Neuromechanical control of locomotion in the rat

From representations to servomechanisms to oscillators: my journey in the study of cognition

The study of comparative cognition bloomed in the 1970s and 1980s with a focus on representations in the heads of animals that undergird what animals can achieve. Even in action-packed domains such as navigation and spatial cognition, a focus on representations prevailed. In the 1990s, I suggested a conception of navigation in terms of navigational servomechanisms. A servomechanism can be said to aim for a goal, with deviations from the goal-directed path registering as an error. The error drives action to reduce the error in a negative-feedback loop. This loop, with the action reducing the very signal that drove action in the first place, is key to defining a servomechanism. Even though actions are crucial components of servomechanisms, my focus was on the representational component that encodes signals and evaluates errors. Recently, I modified and amplified this view in claiming that, in navigation, servomechanisms operate by modulating the performance of oscillators, endogenous units that produce periodic action. The pattern is found from bacteria travelling micrometres to sea turtles travelling thousands of kilometres. This pattern of servomechanisms working with oscillators is found in other realms of cognition and of life. I think that oscillators provide an effective way to organise an organism's own activities while servomechanisms provide an effective means to adjust to the organism's environment, including that of its own body.

Comprehensive dynamic and kinematic analysis of the rodent hindlimb during over ground walking

The rat hindlimb is a frequently utilized pre-clinical model system to evaluate injuries and pathologies impacting the hindlimbs. These studies have demonstrated the translational potential of this model but have typically focused on the force generating capacity of target muscles as the primary evaluative outcome. Historically, human studies investigating extremity injuries and pathologies have utilized biomechanical analysis to better understand the impact of injury and extent of recovery. In this study, we expand that full biomechanical workup to a rat model in order to characterize the spatiotemporal parameters, ground reaction forces, 3-D joint kinematics, 3-D joint kinetics, and energetics of gait in healthy rats. We report data on each of these metrics that meets or exceeds the standards set by the current literature and are the first to report on all these metrics in a single set of animals. The methodology and findings presented in this study have significant implications for the development and clinical application of the improved regenerative therapeutics and rehabilitative therapies required for durable and complete functional recovery from extremity traumas, as well as other musculoskeletal pathologies.

Newly regenerated axons via scaffolds promote sub-lesional reorganization and motor recovery with epidural electrical stimulation

Here, we report the effect of newly regenerated axons via scaffolds on reorganization of spinal circuitry and restoration of motor functions with epidural electrical stimulation (EES). Motor recovery was evaluated for 7 weeks after spinal transection and following implantation with scaffolds seeded with neurotrophin producing Schwann cell and with rapamycin microspheres. Combined treatment with scaffolds and EES-enabled stepping led to functional improvement compared to groups with scaffold or EES, although, the number of axons across scaffolds was not different between groups. Re-transection through the scaffold at week 6 reduced EES-enabled stepping, still demonstrating better performance compared to the other groups. Greater synaptic reorganization in the presence of regenerated axons was found in group with combined therapy. These findings suggest that newly regenerated axons through cell-containing scaffolds with EES-enabled motor training reorganize the sub-lesional circuitry improving motor recovery, demonstrating that neuroregenerative and neuromodulatory therapies cumulatively enhancing motor function after complete SCI.

Total Knee Arthroplasty with a Ti6Al4V/PEEK Prosthesis on an Osteoarthritis Rat Model: Behavioral and Neurophysiological Analysis

Arthroplasty is a surgical procedure to restore the function of the joint of patient suffering from knee osteoarthritis. However, postoperative functional deficits are reported even after a rehabilitation program. In order to determine the origin of functional deficits of patient suffering from knee osteoarthritis and total knee arthroplasty, we developed a rodent model including a chemically-induced-osteoarthritis and designed a knee prosthesis (Ti6Al4V/PEEK) biomechanically and anatomically adapted to rat knee joint. Dynamic Weight-Bearing, gait kinematics, H-reflex from vastus medialis muscle and activities from metabosensitive III and IV afferent fibers in femoral nerve were assessed at 1 and 3 months post-surgery. Results indicate that knee osteoarthritis altered considerably the responses of afferent fibers to their known activators (i.e., lactic acid and potassium chloride) and consequently their ability to modulate the spinal sensorimotor loop, although, paradoxically, motor deficits seemed relatively light. On the contrary, results indicate that, after the total knee arthroplasty, the afferent responses and the sensorimotor function were slightly altered but that motor deficits were more severe. We conclude that neural changes attested by the recovery of the metabosensitive afferent activity and the sensorimotor loop were induced when a total knee replacement was performed and that these changes may disrupt or delay the locomotor recovery.

