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Uniqueness of Human Running Coordination: The Integration of Modern and Ancient Evolutionary Innovations
Abstract and Figures
Running is a pervasive activity across human cultures and a cornerstone of contemporary health, fitness, and sporting activities. Yet for the overwhelming predominance of human existence running was an essential prerequisite for survival. A means to hunt, and a means to escape when hunted. In a very real sense humans have evolved to run. Yet curiously, perhaps due to running's cultural ubiquity and the natural ease with which we learn to run, we rarely consider the uniqueness of human bipedal running within the animal kingdom. Our unique upright, single stance, bouncing running gait imposes a unique set of coordinative difficulties. Challenges demanding we precariously balance our fragile brains in the very position where they are most vulnerable to falling injury while simultaneously retaining stability, steering direction of travel, and powering the upcoming stride: all within the abbreviated time -frames afforded by short, violent ground contacts separated by long flight times. These running coordination challenges are solved through the tightly-integrated blending of primitive evolutionary legacies, conserved from reptilian and vertebrate lineages, and comparatively modern, more exclusively human, innovations. The integrated unification of these top-down and bottom-up control processes bestows humans with an agile control system, enabling us to readily modulate speeds, change direction, negotiate varied terrains and to instantaneously adapt to changing surface conditions. The seamless integration of these evolutionary processes is facilitated by pervasive, neural and biological, activity-dependent adaptive plasticity. Over time, and with progressive exposure, this adaptive plasticity shapes neural and biological structures to best cope with regularly imposed movement challenges. This pervasive plasticity enables the gradual construction of a robust system of distributed coordinated control, comprised of processes that are so deeply collectively entwined that describing their functionality in isolation obscures their true irrevocably entangled nature. Although other species rely on a similar set of coordinated processes to run, the bouncing bipedal nature of human running presents a specific set of coordination challenges, solved using a customized blend of evolved solutions. A deeper appreciation of the foundations of the running coordination phenomenon promotes conceptual clarity, potentially informing future advances in running training and running-injury rehabilitation interventions.
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... Accordingly, it is suggested that smoothness, as it promotes predictability, minimises movement error [39][40][41]. Similarly, more sensitive detection of subtle deviations from predicted trajectories facilitates more sensitive remedial adjustments, thereby offsetting the need for periodic, larger, more disruptive and energetically demanding corrective interventions [42]. ...
... Locomotion is initiated by commands originating in the motor cortex [42]. These descending commands are mediated and modulated by control centres in mid-brain and brain stem, before subsequently activating spinally located central pattern generating (CPG) networks responsible for controlling the rhythmic synchronisation of the arms and legs, thereby delegating much of the coordination burden to lower, less evolutionarily expensive, neural control centres [42]. ...
... Locomotion is initiated by commands originating in the motor cortex [42]. These descending commands are mediated and modulated by control centres in mid-brain and brain stem, before subsequently activating spinally located central pattern generating (CPG) networks responsible for controlling the rhythmic synchronisation of the arms and legs, thereby delegating much of the coordination burden to lower, less evolutionarily expensive, neural control centres [42]. As rhythmic locomotion progresses, streams of sensory feedback return to spinal centres and serve to (a) guide the on-going customization of CPG outputs to current contexts and (b) trigger stabilisation reflexes [42]. ...
Over the expanse of evolutionary history, humans, and predecessor Homo species, ran to survive. This legacy is reflected in many deeply and irrevocably embedded neurological and biological design features, features which shape how we run, yet were themselves shaped by running.
Smoothness is a widely recognised feature of healthy, proficient movement. Nevertheless, although the term ‘smoothness’ is commonly used to describe skilled athletic movement within practical sporting contexts, it is rarely specifically defined, is rarely quantified and remains barely explored experimentally. Elsewhere, however, within various health-related and neuro-physiological domains, many manifestations of movement smoothness have been extensively investigated. Within this literature, smoothness is considered a reflection of a healthy central nervous system (CNS) and is implicitly associated with practiced coordinated proficiency; ‘non-smooth’ movement, in contrast, is considered a consequence of pathological, un-practiced or otherwise inhibited motor control.
