Kinematic and EMG determinants in quadrupedal locomotion of a non-human primate (Rhesus)

A comparative study of muscle activity and synergies during walking in baboons and humans

Bipedal locomotion was a major functional change during hominin evolution, yet, our understanding of this gradual and complex process remains strongly debated. Based on fossil discoveries, it is possible to address functional hypotheses related to bipedal anatomy, however, motor control remains intangible with this approach. Using comparative models which occasionally walk bipedally has proved to be relevant to shed light on the evolutionary transition toward habitual bipedalism. Here, we explored the organization of the neuromuscular control using surface electromyography (sEMG) for six extrinsic muscles in two baboon individuals when they walk quadrupedally and bipedally on the ground. We compared their muscular coordination to five human subjects walking bipedally. We extracted muscle synergies from the sEMG envelopes using the non-negative matrix factorization algorithm which allows decomposing the sEMG data in the linear combination of two non-negative matrixes (muscle weight vectors and activation coefficients). We calculated different parameters to estimate the complexity of the sEMG signals, the duration of the activation of the synergies, and the generalizability of the muscle synergy model across species and walking conditions. We found that the motor control strategy is less complex in baboons when they walk bipedally, with an increased muscular activity and muscle coactivation. When comparing the baboon bipedal and quadrupedal pattern of walking to human bipedalism, we observed that the baboon bipedal pattern of walking is closer to human bipedalism for both baboons, although substantial differences remain. Overall, our findings show that the muscle activity of a non-adapted biped effectively fulfills the basic mechanical requirements (propulsion and balance) for walking bipedally, but substantial refinements are possible to optimize the efficiency of bipedal locomotion. In the evolutionary context of an expanding reliance on bipedal behaviors, even minor morphological alterations, reducing muscle coactivation, could have faced strong selection pressure, ultimately driving bipedal evolution in hominins.

Kinematics and mechanical changes with step frequency at different running speeds

PURPOSE

Running at a given speed can be achieved by taking large steps at a low frequency or on the contrary by taking small steps at a high frequency. The consequences of a change in step frequency, at a fixed speed, affects the stiffness of the lower limb differently. In this study, we compared the running mechanics and kinematics at different imposed step frequencies (from 2 step s-1 to 3.6 step s-1) to understand the relationship between kinematic and kinetic parameters.

METHODS

Eight recreational male runners ran on a treadmill at 5 different speeds and 5 different step frequencies. The lower-limb segment motion and the ground reaction forces were recorded. Mechanical powers, general gait parameters, lower-limb movements and coordination were investigated.

RESULTS

At low step frequencies, in order to limit the magnitude of the ground reaction force, the vertical stiffness is reduced and thus runners deviate from an elastic rebound. At high step frequencies, the stiffness is increased and the elastic rebound is optimised in its ability to absorb and restore energy during the contact phase.

CONCLUSION

We studied the consequences of a change in step frequency on the bouncing mechanics of running. We showed that the lower limb stiffness and the intersegmental coordination of the lower-limb segments are affected by running step frequency rather than speed. The runner rather adapts their lower limb stiffness to match a step frequency for a given speed than the opposite.

A spinal cord neuroprosthesis for locomotor deficits due to Parkinson's disease

People with late-stage Parkinson's disease (PD) often suffer from debilitating locomotor deficits that are resistant to currently available therapies. To alleviate these deficits, we developed a neuroprosthesis operating in closed loop that targets the dorsal root entry zones innervating lumbosacral segments to reproduce the natural spatiotemporal activation of the lumbosacral spinal cord during walking. We first developed this neuroprosthesis in a non-human primate model that replicates locomotor deficits due to PD. This neuroprosthesis not only alleviated locomotor deficits but also restored skilled walking in this model. We then implanted the neuroprosthesis in a 62-year-old male with a 30-year history of PD who presented with severe gait impairments and frequent falls that were medically refractory to currently available therapies. We found that the neuroprosthesis interacted synergistically with deep brain stimulation of the subthalamic nucleus and dopaminergic replacement therapies to alleviate asymmetry and promote longer steps, improve balance and reduce freezing of gait. This neuroprosthesis opens new perspectives to reduce the severity of locomotor deficits in people with PD.

A sensory signal related to left-right symmetry modulates intra- and interlimb cutaneous reflexes during locomotion in intact cats

Introduction

During locomotion, cutaneous reflexes play an essential role in rapidly responding to an external perturbation, for example, to prevent a fall when the foot contacts an obstacle. In cats and humans, cutaneous reflexes involve all four limbs and are task- and phase modulated to generate functionally appropriate whole-body responses.

Methods

To assess task-dependent modulation of cutaneous interlimb reflexes, we electrically stimulated the superficial radial or superficial peroneal nerves in adult cats and recorded muscle activity in the four limbs during tied-belt (equal left-right speeds) and split-belt (different left-right speeds) locomotion.

Results

We show that the pattern of intra- and interlimb cutaneous reflexes in fore- and hindlimbs muscles and their phase-dependent modulation were conserved during tied-belt and split-belt locomotion. Short-latency cutaneous reflex responses to muscles of the stimulated limb were more likely to be evoked and phase-modulated when compared to muscles in the other limbs. In some muscles, the degree of reflex modulation was significantly reduced during split-belt locomotion compared to tied-belt conditions. Split-belt locomotion increased the step-by-step variability of left-right symmetry, particularly spatially.

Discussion

These results suggest that sensory signals related to left-right symmetry reduce cutaneous reflex modulation, potentially to avoid destabilizing an unstable pattern.

Contrasting walking styles map to discrete neural substrates in the mouse brainstem

Walking is a slow gait which is particularly adaptable to meet internal or external needs and is prone to maladaptive alterations that lead to gait disorders. Alterations can affect speed, but also style (the way one walks). While slowed speed may signify the presence of a problem, style represents the hallmark essential for clinical classification of gait disorders. However, it has been challenging to objectively capture key stylistic features while uncovering neural substrates driving these features. Here we revealed brainstem hotspots that drive strikingly different walking styles by employing an unbiased mapping assay that combines quantitative walking signatures with focal, cell type specific activation. We found that activation of inhibitory neurons that mapped to the ventromedial caudal pons induced slow motion-like style. Activation of excitatory neurons that mapped to the ventromedial upper medulla induced shuffle-like style. Contrasting shifts in walking signatures distinguished these styles. Activation of inhibitory and excitatory neurons outside these territories or of serotonergic neurons modulated walking speed, but without walking signature shifts. Consistent with their contrasting modulatory actions, hotspots for slow-motion and shuffle-like gaits preferentially innervated different substrates. These findings lay the basis for new avenues to study mechanisms underlying (mal)adaptive walking styles and gait disorders.