Multifactorial motor behavior assessment for real-time evaluation of emerging therapeutics to treat neurologic impairments

Integrating multiple assessment parameters of motor behavior is critical for understanding neural activity dynamics during motor control in both intact and dysfunctional nervous systems. Here, we described a novel approach (termed Multifactorial Behavioral Assessment (MfBA)) to integrate, in real-time, electrophysiological and biomechanical properties of rodent spinal sensorimotor network activity with behavioral aspects of motor task performance. Specifically, the MfBA simultaneously records limb kinematics, multi-directional forces and electrophysiological metrics, such as high-fidelity chronic intramuscular electromyography synchronized in time to spinal stimulation in order to characterize spinal cord functional motor evoked potentials (fMEPs). Additionally, we designed the MfBA to incorporate a body weight support system to allow bipedal and quadrupedal stepping on a treadmill and in an open field environment to assess function in rodent models of neurologic disorders that impact motor activity. This novel approach was validated using, a neurologically intact cohort, a cohort with unilateral Parkinsonian motor deficits due to midbrain lesioning, and a cohort with complete hind limb paralysis due to T8 spinal cord transection. In the SCI cohort, lumbosacral epidural electrical stimulation (EES) was applied, with and without administration of the serotonergic agonist Quipazine, to enable hind limb motor functions following paralysis. The results presented herein demonstrate the MfBA is capable of integrating multiple metrics of motor activity in order to characterize relationships between EES inputs that modulate mono- and polysynaptic outputs from spinal circuitry which in turn, can be used to elucidate underlying electrophysiologic mechanisms of motor behavior. These results also demonstrate that proposed MfBA is an effective tool to integrate biomechanical and electrophysiology metrics, synchronized to therapeutic inputs such as EES or pharmacology, during body weight supported treadmill or open field motor activities, to target a high range of variations in motor behavior as a result of neurological deficit at the different levels of CNS.

Kinematic and kinetic gait analysis to evaluate functional recovery in thoracic spinal cord injured rats

The recovery of walking function following spinal cord injury (SCI) is of major importance to patients and clinicians. In experimental SCI studies, a rat model is widely used to assess walking function, following thoracic spinal cord lesion. In an effort to provide a resource which investigators can refer to when seeking the most appropriate functional assay, the authors have compiled and categorized the behavioral assessments used to measure the deficits and recovery of the gait in thoracic SCI rats. These categories include kinematic and kinetic measurements. Within this categorization, we discuss the advantages and disadvantages of each type of measurement. The present review includes the type of outcome data that they produce, the technical difficulty and the time required to potentially train the animals to perform them, and the need for expensive or highly specialized equipment. The use of multiple kinematic and kinetic parameters is recommended to identify subtle deficits and processes involved in the compensatory mechanisms of walking function after experimental thoracic SCI in rats.

Vastus lateralis and vastus medialis produce distinct mediolateral forces on the patella but similar forces on the tibia in the rat

Improper activation of the quadriceps muscles vastus medialis (VM) and vastus lateralis (VL) has been implicated in the development of patellofemoral pain (PFP). This explanation of PFP assumes that VM and VL produce opposing mediolateral forces on the patella. Although studies have provided evidence for opposing actions of VM and VL on the patella, other studies have suggested that their actions might be similar. In this study, we took advantage of the experimental accessibility of the rat to directly measure the forces on the patella produced by VM and VL. We found that VM and VL produce opposing mediolateral forces on the patella when the patella was lifted away from the femur. These distinct mediolateral forces were not transmitted to the tibia, however: forces measured at the distal tibia were very similar for VM and VL. Further, when the patella was placed within the trochlear groove, the forces on the patella produced by VM and VL were very similar to one another. These results suggest that mediolateral forces produced by VM and VL are balanced by reaction forces from the trochlear groove and so are not transmitted to the tibia. These results provide a rich characterization of the mechanical actions of VM and VL and have implications about the potential role of these muscles in PFP and their neural control during behavior.

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.

Muscle proprioceptors in adult rat: mechanosensory signaling and synapse distribution in spinal cord

The characteristic signaling and intraspinal projections of muscle proprioceptors best described in the cat are often generalized across mammalian species. However, species-dependent adaptations within this system seem necessary to accommodate asymmetric scaling of length, velocity, and force information required by the physics of movement. In the present study we report mechanosensory responses and intraspinal destinations of three classes of muscle proprioceptors. Proprioceptors from triceps surae muscles in adult female Wistar rats anesthetized with isoflurane were physiologically classified as muscle spindle group Ia or II or as tendon organ group Ib afferents, studied for their firing responses to passive-muscle stretch, and in some cases labeled and imaged for axon projections and varicosities in spinal segments. Afferent projections and the laminar distributions of provisional synapses in rats closely resembled those found in the cat. Afferent signaling of muscle kinematics was also similar to reports in the cat, but rat Ib afferents fired robustly during passive-muscle stretch and Ia afferents displayed an exaggerated dynamic response, even after locomotor scaling was accounted for. These differences in mechanosensory signaling by muscle proprioceptors may represent adaptations for movement control in different animal species.NEW & NOTEWORTHY Muscle sensory neurons signal information necessary for controlling limb movements. The information encoded and transmitted by muscle proprioceptors to networks in the spinal cord is known in detail only for the cat, but differences in size and behavior of other species challenge the presumed generalizability. This report presents the first findings detailing specializations in mechanosensory signaling and intraspinal targets for functionally identified subtypes of muscle proprioceptors in the rat.