Despite the ubiquity of running across human cultures, however, and the apparent importance of smoothness as a fundamental feature of healthy movement control, to date, no theoretical framework linking the phenomenon of movement smoothness to running proficiency has been proposed. Such a framework could, however, provide a novel lens through which to contextualise the deep underlying nature of coordinated running control. Here, we consider the relevant evidence and suggest how running smoothness may integrate with other related concepts such as complexity, entropy and variability. Finally, we suggest that these insights may provide new means of coherently conceptualising running coordination, may guide future research directions, and may productively inform practical coaching philosophies.
... Running is not only an exercise that promotes physical health by enhancing cardiovascular endurance, strengthening muscles and building strong bones as it is a weight-bearing exercise, but also has an implication in lowering mental health burden 9-12 . To execute the appropriate movement, running requires neuronal topdown feedforward control responses to multi-modal sensory information to control coordinated movements and balance [13][14][15] . The prefrontal cortex, a brain region implicated in cognition and mood regulation, is partially involve in running, especially when there is demand for coordinated action 14-21 . ...
... The neural mechanisms for running-elicited cortical activation have remained unclear. Running requires topdown feedforward control responses to multi-modal sensory information that partially involve the prefrontal cortex to execute coordinated movement and balance [13][14][15]17,18 . Specific features of running, such as foot strike, may benefit the brain activation by enhancing blood flow velocity in the middle cerebral artery 24 . ...
Running, compared to pedaling is a whole-body locomotive movement that may confer more mental health via strongly stimulating brains, although running impacts on mental health but their underlying brain mechanisms have yet to be determined; since almost the mechanistic studies have been done with pedaling. We thus aimed at determining the acute effect of a single bout of running at moderate-intensity, the most popular condition, on mood and executive function as well as their neural substrates in the prefrontal cortex (PFC). Twenty-six healthy participants completed both a 10-min running session on a treadmill at 50% V ˙ O 2peak and a resting control session in randomized order. Executive function was assessed using the Stroop interference time from the color-word matching Stroop task (CWST) and mood was assessed using the Two-Dimensional Mood Scale, before and after both sessions. Prefrontal hemodynamic changes while performing the CWST were investigated using functional near-infrared spectroscopy. Running resulted in significant enhanced arousal and pleasure level compared to control. Running also caused significant greater reduction of Stroop interference time and increase in Oxy-Hb signals in bilateral PFCs. Besides, we found a significant association among pleasure level, Stroop interference reaction time, and the left dorsolateral PFCs: important brain loci for inhibitory control and mood regulation. To our knowledge, an acute moderate-intensity running has the beneficial of inducing a positive mood and enhancing executive function coinciding with cortical activation in the prefrontal subregions involved in inhibitory control and mood regulation. These results together with previous findings with pedaling imply the specificity of moderate running benefits promoting both cognition and pleasant mood.
... 717connective tissue networks complicates reductionist approaches, yet they underpin 718 context-sensitive physiological behavior. Tensegrity-like mechanisms support rapid 719 adaptive responses across various organisms[112][113][114][115][116][117][118][119]. While vestibular contributions ...
Contemporary dynamical models of human postural control propose an intermittent controller regulating the postural centre of pressure (CoP) about a stable saddle-shaped manifold along anatomical anteroposterior (AP) and mediolateral (ML) axes, releasing CoP in an outwards spiral when inactive. Experimental manipulations can evoke this saddle-type topology in fractal temporal correlations along the AP axis and reducing correlations along the ML axis. However, true effects of task demands may often manifest within angular space between anatomical AP and ML axes—a space not typically modelled explicitly. We tested how instability and attentional load influence postural control across the full angular range of fractal variability along the two-dimensional (2D) support surface. Forty-eight healthy young adults performed a suprapostural Trail Making Test (TMT) while standing on a wobble board, inducing continuous perturbations along the ML axis. Stable, quiet standing exhibited classic saddle-like topology, with stronger fractal temporal correlations in CoP displacements along AP axes. The attentional demand of the TMT did not affect angular variation or strength of fractal temporal correlations across the 2Dsupport surface. However, maintaining upright balance on the wobble board reshaped and reoriented the angular distribution of fractal temporal correlations, accentuating saddle-like angular variation and rotating the strongest fractal temporal correlations predominantly along the ML axis. Stabilizing posture in the face of wobble board instability prompted the saddle-type angular distribution of fractal temporal correlations. These findings challenge the traditional dependence of postural control theories exclusively on external force-plate axes and underscore the significance of multifractality in defining control parameters that govern postural stability across the full angular range of the 2D support surface.