Graphical abstract

Knuckle-walking in Sahelanthropus? Locomotor inferences from the ulnae of fossil hominins and other hominoids

Because the ulna supports and transmits forces during movement, its morphology can signal aspects of functional adaptation. To test whether, like extant apes, some hominins habitually recruit the forelimb in locomotion, we separate the ulna shaft and ulna proximal complex for independent shape analyses via elliptical Fourier methods to identify functional signals. We examine the relative influence of locomotion, taxonomy, and body mass on ulna contours in Homo sapiens (n = 22), five species of extant apes (n = 33), two Miocene apes (Hispanopithecus and Danuvius), and 17 fossil hominin specimens including Sahelanthropus, Ardipithecus, Australopithecus, Paranthropus, and early Homo. Ulna proximal complex contours correlate with body mass but not locomotor patterns, while ulna shafts significantly correlate with locomotion. African apes' ulna shafts are more robust and curved than Asian apes and are unlike other terrestrial mammals (including other primates), curving ventrally rather than dorsally. Because this distinctive curvature is absent in orangutans and hylobatids, it is likely a function of powerful flexors engaged in wrist and hand stabilization during knuckle-walking, and not an adaptation to climbing or suspensory behavior. The OH 36 (purported Paranthropus boisei) and TM 266 (assigned to Sahelanthropus tchadensis) fossils differ from other hominins by falling within the knuckle-walking morphospace, and thus appear to show forelimb morphology consistent with terrestrial locomotion. Discriminant function analysis classifies both OH 36 and TM 266 with Pan and Gorilla with high posterior probability. Along with its associated femur, the TM 266 ulna shaft contours and its deep, keeled trochlear notch comprise a suite of traits signaling African ape-like quadrupedalism. While implications for the phylogenetic position and hominin status of S. tchadensis remain equivocal, this study supports the growing body of evidence indicating that S. tchadensis was not an obligate biped, but instead represents a late Miocene hominid with knuckle-walking adaptations.

Non-human primate models and systems for gait and neurophysiological analysis

Brain-computer interfaces (BCIs) have garnered extensive interest and become a groundbreaking technology to restore movement, tactile sense, and communication in patients. Prior to their use in human subjects, clinical BCIs require rigorous validation and verification (V&V). Non-human primates (NHPs) are often considered the ultimate and widely used animal model for neuroscience studies, including BCIs V&V, due to their proximity to humans. This literature review summarizes 94 NHP gait analysis studies until 1 June, 2022, including seven BCI-oriented studies. Due to technological limitations, most of these studies used wired neural recordings to access electrophysiological data. However, wireless neural recording systems for NHPs enabled neuroscience research in humans, and many on NHP locomotion, while posing numerous technical challenges, such as signal quality, data throughout, working distance, size, and power constraint, that have yet to be overcome. Besides neurological data, motion capture (MoCap) systems are usually required in BCI and gait studies to capture locomotion kinematics. However, current studies have exclusively relied on image processing-based MoCap systems, which have insufficient accuracy (error: ≥4° and 9 mm). While the role of the motor cortex during locomotion is still unclear and worth further exploration, future BCI and gait studies require simultaneous, high-speed, accurate neurophysiological, and movement measures. Therefore, the infrared MoCap system which has high accuracy and speed, together with a high spatiotemporal resolution neural recording system, may expand the scope and improve the quality of the motor and neurophysiological analysis in NHPs.

Modulation of the gait pattern during split-belt locomotion after lateral spinal cord hemisection in adult cats

Most previous studies investigated the recovery of locomotion in animals and people with incomplete spinal cord injury (SCI) during relatively simple tasks (e.g., walking in a straight line on a horizontal surface or a treadmill). We know less about the recovery of locomotion after incomplete SCI in left-right asymmetric conditions, such as turning or stepping along circular trajectories. To investigate this, we collected kinematic and electromyography data during split-belt locomotion at different left-right speed differences before and after a right thoracic lateral spinal cord hemisection in nine adult cats. After hemisection, although cats still performed split-belt locomotion, we observed several changes in the gait pattern compared with the intact state at early (1-2 wk) and late (7-8 wk) time points. Cats with larger lesions showed new coordination patterns between the fore- and hindlimbs, with the forelimbs taking more steps. Despite this change in fore-hind coordination, cats maintained consistent phasing between the fore- and hindlimbs. Adjustments in cycle and phase (stance and swing) durations between the slow and fast sides allowed animals to maintain 1:1 left-right coordination. Periods of triple support involving the right (ipsilesional) hindlimb decreased in favor of quad support and triple support involving the other limbs. Step and stride lengths decreased with concurrent changes in the right fore- and hindlimbs, possibly to avoid interference. The above adjustments in the gait pattern allowed cats to retain the ability to locomote in asymmetric conditions after incomplete SCI. We discuss potential plastic neuromechanical mechanisms involved in locomotor recovery in these conditions.NEW & NOTEWORTHY Everyday locomotion often involves left-right asymmetries, when turning, walking along circular paths, stepping on uneven terrains, etc. To show how incomplete spinal cord injury affects locomotor control in asymmetric conditions, we collected data before and after a thoracic lateral spinal hemisection on a split-belt treadmill with one side stepping faster than the other. We show that adjustments in kinematics and muscle activity allowed cats to retain the ability to perform asymmetric locomotion after hemisection.

The neurons that restore walking after paralysis

A spinal cord injury interrupts pathways from the brain and brainstem that project to the lumbar spinal cord, leading to paralysis. Here we show that spatiotemporal epidural electrical stimulation (EES) of the lumbar spinal cord1–3 applied during neurorehabilitation4,5 (EESREHAB) restored walking in nine individuals with chronic spinal cord injury. This recovery involved a reduction in neuronal activity in the lumbar spinal cord of humans during walking. We hypothesized that this unexpected reduction reflects activity-dependent selection of specific neuronal subpopulations that become essential for a patient to walk after spinal cord injury. To identify these putative neurons, we modelled the technological and therapeutic features underlying EESREHAB in mice. We applied single-nucleus RNA sequencing6–9 and spatial transcriptomics10,11 to the spinal cords of these mice to chart a spatially resolved molecular atlas of recovery from paralysis. We then employed cell type12,13 and spatial prioritization to identify the neurons involved in the recovery of walking. A single population of excitatory interneurons nested within intermediate laminae emerged. Although these neurons are not required for walking before spinal cord injury, we demonstrate that they are essential for the recovery of walking with EES following spinal cord injury. Augmenting the activity of these neurons phenocopied the recovery of walking enabled by EESREHAB, whereas ablating them prevented the recovery of walking that occurs spontaneously after moderate spinal cord injury. We thus identified a recovery-organizing neuronal subpopulation that is necessary and sufficient to regain walking after paralysis. Moreover, our methodology establishes a framework for using molecular cartography to identify the neurons that produce complex behaviours.

Overground gait kinematics and muscle activation patterns in the Yucatan mini pig

Objective The objectives of this study were to assess gait biomechanics and the effect of overground walking speed on gait parameters, kinematics, and electromyographic (EMG) activity in the hindlimb muscles of Yucatan Minipigs (YMPs). Approach Nine neurologically-intact, adult YMPs were trained to walk overground in a straight line. Whole-body kinematics and EMG activity of hindlimb muscles were recorded and analyzed at 6 different speed ranges (0.4-0.59, 0.6-0.79, 0.8-0.99, 1.0-1.19, 1.2-1.39, and 1.4-1.6 m/s). A MATLAB program was developed to detect strides and gait events automatically from motion-captured data. The kinematics and EMG activity were analyzed for each stride based on the detected events. Main results Significant decreases in stride duration, stance and swing times and an increase in stride length were observed with increasing speed. A transition in gait pattern occurred at the 1.0m/s walking speed. Significant increases in the range of motion of the knee and ankle joints were observed at higher speeds. Also, the points of minimum and maximum joint angles occurred earlier in the gait cycle as the walking speed increased. The onset of EMG activity in the biceps femoris muscle occurred significantly earlier in the gait cycle with increasing speed. Significance YMPs are becoming frequently used as large animal models for preclinical testing and translation of novel interventions to humans. A comprehensive characterization of overground walking in neurologically-intact YMPs is provided in this study. These normative measures set the basis against which the effects of future interventions on locomotor capacity in YMPs can be compared.

