Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. II. Stepping in spinal kittens

Serotonergic modulation of post-synaptic inhibition and locomotor alternating pattern in the spinal cord

The central pattern generators (CPGs) for locomotion, located in the lumbar spinal cord, are functional at birth in the rat. Their maturation occurs during the last few days preceding birth, a period during which the first projections from the brainstem start to reach the lumbar enlargement of the spinal cord. Locomotor burst activity in the mature intact spinal cord alternates between flexor and extensor motoneurons through reciprocal inhibition and between left and right sides through commisural inhibitory interneurons. By contrast, all motor bursts are in phase in the fetus. The alternating pattern disappears after neonatal spinal cord transection which suppresses supraspinal influences upon the locomotor networks. This article will review the role of serotonin (5-HT), in particular 5-HT₂ receptors, in shaping the alternating pattern. For instance, pharmacological activation of these receptors restores the left-right alternation after injury. Experiments aimed at either reducing the endogenous level of serotonin in the spinal cord or blocking the activation of 5-HT₂ receptors. We then describe recent evidence that the action of 5-HT₂ receptors is mediated, at least in part, through a modulation of chloride homeostasis. The postsynaptic action of GABA and glycine depends on the intracellular concentration of chloride ions which is regulated by a protein in the plasma membrane, the K⁺-Cl⁻ cotransporter (KCC2) extruding both K⁺ and Cl⁻ ions. Absence or reduction of KCC2 expression leads to a depolarizing action of GABA and glycine and a marked reduction in the strength of postsynaptic inhibition. This latter situation is observed early during development and in several pathological conditions, such as after spinal cord injury, thereby causing spasticity and chronic pain. It was recently shown that specific activation of 5-HT_2A receptors is able to up-regulate KCC2, restore endogenous inhibition and reduce spasticity.

Kinematic analysis of overground locomotion in chicks incubated under different light conditions

Domestic chicks walk within 3-4 hr after hatching following 21 days of incubation. However, differences in light exposure can vary incubation duration. Based on pilot studies, we predicted that there would be a positive relationship between incubation duration and locomotor competence at hatching. Embryos were incubated in one of three conditions that varied light duration and intensity, and overground locomotor performance was tested on the day of hatching. Chicks incubated in continuous bright light hatched 1-2 days earlier than chicks incubated in less or no light. Kinematic findings indicated that locomotor skill was similar across incubation conditions and led us to reject our hypothesis. We propose that light may accelerate locomotor development without adversely affecting skill. Our findings raise two important implications for future studies: whether light exposure accelerates locomotor circuit development; and/or it unmasks adaptive motor skill by accelerating development of other physiological systems.

Interlimb coordination in rhythmic leg movements: spontaneous and training-induced manifestations in human infants

Different rhythmic leg movements in vertebrates can share coordinating neural circuitry. These movements are often similar kinematically, and smooth transitions between the different movements are common. We focused on interlimb coordination of the legs in young infants to determine whether weight bearing and non-weight bearing movements might share coordinating circuitry. If interlimb coordination is controlled by the same circuitry, the same coordination (i.e., either synchronous or alternate) should be seen in different rhythmic movements. Moreover, if we altered the interlimb coordination in one movement through exercise, it should translate to a change in coordination in another rhythmic movement that received no exercise. Video and electrogoniometry were recorded while 46 infants (age, 6.2+/-1.4 mo) performed non-weight bearing and weight bearing movements. Interlimb coordination was quantified by the phase lag between the movement cycles of each leg. Most infants (83%) showed the same coordination in weight bearing and non-weight bearing movements. Ten infants practiced the form of coordination they did not exhibit in the first visit, in weight bearing for 4 wk. Following practice, 8 of 10 infants changed their interlimb coordination in weight bearing to that practiced. Some who practiced synchronous coordination also changed their coordination in non-weight bearing activity. More infants showed both forms of coordination after practice and smooth transitions between the two forms. The results suggest that interlimb coordination is malleable in infants, and there is a partial sharing of the neural substrates for interlimb coordination between different rhythmic leg movements in infants.