Dynamic feet distance: A new functional assessment during treadmill locomotion in normal and thoracic spinal cord injured rats

Of all the detrimental effects of spinal cord injury (SCI), one of the most devastating is the disruption of the ability to perform functional movement. Very little is known on the recovery of hindlimb joint kinematics after clinically-relevant contusive thoracic lesion in experimental animal models. A new functional assessment instrument, the dynamic feet distance (DFD) was used to describe the distance between the two feet throughout the gait cycle in normal and affected rodents. The purpose of this investigation was the evaluation and characterization of the DFD during treadmill locomotion in normal and T9 contusion injured rats, using three-dimensional (3D) instrumented gait analysis. Despite that normal and injured rats showed a similar pattern in the fifth metatarsal head joints distance excursion, we found a significantly wider distance between the feet during the entire gait cycle following spinal injury. This is the first study to quantify the distance between the two feet, throughout the gait cycle, and the biomechanical adjustments made between limbs in laboratory rodents after nervous system injury.

Validity and reliability of the CatWalk system as a static and dynamic gait analysis tool for the assessment of functional nerve recovery in small animal models

INTRODUCTION

A range of behavioral testing paradigms have been developed for the research of central and peripheral nerve injuries with the help of small animal models. Following any nerve repair strategy, improved functional outcome may be the most important evidence of axon regeneration. A novel automated gait analysis system, the CatWalk^™, can measure dynamic as well as static gait patterns of small animals. Of most interest in detecting functional recovery are in particular dynamic gait parameters, coordination measures, and the intensity of the animals paw prints. This article is designed to lead to a more efficient choice of CatWalk parameters in future studies concerning the functional evaluation of nerve regeneration and simultaneously add to better interstudy comparability.

METHODS

The aims of the present paper are threefold: (1) to describe the functional method of CatWalk gait analysis, (2) to characterize different parameters acquired by CatWalk gait analysis, and to find the most frequently used parameters as well as (3) to compare their reliability and validity throughout the different studies.

RESULTS

In the reviewed articles, the most frequently used parameters were Swing Duration (30), Print Size (27), Stride Length (26), and Max Contact Area (24). Swing Duration was not only frequently used but was also the most reliable and valid parameter. Therefore, we hypothesize that Swing Duration constitutes an important parameter to be chosen for future studies, as it has the highest level of reliability and validity.

CONCLUSION

In conclusion, CatWalk can be used as a complementary approach to other behavioral testing paradigms to assess clinically relevant behavioral benefits, with the main advantage that this system demonstrates both static and dynamic gait parameters at the same time. Due to limited reliability and validity of certain parameters, we recommend that only the most frequently assessed parameters should be used in the future.

Evidence of adaptations of locomotor neural drive in response to enhanced intermuscular connectivity between the triceps surae muscles of the rat

The aims of this study were to investigate changes 1) in the coordination of activation of the triceps surae muscle group, and 2) in muscle belly length of soleus (SO) and lateral gastrocnemius (LG) during locomotion (trotting) in response to increased stiffness of intermuscular connective tissues in the rat. We measured muscle activation and muscle belly lengths, as well as hindlimb kinematics, before and after an artificial enhancement of the connectivity between SO and LG muscles obtained by implanting a tissue-integrating surgical mesh at the muscles' interface. We found that SO muscle activation decreased to 62%, while activation of LG and medial gastrocnemius muscles increased to 134 and 125%, respectively, compared with the levels measured preintervention. Although secondary additional or amplified activation bursts were observed with enhanced connectivity, the primary pattern of activation over the stride and the burst duration were not affected by the intervention. Similar muscle length changes after manipulation were observed, suggesting that length feedback from spindle receptors within SO and LG was not affected by the connectivity enhancement. We conclude that peripheral mechanical constraints given by morphological (re)organization of connective tissues linking synergists are taken into account by the central nervous system. The observed shift in activity toward the gastrocnemius muscles after the intervention suggests that these larger muscles are preferentially recruited when the soleus has a similar mechanical disadvantage in that it produces an unwanted flexion moment around the knee.NEW & NOTEWORTHY Connective tissue linkages between muscle-tendon units may act as an additional mechanical constraint on the musculoskeletal system, thereby reducing the spectrum of solutions for performing a motor task. We found that intermuscular coordination changes following intermuscular connectivity enhancement. Besides showing that the extent of such connectivity is taken into account by the central nervous system, our results suggest that recruitment of triceps surae muscles is governed by the moments produced at the ankle-knee joints.