State- and Condition-Dependent Modulation of the Hindlimb Locomotor Pattern in Intact and Spinal Cats Across Speeds

Locomotion after complete spinal cord injury (spinal transection) in animal models is usually evaluated in a hindlimb-only condition with the forelimbs suspended or placed on a stationary platform and compared with quadrupedal locomotion in the intact state. However, because of the quadrupedal nature of movement in these animals, the forelimbs play an important role in modulating the hindlimb pattern. This raises the question: whether changes in the hindlimb pattern after spinal transection are due to the state of the system (intact versus spinal) or because the locomotion is hindlimb-only. We collected kinematic and electromyographic data during locomotion at seven treadmill speeds before and after spinal transection in nine adult cats during quadrupedal and hindlimb-only locomotion in the intact state and hindlimb-only locomotion in the spinal state. We attribute some changes in the hindlimb pattern to the spinal state, such as convergence in stance and swing durations at high speed, improper coordination of ankle and hip joints, a switch in the timing of knee flexor and hip flexor bursts, modulation of burst durations with speed, and incidence of bi-phasic bursts in some muscles. Alternatively, some changes relate to the hindlimb-only nature of the locomotion, such as paw placement relative to the hip at contact, magnitude of knee and ankle yield, burst durations of some muscles and their timing. Overall, we show greater similarity in spatiotemporal and EMG variables between the two hindlimb-only conditions, suggesting that the more appropriate pre-spinal control is hindlimb-only rather than quadrupedal locomotion.

Functional reorganization of locomotor kinematic synergies reflects the neuropathology in a mouse model of spinal cord injury

Spinal cord injury (SCI) disrupts motor commands to modular structures of the spinal cord, limiting the ability to walk. Evidence suggests that these modules are conserved across species from rodent to human and subserve adaptive walking by controlling coordinated joint movements (kinematic synergies). Since SCI causes uncoordinated joint movements of the lower limbs during walking, there may be a disorder of the modular structures that control them. To gain insights into this complex process, we recorded the kinematics of intact and SCI mice when walking on a treadmill and applied principal component analysis to extract kinematic synergies. Most SCI mice walked stably on the treadmill, but their kinematic synergies were generally different from those of intact mice. We classified the kinematic synergies of SCI mice into three groups based on the similarity of the extracted first three synergy components. We found that these three groups had different degrees of spinal cord damage. This suggests that differences in kinematic synergies reflect underlying SCI neuropathology. These results may help guide the development of different rehabilitation approaches and future physiological experiments to understand the mechanisms of motor control and recovery.

A Validation of Supervised Deep Learning for Gait Analysis in the Cat

Gait analysis in cats and other animals is generally performed with custom-made or commercially developed software to track reflective markers placed on bony landmarks. This often involves costly motion tracking systems. However, deep learning, and in particular DeepLabCut^TM (DLC), allows motion tracking without requiring placing reflective markers or an expensive system. The purpose of this study was to validate the accuracy of DLC for gait analysis in the adult cat by comparing results obtained with DLC and a custom-made software (Expresso) that has been used in several cat studies. Four intact adult cats performed tied-belt (both belts at same speed) and split-belt (belts operating at different speeds) locomotion at different speeds and left-right speed differences on a split-belt treadmill. We calculated several kinematic variables, such as step/stride lengths and joint angles from the estimates made by the two software and assessed the agreement between the two measurements using intraclass correlation coefficient or Lin's concordance correlation coefficient as well as Pearson's correlation coefficients. The results showed that DLC is at least as precise as Expresso with good to excellent agreement for all variables. Indeed, all 12 variables showed an agreement above 0.75, considered good, while nine showed an agreement above 0.9, considered excellent. Therefore, deep learning, specifically DLC, is valid for measuring kinematic variables during locomotion in cats, without requiring reflective markers and using a relatively low-cost system.

The quadrupedal walking gait of the olive baboon, Papio anubis: an exploratory study integrating kinematics and EMG

Primates exhibit unusual quadrupedal features (e.g. diagonal gaits, compliant walk) compared with other quadrupedal mammals. Their origin and diversification in arboreal habitats have certainly shaped the mechanics of their walking pattern to meet the functional requirements necessary for balance control in unstable and discontinuous environments. In turn, the requirements for mechanical stability probably conflict with mechanical energy exchange. In order to investigate these aspects, we conducted an integrative study on quadrupedal walking in the olive baboon (Papio anubis) at the Primatology station of the CNRS in France. Based on kinematics, we describe the centre of mass mechanics of the normal quadrupedal gait performed on the ground, as well as in different gait and substrate contexts. In addition, we studied the muscular activity of six hindlimb muscles using non-invasive surface probes. Our results show that baboons can rely on an inverted pendulum-like exchange of energy (57% on average, with a maximal observed value of 84%) when walking slowly (<0.9 m s-1) with a tight limb phase (∼55%) on the ground using diagonal sequence gaits. In this context, the muscular activity is similar to that of other quadrupedal mammals, thus reflecting the primary functions of the muscles for limb movement and support. In contrast, walking on a suspended branch generates kinematic and muscular adjustments to ensure better control and to maintain stability. Finally, walking using the lateral sequence gait increases muscular effort and reduces the potential for high recovery rates. The present exploratory study thus supports the assumption that primates are able to make use of an inverted pendulum mechanism on the ground using a diagonal walking gait, yet a different footfall pattern and substrate appear to influence muscular effort and efficiency.

Oscillatory Activity in Mouse Lemur Primary Motor Cortex During Natural Locomotor Behavior

In arboreal environments, substrate orientation determines the biomechanical strategy for postural maintenance and locomotion. In this study, we investigated possible neuronal correlates of these mechanisms in an ancestral primate model, the gray mouse lemur. We conducted telemetric recordings of electrocorticographic activity in left primary motor cortex of two mouse lemurs moving on a branch-like small-diameter pole, fixed horizontally, or vertically. Analysis of cortical oscillations in high β (25–35 Hz) and low γ (35–50 Hz) bands showed stronger resting power on horizontal than vertical substrate, potentially illustrating sensorimotor processes for postural maintenance. Locomotion on horizontal substrate was associated with stronger event-related desynchronization than vertical substrate, which could relate to locomotor adjustments and/or derive from differences in baseline activity. Spectrograms of cortical activity showed modulation throughout individual locomotor cycles, with higher values in the first than second half cycle. However, substrate orientation did not significantly influence these variations. Overall, these results confirm that specific cortical mechanisms are solicited during arboreal locomotion, whereby mouse lemurs adjust cortical activity to substrate orientation during static posture and locomotion, and modulate this activity throughout locomotor cycles.