Functional plasticity following spinal cord lesions

Spinal cord injury results in marked modification and reorganization of several reflex pathways caudal to the injury. The sudden loss or disruption of descending input engenders substantial changes at the level of primary afferents, interneurons, and motoneurons thus dramatically influencing sensorimotor interactions in the spinal cord. As a general rule reflexes are initially depressed following spinal cord injury due to severe reductions in motoneuron excitability but recover and in some instances become exaggerated. It is thought that modified inhibitory connections and/or altered transmission in some of these reflex pathways after spinal injury as well as the recovery and enhancement of membrane properties in motoneurons underlie several symptoms such as spasticity and may explain some characteristics of spinal locomotion observed in spinally transected animals. Indeed, after partial or complete spinal lesions at the last thoracic vertebra cats recover locomotion when the hindlimbs are placed on a treadmill. Although some deficits in spinal locomotion are related to lesion of specific descending motor pathways, other characteristics can also be explained by changes in the excitability of reflex pathways mentioned above. Consequently it may be the case that to reestablish a stable walking pattern that modified afferent inflow to the spinal cord incurred after injury must be normalized to enable a more normal re-expression of locomotor rhythm generating networks. Indeed, recent evidence demonstrates that step training, which has extensively been shown to facilitate and ameliorate locomotor recovery in spinal animals, directly influences transmission in simple reflex pathways after complete spinal lesions.

Reversible disorganization of the locomotor pattern after neonatal spinal cord transection in the rat

The central pattern generators (CPGs) for locomotion, located in the lumbar spinal cord, are functional at birth in the rat. Their maturation occurs during the last few days preceding birth, a period during which the first projections from the brainstem start to reach the lumbar enlargement of the spinal cord. The goal of the present study was to investigate the effect of suppressing inputs from supraspinal structures on the CPGs, shortly after their formation. The spinal cord was transected at the thoracic level at birth [postnatal day 0 (P0)]. We examined during the first postnatal week the capacity of the CPGs to produce rhythmic motor activity in two complementary experimental conditions. Left and right ankle extensor muscles were recorded in vivo during airstepping, and lumbar ventral roots were recorded in vitro during pharmacologically evoked fictive locomotion. Mechanical stimulation of the tail elicited long-lasting sequences of airstepping in the spinal neonates and only a few steps in sham-operated rats. In vitro experiments made simultaneously on spinal and sham animals confirmed the increased excitability of the CPGs after spinalization. A left-right alternating locomotor pattern was observed at P1-P3. Both types of experiments showed that the pattern was disorganized at P6-P7, and that the left-right alternation was lost. Alternation was restored after the activation of serotonergic 5-HT(2) receptors in vivo. These results suggest that descending pathways, in particular serotonergic projections, control the strength of reciprocal inhibition and therefore shape the locomotor pattern in the neonatal rat.

Electromyographic activity patterns of ankle flexor and extensor muscles during spontaneous and L-DOPA-induced locomotion in freely moving neonatal rats

In rats, hindlimb postural and locomotor functions mature during the first 3 postnatal weeks. Previous evidence indicates that maturation of descending monoaminergic pathways is important for the postnatal emergence of locomotion with adequate antigravity postural support. Here we have studied the effect of the monoamine precursor L-DOPA on locomotor activity in freely moving postnatal rats (7-9 days old) using electromyographic recordings from ankle extensor (soleus) and flexor (tibialis anterior or extensor digitorum longus) muscles. Before pharmacological treatment, both muscles were usually silent at rest, and during spontaneous movements there was a high degree of coactivation between the two antagonists. This was due to a longer electromyographic (EMG) burst duration in flexors, which partly overlapped with the extensor burst. L-DOPA administration (150 mg/kg) resulted in a marked increase in postural tonic EMG activity in extensors which appeared gradually within 10 min after injection and was sufficient for the pups to maintain a standing posture with the pelvis raised above ground. Thereafter, episodes of locomotion characterized by rhythmic reciprocal bursts of EMG activity in flexor and extensor muscles were seen. The L-DOPA-induced rhythmic EMG pattern was also seen in postnatal rats subjected to a midthoracic spinal cord transection, indicating that the effect of L-DOPA on motor coordination is exerted primarily at the level of the spinal pattern generator. Analysis of EMG burst characteristics showed that the pattern of L-DOPA-induced locomotion in both intact and spinalized postnatal rats resembled in some respects that observed in adults during spontaneous locomotion. The appearance of reciprocal activation during L-DOPA-induced locomotion in neonates was primarily due to a shortening of the EMG burst duration in flexors, which reduced the degree of antagonist coactivation. These results show that the spinal cord has the potential to produce coordinated overground locomotion several days before such movements are normally expressed in the freely moving animal.