Kinematic response in limb and body posture to sensory feedback from carpal sinus hairs in the rat (Rattus norvegicus)

One of the most challenging adaptations within the therians has been to ensure dynamic stability of the trunk during rapid locomotion in highly structured environments. A reorganization of limb mechanics and development of new sensors has taken place within their stem lineage. Rats, which have a similar lifestyle to the first therians, possess sinus hairs specialized for tactile sensing. It is supposed that carpal sinus hairs have a role in sensing substrate properties and can thus induce adjustments in limb kinematics and body posture according to the different surface diameters and structures detected. This implies a shared sensorimotor control loop of sinus hairs and body posture. To investigate the role of the carpal sinus hairs during locomotion and to explore a possible interaction between limb and spine motion, spatiotemporal and kinematic parameters as well as the contact mechanics of the hairs with regard to the surface were quantified. Locomotion on a treadmill with continuous and discontinuous substrates was compared in the presence/absence of the carpal sinus hairs across a speed range from 0.2m/s to 0.6m/s. Recordings were taken synchronously using x-ray fluoroscopy and normal-light high-speed cameras. Our investigation revealed that the three tactile hairs made a triangle-like contact with the ground approximately 30ms before touchdown of the forelimb. Within that time, it is likely that both the body posture and its oscillation are adjusted according to the different surface textures. The sensory input of the carpal sinus hairs induces a stabilization of the trajectory of the center of mass and, therefore, improves the dynamic stability of the trunk; conversely, the absence of the sensors results in a more crouched frontal body posture, similar to that seen in rats when they are moving in an unknown terrain. The carpal sinus hairs also sense the animal's speed during surface contact. This implicates an adjustment of the limb and spine kinematics, by increasing the speed-dependent effect or by increasing the distance between the trunk and the ground when the rat is walking faster.

Longitudinal and transversal displacements between triceps surae muscles during locomotion of the rat

The functional consequences of differential muscle activation and contractile behavior between mechanically coupled synergists are still poorly understood. Even though synergistic muscles exert similar mechanical effects at the joint they span, differences in the anatomy, morphology and neural drive may lead to non-uniform contractile conditions. This study aimed to investigate the patterns of activation and contractile behavior of triceps surae muscles, to understand how these contribute to the relative displacement between the one-joint soleus (SO) and two-joint lateral gastrocnemius (LG) muscle bellies and their distal tendons during locomotion in the rat. In seven rats, muscle belly lengths and muscle activation during level and upslope trotting were measured by sonomicrometry crystals and electromyographic electrodes chronically implanted in the SO and LG. Length changes of muscle–tendon units (MTUs) and tendon fascicles were estimated based on joint kinematics and muscle belly lengths. Distances between implanted crystals were further used to assess longitudinal and transversal deformations of the intermuscular volume between the SO and LG. For both slope conditions, we observed differential timing of muscle activation as well as substantial differences in contraction speeds between muscle bellies (maximal relative speed 55.9 mm s−1). Muscle lengths and velocities did not differ significantly between level and upslope locomotion, only EMG amplitude of the LG was affected by slope. Relative displacements between SO and LG MTUs were found in both longitudinal and transversal directions, yielding an estimated maximal length change difference of 2.0 mm between their distal tendons. Such relative displacements may have implications for the force exchanged via intermuscular and intertendinous pathways.

Gait analysis methods for rodent models of arthritic disorders: reviews and recommendations

Gait analysis is a useful tool to understand behavioral changes in preclinical arthritis models. While observational scoring and spatiotemporal gait parameters are the most widely performed gait analyses in rodents, commercially available systems can now provide quantitative assessments of spatiotemporal patterns. However, inconsistencies remain between testing platforms, and laboratories often select different gait pattern descriptors to report in the literature. Rodent gait can also be described through kinetic and kinematic analyses, but systems to analyze rodent kinetics and kinematics are typically custom made and often require sensitive, custom equipment. While the use of rodent gait analysis rapidly expands, it is important to remember that, while rodent gait analysis is a relatively modern behavioral assay, the study of quadrupedal gait is not new. Nearly all gait parameters are correlated, and a collection of gait parameters is needed to understand a compensatory gait pattern used by the animal. As such, a change in a single gait parameter is unlikely to tell the full biomechanical story; and to effectively use gait analysis, one must consider how multiple different parameters contribute to an altered gait pattern. The goal of this article is to review rodent gait analysis techniques and provide recommendations on how to use these technologies in rodent arthritis models, including discussions on the strengths and limitations of observational scoring, spatiotemporal, kinetic, and kinematic measures. Recognizing rodent gait analysis is an evolving tool, we also provide technical recommendations we hope will improve the utility of these analyses in the future.