The forearm and hand musculature of semi-terrestrial rhesus macaques (Macaca mulatta) and arboreal gibbons (fam.Hylobatidae). Part II. Quantitative analysis

Nonhuman primates have a highly diverse locomotor repertoire defined by an equally diverse hand use. Based on how primates use their hands during locomotion, we can distinguish between terrestrial and arboreal taxa. The 'arboreal' hand is likely adapted towards high wrist mobility and grasping, whereas the 'terrestrial' hand will show adaptations to loading. While the morphology of the forearm and hand bones have been studied extensively, functional adaptations in the forearm and hand musculature to locomotor behaviour have been documented only scarcely. In this paper, we investigate the forelimb musculature of the highly arboreal gibbons (including Hylobates lar,Hylobates pileatus,Nomascus leucogenys,Nomascus concolor and Symphalangus syndactylus) and compare this with the musculature of the semi-terrestrial rhesus macaques (Macaca mulatta). Anatomical data from previous dissections on knuckle-walking bonobos (Pan paniscus) and bipedal humans (Homo sapiens) are also included to further integrate the analyses in the scope of catarrhine hand adaptation. This study indicates that the overall configuration of the arm and hand musculature of these primates is very similar but there are some apparent differences in relative size which can be linked to differences in forelimb function and which might be related to their specific locomotor behaviour. In macaques, there is a large development of wrist deviators, wrist and digital flexors, and m. triceps brachii, as these muscles are important during the different phases of palmi- and digitigrade quadrupedal walking to stabilize the wrist and elbow. In addition, their m. flexor carpi ulnaris is the most important contributor to the total force-generating capacity of the wrist flexors and deviators, and is needed to counteract the adducting torque at the elbow joint during quadrupedal walking. Gibbons show a relatively high force-generating capacity in their forearm rotators, wrist and digital flexors, which are important muscles in brachiation to actively regulate forward movement of the body. The results also stress the importance of the digital flexors in bonobos, during climbing and clambering, and in humans, which is likely linked to our advanced manipulation skills.

Modularity in Motor Control: Similarities in Kinematic Synergies Across Varying Locomotion Tasks

Kinematic synergies (kSYN) provide an approach to quantify the covariation of joint motions and to explain the mechanisms underlying human motor behavior. A low-dimensional control strategy by means of the activation of a moderate number of kSYN would simplify the performance of complex motor tasks. The purpose of this study was to examine similarities between the kSYN of varying locomotion tasks: straight-line walking, walking a 90° spin turn and walking upstairs. Task-specific kSYN were extracted from full body kinematic recordings of 13 participants by principal component analysis. The first five kSYN accounting for most of the variance within each task were selected for further analysis following previous studies. The similarities between the kSYN of the three different locomotion tasks were quantified by calculating cosine similarities (SIM), as a vector-based similarity measure ranging from 0 (no similarity) to 1 (high similarity), between absolute principal component loading vectors. A SIM between two kSYN > 0.8 was interpreted as highly similar. Two to three highly similar kSYN were identified when comparing two individual tasks with each other. One kSYN, primarily characterized by anteversion and retroversion of the arms and legs, were found to be similar in all three tasks. Additional kSYN that occurred between individual tasks reflected mainly an upwards/downwards movement of the body or a countercyclical knee flexion/extension. The results demonstrate that the three investigated locomotion tasks are characterized by kSYN and that certain kSYN repeatedly occur across the three locomotion tasks. PCA yields kSYN which are in descent order according to their amount of total variance accounted for. Referring to the placing of a kSYN within the order as priorization, we found a change in priorization of repeatedly occurring kSYN across the individual tasks. The findings support the idea that movements can be efficiently performed through a flexible combination of a lower number of control-relevant variables.

The Spinal Control of Backward Locomotion

Animal locomotion requires changing direction, from forward to backward. Here, we tested the hypothesis that sensorimotor circuits within the spinal cord generate backward locomotion and adjust it to task demands. We collected kinematic and electromyography (EMG) data during forward and backward locomotion at different treadmill speeds before and after complete spinal transection in six adult cats (three males and three females). After spinal transection, five/six cats performed backward locomotion, which required tonic somatosensory input in the form of perineal stimulation. One spinal cat performed forward locomotion but not backward locomotion while two others stepped backward but not forward. Spatiotemporal adjustments to increasing speed were similar in intact and spinal cats during backward locomotion and strategies were similar to forward locomotion, with shorter cycle and stance durations and longer stride lengths. Patterns of muscle activations, including muscle synergies, were similar for forward and backward locomotion in spinal cats. Indeed, we identified five muscle synergies that were similar during forward and backward locomotion. Lastly, spinal cats also stepped backward on a split-belt treadmill, with the left and right hindlimbs stepping at different speeds. Therefore, our results show that spinal sensorimotor circuits generate backward locomotion but require additional excitability compared with forward locomotion. Similar strategies for speed modulation and similar patterns of muscle activations and muscle synergies during forward and backward locomotion are consistent with a shared spinal locomotor network, with sensory feedback from the limbs controlling the direction.SIGNIFICANCE STATEMENT Animal locomotion requires changing direction, including forward, sideways and backward. This paper shows that the center controlling locomotion within the spinal cord can produce a backward pattern when instructed by sensory signals from the limbs. However, the spinal locomotor network requires greater excitability to produce backward locomotion compared with forward locomotion. The paper also shows that the spinal network controlling locomotion in the forward direction also controls locomotion in the backward direction.

The biomechanics of knuckle-walking: 3-D kinematics of the chimpanzee and macaque wrist, hand and fingers

The origin and evolution of knuckle-walking has long been a key focus in understanding African ape, including human, origins. Yet, despite numerous studies documenting morphological characteristics potentially associated with knuckle-walking, little quantitative three-dimensional (3-D) data exist of forelimb motion during knuckle-walking. Nor do any comparative 3-D data exist for hand postures used during quadrupedalism in monkeys. This lack of data has limited the testability of proposed adaptations for knuckle-walking in African apes. This study presents the first 3-D kinematic data of the wrist, hand and metacarpophalangeal joints during knuckle-walking in chimpanzees and in macaques using digitigrade and palmigrade hand postures. These results clarify the unique characteristics of, and commonalities between, knuckle-walking and digitigrady/palmigrady in multiple planes of motion. Notably, chimpanzees utilized more wrist ulnar deviation than any macaque hand posture. Maximum extension of the chimpanzee wrist was slight (5-20 deg) and generally overlapped with macaque digitigrady. Metacarpophalangeal joint motion displayed distinct differences between digits in both species, likely related to the timing of force application. These data also reveal that maximum metacarpophalangeal extension angles during knuckle-walking (26-59 deg) were generally higher than previously considered. In macaques, maximum metacarpophalangeal extension during digitigrady and palmigrady overlapped for most digits, highlighting additional complexity in the interpretation of skeletal features that may be related to limiting metacarpophalangeal motion. Most importantly, however, these new 3-D data serve as a fundamental dataset with which evaluation of proposed musculoskeletal adaptations for knuckle-walking can be tested.

Wireless recording from unrestrained monkeys reveals motor goal encoding beyond immediate reach in frontoparietal cortex

System neuroscience of motor cognition regarding the space beyond immediate reach mandates free, yet experimentally controlled movements. We present an experimental environment (Reach Cage) and a versatile visuo-haptic interaction system (MaCaQuE) for investigating goal-directed whole-body movements of unrestrained monkeys. Two rhesus monkeys conducted instructed walk-and-reach movements towards targets flexibly positioned in the cage. We tracked 3D multi-joint arm and head movements using markerless motion capture. Movements show small trial-to-trial variability despite being unrestrained. We wirelessly recorded 192 broad-band neural signals from three cortical sensorimotor areas simultaneously. Single unit activity is selective for different reach and walk-and-reach movements. Walk-and-reach targets could be decoded from premotor and parietal but not motor cortical activity during movement planning. The Reach Cage allows systems-level sensorimotor neuroscience studies with full-body movements in a configurable 3D spatial setting with unrestrained monkeys. We conclude that the primate frontoparietal network encodes reach goals beyond immediate reach during movement planning.