Ankle restraint modifies motility at E12 in chick embryos

The chick's relationship to its environment changes dramatically over 21 days of embryonic development. At early ages embryos are buoyant; their posture and movements are relatively unconstrained. As embryos grow and fluid level in ovo decreases, movements are increasingly constrained by gravitational forces and reactive forces due to body contact with the shell wall. The issue of how age-related changes in the constraints on movement in ovo may affect embryonic motility is addressed in this paper. Our long-term goal is to determine whether experience imposed by these conditions contributes to development of posthatching motor behaviors. Because previous work indicated that parameters of motility can be modified by a reduction in buoyancy at embryonic day (E) 9, we sought to determine whether a restraint localized to a single joint could also alter either the episodic distribution of activity or the spatiotemporal patterns of limb movement at either E9 or E12. Thus a restraint was applied to the right ankle of embryos prepared for kinematic recordings. Video and kinematic analyses indicated that the restraint had minimal effect at E9, but significantly modified several motility parameters in both the wing and leg at E12. Ankle restraint decreased episode duration. Restraint also decreased most joint excursion parameters, including excursion range, cycles per sequence, and excursion velocity. Restraint increased cycle period duration and signal frequency content under 1.0 Hz. Parameters of intralimb and interlimb coordination exhibited small mixed effects. Results provide support for the hypothesis that environmental conditions contribute to features of embryonic motility. Further, significant modifications of wing excursions in ankle restrained embryos suggest that sensory feedback arising from mechanical perturbations of leg movements may entrain rostral spinal circuits for preservation of interlimb coordination at E12. Potential mechanisms and implications are discussed.

Weight support and balance during perturbed stance in the chronic spinal cat

The intact cat maintains balance during unexpected disturbances of stance through automatic postural responses that are stereotyped and rapid. The extent to which the chronic spinal cat can maintain balance during stance is unclear, and there have been no quantitative studies that examined this question directly. This study examined whether the isolated lumbosacral cord of the chronic spinal cat can generate automatic postural responses in the hindlimbs during translation of the support surface. Responses to 16 directions of linear translation in the horizontal plane were quantified before and after spinalization at the T(6) level in terms of forces exerted by each paw against the support, motion of the body segments (kinematics), and electromyographic (EMG) activity. After spinalization, the cats were trained on a daily basis to stand on the force platform, and all four cats were able to support their full body weight. The cats usually required assistance for balance or stability in the horizontal plane, which was provided by an experimenter exerting gentle lateral force at the level of the hips. Three of the four animals could maintain independent stance for a brief period (10 s) after the experimenter stabilized them. The fourth cat maintained weight support but always required assistance with balance. Perturbations were delivered during the periods of independent stance in three cats and during assisted stance in the fourth. A response to translation in the spinal cats was observed only in those muscles that were tonically active to maintain stance and never in the flexors. Moreover, latencies were increased and amplitudes of activation were diminished compared with control. Nevertheless, flexors and extensors were recruited easily during behaviors such as paw shake and stepping. It is concluded that centers above the lumbosacral cord are required for the full elaboration of automatic postural responses. Although the spinal cat can achieve good weight support, it cannot maintain balance during stance except for brief periods and within narrow limits. This limited stability is probably achieved through spinal reflex mechanisms and the stiffness characteristics of the tonically active extensors.