... The extensive reach of 663 connective tissue networks complicates reductionist approaches, yet they underpin 664 context-sensitive physiological behavior. Tensegrity-like mechanisms support rapid 665 adaptive responses across various organisms [111][112][113][114][115][116][117][118]. While vestibular contributions 666 are likely, other systems also exhibit sensitivity to mechanical rotation [119][120][121]. ...
Contemporary dynamical models of human postural control propose an intermittent controller regulating the postural center of pressure (CoP) about a stable saddle-shaped manifold along anatomical anteroposterior (AP) and mediolateral (ML) axes, releasing CoP in an outwards spiral when inactive. Experimental manipulations can evoke this saddle-type topology in fractal temporal correlations along the AP axis and reducing correlations along the ML axis. However, true effects of task demands may often manifest within angular space between anatomical AP and ML axes—a space not typically modeled explicitly. We tested how instability and attentional load influence postural control across the full angular range of fractal variability along the 2D support surface. Forty-eight healthy young adults performed a suprapostural Trail Making Test (TMT) while standing on a wobble board, inducing continuous perturbations along the ML axis. Stable, quiet standing exhibited classic saddle-like topology, with stronger fractal temporal correlations in CoP displacements along AP axes. The attentional demand of TMT did not affect angular variation or strength of fractal temporal correlations across the 2D support surface. However, maintaining an upright balance on the wobble board reshaped and reoriented the angular distribution of fractal temporal correlations, accentuating saddle-like angular variation and rotating the strongest fractal temporal correlations predominantly along the ML axis. Stabilizing posture in the face of wobble board instability prompted the saddle-type angular distribution of fractal temporal correlations. These findings challenge the traditional dependence of postural control theories exclusively on external force-plate axes and underscore the significance of multifractality in defining control parameters that govern postural stability across the full angular range of the 2D support surface.
... They also provide a plausible explanation for the non-linear behavior inherent in living tissues [113] and have evolutionary consequences [114][115][116]. Such configurations would enable the tissues to respond instantly to rapidly changing conditions with the anatomy itself controlling complex movements [112,117,118] and acting in synergy with the nervous system (where befits) [119][120][121]. ...
The improvement of the human condition is the driver behind a vast amount of ongoing research and naturally employs the most up-to-date methods in its endeavours. It has contributed greatly to our understanding of the body and benefitted our healthcare systems in remarkable ways, but there is a problem. The mapping of anatomy to its physiological functions is essentially derived from the work of Vesalius and traditionally favoured mobility over stability, and as a consequence has allowed the entrenched and simplifying assumptions of the musculoskeletal duality to persist to the present day, despite advances in technology.
The lever model of motion, for example, assumes that the body is an intrinsically unstable system that requires an external controller (e.g. neural) to provide the necessary ‘catch-up’ stability for transient muscular latencies, and it is likely that the vulnerabilities inherent within such a mechanism would severely compromise living tissues. The foundational biomechanical assumptions of steady-state forces and kinematics has meant that the disproportionate and potentially damaging consequences of transient peak loadings have been largely overlooked, and which added to the long healing times required for post-traumatic recovery, suggests that such a mechanism would lead to material fatigue and destructive tissue failure.
The musculoskeletal duality, however, was not always so dominant but conceptually rivalled in the 17th and 18th centuries by Hooke’s ‘cells’ and Malpighi’s ‘cellular tissues’, both of which have been largely forgotten but now deserve a re-evaluation. The definition of the term ‘cell’ as a small compartment within a larger structure had quite different connotations then than it does today, but this compartmental aspect of connective tissue anatomy gradually faded and is now only recognized for its pathological significance.