Variant and Invariant Spatiotemporal Structures in Kinematic Coordination to Regulate Speed During Walking and Running

Humans walk, run, and change their speed in accordance with circumstances. These gaits are rhythmic motions generated by multi-articulated movements, which have specific spatiotemporal patterns. The kinematic characteristics depend on the gait and speed. In this study, we focused on the kinematic coordination of locomotor behavior to clarify the underlying mechanism for the effect of speed on the spatiotemporal kinematic patterns for each gait. In particular, we used seven elevation angles for the whole-body motion and separated the measured data into different phases depending on the foot-contact condition, that is, single-stance phase, double-stance phase, and flight phase, which have different physical constraints during locomotion. We extracted the spatiotemporal kinematic coordination patterns with singular value decomposition and investigated the effect of speed on the coordination patterns. Our results showed that most of the whole-body motion could be explained by only two sets of temporal and spatial coordination patterns in each phase. Furthermore, the temporal coordination patterns were invariant for different speeds, while the spatial coordination patterns varied. These findings will improve our understanding of human adaptation mechanisms to tune locomotor behavior for changing speed.

Functional organization of motor networks in the lumbosacral spinal cord of non-human primates

Implantable spinal-cord-neuroprostheses aiming to restore standing and walking after paralysis have been extensively studied in animal models (mainly cats) and have shown promising outcomes. This study aimed to take a critical step along the clinical translation path of these neuroprostheses, and investigated the organization of the neural networks targeted by these implants in a non-human primate. This was accomplished by advancing a microelectrode into various locations of the lumbar enlargement of the spinal cord, targeting the ventral horn of the gray matter. Microstimulation in these locations produced a variety of functional movements in the hindlimb. The resulting functional map of the spinal cord in monkeys was found to have a similar overall organization along the length of the spinal cord to that in cats. This suggests that the human spinal cord may also be organized similarly. The obtained spinal cord maps in monkeys provide important knowledge that will guide the very first testing of these implants in humans.

Neuromuscular control pattern in rhesus monkeys during bipedal walking

Walking is characterized by repetitive limb movements associated with highly structured patterns of muscle activity. The causal relationships between the muscle activities and hindlimb segments of walking are difficult to decipher. This study investigated these particular relationships and clarified whether they are correlated with speed to further understand the neuromuscular control pattern. Four adult female rhesus monkeys (Macaca mulatta) were selected to record gait parameters while walking on a bipedal treadmill at speeds of 0.2, 0.8, 1.4, and 2.0 km/h. We recorded 3 ipsilateral hindlimb muscles by surface recording. In this study, we calculated the correlations between electromyography (EMG) and kinematic parameters (24 EMG*17 kinematic parameters). Of the 408 calculated coefficients, 71.6% showed significant linear correlations. Significant linear correlations were found between muscle activity, such as burst amplitudes and the integral of muscle activity, and the corresponding kinematic parameters of each joint. Most of these relationships were speed independent (91.7% of all variables). Through correlation analysis, this study demonstrated a causal association between kinematic and EMG patterns of rhesus monkey locomotion. Individuals have particular musculoskeletal control patterns, and most of the relationships between hindlimb segments and muscles are speed independent. The current findings may enhance our understanding of neuromusculoskeletal control strategies.

Locomotor kinematics and EMG activity during quadrupedal versus bipedal gait in the Japanese macaque

Several qualitative features distinguish bipedal from quadrupedal locomotion in mammals. In this study we show quantitative differences between quadrupedal and bipedal gait in the Japanese monkey in terms of gait patterns, trunk/hindlimb kinematics, and electromyographic (EMG) activity, obtained from 3 macaques during treadmill walking. We predicted that as a consequence of an almost upright body axis, bipedal gait would show properties consistent with temporal and spatial optimization countering higher trunk/hindlimb loads and a less stable center of mass (CoM). A comparatively larger step width, an ~9% longer duty cycle, and ~20% increased relative duration of the double-support phase were all in line with such a strategy. Bipedal joint kinematics showed the strongest differences in proximal, and least in distal, hindlimb joint excursions compared with quadrupedal gait. Hindlimb joint coordination (cyclograms) revealed more periods of single-joint rotations during bipedal gait and predominance of proximal joints during single support. The CoM described a symmetrical, quasi-sinusoidal left/right path during bipedal gait, with an alternating shift toward the weight-supporting limb during stance. Trunk/hindlimb EMG activity was nonuniformally increased during bipedal gait, most prominently in proximal antigravity muscles during stance (up to 10-fold). Non-antigravity hindlimb EMG showed altered temporal profiles during liftoff or touchdown. Muscle coactivation was more, but muscle synergies less, frequent during bipedal gait. Together, these results show that behavioral and EMG properties of bipedal vs. quadrupedal gait are quantitatively distinct and suggest that the neural control of bipedal primate locomotion underwent specific adaptations to generate these particular behavioral features to counteract increased load and instability.

NEW & NOTEWORTHY Bipedal locomotion imposes particular biomechanical constraints on motor control. In a within-species comparative study, we investigated joint kinematics and electromyographic characteristics of bipedal vs. quadrupedal treadmill locomotion in Japanese macaques. Because these features represent (to a large extent) emergent properties of the underlying neural control, they provide a comparative, behavioral, and neurophysiological framework for understanding the neural system dedicated to bipedal locomotion in this nonhuman primate, which constitutes a critical animal model for human bipedalism.

Neurological functional evaluation based on accurate motions in big animals with traumatic brain injury

An accurate and effective neurological evaluation is indispensable in the treatment and rehabilitation of traumatic brain injury. However, most of the existing evaluation methods in basic research and clinical practice are not objective or intuitive for assessing the neurological function of big animals, and are also difficult to use to qualify the extent of damage and recovery. In the present study, we established a big animal model of traumatic brain injury by impacting the cortical motor region of beagles. At 2 weeks after successful modeling, we detected neurological deficiencies in the animal model using a series of techniques, including three-dimensional motion capture, electromyogram and ground reaction force. These novel technologies may play an increasingly important role in the field of traumatic brain injury diagnosis and rehabilitation in the future. The experimental protocol was approved by the Animal Care and Use Committee of Logistics University of People’s Armed Police Force (approval No. 2017-0006.2).

Motor neuron degeneration, severe myopathy and TDP-43 increase in a transgenic pig model of SOD1-linked familiar ALS

Amyotrophic Lateral Sclerosis (ALS) is a neural disorder gradually leading to paralysis of the whole body. Alterations in superoxide dismutase SOD1 gene have been linked with several variants of familial ALS. Here, we investigated a transgenic (Tg) cloned swine model expressing the human pathological hSOD1^G93A allele. As in patients, these Tg pigs transmitted the disease to the progeny with an autosomal dominant trait and showed ALS onset from about 27 months of age. Post mortem analysis revealed motor neuron (MN) degeneration, gliosis and hSOD1 protein aggregates in brainstem and spinal cord. Severe skeletal muscle pathology including necrosis and inflammation was observed at the end stage, as well. Remarkably, as in human patients, these Tg pigs showed a quite long presymptomatic phase in which gradually increasing amounts of TDP-43 were detected in peripheral blood mononuclear cells. Thus, this transgenic swine model opens the unique opportunity to investigate ALS biomarkers even before disease onset other than testing novel drugs and possible medical devices.