Influence of ovarian hormones on strength loss in healthy and dystrophic female mice

PURPOSE

The primary objective of this study was to determine whether strength loss and recovery after eccentric contractions are impaired in healthy and dystrophic female mice with low levels of ovarian hormones.

METHODS

Female C57BL/6 (wild-type) or mdx mice were randomly assigned to ovarian-intact (Sham) and ovariectomized (Ovx) groups. Anterior crural muscles were tested for susceptibility to injury from 150 or 50 eccentric contractions in wild-type and mdx mice, respectively. An additional experiment challenged mdx mice with a 2-wk treadmill running protocol followed by an eccentric contraction injury to posterior crural muscles. Functional recovery from injury was evaluated in wild-type mice by measuring isometric torque 3, 7, 14, or 21 d after injury.

RESULTS

Ovarian hormone deficiency in wild-type mice did not affect susceptibility to injury because the ∼50% isometric torque loss after eccentric contractions did not differ between Sham and Ovx mice (P = 0.121). Similarly, in mdx mice, hormone deficiency did not affect the percent of preinjury isometric torque lost by anterior crural muscles after eccentric contractions (P = 0.952), but the percent of preinjury torque in posterior crural muscles was lower in Ovx than in Sham mice (P = 0.014). Recovery from injury in wild-type mice was affected by hormone deficiency. Sham mice recovered preinjury isometric strength by 14 d (96% ± 2%), whereas Ovx mice maintained deficits at 14 and 21 d after injury (80% ± 3% and 84% ± 2%, P < 0.001).

CONCLUSIONS

Ovarian hormone status did not affect the vulnerability of skeletal muscle to strength loss after eccentric contractions. However, ovarian hormone deficiency did impair the recovery of muscle strength in female mice.

The efficacy of neuromuscular electrical stimulation with alternating currents in the kilohertz frequency to stimulate gait rhythm in rats following spinal cord injury

Background

Rehabilitation facilitates the reorganization of residual/regenerated neural pathways and is key in improving motor function following spinal cord injury. Neuromuscular electrical stimulation (NMES) has been reported as being clinically effective. Although it can be used after the acute phase post-injury, the optimal stimulation conditions to improve motor function remain unclear. In this paper, we examined the effectiveness of NMES with alternating currents in the kilohertz (kHz) frequency in gait rhythm stimulation therapy.

Methods

Tests were performed using 20 mature female Fischer rats. Incomplete spinal cord injuries (T9 level) were made with an IH impactor using a force of 150 kdyn, and NMES was administered for 3 days from the 7th day post-injury. The needle electrodes were inserted percutaneously near the motor point of each muscle in conscious rats, and each muscle on the left and right leg was stimulated for 15 min at two frequencies, 75 Hz and 8 kHz, to induce a gait rhythm. Motor function was evaluated using Basso, Beattie, Bresnahan (BBB) scores and three-dimensional (3D) gait analysis. Rats were divided into four groups (5 rats/group), including the NMES treatment 75-Hz group (iSCI-NMES 75 Hz), 8-kHz group (iSCI-NMES 8 kHz), injury control group (iSCI-NT), and normal group (Normal-CT), and were compared.

Results

There was no significant difference in BBB scores among the three groups. In 3D gait analysis, compared with the injury control group, the 8-kHz group showed a significant improvement in synergistic movement of both hindlimbs.

Conclusion

We suggest that kHz stimulation is effective in gait rhythm stimulation using NMES.

Intermittent application of hypergravity by centrifugation attenuates disruption of rat gait induced by 2 weeks of simulated microgravity

The effects of intermittent hypergravity on gait alterations and hindlimb muscle atrophy in rats induced by 2 weeks of simulated microgravity were investigated. Rats were submitted to hindlimb unloading for 2 weeks (unloading period), followed by 2 weeks of reloading (recovery period). During the unloading period, animals were subjected to the following treatments: (1) free in cages (Control); (2) continuous unloading (UL); (3) released from unloading for 1 hour per day (UL+1G); (4) hypergravity for 1h per day using a centrifuge for small animals (UL+2G). The relative weights of muscles to the whole body weight and kinematics properties of hindlimbs during gait were evaluated. UL rats walked with their hindlimbs overextended, and the oscillation of their limb motion had become narrowed and forward-shifted after the unloading period, and this persisted for at least 2 weeks after the termination of unloading. However, these locomotor alterations were attenuated in rats subjected to UL+2G centrifugation despite minor systematic changes in muscle recovery. These findings indicate hypergravity application could counteract the adverse effects of simulated or actual microgravity environments.

Discordance in recovery between altered locomotion and muscle atrophy induced by simulated microgravity in rats

Exposure to a microgravity environment leads to adverse effects in motion and musculoskeletal properties. However, few studies have investigated the recovery of altered locomotion and muscle atrophy simultaneously. The authors investigated altered locomotion in rats submitted to simulated microgravity by hindlimb unloading for 2 weeks. Motion deficits were characterized by hyperextension of the knees and ankle joints and forward-shifted limb motion. Furthermore, these locomotor deficits did not revert to their original form after a 2-week recovery period, although muscle atrophy in the hindlimbs had recovered, implying discordance in recovery between altered locomotion and muscle atrophy, and that other factors such as neural drives might control behavioral adaptations to microgravity.

