drives that operate locally in the trunk-tail junctional region.

Locomotion-induced ocular motor behavior in larval Xenopus is developmentally tuned by visuo-vestibular reflexes

Locomotion in vertebrates is accompanied by retinal image-stabilizing eye movements that derive from sensory-motor transformations and predictive locomotor efference copies. During development, concurrent maturation of locomotor and ocular motor proficiency depends on the structural and neuronal capacity of the motion detection systems, the propulsive elements and the computational capability for signal integration. In developing Xenopus larvae, we demonstrate an interactive plasticity of predictive locomotor efference copies and multi-sensory motion signals to constantly elicit dynamically adequate eye movements during swimming. During ontogeny, the neuronal integration of vestibulo- and spino-ocular reflex components progressively alters as locomotion parameters change. In young larvae, spino-ocular motor coupling attenuates concurrent angular vestibulo-ocular reflexes, while older larvae express eye movements that derive from a combination of the two components. This integrative switch depends on the locomotor pattern generator frequency, represents a stage-independent gating mechanism, and appears during ontogeny when the swim frequency naturally declines with larval age.

SLC3A2 N‐glycosylation and alternate evolutionary trajectories for amino acid metabolism

Cancer and other proliferative diseases are driven by cooperative rewiring of growth signaling and metabolism, where posttranslational modifications to proteins play a critical role in maintaining the disease state. Most PTMs require metabolites as donor substrates, providing underappreciated link between metabolism and signaling. Protein N‐glycosylation and Golgi remodeling of N‐glycan on receptor kinases, T cell receptor, glucagon receptor, and glucose transporters provides feedback between signaling and metabolism. In turn, N‐glycosylation requires uridine diphosphate N‐acetylglucosamine (UDP‐GlcNAc), generated by the hexosamine biosynthesis pathway (HBP) from glucose, glutamine and acetyl‐CoA, where the flux of these substrates into UDP‐GlcNAc are in competition with glycolysis, glutaminolysis and fatty‐acid turnover. Thus, N‐glycan branching, a pathway of N‐acetylglucosaminyltransferases (MGAT1, 2, 4 and 5, avian 6) in the Golgi, is a sensor of central metabolism.In a search for additional glycoproteins that link metabolism, N‐glycosylation and signaling, we identified SLC3A2 (4F2hc, CD98),an adaptor to amino acid (AA) transporters SLC7A5‐11, ‐13 which stabilizes cell surface residency. The SLC3A2*SLC7A5 exchanger imports essential AA that stimulate mTOR signaling and anabolic metabolism, while SLC3A2*SLC7A11 supports glutathione synthesis and mitigation of oxidative stress. Analysis of SLC3A2 N‐glycans revealed stable site‐specific profiles of Golgi remodeling, with the exception of the conserved N365 where branching and poly‐N‐acetyllactosamine content were sensitive to the insertion of lost ancestral sites and to HBP. The N‐glycans are positioned to promote galectin‐mediated clustering with N‐glycosylated Na++/AA symporters and enhance diffusion‐limited flux between exchangers and AA /Na+ symporters. In a successful experiment of Nature, SLC3A2 has been deleted in Neoaves, a clade representing 95% of Ave species that are adapted for flapping flight, the most metabolically intense form of vertebrate locomotion. Neoaves have also acquired a unique N‐glycan branching specificity GnT‐VI (Mgat6, Q9DGD1), which may support an alternate clustering of N‐glycosylated transporters. KEAP1, and four SLC7A family exchangers are also absent, and two others have duplicated, consistent with a shift in metabolic imperatives of Neoaves. The fate of these genes in Primates, Neoaves and Naked mole rat highlights the tension between metabolic rates and oxidative stress that govern lifestyle and longevity.

Control of mammalian locomotion by ventral spinocerebellar tract neurons

Locomotion is a complex behavior required for animal survival. Vertebrate locomotion depends on spinal interneurons termed the central pattern generator (CPG), which generates activity responsible for the alternation of flexor and extensor muscles and the left and right side of the body. It is unknown whether multiple or a single neuronal type is responsible for the control of mammalian locomotion. Here, we show that ventral spinocerebellar tract neurons (VSCTs) drive generation and maintenance of locomotor behavior in neonatal and adult mice. Using mouse genetics, physiological, anatomical, and behavioral assays, we demonstrate that VSCTs exhibit rhythmogenic properties and neuronal circuit connectivity consistent with their essential role in the locomotor CPG. Importantly, optogenetic activation and chemogenetic silencing reveals that VSCTs are necessary and sufficient for locomotion. These findings identify VSCTs as critical components for mammalian locomotion and provide a paradigm shift in our understanding of neural control of complex behaviors.

Kinematics of sea star legged locomotion.

Sea stars have slower crawling and faster bouncing gaits. Both speed and oscillation amplitude increase during the transition from crawling to oscillating. In the bouncy gait, oscillating vertical velocities precede oscillating horizontal velocities by 90 deg, as reflected by clockwise circular hodographs. Potential energy precedes horizontal kinetic energy by 9.6 deg and so they are nearly in phase. These phase relationships resemble terrestrial running gaits, except that podia are always on the ground. Kinetic and potential energy scale with body mass as Mb 1.1, with the change in kinetic energy consistently two orders of magnitude less, indicating that efficient exchange is not feasible. Frequency of the bouncy gait scales with Mb-0.14, which is similar to continuously running vertebrates and indicates that gravitational forces are important. This scaling differs from the Hill model, in which scaling of muscle forces determine frequency. We propose a simple torque-stabilized inverted pendulum (TS-IP) model to conceptualize the dynamics of this gait. The TS-IP model incorporates mathematics equivalent to an angular spring, but implemented by a nearly constant upward force generated by the podia in each step. That upward force is just larger than the force required to sustain the underwater weight of the sea star. Even though the bouncy gait is the rapid gait for these sea stars, the pace of movement is still very slow. In fact, the observed Froude numbers (10-2 to 10-3) are much lower than those typical of vertebrate locomotion and are as low or lower than those reported for slow-walking fruit flies, which are the lowest values for pedestrian Froude numbers of which we are aware.

Great power comes at a high (locomotor) cost: the role of muscle fascicle length in the power versus economy performance trade-off.

Muscle design constraints preclude simultaneous specialization of the vertebrate locomotor system for explosive and economical force generation. The resulting performance trade-off between power and economy has been attributed primarily to individual differences in muscle fiber type composition. While certainly crucial for performance specialization, fiber type likely interacts with muscle architectural parameters, such as fascicle length, to produce this trade-off. Longer fascicles composed of more serial sarcomeres can achieve faster shortening velocities, allowing for greater power production. Long fascicles likely reduce economy, however, because more energy-consuming contractile units are activated for a given force production. We hypothesized that longer fascicles are associated with both increased power production and locomotor cost. In 11 power-trained and 13 endurance-trained recreational athletes, we measured (1) muscle fascicle length via ultrasound in the gastrocnemius lateralis, gastrocnemius medialis and vastus lateralis, (2) maximal power during cycling and countermovement jumps, and (3) running cost of transport. We estimated muscle fiber type non-invasively based on the pedaling rate at which maximal cycling power occurred. As predicted, longer gastrocnemius muscle fascicles were correlated with greater lower-body power production and cost of transport. Multiple regression analyses revealed that variability in maximal power was explained by fiber type (46% for cycling, 24% for jumping) and average fascicle length (20% for cycling, 13% for jumping), while average fascicle length accounted for 15% of the variation in cost of transport. These results suggest that, at least for certain muscles, fascicle length plays an important role in the power versus economy performance trade-off.