Control of functional electrical stimulation with extended physiological proprioception

The use of functional electrical stimulation (FES) of muscle for paraplegic locomotion, or grasp augmentation in tetraplegia, is limited by the variability in muscle response to stimulation as a result of several external and internal factors. Previous approaches to this problem have used position-servo controllers, which have been shown to function satisfactorily in the laboratory. However, such systems will fail should obstacles be encountered or should the stimulation hardware develop a fault. To prevent such potentially dangerous failures some form of sensory feedback is required. This paper describes the first application of a technique known as extended physiological proprioception (EPP) to the control of FES to compensate for muscle response variability and provide proprioceptive feedback via the appropriate sensory pathways. In the experimental system described, a paraplegic subject controlled the extension of his paralysed knee by shoulder protraction. A Bowden cable linked the two joints, and a dynamometer in this cable was used to derive the control signal for a computer-controlled stimulator which delivered surface stimulation to the quadriceps muscle group. Modelling and parameter identification were performed by analysis of the step response, and the controller was designed from consideration of the root locus. The advantages of the system, in terms of improved proprioceptive feedback and reduced limb-positioning error were assessed in a test of joint positioning accuracy with vision occluded. The EPP system showed improvements over both open and closed-loop position-servo controllers.     

Desert ants: is active locomotion a prerequisite for path integration?

Desert ants Cataglyphis fortis have been shown to be able to employ two mechanisms of distance estimation: exploiting both optic flow and proprioceptive information. This study aims at understanding possible interactions between the two possibly redundant mechanisms of distance estimation. We ask whether in Cataglyphis the obviously minor contribution of optic flow would increase or even take over completely if the ants were deprived of reliable proprioceptive information. In various experimental paradigms ants were subjected to passive horizontal displacements during which they perceived optic flow, but were prohibited from active locomotion. The results show that in desert ants active locomotion is essential for providing the ants’ odometer and hence its path integrator with the necessary information.     

Micro movements of the upper limb in fibromyalgia: The relation to proprioceptive accuracy and visual feedback

The purpose of this study was to explore the role of visual and proprioceptive feedback in upper limb posture control in fibromyalgia (FM) and to assess the coherence between acceleration measurements of upper limb micro movements and surface electromyography (sEMG) of shoulder muscle activity (upper trapezius and deltoid). Twenty-five female FM patients and 25 age- and sex-matched healthy controls (HCs) performed three precision motor tasks: (1) maintain a steady shoulder abduction angle of 45° while receiving visual feedback about upper arm position and supporting external loads (0.5, 1, or 2 kg), (2) maintain the same shoulder abduction angle without visual feedback (eyes closed) and no external loading, and (3) a joint position sense test (i.e., assessment of proprioceptive accuracy). Patients had more extensive increase in movement variance than HCs when visual feedback was removed (P < 0.03). Proprioceptive accuracy was related to movement variance in HCs (R ⩾ 0.59, P ⩽ 0.002), but not in patients (R ⩽ 0.25, P ⩾ 0.24). There was no difference between patients and HCs in coherence between sEMG and acceleration data. These results may indicate that FM patients are more dependent on visual feedback and less reliant on proprioceptive information for upper limb posture control compared to HCs.     

Corollary discharge enables proprioception from lateral line sensory feedback

Animals modulate sensory processing in concert with motor actions. Parallel copies of motor signals, called corollary discharge (CD), prepare the nervous system to process the mixture of externally and self-generated (reafferent) feedback that arises during locomotion. Commonly, CD in the peripheral nervous system cancels reafference to protect sensors and the central nervous system from being fatigued and overwhelmed by self-generated feedback. However, cancellation also limits the feedback that contributes to an animal’s awareness of its body position and motion within the environment, the sense of proprioception. We propose that, rather than cancellation, CD to the fish lateral line organ restructures reafference to maximize proprioceptive information content. Fishes’ undulatory body motions induce reafferent feedback that can encode the body’s instantaneous configuration with respect to fluid flows. We combined experimental and computational analyses of swimming biomechanics and hair cell physiology to develop a neuromechanical model of how fish can track peak body curvature, a key signature of axial undulatory locomotion. Without CD, this computation would be challenged by sensory adaptation, typified by decaying sensitivity and phase distortions with respect to an input stimulus. We find that CD interacts synergistically with sensor polarization to sharpen sensitivity along sensors’ preferred axes. The sharpening of sensitivity regulates spiking to a narrow interval coinciding with peak reafferent stimulation, which prevents adaptation and homogenizes the otherwise variable sensor output. Our integrative model reveals a vital role of CD for ensuring precise proprioceptive feedback during undulatory locomotion, which we term external proprioception.