Intersegmental coordination patterns are differently affected in Parkinson's disease and cerebellar ataxia

The law of intersegmental coordination (Borghese et al. 1996) may be altered in pathological conditions. Here we investigated the contribution of the basal ganglia (BG) and the cerebellum to lower limb intersegmental coordination by inspecting the plane's orientation and other parameters pertinent to this law in patients with idiopathic Parkinson's disease (PD) or cerebellar ataxia (CA). We also applied a mathematical model that successfully accounts for the intersegmental law of coordination observed in control subjects (Barliya et al. 2009). In the present study, we compared the planarity index (PI), covariation plane (CVP) orientation, and CVP orientation predicted by the model in 11 PD patients, 8 CA patients, and two groups of healthy subjects matched for age, height, weight, and gender to each patient group (Ctrl_PD and Ctrl_CA). Controls were instructed to alter their gait speed to match those of their respective patient group. PD patients were examined after overnight withdrawal of anti-parkinsonian medications (PD-off-med) and then on medication (PD-on-med). PI was above 96% in all gait conditions in all groups suggesting that the law of intersegmental coordination is preserved in both BG and cerebellar pathology. However, the measured and predicted CVP orientations rotated in PD-on-med and PD-off-med compared with Ctrl_PD and in CA vs. Ctrl_CA. These rotations caused by PD and CA were in opposite directions suggesting differences in the roles of the BG and cerebellum in intersegmental coordination during human locomotion. NEW & NOTEWORTHY Kinematic and muscular synergies may have a role in overcoming motor redundancies, which may be reflected in intersegmental covariation. Basal ganglia and cerebellar networks were suggested to be involved in crafting and modulating synergies. We thus compared intersegmental coordination in Parkinson's disease and cerebellar disease patients and found opposite effects in some aspects. Further research integrating muscle activities as well as biomechanical and neural control modeling are needed to account for these findings.

A kinematic synergy for terrestrial locomotion shared by mammals and birds

Locomotion of tetrapods on land adapted to different environments and needs resulting in a variety of different gait styles. However, comparative analyses reveal common principles of limb movement control. Here, we report that a kinematic synergy involving the planar covariation of limb segment motion holds in 54 different animal species (10 birds and 44 mammals), despite large differences in body size, mass (ranging from 30 g to 4 tonnes), limb configuration, and amplitude of movements. This kinematic synergy lies at the interface between the neural command signals output by locomotor pattern generators, the mechanics of the body center of mass and the external environment, and it may represent one neuromechanical principle conserved in evolution to save mechanical energy.

Animals have evolved very different body shapes and styles of movement that are adapted to their needs in the habitats they live in. For example, mice, lions and many other animals use four limbs to walk, while humans and birds only use two limbs.

The styles animals use to walk also differ in terms of how long each foot is on the ground during a single stride, and for four-legged animals, in how long a forefoot lags behind the hindfoot on the same side of the body during the stride. Yet, there are general principles in how walking is organized that are shared between animals of vastly different shapes and sizes. Many animals save energy during walking by swinging the center of their body mass back and forth like a pendulum.

Networks of neurons are responsible for controlling how and when animals move, and these networks have similar architectures and patterns of activity in many different mammals and birds. How do signals from the nervous system regulate the position of the center of body mass while an animal walks?

Here, Catavitello et al. addressed this question by analyzing how over 50 different species of birds and mammals walked around in zoo enclosures and other semi-natural or natural environments. The species studied ranged in size from mice weighing around 30 grams to elephants weighing around 4 tonnes. The team also studied human volunteers walking on treadmills.

The experiments show that all the species studied coordinate their limbs in the same way, so that the angle to which a particular segment of a limb can bend varies together with the angles that the other limb segments bend. This coordination implies that the movement of the center of body mass is regulated and energy is saved.

Along with providing new insight into how walking evolved, these findings may aid research into new approaches to treat walking impairments in humans and other animals.

Establishment of a precise novel brain trauma model in a large animal based on injury of the cerebral motor cortex

BACKGROUND

Animal models are essential in simulating clinical diseases and facilitating relevant studies.

NEW METHOD

We established a precise canine model of traumatic brain injury (TBI) based on cerebral motor cortex injury which was confirmed by neuroimaging, electrophysiology, and a series of motor function assessment methods. Twelve beagles were divided into control, sham, and model groups. The cerebral motor cortex was identified by diffusion tensor imaging (DTI), a simple marker method, and intraoperative electrophysiological measurement. Bony windows were designed by magnetic resonance imaging (MRI) scan and DTI. During the operation, canines in the control group were under general anesthesia. The canines were operated via bony window craniotomy and dura mater opening in the sham group. After opening of the bony window and dura mater, the motor cortex was impacted by a modified electronic cortical contusion impactor (eCCI) in the model group.

RESULTS

Postoperative measurements revealed damage to the cerebral motor cortex and functional defects. Comparisons between preoperative and postoperative results demonstrated that the established model was successful.

COMPARISON WITH EXISTING METHOD(S)

Compared with conventional models, this is the first brain trauma model in large animal that was constructed based on injury to the cerebral motor cortex under the guidance of DTI, a simple marker method, and electrophysiology.

CONCLUSION

The method used to establish this model can be standardized to enhance reproducibility and provide a stable and precise large animal model of TBI for clinical and basic research.

Spinal Control of Locomotion: Individual Neurons, Their Circuits and Functions

Systematic research on the physiological and anatomical characteristics of spinal cord interneurons along with their functional output has evolved for more than one century. Despite significant progress in our understanding of these networks and their role in generating and modulating movement, it has remained a challenge to elucidate the properties of the locomotor rhythm across species. Neurophysiological experimental evidence indicates similarities in the function of interneurons mediating afferent information regarding muscle stretch and loading, being affected by motor axon collaterals and those mediating presynaptic inhibition in animals and humans when their function is assessed at rest. However, significantly different muscle activation profiles are observed during locomotion across species. This difference may potentially be driven by a modified distribution of muscle afferents at multiple segmental levels in humans, resulting in an altered interaction between different classes of spinal interneurons. Further, different classes of spinal interneurons are likely activated or silent to some extent simultaneously in all species. Regardless of these limitations, continuous efforts on the function of spinal interneuronal circuits during mammalian locomotion will assist in delineating the neural mechanisms underlying locomotor control, and help develop novel targeted rehabilitation strategies in cases of impaired bipedal gait in humans. These rehabilitation strategies will include activity-based therapies and targeted neuromodulation of spinal interneuronal circuits via repetitive stimulation delivered to the brain and/or spinal cord.