This paper examines musculoskeletal anatomy from both historical and more recent viewpoints and highlights the concept of the fascial system as a distinct and intrinsically stable functional entity. It is a perspective that enables every anatomical ‘part’ to be included within a ‘cellular’ framework that differs substantially from the mobility-driven machine model: a tensioned fibrous network encompassing a complex heterarchy of regionally specialized compartments under compression, each of which has its own physical and parenchyma-driven characteristics that contribute to the functional whole. In other words, an updated fascia-centric interpretation of architectural anatomy that maps muscles and bones in a substantially different way from traditional models, renders the term musculoskeletal obsolete and greatly expands the meaning of compartment syndrome.
... Interestingly, players with the larger ASR also showed lower neuromuscular impairments and ratings of perceived exertion (RPE) during this session (due to using a lower % of ASR [43,44]). These are pertinent findings with regards to ASR application in a team sport context, where a greater use of the ASR is associated with a greater rate of fatigue development, which in theory relates to alterations in neuromuscular co-ordination patterns and movement execution [45][46][47][48], both of which would directly impact decision making and technical performances [49,50]. However, whether this concept can be directly transferred to the majority of team sport actions during matches and training remains to be elucidated, since the speed of movement often does not reach the 'ASR domain' during the majority of actions in team sports. ...
Many individual and team sport events require extended periods of exercise above the speed or power associated with maximal oxygen uptake (i.e., maximal aerobic speed/power, MAS/MAP). In the absence of valid and reliable measures of anaerobic metabolism, the anaerobic speed/power reserve (ASR/APR) concept, defined as the difference between an athlete’s MAS/MAP and their maximal sprinting speed (MSS)/peak power (MPP), advances our understanding of athlete tolerance to high speed/power efforts in this range. When exercising at speeds above MAS/MAP, what likely matters most, irrespective of athlete profile or locomotor mode, is the proportion of the ASR/APR used, rather than the more commonly used reference to percent MAS/MAP. The locomotor construct of ASR/APR offers numerous underexplored opportunities. In particular, how differences in underlying athlete profiles (e.g., fiber typology) impact the training response for different ‘speed’, ‘endurance’ or ‘hybrid’ profiles is now emerging. Such an individualized approach to athlete training may be necessary to avoid ‘maladaptive’ or ‘non-responses’. As a starting point for coaches and practitioners, we recommend upfront locomotor profiling to guide training content at both the macro (understanding athlete profile variability and training model selection, e.g., annual periodization) and micro levels (weekly daily planning of individual workouts, e.g., short vs long intervals vs repeated sprint training and recovery time between workouts). More specifically, we argue that high-intensity interval training formats should be tailored to the locomotor profile accordingly. New focus and appreciation for the ASR/APR is required to individualize training appropriately so as to maximize athlete preparation for elite competition.
... We offer an alternative hypothesis that anti-phase motion at peak vertical ground reaction force creates a stable system, taking advantage of passive resistance to more transfer force through the lower extremity. In agreement, Kiely and Collins (Kiely & Collins, 2016) suggest that "biotensegrity"when tension and compression resistant tissues are organized in a specific configuration creating a selfstabilizing system -is pivotal in the coordination of human running. It is possible that protective adaptations in older runners (i.e. ...
Older runners are at greater risk of certain running-related injuries. Previous work demonstrated that aging influences running biomechanics, and suggest a compensatory relation between changes in the proximal and distal joints. Previous comparisons of interjoint coordination strategies between young and older runners could potentially have missed relevant differences by averaging coordination measures across time.
Objective
To compare coordination strategies between male runners under the age of 30 to those over the age of 60.
Methods
Twelve young (22 ± 3 yrs, 1.80 ± 0.07 m, 78.0 ± 12.1 kg) and 12 older (63 ± 3 yrs, 1.78 ± 0.06 m, 73.2 ± 15.8 kg) male runners ran at 3.35 m/s on an instrumented treadmill. Ankle frontal plane, tibial transverse plane, knee sagittal plane, and hip frontal plane motion were measured. Inter-joint coordination was calculated using a modified vector coding technique. Coordination patterns and variability time series were compared between groups throughout stance using ANOVA for circular data.