Animals modulate sensory processing in concert with motor actions. A study of the corollary discharge in zebrafish reveals that it modulates the sensitivity of the lateral line during swimming to prevent sensor adaptation and maintain the high-quality feedback necessary for kinematic control.     

Differential control of temporal and spatial aspects of cockroach leg coordination

Ensembles of neuronal networks and sensory pathways participate in controlling the kinematic and dynamic parameters of animal movement necessary to achieve motor coordination. Determining the relative contribution of proprioceptive feedback is essential for understanding how animals sustain stable, coordinated locomotion in complex natural environments. Here, we focus on the role of chordotonal organs (COs), proprioceptors found in insect legs, in the spatial and temporal regulation of walking. We compare gait parameters of intact cockroaches (Periplaneta americana) and sensory-impaired ones, injected with pymetrozine, a chemical previously shown to abolish CO function in locusts. We verify that afferent CO activity in pymetrozine-treated cockroaches is inhibited, and analyze the effect of this sensory deprivation on inter-leg coordination. We find significant changes in tarsi placement and leg path trajectories after pymetrozine treatment. Leg touchdown accuracy, measured from relative tarsi positions of adjacent legs, is reduced in treated animals. Interestingly, despite poorer spatial coordination in both stance and swing, temporal properties of the gait remain largely the same as in the intact preparations, apart from changes in ipsilateral phase differences between front and middle legs. These findings provide insights into the role of COs in insect gait control and establish pymetrozine as a useful tool for further studies of insect locomotion.     

Internal noise-sustained circadian rhythms in a Drosophila model

Circadian rhythmic processes, mainly regulated by gene expression at the molecular level, have inherent stochasticity. Their robustness or resistance to internal noise has been extensively investigated by most of the previous studies. This work focuses on the constructive roles of internal noise in a reduced Drosophila model, which incorporates negative and positive feedback loops, each with a time delay. It is shown that internal noise sustains reliable oscillations with periods close to 24 h in a region of parameter space, where the deterministic kinetics would evolve to a stable steady state. The amplitudes of noise-sustained oscillations are significantly affected by the variation of internal noise level, and the best performance of the oscillations could be found at an optimal noise intensity, indicating the occurrence of intrinsic coherence resonance. In the oscillatory region of the deterministic model, the coherence of noisy circadian oscillations is suppressed by internal noise, while the period remains nearly constant over a large range of noise intensity, demonstrating robustness of the Drosophila model for circadian rhythms to intrinsic noise. In addition, the effects of time delay in the positive feedback on the oscillations are also investigated. It is found that the time delay could efficiently tune the performance of the noise-sustained oscillations. These results might aid understanding of the exploitation of intracellular noise in biochemical and genetic regulatory systems.     

Proprioceptors of the thoracic limbs of Squilla mantis (Crustacea,Stomatopoda)

Summary The distribution and morphology of the internal proprioceptive organs in the thoracic appendages of the stomatopod crustacean, Squilla mantis, are described. Five distinct types of proprioceptor are recognised; connective chordotonal organ, myochordotonal organ, cuticular stress detector, multiterminal stretch receptor and apodeme stretch receptor. The connective chordotonal organs and multiterminal receptors exhibit a wide variety of structural and functional complexity. The stomatopods resemble the decapods in that the basic proprioceptive unit of the thoracic appendages appears to be the connective chordotonal organ, but differs in the higher occurrence of multiterminal stretch receptors. As described for decapods, the pereiopods of stomatopods possess the largest number and greatest variety of proprioceptors, suggesting that locomotion exerts greater selective pressure on proprioceptor development than do the other thoracic limb functions, including the raptorial strike.     