An Engineering Model to Test for Sensory Reweighting: Nonhuman Primates Serve as a Model for Human Postural Control and Vestibular Dysfunction

Quantitative animal models are critically needed to provide proof of concept for the investigation of rehabilitative balance therapies (e.g., invasive vestibular prostheses) and treatment response prior to, or in conjunction with, human clinical trials. This paper describes a novel approach to modeling the nonhuman primate postural control system. Our observation that rhesus macaques and humans have even remotely similar postural control motivates the further application of the rhesus macaque as a model for studying the effects of vestibular dysfunction, as well as vestibular prosthesis-assisted states, on human postural control. Previously, system identification methodologies and models were only used to describe human posture. However, here we utilized pseudorandom, roll-tilt balance platform stimuli to perturb the posture of a rhesus monkey in normal and mild vestibular (equilibrium) loss states. The relationship between rhesus monkey trunk sway and platform roll-tilt was determined via stimulus-response curves and transfer function results. A feedback controller model was then used to explore sensory reweighting (i.e., changes in sensory reliance), which prevented the animal from falling off of the tilting platform. Conclusions involving sensory reweighting in the nonhuman primate for a normal sensory state and a state of mild vestibular loss led to meaningful insights. This first-phase effort to model the balance control system in nonhuman primates is essential for future investigations toward the effects of invasive rehabilitative (balance) technologies on postural control in primates, and ultimately, humans.

Planar covariance of upper and lower limb elevation angles during hand-foot crawling in healthy young adults

Habitual quadrupeds have been shown to display a planar covariance of segment elevation angle waveforms in the fore and hind limbs during many forms of locomotion. The purpose of the current study was to determine if humans generate similar patterns in the upper and lower limbs during hand-foot crawling. Nine healthy young adults performed hand-foot crawling on a treadmill at speeds of 1, 2, and 3 km/h. A principal component analysis (PCA) was applied to the segment elevation angle waveforms for the upper (upper arm, lower arm, and hand) and lower (thigh, shank, and foot) limbs separately. The planarity of the elevation angle waveforms was determined using the sum of the variance explained by the first two PCs and the orientation of the covariance plane was quantified using the direction cosines of the eigenvector orthogonal to the plane, projected upon each of the segmental semi-axes. Results showed that planarity of segment elevation angles was maintained in the upper and lower limbs (explained variance >97%), although a slight decrease was present in the upper limb when crawling at 3 km/h. The orientation of the covariance plane was highly limb-specific, consistent with animal studies and possibly related to the functional neural control differences between the upper and lower limbs. These results may suggest that the motor patterns stored in the central nervous system for quadrupedal locomotion may be retained through evolution and may still be exploited when humans perform such tasks.

Three-dimensional kinematic and kinetic analysis of quadrupedal walking in the common marmoset (Callithrix jacchus)

The common marmoset has recently gained a great deal of attention as an experimental primate model for biological science and medical research. To use the common marmoset for development of novel treatments and rehabilitation for locomotor disorders, it is crucial to understand fundamental baseline characteristics of locomotion in this species. Therefore, in the present study we performed kinematic and kinetic analyses of quadrupedal locomotion in this animal. A total of 14 common marmosets walking quadrupedally along a walkway were analyzed using synchronized high-speed cameras, with two force platforms set in the walkway. Our results demonstrated that the marmoset uses a lateral sequence walking pattern, in contrast to the macaque and other primates, which usually adopt a diagonal sequence pattern. Furthermore, peak vertical ground reaction force on the forelimb was larger than that on the hindlimb. The rate of energy recovery for quadrupedal walking in the common marmoset was much smaller than that in the macaque, indicating that the marmoset generally utilizes bouncing mechanics in locomotion, even though the duty factor is >0.5. This description of locomotor characteristics of intact marmosets may serve as a basis for comparative analyses of changes in gait due to rehabilitation and regenerative treatments.

Combination of kinematic analyses and diffusion tensor tractrography to evaluate the residual motor functions in spinal cord-hemisected monkeys

BACKGROUND

Spinal cord injury (SCI) causes loss of locomotor functions. Nowadays, the relationship between the residual locomotion after SCI and the diffusion tensor tractography (DTT) results still remains unclear.

METHODS

Four rhesus monkeys were suffered thoracic cord hemisection. Kinematic evaluation and DTT were performed prior- and post-SCI (6 and 12 weeks). The longitudinal changes of gait parameters and the DTT parameters were analyzed for the injury-contralateral hindlimb. The correlations between gaits and DTT parameters were also investigated.

RESULTS

Almost gait parameters significantly changed after SCI, meanwhile, the caudal-rostral connection rate of DTT showed negative correlation with all gait parameters, demonstrating that the locomotor changes of the injury-contralateral hindlimb were associated with the ratio of residual fibers.

CONCLUSIONS

The combinatory use of gait analysis and DTT has been demonstrated to be sensitive to locomotion changes after SCI, and may therefore have potential applications in the pre-clinical studies of SCI.

Investigation on Inter-Limb Coordination and Motion Stability, Intensity and Complexity of Trunk and Limbs during Hands-Knees Crawling in Human Adults

This study aimed to investigate the inter-limb coordination pattern and the stability, intensity, and complexity of the trunk and limbs motions in human crawling under different speeds. Thirty healthy human adults finished hands-knees crawling trials on a treadmill at six different speeds (from 1 km/h to 2.5 km/h). A home-made multi-channel acquisition system consisting of five 3-axis accelerometers (ACC) and four force sensors was used for the data collection. Ipsilateral phase lag was used to represent inter-limb coordination pattern during crawling and power, harmonic ratio, and sample entropy of acceleration signals were adopted to depict the motion intensity, stability, and complexity of trunk and limbs respectively. Our results revealed some relationships between inter-limb coordination patterns and the stability and complexity of trunk movement. Trot-like crawling pattern was found to be the most stable and regular one at low speed in the view of trunk movement, and no-limb-pairing pattern showed the lowest stability and the greatest complexity at high speed. These relationships could be used to explain why subjects tended to avoid no-limb-pairing pattern when speed was over 2 km/h no matter which coordination type they used at low speeds. This also provided the evidence that the central nervous system (CNS) chose a stable inter-limb coordination pattern to keep the body safe and avoid tumbling. Although considerable progress has been made in the study of four-limb locomotion, much less is known about the reasons for the variety of inter-limb coordination. The research results of the exploration on the inter-limb coordination pattern choice during crawling from the standpoint of the motion stability, intensity, and complexity of trunk and limbs sheds light on the underlying motor control strategy of the human CNS and has important significance in the fields of clinical diagnosis, rehabilitation engineering, and kinematics research.

The variability of co-activation pattern of antagonist muscles in human infant crawling

Infant crawling is part of normal human gross motor development, and a 4-beat gait that involves rhythmical flexion and extension of limbs and the underlying muscle co-activation of antagonist muscle around the joint. However, detection the co-activation pattern of antagonist muscle are sparse due to the general difficulty of measuring locomotion in human infants. In this paper, sEMG of antagonist muscles and the corresponding kinematics data of limbs were collected when infants were crawling on hands and knees at their self-selected speed. The infant's gross motor developmental status was assessed by the global Gross Motor Function Measure Scale (GMFM-88) as well. The method based on EMG-EMG plots was used to quantify the variability of co-activation pattern of antagonist muscle. After that, we observed that antagonist muscles of upper limb (triceps brachii and biceps brachii) showed less variability of co-activation pattern of muscles than lower limb(quadriceps femoris and hamstrings) during crawling, and this variability was also varied in different crawling phases (stance and swing). Furthermore, we found some varied behaviors in the co-activation patterns of antagonist muscles when gross motor developmental level increased. The preliminary work suggests that such adaptive changes may be related to the adjustment of neuromuscular in the early stage of gross motor development.