Results
At the ankle, older runners use in-phase propulsion (inversion, tibia external rotation) pattern following midstance (46–47% stance) while young runners are still in an in-phase collapse pattern (eversion, tibia external rotation). In coordination of the knee and hip, older runners maintained an in-phase collapse pattern (knee flexion, hip adduction) approaching midstance (35–37% stance), while younger runners use an out of phase strategy (knee extension, hip adduction). In coordination of the ankle and hip in the frontal plane, older runners again maintained an in phase collapse pattern up to midstance (34–39% stance), while younger runners used an out of phase strategy (ankle inversion, hip adduction). Variability was similar between age groups.
Conclusion
Older runners appear to display altered coordination patterns during mid-stance, which may indicate protective biomechanical adaptations. These changes may also have implications for performance in older runners.
... Neuromuscular aspects refer to the nervous system and coordination of muscle contraction needed to perform the running task [34]. Importantly, having coordination across a mechanical bandwidth of speeds in and around race pace, and the ability to smoothly selfadjust, will enable efficiency for race surges under fatigue [35,36]. Sections 3.1 and 3.2 discuss the neuromuscular, biomechanical and motor qualities that underpin race pace speed. ...
Recent evidence indicates that the modern-day men’s 800 m runner requires a speed capability beyond that of previous eras. In addition, the appreciation of different athlete subgroups (400–800, 800, 800–1500 m) implies a complex interplay between the mechanical (aerial or terrestrial) and physiological characteristics that enable success in any individual runner. Historically, coach education for middle-distance running often emphasises aerobic metabolic conditioning, while it relatively lacks consideration for an important neuromuscular and mechanical component. Consequently, many 800 m runners today may lack the mechanical competence needed to achieve the relaxed race pace speed required for success, resulting in limited ability to cope with surges, run faster first laps or close fast. Mechanical competence may refer to the skilled coordination of neuromuscular/mechanical (stride length/frequency/impulse) and metabolic components needed to sustain middle-distance race pace and adjust to surges efficiently. The anaerobic speed reserve (ASR) construct (difference between an athlete’s velocity at maximal oxygen uptake [vO2max]—the first speed at which maximal oxygen uptake [O2max] is attained) and their maximal sprint speed (MSS) offers a framework to assess a runner’s speed range relative to modern-day race demands. While the smooth and relaxed technique observed in middle-distance runners is often considered causal to running economy measured during submaximal running, little empirical evidence supports such an assumption. Thus, a multidisciplinary approach is needed to examine the underpinning factors enabling elite 800 m running race pace efficiency. Here, we argue for the importance of utilising the ASR and MSS measurement to ensure middle-distance runners have the skills to compete in the race-defining surges of modern-day 800 m running.
An adaptive response to unexpected perturbations requires near-term and long-term adjustments over time. We used multifractal analysis to test how nonlinear interactions across timescales might support an adaptive response following an unpredictable perturbation. We reanalyzed torque data from 44 young and 24 older adults who performed a single-leg squat task challenged by an unexpected mechanical perturbation and a secondary visual-cognitive task. We report three findings: (a) multifractal nonlinearity interacted with pre-perturbation torque production and task error to presage greater pre-voluntary feedforward increases and greater voluntary reductions, respectively, in post-perturbation task error; (b) multifractal nonlinearity presaged relatively smaller task error than standard deviations of both pre-perturbation torques and pre-perturbation task error; and (c) increased task demand (e.g., age-related changes in dexterity and dual-task settings) led to multifractal nonlinearity presaging reduced task error. All these results were consistent with our expectations, except that a pre-perturbation knee torque-dependent increase in post-perturbation task error appeared later for older than for younger participants. This correlational multifractal modeling offered theoretical clarity on the possible roles of nonlinear interactions across timescales, moderating both feedforward and feedback processes, and presaging greater stability when the standard deviation is relatively large and task demands are strong. Thus, multifractal nonlinearity usefully describes movement variability even when paired with classical descriptors like the standard deviation. We discuss potential insights from these findings for understanding suprapostural dexterity and developing rehabilitative interventions.