Rhizobium meliloti exopolysaccharide Mutants Elicit Feedback Regulation of Nodule Formation in Alfalfa

Nodule formation by wild-type Rhizobium meliloti is strongly suppressed in younger parts of alfalfa (Medicago sativum L.) root systems as a feedback response to development of the first nodules (G Caetano-Anollés, WD Bauer [1988] Planta 175: 546-557). Mutants of R. meliloti deficient in exopolysaccharide synthesis can induce the formation of organized nodular structures (pseudonodules) on alfalfa roots but are defective in their ability to invade and multiply within host tissues. The formation of empty pseudonodules by exo mutants was found to elicit a feedback suppression of nodule formation similar to that elicited by the wild-type bacteria. Inoculation of an exo mutant onto one side of a split-root system 24 hours before inoculation of the second side with wild-type cells suppressed wild-type nodule formation on the second side in proportion to the extent of pseudonodule formation by the exo mutants. The formation of pseudonodules is thus sufficient to elicit systemic feedback control of nodulation in the host root system: infection thread development and internal proliferation of the bacteria are not required for elicitation of feedback. Pseudonodule formation by the exo mutants was found to be strongly suppressed in split-root systems by prior inoculation on the opposite side with the wild type. Thus, feedback control elicited by the wild-type inhibits Rhizobium-induced redifferentiation of host root cells.     

Mechanosensitive body–brain interactions in Caenorhabditis elegans

Proprioception and visceral mechanosensation provide important information about the location and deformation of the body parts in space and time. These deformations arise from muscle contraction during locomotion, but also from volume changes in organs that are subjected to stresses as a part of their physiological function. These internal morphodynamics give rise to periodic contraction–relaxation cycles with surprisingly constant amplitudes and the maintenance of these optimal driving patterns is remarkably robust against external and internal perturbations. One of the underlying reason for this robustness is an internal feedback mechanism in which specialized sensory cells and neurons signal the mechanical deformation of the inner workings of our organs, from the body to the brain, which subsequently adjust the driver to a predetermined physiological setpoint. Here, we review recent progress in the field of visceral mechanosensation and proprioception in Caenorhabditis elegans and discuss how future studies with this model can be used to gain insight into mechanosensory body–brain interactions in mammals.     

Role of Sensory Experience in Functional Development of Drosophila Motor Circuits

Neuronal circuits are formed according to a genetically predetermined program and then reconstructed in an experience-dependent manner. While the existence of experience-dependent plasticity has been demonstrated for the visual and other sensory systems, it remains unknown whether this is also the case for motor systems. Here we examined the effects of eliminating sensory inputs on the development of peristaltic movements in Drosophila embryos and larvae. The peristalsis is initially slow and uncoordinated, but gradually develops into a mature pattern during late embryonic stages. We tested whether inhibiting the transmission of specific sensory neurons during this period would have lasting effects on the properties of the sensorimotor circuits. We applied Shibire-mediated inhibition for six hours during embryonic development (15–21 h after egg laying [AEL]) and studied its effects on peristalsis in the mature second- and third-instar larvae. We found that inhibition of chordotonal organs, but not multidendritic neurons, led to a lasting decrease in the speed of larval locomotion. To narrow down the sensitive period, we applied shorter inhibition at various embryonic and larval stages and found that two-hour inhibition during 16–20 h AEL, but not at earlier or later stages, was sufficient to cause the effect. These results suggest that neural activity mediated by specific sensory neurons is involved in the maturation of sensorimotor circuits in Drosophila and that there is a critical period for this plastic change. Consistent with a role of chordotonal neurons in sensory feedback, these neurons were activated during larval peristalsis and acute inhibition of their activity decreased the speed of larval locomotion.     

Dispensable,Redundant, Complementary,and Cooperative Roles of Dopamine,Octopamine, and Serotonin in Drosophila melanogaster

To investigate the regulation of Drosophila melanogaster behavior by biogenic amines, we have exploited the broad requirement of the vesicular monoamine transporter (VMAT) for the vesicular storage and exocytotic release of all monoamine neurotransmitters. We used the Drosophila VMAT (dVMAT) null mutant to globally ablate exocytotic amine release and then restored DVMAT activity in either individual or multiple aminergic systems, using transgenic rescue techniques. We find that larval survival, larval locomotion, and female fertility rely predominantly on octopaminergic circuits with little apparent input from the vesicular release of serotonin or dopamine. In contrast, male courtship and fertility can be rescued by expressing DVMAT in octopaminergic or dopaminergic neurons, suggesting potentially redundant circuits. Rescue of major aspects of adult locomotion and startle behavior required octopamine, but a complementary role was observed for serotonin. Interestingly, adult circadian behavior could not be rescued by expression of DVMAT in a single subtype of aminergic neurons, but required at least two systems, suggesting the possibility of unexpected cooperative interactions. Further experiments using this model will help determine how multiple aminergic systems may contribute to the regulation of other behaviors. Our data also highlight potential differences between behaviors regulated by standard exocytotic release and those regulated by other mechanisms.     