Biodegradable scaffolds promote tissue remodeling and functional improvement in non-human primates with acute spinal cord injury

Tissue loss significantly reduces the potential for functional recovery after spinal cord injury. We previously showed that implantation of porous scaffolds composed of a biodegradable and biocompatible block copolymer of Poly-lactic-co-glycolic acid and Poly-l-lysine improves functional recovery and reduces spinal cord tissue injury after spinal cord hemisection injury in rats. Here, we evaluated the safety and efficacy of porous scaffolds in non-human Old-World primates (Chlorocebus sabaeus) after a partial and complete lateral hemisection of the thoracic spinal cord. Detailed analyses of kinematics and muscle activity revealed that by twelve weeks after injury fully hemisected monkeys implanted with scaffolds exhibited significantly improved recovery of locomotion compared to non-implanted control animals. Twelve weeks after injury, histological analysis demonstrated that the spinal cords of monkeys with a hemisection injury implanted with scaffolds underwent appositional healing characterized by a significant increase in remodeled tissue in the region of the hemisection compared to non-implanted controls. The number of glial fibrillary acidic protein immunopositive astrocytes was diminished within the inner regions of the remodeled tissue layer in treated animals. Activated macrophage and microglia were present diffusely throughout the remodeled tissue and concentrated at the interface between the preserved spinal cord tissue and the remodeled tissue layer. Numerous unphosphorylated neurofilament H and neuronal growth associated protein positive fibers and myelin basic protein positive cells may indicate neural sprouting inside the remodeled tissue layer of treated monkeys. These results support the safety and efficacy of polymer scaffolds in a primate model of acute spinal cord injury. A device substantially similar to the device described here is the subject of an ongoing human clinical trial.

Role of the pedunculopontine nucleus in controlling gait and sleep in normal and parkinsonian monkeys

Patients with Parkinson's disease (PD) develop cardinal motor symptoms, including akinesia, rigidity, and tremor, that are alleviated by dopaminergic medication and/or subthalamic deep brain stimulation. Over the time course of the disease, gait and balance disorders worsen and become resistant to pharmacological and surgical treatments. These disorders generate debilitating motor symptoms leading to increased dependency, morbidity, and mortality. PD patients also experience sleep disturbance that raise the question of a common physiological basis. An extensive experimental and clinical body of work has highlighted the crucial role of the pedunculopontine nucleus (PPN) in the control of gait and sleep, and its potential major role in PD. Here, we summarise our investigations in the monkey PPN in the normal and parkinsonian states. We first examined the anatomy and connectivity of the PPN and the cuneiform nucleus which both belong to the mesencephalic locomotor region. Second, we conducted experiments to demonstrate the specific effects of PPN cholinergic lesions on locomotion in the normal and parkinsonian monkey. Third, we aimed to understand how PPN cholinergic lesions impair sleep in parkinsonian monkeys. Our final goal was to develop a novel model of advanced PD with gait and sleep disorders. We believe that this monkey model, even if it does not attempt to reproduce the exact human disease with all its complexities, represents a good biomedical model to characterise locomotion and sleep in the context of PD.

A brain-spine interface alleviating gait deficits after spinal cord injury in primates

Spinal cord injury disrupts the communication between the brain and the spinal circuits that orchestrate movement. To bypass the lesion, brain–computer interfaces1–3 have directly linked cortical activity to electrical stimulation of muscles, which have restored grasping abilities after hand paralysis1,4. Theoretically, this strategy could also restore control over leg muscle activity for walking5. However, replicating the complex sequence of individual muscle activation patterns underlying natural and adaptive locomotor movements poses formidable conceptual and technological challenges6,7. Recently, we showed in rats that epidural electrical stimulation of the lumbar spinal cord can reproduce the natural activation of synergistic muscle groups producing locomotion8–10. Here, we interfaced leg motor cortex activity with epidural electrical stimulation protocols to establish a brain–spinal interface that alleviated gait deficits after a spinal cord injury in nonhuman primates. Rhesus monkeys were implanted with an intracortical microelectrode array into the leg area of motor cortex; and a spinal cord stimulation system composed of a spatially selective epidural implant and a pulse generator with real-time triggering capabilities. We designed and implemented wireless control systems that linked online neural decoding of extension and flexion motor states with stimulation protocols promoting these movements. These systems allowed the monkeys to behave freely without any restrictions or constraining tethered electronics. After validation of the brain–spinal interface in intact monkeys, we performed a unilateral corticospinal tract lesion at the thoracic level. As early as six days post-injury and without prior training of the monkeys, the brain–spinal interface restored weight-bearing locomotion of the paralyzed leg on a treadmill and overground. The implantable components integrated in the brain–spinal interface have all been approved for investigational applications in similar human research, suggesting a practical translational pathway for proof-of-concept studies in people with spinal cord injury.

Neuromechanical interactions between the limbs during human locomotion: an evolutionary perspective with translation to rehabilitation

During bipedal locomotor activities, humans use elements of quadrupedal neuronal limb control. Evolutionary constraints can help inform the historical ancestry for preservation of these core control elements support transfer of the huge body of quadrupedal non-human animal literature to human rehabilitation. In particular, this has translational applications for neurological rehabilitation after neurotrauma where interlimb coordination is lost or compromised. The present state of the field supports including arm activity in addition to leg activity as a component of gait retraining after neurotrauma.

Influence of walking speed on gait parameters of bipedal locomotion in rhesus monkeys

BACKGROUND

The primate model of bipedal locomotion has been extensively used to study human evolution and played a critical role in exploring the pathological mechanisms of human neurologic disease and spinal cord injury. Speed has great influence on both walking posture and gait parameters in human walking; however, how speed changes the gait pattern of bipedal locomotion in primates remains unclear.

METHODS

We chose six adult female rhesus monkeys (Macaca mulatta) and collected the gait parameters of these animals during their treadmill locomotion over a wide range of speed. Using a 3D motion analysis system, we studied the spatiotemporal characteristics of the gait pattern.

RESULTS AND CONCLUSIONS

We have built an efficient and time-saving primate model and shown that speed significantly impacts kinematic parameters. This may present a thorough description of speed-related changes in the gait pattern of rhesus and shed light on the control of bipedal locomotion in primates.

Closed-loop neuromodulation of spinal sensorimotor circuits controls refined locomotion after complete spinal cord injury

Neuromodulation of spinal sensorimotor circuits improves motor control in animal models and humans with spinal cord injury. With common neuromodulation devices, electrical stimulation parameters are tuned manually and remain constant during movement. We developed a mechanistic framework to optimize neuromodulation in real time to achieve high-fidelity control of leg kinematics during locomotion in rats. We first uncovered relationships between neuromodulation parameters and recruitment of distinct sensorimotor circuits, resulting in predictive adjustments of leg kinematics. Second, we established a technological platform with embedded control policies that integrated robust movement feedback and feed-forward control loops in real time. These developments allowed us to conceive a neuroprosthetic system that controlled a broad range of foot trajectories during continuous locomotion in paralyzed rats. Animals with complete spinal cord injury performed more than 1000 successive steps without failure, and were able to climb staircases of various heights and lengths with precision and fluidity. Beyond therapeutic potential, these findings provide a conceptual and technical framework to personalize neuromodulation treatments for other neurological disorders.