As research into the postural and cognitive effects of ultramarathon running is sparse and still needed, we investigated the effect of ultramarathon running on runners’ postural control, dual task postural control and a measure of executive function—the flanker test, expecting fatigue-related deterioration on each measure. We used a pre- and post-test research design with 14 runners who completed (a) postural assessment with eyes open and closed, on a flat surface and on foam during (b) a two-choice reaction time dual task postural assessment, and (c) an executive function modified flanker task. With regard to postural stability, we observed, after running, increased anterior-posterior (AP) path length and AP root mean square (RMS) and reductions in both mediolateral (ML) RMS and ML median frequency. Dual task analysis showed reduced ML RMS prior to the race, whereas the effect was absent afterwards. Reaction times were not significantly altered between pre-post or surface conditions assessments. There were no statistically significant differences in mean modified flanker scores before and after the race. These data demonstrated that, following an endurance run, there were plane specific movement adaptations in postural sway that may have resulted from neuroprotective changes under extreme fatigue.
The human leg with segments, joints and many muscles is a complicated device. Yet, in dynamic situations such as running, hopping or jumping we behave with ease and without being overwhelmed by the complicated task. We argue that this is possible due to a careful arrangement and fine tuning of all properties from which stability and robustness emerges. Robust and stable systems are easy to control.
There are many possible approaches for developing models of physical systems. At one extreme exists the black-box model in which only the inputs and outputs of the system are considered important aspects of the model (see Chapter 9). Alternatively one can divide a system into separate components and model each component separately (see Chapter 8). The most obvious difference between these approaches is that there is more information in the latter model than just the inputs and outputs of the system. This latter approach is loosely termed reductionism—the form of the model for the system is ‘reduced’ into smaller components, each of which should have some testable relationship to a corresponding physical structure.
Muscle spindles are commonly considered as stretch receptors encoding movement, but the functional consequence of their efferent control has remained unclear. The "α-γ coactivation" hypothesis states that activity in a muscle is positively related to the output of its spindle afferents. However, in addition to the above, possible reciprocal inhibition of spindle controllers entails a negative relationship between contractile activity in one muscle and spindle afferent output from its antagonist. By recording spindle afferent responses from alert humans using microneurography, I show that spindle output does reflect antagonistic muscle balance. Specifically, regardless of identical kinematic profiles across active finger movements, stretch of the loaded antagonist muscle (i.e., extensor) was accompanied by increased afferent firing rates from this muscle compared with the baseline case of no constant external load. In contrast, spindle firing rates from the stretching antagonist were lowest when the agonist muscle powering movement (i.e., flexor) acted against an additional resistive load. Stepwise regressions confirmed that instantaneous velocity, extensor, and flexor muscle activity had a significant effect on spindle afferent responses, with flexor activity having a negative effect. Therefore, the results indicate that, as consequence of their efferent control, spindle sensitivity (gain) to muscle stretch reflects the balance of activity between antagonistic muscles rather than only the activity of the spindle-bearing muscle.
The commonly accepted "tower of blocks" model for vertebrate spine mechanics is only useful when modeling a perfectly balanced, upright, immobile spine. Using that model, in any other position than perfectly upright, the forces generated will tear muscle, crush bone and exhaust energy. A new model of the spine uses a tensegrity-truss system that will model the spine right side up, upside-down or in any position, static or dynamic. In a tensegrity-truss model, the loads distribute through the system only in tension or compression. As in all truss systems, there are no levers and no moments at the joints. The model behaves non-linearly and is energy efficient. Unlike a tower of blocks, it is independent of gravity and functions equally well on land, at sea, in the air or in space and models the spines of fish and fowl, bird and beast.
Although the concept of central pattern generators (CPGs) controlling locomotion in vertebrates is widely accepted, the presence of specialized CPGs in human locomotion is still a matter of debate. An interesting numerical model developed in the 90s’ demonstrated the important role CPGs could play in human locomotion, both in terms of stability against perturbations, and in terms of speed control. Recently, a reflex-based neuro-musculo-skeletal model has been proposed, showing a level of stability to perturbations similar to the previous model, without any CPG components. Although exhibiting striking similarities with human gaits, the lack of CPG makes the control of speed/step length in the model difficult. In this paper, we hypothesize that a CPG component will offer a meaningful way of controlling the locomotion speed. After introducing the CPG component in the reflex model, and taking advantage of the resulting properties, a simple model for gait modulation is presented. The results highlight the advantages of a CPG as feedforward component in terms of gait modulation.