Adaptive Dynamics of the Leg Movement Patterns of Human Infants: II. Treadmill stepping in Infants and Adults

Infant treadmill steps have many temporal and kinematic similarities to adult walking. Kinematic similarities can result from different patterns of underlying torque, however. In this study, we used inverse dynamics to compare the patterns and contributions of active (muscle) and passive (gravity and motion-dependent) torques in the swing phase of treadmill stepping in 7-month-old infants and adults. Results indicated that adults consistently used muscle torque to initiate and terminate swing, but that passive torques accounted for leg motion during most of the swing phase. Infants, in contrast, displayed multiple patterns of torque contributions during swing. In the most frequently occurring infant pattern, muscle torque remained flexor throughout swing and joint reversals were due to the dominant passive gravitational torque. The kinetic data suggest that the temporally and kinematically similar treadmill steps produced by adults and infants do not emanate from a unique set of neural commands to the muscles, but from a flexible interplay between multiple internal as well as external elements. These data suggest that the intrinsic dynamics of the human system provide a medium out of which, given a supportive context, stable patterns can emerge spontaneously. During development, voluntary controlled movement patterns must build on these intrinsic dynamics.

Development of the EMG of the soleus muscle in the rat

The EMG of the soleus muscle was recorded with bipolar electrodes chronically implanted in rats aged 10-30 days. Changes in the activity pattern were studied in relation to motor development. The firing pattern and shape of motor units were studied at higher resolution. EMG activity was closely related to motor behaviour and posture. At P11, soleus was only phasically active during movements. Tonic EMG activity appeared with age, and by P16 the activity pattern was similar to that of the adult. The activity level also increased markedly with age. This was paralleled by a change in hindlimb posture. The development of tonic activity precedes the development of dendrite bundles in the motoneuron pool of the soleus muscle, which does not support the hypothesis that bundle formation is causally related to tonic activity in postural muscles. At young ages, the tonic EMG was characterized by a clear oscillation pattern with a frequency ranging from 7 to 20 Hz, the higher frequencies occurring at older ages. This oscillation disappeared around P16, which suggests that it is related to functional development of the stretch reflex. Periods of high frequency firing were observed in the EMG; the highest firing frequencies occurred during locomotion. The maximum firing frequency increased steeply to about 80 Hz at P18. It is suggested that bursts of high frequency firing are related to the development of monoaminergic innervation of the spinal cord. During the first three weeks, motor unit action potentials often showed complex shapes of long duration and considerable spike-to-spike variability. Computer simulation of the summation process of fibre potentials at the electrode showed that the small fibre diameters and high variability in diameter at young ages are major factors with respect to the generation of complex potentials.

Infantile chorea in an infant with severe bronchopulmonary dysplasia: an EMG study

Recently a new movement disorder has been described which develops in some preterm infants with bronchopulmonary dysplasia. The present report provides a detailed description (including polyelectromyographical and serial cranial ultrasound findings) of this syndrome in a single infant. The authors suggest that the movement disorder be called infantile chorea, because the movement characteristics and EMG bursting pattern resemble those of adult chorea. It is hypothesized that 'infantile chorea', which is assumed to be caused by dysfunction of the striato-thalamic circuitry, emerges at the age of two to three months post-erm as thalamo-cortical connections are supposed to become functionally active.

General movements in early infancy: what do they tell us about the nervous system?

Developmental changes in muscle coordination patterns of normal and abnormal general movements (GMs) are described. GMs were studied simultaneously with video-recording and polymyography (polyEMG). GMs of normal full-term infants gradually lose their neonatally slow and 'writhing' character, to be turned into the elegant flow of small movements of 'fidgety' GMs at the age of 2-3 months. This transformation coincides with changes in the polyEMG. Tonic background activity decreases concurrent with a reduction in amplitude and duration of phasic bursts. The coordination between antagonistic muscles does not change, co-activation remains the prevailing pattern. Secondly, preliminary results on healthy preterms (n = 6) are presented. At the examination age of 33-34 weeks postmenstrual age the preterms showed so-called 'preterm' GMs, which are characterized by variation and graceful complexity. EMG burst duration was significantly longer during 'preterm' GMs than during 'writhing' GMs of full-term newborns. The percentage of co-activation during 'preterm' GMs varied considerably. The polyEMG of 'writhing' GMs of two preterms, who were followed longitudinally, differed from that of 'writhing' GMs of full-terms. At 'fidgety' age the EMG differences between preterms and full-terms had disappeared. Finally the first results on abnormal GMs are reported. A synchronized onset of muscle activity in all extremity muscles and 'packaging' of EMG-bursts into subunits of 5-8 Hz. seemed to be specific properties of these abnormal GMs. This could indicate a loss of supraspinal control.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