FIM,a Novel FTIR-Based Imaging Method for High Throughput Locomotion Analysis

We designed a novel imaging technique based on frustrated total internal reflection (FTIR) to obtain high resolution and high contrast movies. This FTIR-based Imaging Method (FIM) is suitable for a wide range of biological applications and a wide range of organisms. It operates at all wavelengths permitting the in vivo detection of fluorescent proteins. To demonstrate the benefits of FIM, we analyzed large groups of crawling Drosophila larvae. The number of analyzable locomotion tracks was increased by implementing a new software module capable of preserving larval identity during most collision events. This module is integrated in our new tracking program named FIMTrack which subsequently extracts a number of features required for the analysis of complex locomotion phenotypes. FIM enables high throughput screening for even subtle behavioral phenotypes. We tested this newly developed setup by analyzing locomotion deficits caused by the glial knockdown of several genes. Suppression of kinesin heavy chain (khc) or rab30 function led to contraction pattern or head sweeping defects, which escaped in previous analysis. Thus, FIM permits forward genetic screens aimed to unravel the neural basis of behavior.     

Proprioceptive input to a descending pathway conveying antennal postural information: Terminal organisation of antennal hair field afferents

Like several other arthropod species, stick insects use their antennae for tactile exploration of the near-range environment and for spatial localisation of touched objects. More specifically, Carausius morosus continuously moves its antennae during locomotion and reliably responds to antennal contact events with directed movements of a front leg. Here we investigate the afferent projection patterns of antennal hair fields (aHF), proprioceptors known to encode antennal posture and movement, and to be involved in antennal movement control. We show that afferents of all seven aHF of C. morosus have terminal arborisations in the dorsal lobe (DL) of the cerebral (=supraoesophageal) ganglion, and descending collaterals that terminate in a characteristic part of the gnathal (=suboesophageal) ganglion. Despite differences of functional roles among aHF, terminal arborisation patterns show no topological arrangement according to segment specificity or direction of movement. In the DL, antennal motoneuron neurites show arborizations in proximity to aHF afferent terminals. Despite the morphological similarity of single mechanoreceptors of aHF and adjacent tactile hairs on the pedicel and flagellum, we find a clear separation of proprioceptive and exteroceptive mechanosensory neuropils in the cerebral ganglion. Moreover, we also find this functional separation in the gnathal ganglion.     

Predictability of visual perturbation during locomotion: implications for corrective efference copy signaling

In guiding adaptive behavior, efference copy signals or corollary discharge are traditionally considered to serve as predictors of self-generated sensory inputs and by interfering with their central processing are able to counter unwanted consequences of an animal??s own actions. Here, in a speculative reflection on this issue, we consider a different functional role for such intrinsic predictive signaling, namely in stabilizing gaze during locomotion where resultant changes in head orientation in space require online compensatory eye movements in order to prevent retinal image slip. The direct activation of extraocular motoneurons by locomotor-related efference copies offers a prospective substrate for assisting self-motion derived sensory feedback, rather than being subtracted from the sensory signal to eliminate unwanted reafferent information. However, implementing such a feed-forward mechanism would be critically dependent on an appropriate phase coupling between rhythmic propulsive movement and resultant head/visual image displacement. We used video analyzes of actual locomotor behavior and basic theoretical modeling to evaluate head motion during stable locomotion in animals as diverse as Xenopus laevis tadpoles, teleost fish and horses in order to assess the potential suitability of spinal efference copies to the stabilization of gaze during locomotion. In all three species, and therefore regardless of aquatic or terrestrial environment, the head displacements that accompanied locomotor action displayed a strong correlative spatio-temporal relationship in correspondence with a potential predictive value for compensatory eye adjustments. Although spinal central pattern generator-derived efference copies offer appropriately timed commands for extraocular motor control during self-generated motion, it is likely that precise image stabilization requires the additional contributions of sensory feedback signals. Nonetheless, the predictability of the visual consequences of stereotyped locomotion renders intrinsic efference copy signaling an appealing mechanism for offsetting these disturbances, thus questioning the exclusive role traditionally ascribed to sensory-motor transformations in stabilizing gaze during vertebrate locomotion.     