The spinal stretch reflex and its electrical analog the Hoffmann (H)-reflex are simple behaviors mediated by spinal cord pathways. Because these pathways are affected by descending activity from the brain, monkeys, humans, rats, and mice can gradually increase or decrease these reflexes when rewarded for doing so. This operant conditioning depends on the corticospinal tract and produces a complex hierarchy of spinal and supraspinal plasticity. It is related to processes that occur during development, during skill acquisition throughout life, and in response to trauma and disease. It is a simple model for exploring the mechanisms of skill acquisition and may be the basis for new rehabilitation methods for people with spinal cord injuries or other disorders of motor control.
The problem of controlling locomotion is an area in which neuroscience and robotics can fruitfully interact. In this article, I will review research carried out on locomotor central pattern generators (CPGs), i.e. neural circuits capable of producing coordinated patterns of high-dimensional rhythmic output signals while receiving only simple, lowdimensional, input signals. The review will first cover neurobiological observations concerning locomotor CPGs and their numerical modelling, with a special focus on vertebrates. It will then cover how CPG models implemented as neural networks or systems of coupled oscillators can be used in robotics for controlling the locomotion of articulated robots. The review also presents how robots can be used as scientific tools to obtain a better understanding of the functioning of biological CPGs. Finally, various methods for designing CPGs to control specific modes of locomotion will be briefly reviewed. In this process, I will discuss different types of CPG models, the pros and cons of using CPGs with robots, and the pros and cons of using robots as scientific tools. Open research topics both in biology and in robotics will also be discussed.
Striding bipedalism is a key derived behaviour of hominids that possibly originated soon after the divergence of the chimpanzee and human lineages. Although bipedal gaits include walking and running, running is generally considered to have played no major role in human evolution because humans, like apes, are poor sprinters compared to most quadrupeds. Here we assess how well humans perform at sustained long-distance running, and review the physiological and anatomical bases of endurance running capabilities in humans and other mammals. Judged by several criteria, humans perform remarkably well at endurance running, thanks to a diverse array of features, many of which leave traces in the skeleton. The fossil evidence of these features suggests that endurance running is a derived capability of the genus Homo, originating about 2 million years ago, and may have been instrumental in the evolution of the human body form.
Often relegated to the methods section of genetic research articles, the term “degeneracy” is regularly misunderstood and its theoretical significance widely understated. Degeneracy describes the ability of different structures to be conditionally interchangeable in their contribution to system functions. Frequently mislabeled redundancy, degeneracy refers to structural variation whereas redundancy refers to structural duplication. Sources of degeneracy include, but are not limited to, (1) duplicate structures that differentiate yet remain isofunctional, (2) unrelated isofunctional structures that are dispersed endogenously or exogenously, (3) variable arrangements of interacting structures that achieve the same output through multiple pathways, and (4) parcellation of a structure into subunits that can still variably perform the same initial function. The ability to perform the same function by drawing upon an array of dissimilar structures contributes advantageously to the integrity of a system. Drawing attention to the heterogeneous construction of living systems by highlighting the concept of degeneracy valuably enhances the ways scientists think about self-organization, robustness, and complexity. Labels in science, however, can sometimes be misleading. In scientific nomenclature, the word “degeneracy” has calamitous proximity to the word “degeneration” used by pathologists and the shunned theory of degeneration once promoted by eugenicists. This article disentangles the concept of degeneracy from its close etymological siblings and offers a brief overview of the historical and contemporary understandings of degeneracy in science. Distinguishing the importance of degeneracy will hopefully allow systems theorists to more strategically operationally conceptualize the distributed intersecting networks that comprise complex living systems. © 2014 Wiley Periodicals, Inc. Complexity, 2014