Emergence of pattern in the development of mammalian movement sequences

The manner in which behavior is patterned in space and over time represents a fundamental problem in both ethology and neuroscience. Prior to the analysis of mechanism it is important to be sensitive to issues involved in the provision of descriptive taxonomies. Often alternative modes of description lead to different perspectives and research strategies. In both the development of behavioral patterns and their expression a major question is how underlying organizational systems become self-organizing through the process of mutual interactions. It is clear that simple static dichotomies in both behavioral and developmental science must be replaced by more sophisticated models that emphasize the dynamics of pattern formation and control. Some of these perspectives are illustrated from our ongoing research on rodent movement patterns.

Fundamental motor patterns of the mammalian fetus

Techniques that permit direct observation of fetuses in vivo recently have expanded our understanding of prenatal behavioral development in mammals. Although fetal motor activity seems to lack the dynamic, goal-directed character of postnatal behavior, the dimensions that define behavioral organization after birth are applicable to the movements expressed by fetuses. Fetal activity exhibits temporal, sequential, and spatial organization that emerges between the inception of movement and term. Fetal rodents, for example, exhibit coordinated motor patterns antecedent to postnatal righting, locomotion, suckling, maternal-infant communication and grooming behavior, while other action patterns appear to be functional adaptations to the intrauterine niche. Fetuses also are behaviorally responsive to sensory stimulation and changes in environmental conditions in utero. Expression of these behavioral properties emphasizes continuity between prenatal and postnatal life while implying an adaptive role for behavior before birth.

Development of coordinated movement in chicks: II. Temporal analysis of hindlimb muscle synergies at embryonic day 10 in embryos with spinal gap transections

Spinal neural circuits can recruit muscles to produce organized patterns of activity early in embryonic development. In a previous study, using multichannel electromyographic (EMG) recordings, we characterized burst parameters for these patterns in the legs of chick embryos during spontaneous motility in ovo at embryonic days (E) 9 and E10 (Bradley and Bekoff, 1990). Results of the study suggested both neural and biomechanical factors play an important role in the development of coordinated limb movements. In this study, to explore the contribution of descending neural inputs to the control of leg movements during motility, we applied similar methods to characterize motor patterns produced by the spinal cord in the absence of descending inputs. Thoracic spinal gap transections were performed at E2 and EMG patterns were recorded at E10. Several EMG features for chronic spinal embryos were similar to those for normal embryos and demonstrate that lumbar spinal circuits can be correctly assembled to control limb movements in the absence of connectivity with more rostral neural structures during early differentiation processes. However, certain aspects of the EMG patterns in chronic spinal embryos were different from patterns in normal embryos and provide support for conclusions drawn earlier by Oppenheim (1975). Specifically, our data support the view that propriospinal and/or supraspinal inputs function to regulate the timing of cyclic limb movements controlled by spinal neural circuits. Finally, we consider the possible long-term effects of chronic spinal gap transections as compared to acute spinal transections on the development of motility.

Alternating stepping patterns: hidden abilities of 11-month-old infants with Down syndrome

Normally developing infants can produce organized alternating stepping patterns long before they stand alone or attempt to walk, if supported upright on a motorized treadmill. The purpose of this study was to examine whether infants with Down syndrome, who begin to walk at a much later age than non-disabled infants, could produced alternating steps in a similar way. Six of the seven 11-month-old infants studied responded to the treadmill stimulus by producing alternating steps. This suggests that the basic neural substrate necessary for upright locomotion is available long before walking occurs in infants with Down syndrome, as it is in normally developing infants. The infants in this study began to walk at an average of 13.3 months after demonstrating the ability to produce treadmill steps.