Underwater locomotion strategy by a benthic pennate diatom Navicula sp.

The mechanism of diatom locomotion has been widely researched but still remains a hypothesis. There are several questionable points on the prevailing model proposed by Edgar, and some of the observed phenomena cannot be completely explained by this model. In this paper, we undertook detailed investigations of cell structures, locomotion, secreted mucilage, and bending deformation for a benthic pennate diatom Navicula species. According to these broad evidences, an updated locomotion model is proposed. For Navicula sp., locomotion is realized via two or more pseudopods or stalks protruded out of the frustules. The adhesion can be produced due to the pull-off of one pseudopod or stalk from the substratum through extracellular polymeric substances. And the positive pressure is generated to balance the adhesion because of the push-down of another pseudopod or stalk onto the substratum. Because of the positive pressure, friction is generated, acting as a driving force of locomotion, and the other pseudopod or stalk can detach from the substratum, resulting in the locomotion. Furthermore, this model is validated by the force evaluation and can better explain observed phenomena. This updated model would provide a novel aspect on underwater locomotion strategy, hence can be useful in terms of artificial underwater locomotion devices.     

Diurnal locomotion and feeding activities of two rice-ear bugs,Trigonotylus caelestialium and Stenotus rubrovittatus (Hemiptera: Heteroptera: Miridae)

Diurnal locomotion and feeding activities of Trigonotylus caelestialium (Kirkaldy) and Stenotus rubrovittatus (Matsumura) were investigated using a video camera and electrical penetration graph. Diurnal locomotion activity of T. caelestialium was higher in photophase than in scotophase, whereas that of S. rubrovittatus was higher in scotophase than in photophase. No difference was observed in the locomotion activity of T. caelestialium between mated and unmated bugs or between males and females. Locomotion activity of S. rubrovittatus was different between the sexes. The activity of females was higher than that of males. Diurnal rhythms of feeding activity were obscure compared with those of the locomotion activities in both mirids. The feeding behavior of T. caelestialium was significantly more active in the photophase than in the scotophase. In S. rubrovittatus, feeding activity of males was higher in the scotophase than in the photophase, whereas females showed no such difference. It is thought that both species of mirid bugs are active during the daytime and nighttime, although the locomotion and feeding activities of T. caelestialium were high in the photophase, while those of S. rubrovittatus were high in the scotophase.     

Identification of Inhibitory Premotor Interneurons Activated at a Late Phase in a Motor Cycle during Drosophila Larval Locomotion

Rhythmic motor patterns underlying many types of locomotion are thought to be produced by central pattern generators (CPGs). Our knowledge of how CPG networks generate motor patterns in complex nervous systems remains incomplete, despite decades of work in a variety of model organisms. Substrate borne locomotion in Drosophila larvae is driven by waves of muscular contraction that propagate through multiple body segments. We use the motor circuitry underlying crawling in larval Drosophila as a model to try to understand how segmentally coordinated rhythmic motor patterns are generated. Whereas muscles, motoneurons and sensory neurons have been well investigated in this system, far less is known about the identities and function of interneurons. Our recent study identified a class of glutamatergic premotor interneurons, PMSIs (period-positive median segmental interneurons), that regulate the speed of locomotion. Here, we report on the identification of a distinct class of glutamatergic premotor interneurons called Glutamatergic Ventro-Lateral Interneurons (GVLIs). We used calcium imaging to search for interneurons that show rhythmic activity and identified GVLIs as interneurons showing wave-like activity during peristalsis. Paired GVLIs were present in each abdominal segment A1-A7 and locally extended an axon towards a dorsal neuropile region, where they formed GRASP-positive putative synaptic contacts with motoneurons. The interneurons expressed vesicular glutamate transporter (vGluT) and thus likely secrete glutamate, a neurotransmitter known to inhibit motoneurons. These anatomical results suggest that GVLIs are premotor interneurons that locally inhibit motoneurons in the same segment. Consistent with this, optogenetic activation of GVLIs with the red-shifted channelrhodopsin, CsChrimson ceased ongoing peristalsis in crawling larvae. Simultaneous calcium imaging of the activity of GVLIs and motoneurons showed that GVLIs’ wave-like activity lagged behind that of motoneurons by several segments. Thus, GVLIs are activated when the front of a forward motor wave reaches the second or third anterior segment. We propose that GVLIs are part of the feedback inhibition system that terminates motor activity once the front of the motor wave proceeds to anterior segments.     