Effects of spinal cord injury-induced changes in muscle activation on foot drag in a computational rat ankle model

Spinal cord injury (SCI) can lead to changes in muscle activation patterns and atrophy of affected muscles. Moderate levels of SCI are typically associated with foot drag during the swing phase of locomotion. Foot drag is often used to assess locomotor recovery, but the causes remain unclear. We hypothesized that foot drag results from inappropriate muscle coordination preventing flexion at the stance-to-swing transition. To test this hypothesis and to assess the relative contributions of neural and muscular changes on foot drag, we developed a two-dimensional, one degree of freedom ankle musculoskeletal model with gastrocnemius and tibialis anterior muscles. Anatomical data collected from sham-injured and incomplete SCI (iSCI) female Long-Evans rats as well as physiological data from the literature were used to implement an open-loop muscle dynamics model. Muscle insertion point motion was calculated with imposed ankle trajectories from kinematic analysis of treadmill walking in sham-injured and iSCI animals. Relative gastrocnemius deactivation and tibialis anterior activation onset times were varied within physiologically relevant ranges based on simplified locomotor electromyogram profiles. No-atrophy and moderate muscle atrophy as well as normal and injured muscle activation profiles were also simulated. Positive moments coinciding with the transition from stance to swing phase were defined as foot swing and negative moments as foot drag. Whereas decreases in activation delay caused by delayed gastrocnemius deactivation promote foot drag, all other changes associated with iSCI facilitate foot swing. Our results suggest that even small changes in the ability to precisely deactivate the gastrocnemius could result in foot drag after iSCI.

The need for speed in rodent locomotion analyses

Locomotion analysis is now widely used across many animal species to understand the motor defects in disease, functional recovery following neural injury, and the effectiveness of various treatments. More recently, rodent locomotion analysis has become an increasingly popular method in a diverse range of research. Speed is an inseparable aspect of locomotion that is still not fully understood, and its effects are often not properly incorporated while analyzing data. In this hybrid manuscript, we accomplish three things: (1) review the interaction between speed and locomotion variables in rodent studies, (2) comprehensively analyze the relationship between speed and 162 locomotion variables in a group of 16 wild-type mice using the CatWalk gait analysis system, and (3) develop and test a statistical method in which locomotion variables are analyzed and reported in the context of speed. Notable results include the following: (1) over 90% of variables, reported by CatWalk, were dependent on speed with an average R(2) value of 0.624, (2) most variables were related to speed in a nonlinear manner, (3) current methods of controlling for speed are insufficient, and (4) the linear mixed model is an appropriate and effective statistical method for locomotion analyses that is inclusive of speed-dependent relationships. Given the pervasive dependency of locomotion variables on speed, we maintain that valid conclusions from locomotion analyses cannot be made unless they are analyzed and reported within the context of speed.

Gait compensations in rats after a temporary nerve palsy quantified using temporo-spatial and kinematic parameters

BACKGROUND

The aim of this work was to test a method for measuring the gait of rats with sufficient sensitivity to detect subtle locomotor changes due to pathology, injury and recovery.

METHOD

The gait of female Sprague-Dawley rats was assessed using an optical motion tracking system and the DigiGait™ imaging system during normal locomotion, shortly after temporary nerve block to the left hind limb and after full recovery.

RESULTS

The effect of low treadmill speeds (10-30 cm/s) was initially investigated. Significant changes were detected in the spatiotemporal gait parameters, consistent with those previously reported. The overall ranges of motion in the hip, knee and ankle joints were 37.5° (±7.1°), 50.2° (±9.4°) and 61.6° (±9.1°) and did not appear to change with speed, indicating that for low speed variations, kinematic comparisons across speeds may be possible. Following the induction of a temporary sciatic nerve block, the range of motion of the left ankle and knee during swing decreased by 23° and 33°, respectively (p<0.05). A compensatory change of a greater range of motion at the hip was noted in the contralateral limb (p<0.01). 90 min post injection, most of the gait parameters had returned to normal, however, minor walking deficits were still present.

COMPARISON WITH EXISTING METHOD(S)

Discriminant analysis showed that a combination of dynamic and kinematic parameters provides a more robust method for the classification of gait changes.

CONCLUSIONS

This more detailed method, employing both dynamic analysis and joint kinematics simultaneously, was found to be a reliable approach for the quantification of gait in rats.

A pelvic implant orthosis in rodents, for spinal cord injury rehabilitation, and for brain machine interface research: construction, surgical implantation and validation

BACKGROUND

Rodents are important model systems used to explore spinal cord injury (SCI) and rehabilitation, and brain machine interfaces (BMI). We present a new method to provide mechanical interaction for BMI and rehabilitation in rat models of SCI.