Locomotion induced by spinal cord stimulation in the neonate rat in vitro

The present studies employed the neonate rat brain stem-spinal cord preparation to determine whether electrical stimulation of the lumbosacral enlargement (LE) of the spinal cord itself can be used to elicit locomotion, and whether or not such stimulation persists in inducing locomotion following midthoracic spinal cord transection or hindlimb deafferentation. Results suggest that (1) stimulation of the dorsal columns or ventral funiculus of the LE is effective in inducing airstepping in the neonatal rat brain stem-spinal cord limb-attached preparation; (2) central disconnection by midthoracic spinal cord transection does not alter LE-stimulation-induced airstepping and may lead to an increase in stepping frequency if suprathreshold stimulation is used; and (3) dorsal root section also leads to an increase in the frequency of suprathreshold LE-stimulation-induced locomotion, but there is not further increase in frequency if a spinal cord transection is performed in addition to dorsal rhizotomy.

Development of coordinated movement in chicks: I. Temporal analysis of hindlimb muscle synergies at embryonic days 9 and 10

In this study, we examined electromyographic activity for an ensemble of hindlimb muscles during spontaneous activity in chick embryos to advance understanding of early motor coordination and its relationship to later emerging behaviors. Four-channel recordings were obtained from 6 muscles in ovo at embryonic Days 9 and 10. Analyses indicated that when muscles are repetitively active, patterns during embryonic motility are distinct from those for other behaviors. For example, unlike the muscle patterns for locomotion, extensor muscles and flexor muscles are synchronously activated at 50% of the extensor cycle period. Furthermore, flexor and extensor bursts are similar in duration and show little correlation with extensor cycle period. Finally, our data suggest that the ensemble of muscles active can vary from cycle to cycle. This study provides the basis for future studies that will examine neural and biomechanical interactions underlying the development of coordinated movement.

An Analysis of Air-Stepping in Normal Infant Vervet Monkeys

Limb movements during air-stepping were analyzed in three neonatal vervet monkeys over a three-week period. The movements had similar temporal organization both across animals and across time. For example, the duration of both the hind and the forelimb cycle equaled about 500 ms, with hind limb return strokes lasting much longer than the hind limb power strokes. Furthermore, there were clear indications of both intra- and interlimb coordination. Specifically, all the joints of a limb tended to flex and extend simultaneously, and contralateral and ipsilateral limb pairs had an average phase relationship of approximately 50% of cycle duration. Despite a qualitative similarity between limb movements during air-stepping in the neonates and overground locomotion in older animals, there were notable differences both in temporal relationships and joint displacement patterns. Finally, there appeared to be important similarities between air-stepping in these monkeys and stepping in newborn humans. Most notably, both tended to disappear after a limited period. The implications of these similarities, as well as the overall results, are discussed in relation to the understanding of the development of locomotor behavior in human and nonhuman primates, using approaches based both upon the hard-wired and dynamic models.

Neuromuscular patterns of stereotypic hindlimb behaviors in the first two postnatal months. I. Stepping in normal kittens

Neuromuscular patterns associated with the development of hindlimb stepping behaviors were studied from birth to postnatal day 60 in normal kittens. Hindlimb muscles were chronically implanted with EMG electrodes at birth to characterize interlimb coordination and intralimb synergies during development of overground and treadmill stepping. Airstepping was also examined but seldom occurred after the second postnatal week. All kittens performed stepping under each condition, including weight-supported stepping, by postnatal day 3. The number of sequential steps on the treadmill and overground increased with age and cycle periods decreased. At onset, stepping behaviors were characterized by adult-like EMG patterns. Interlimb coordination was typified by alternating extensor bursts of similar duration. Extensors at the knee and ankle were coactive during the stance phase, and extensor burst durations were strongly correlated with the cycle periods over a wide range of stepping frequency. Ankle flexor and extensor muscles were reciprocally active during postural tremor, bouts of airstepping, and weight-supported steps on the treadmill and overground. The duration of the reciprocal flexor bust did not vary with cycle period or age. Observations of stepping behaviors and adult-like EMG patterns during initial postnatal development were contingent on optimal testing conditions. Taken together, the data suggest that pattern-generating circuits for regulating interlimb coordination and intralimb muscle synergies are potentially functional prior to the normal ontogenetic onset of locomotion. Perhaps the prolonged postnatal development of locomotion reflects the time required to establish adaptive mechanisms, such as postural control and agility, rather than spinal pattern-generating circuits for locomotion.