System identification of muscle–joint interactions of the cat hind limb during locomotion

Neurophysiological experiments in walking cats have shown that a number of neural control mechanisms are involved in regulating the movements of the hind legs during locomotion. It is experimentally hard to isolate individual mechanisms without disrupting the natural walking pattern and we therefore introduce a different approach where we use a model to identify what control is necessary to maintain stability in the musculo-skeletal system. We developed a computer simulation model of the cat hind legs in which the movements of each leg are produced by eight limb muscles whose activations follow a centrally generated pattern with no proprioceptive feedback. All linear transfer functions, from each muscle activation to each joint angle, were identified using the response of the joint angle to an impulse in the muscle activation at 65 postures of the leg covering the entire step cycle. We analyzed the sensitivity and stability of each muscle action on the joint angles by studying the gain and pole plots of these transfer functions. We found that the actions of most of the hindlimb muscles display inherent stability during stepping, even without the involvement of any proprioceptive feedback mechanisms, and that those musculo-skeletal systems are acting in a critically damped manner, enabling them to react quickly without unnecessary oscillations. We also found that during the late swing, the activity of the posterior biceps/semitendinosus (PB/ST) muscles causes the joints to be unstable. In addition, vastus lateralis (VL), tibialis anterior (TA) and sartorius (SAT) muscle-joint systems were found to be unstable during the late stance phase, and we conclude that those muscles require neuronal feedback to maintain stable stepping, especially during late swing and late stance phases. Moreover, we could see a clear distinction in the pole distribution (along the step cycle) for the systems related to the ankle joint from that of the other two joints, hip or knee. A similar pattern, i.e., a pattern in which the poles were scattered over the s-plane with no clear clustering according to the phase of the leg position, could be seen in the systems related to soleus (SOL) and TA muscles which would indicate that these muscles depend on neural control mechanisms, which may involve supraspinal structures, over the whole step cycle.     

Predictive and reactive tuning of the locomotor CPG

The neural control of locomotion involves a constant interplaybetween the actions of a central pattern generator (CPG) andsensory input elicited by bodily movement. With respect to theCPG, recent analysis of fictive locomotion has shown that durationsof flexion and extension tend to covary along specific linesin plots of phase duration versus cycle duration. The slopesof these lines evidently depend on internal states that varyamong preparations, but, within a preparation, remain rathersteady from one sequence to the next. These relationships canbe reproduced in a simple oscillator model having two pairsof preset parameters, suggesting that steady internal drivesto flexor and extensor half-centers determine how phase durationscovary. Regarding the role of sensory inputs, previous experimentshave revealed state-dependent rules that govern phase-switchingindependently of the CPG rhythm. In addition, sensory inputis known to modulate motoneuronal activation through stretchreflexes. To explore how sensory input combines with the locomotorCPG, we used a neuromechanical model with muscle actuators,proprioceptive feedback, sensory phase-switching rules, anda CPG. Interestingly, sequences of stable locomotion were alwaysassociated with phase durations that conformed to an extensor-dominatedphase-duration characteristic (where extension durations varymore than flexion durations). This is the characteristic seenin normal animals, but not necessarily in fictive locomotion,where movement and associated sensory input are absent. Thissuggests that to produce the biomechanical events required forstability, an extensor-dominated phase-duration characteristicis required. In the model, when the preset CPG phase durationswere well matched to coincide the biomechanical requirements,CPG-mediated phase switching produced stable cycles. When CPGphase durations were too short, phases switched prematurelyand the model soon fell. When CPG phase durations were too long,sensory rules fired and overrode the CPG, maintaining stability.We posit that under normal circumstances, descending input fromhigher centers continually adjusts the operating point of theCPG on the preset phase-duration characteristic according toanticipated biomechanical requirements. When the predictionsare good, CPG-generated phase durations closely match thoserequired by the kinetics and kinematics, and little or no sensoryadjustment occurs. We propose the term "neuromechanical tuning"to describe this process of matching the CPG to the biomechanicalrequirements.