NEW METHOD

We present the design and implantation procedures for a pelvic orthosis that allows direct force application to the skeleton in brain machine interface and robot rehabilitation applications in rodents. We detail the materials, construction, machining, surgery and validation of the device.

RESULTS

We describe the statistical validation of the implant procedures by comparing stepping parameters of 8 rats prior to and after implantation and surgical recovery. An ANOVA showed no effects of the implantation on stepping. Paired tests in the individual rats also showed no effect in 7/8 rats and minor effects in the last rat, within the group's variance.

COMPARISON WITH EXISTING METHODS

Our method allows interaction with rats at the pelvis without any perturbation of normal stepping in the intact rat. The method bypasses slings, and cuffs, avoiding cuff or slings squeezing the abdomen, or other altered sensory feedback. Our implant osseointegrates, and thus allows an efficient high bandwidth mechanical coupling to a robot. The implants support quadrupedal training and are readily integrated into either treadmill or overground contexts.

CONCLUSIONS

Our novel device and procedures support a range of novel experimental designs and motor tests for rehabilitative and augmentation devices in intact and SCI model rats, with the advantage of allowing direct force application at the pelvic bones.

Three-dimensional rodent motion analysis and neurodegenerative disorders

BACKGROUND

Three-dimensional (3D) motion analysis is established in investigating, human pathological motion. In the field of gait, its use results in the objective identification of primary, and secondary causes of deviations, many current interventions are the result of pre- and post-testing, and it was shown recently that it can result in decreased number of surgeries and overall cost of care. Consequently, recent attempts have implemented 3D motion analysis using rat models to study, parkinsonism. However, to-date, a 3D user friendly analytical approach using rodent models to, identify etiologies of age-related motor impairment and accompanying pathologies has not been, implemented.

NEW METHOD

We have developed and presented all aspects of a 3D, three body-segment rodent model, to analyze motions of the lower, upper and head segments between rodents of parkinsonism-type and, normal aging during free walking. Our model does not require transformation matrices to describe the, position of each body-segment. Because body-segment positions are not considered to consist of three, rotations about the laboratory axes, the rotations are not sequence dependent.

RESULTS

Each body-segment demonstrated distinct 3D movement patterns. The parkinsonism-type, genotype walked slower with less range of motion, similarly to patients with parkinsonism.

COMPARISON WITH EXISTING METHODS

This is the first model considering the rodent's body as three, distinct segments. To the best of our knowledge, it is the first model to ever consider and report the 3D, head motion patterns.

CONCLUSIONS

This novel approach will allow unbiased analysis of spontaneous locomotion in mouse, models of parkinsonism or normal aging.

Joint-specific changes in locomotor complexity in the absence of muscle atrophy following incomplete spinal cord injury

Background

Following incomplete spinal cord injury (iSCI), descending drive is impaired, possibly leading to a decrease in the complexity of gait. To test the hypothesis that iSCI impairs gait coordination and decreases locomotor complexity, we collected 3D joint angle kinematics and muscle parameters of rats with a sham or an incomplete spinal cord injury.

Methods

12 adult, female, Long-Evans rats, 6 sham and 6 mild-moderate T8 iSCI, were tested 4 weeks following injury. The Basso Beattie Bresnahan locomotor score was used to verify injury severity. Animals had reflective markers placed on the bony prominences of their limb joints and were filmed in 3D while walking on a treadmill. Joint angles and segment motion were analyzed quantitatively, and complexity of joint angle trajectory and overall gait were calculated using permutation entropy and principal component analysis, respectively. Following treadmill testing, the animals were euthanized and hindlimb muscles removed. Excised muscles were tested for mass, density, fiber length, pennation angle, and relaxed sarcomere length.

Results

Muscle parameters were similar between groups with no evidence of muscle atrophy. The animals showed overextension of the ankle, which was compensated for by a decreased range of motion at the knee. Left-right coordination was altered, leading to left and right knee movements that are entirely out of phase, with one joint moving while the other is stationary. Movement patterns remained symmetric. Permutation entropy measures indicated changes in complexity on a joint specific basis, with the largest changes at the ankle. No significant difference was seen using principal component analysis. Rats were able to achieve stable weight bearing locomotion at reasonable speeds on the treadmill despite these deficiencies.

Conclusions

Decrease in supraspinal control following iSCI causes a loss of complexity of ankle kinematics. This loss can be entirely due to loss of supraspinal control in the absence of muscle atrophy and may be quantified using permutation entropy. Joint-specific differences in kinematic complexity may be attributed to different sources of motor control. This work indicates the importance of the ankle for rehabilitation interventions following spinal cord injury.

Exercise training improves plantar flexor muscle function in mdx mice

PURPOSE

We tested the hypothesis that low-intensity exercise in mdx mice improves plantar flexor muscle contractile function, resistance to fatigue, and mitochondrial adaptations without exacerbating muscular dystrophy.

METHODS

We subjected mdx mice to 12 wk of voluntary low-resistance wheel running (Run, n = 17) or normal cage activities (sedentary (Sed), n = 16) followed by in vivo analyses for plantar flexor torque generation and fatigue resistance or running capacity on a treadmill. Gastrocnemius muscles were further evaluated for exercise-induced mitochondrial adaptations and fiber type distribution and central nuclei. t-tests were used to determine differences between the Sed and Run groups.

RESULTS

Plantar flexor submaximal isometric torques and maximal isometric torque at multiple ankle joint angles and resistance to fatigue were greater in Run compared with Sed mdx mice (P G 0.05). Citrate synthase and A-hydroxyacyl-CoA dehydrogenase enzyme activities and cytochrome c oxidase IV protein expression in gastrocnemius muscles were greater in Run than in Sed mdx mice(P e 0.04), along with a trend of fiber type transformation from Type IIb to Type IIx fibers. Exercise training in mdx mice did not elevate serum creatine kinase levels but led to a significant reduction of centrally nucleated myofibers.

CONCLUSIONS

Voluntary low-resistance wheel running in mdx mice can result in skeletal muscle adaptation, leading to improved contractile function and reduced fatigability,with no indication that exercise was detrimental. This study supports the need for further investigation of low-intensity exercise as an early therapeutic intervention in ambulatory boys with Duchenne muscular dystrophy.

Accelerating locomotor recovery after incomplete spinal injury

A traumatic spinal injury can destroy cells, irreparably damage axons, and trigger a cascade of biochemical responses that increase the extent of injury. Although damaged central nervous system axons do not regrow well naturally, the distributed nature of the nervous system and its capacity to adapt provide opportunities for recovery of function. It is apparent that activity-dependent plasticity plays a role in this recovery and that the endogenous response to injury heightens the capacity for recovery for at least several weeks postinjury. To restore locomotor function, researchers have investigated the use of treadmill-based training, robots, and electrical stimulation to tap into adaptive activity-dependent processes. The current challenge is to maximize the degree of functional recovery. This manuscript reviews the endogenous neural system response to injury, and reviews data and presents novel analyses of these from a rat model of contusion injury that demonstrates how a targeted intervention can accelerate recovery, presumably by engaging processes that underlie activity-dependent plasticity.

Effects of treadmill training on functional recovery following peripheral nerve injury in rats

Exercise, in the form of moderate daily treadmill training following nerve transection and repair leads to enhanced axon regeneration, but its effect on functional recovery is less well known. Female rats were exercised by walking continuously, at a slow speed (10 m/min), for 1 h/day on a level treadmill, beginning 3 days after unilateral transection and surgical repair of the sciatic nerve, and conducted 5 days/wk for 2 wk. In Trained rats, both direct muscle responses to tibial nerve stimulation and H reflexes in soleus reappeared earlier and increased in amplitude more rapidly over time than in Untrained rats. The efficacy of the restored H reflex was greater in Trained rats than in Untrained controls. The reinnervated tibialis anterior and soleus were coactivated during treadmill locomotion in Untrained rats. In Trained animals, the pattern of activation of soleus, but not tibialis anterior, was not significantly different from that found in Intact rats. The overall length of the hindlimb during level and up- and downslope locomotion was conserved after nerve injury in both groups. This conservation was achieved by changes in limb orientation. Limb length was conserved effectively in all rats during downslope walking but only in Trained rats during level and upslope walking. Moderate daily exercise applied immediately after sciatic nerve transection is sufficient to promote axon regeneration, to restore muscle reflexes, and to improve the ability of rats to cope with different biomechanical demands of slope walking.

Kinematics of treadmill locomotion in mice raised in hypergravity

The study compared the motor performance of adult C57Bl/6J mice previously exposed to a 2G gravity environment during different periods of their development. 12 mice were housed in a large diameter centrifuge from the conception to Postnatal day 10 (P10). Another group of 10 mice was centrifuged form P10 to P30, and a third group of 9 mice was centrifuged from conception to P30. Their gait parameters, and kinematics of joint excursions were compared with 11 control mice, at the age of 2 months using a video-radiographic apparatus connected to a motorized treadmill. The mice that returned to Earth gravity level at the age of P10 showed a motor pattern similar to control mice. At variance the two groups that were centrifuged from P10 to P30 showed a different motor pattern with smaller and faster strides to walk at the same velocity as controls. On the other hand all the centrifuged mice showed significant postural changes, particularly with a more extended ankle joint, but the mice centrifuged during the whole experimental period differed even more. Our results showed that the exposure to hypergravity before P10 sufficed to modify the posture, suggesting that postural control starts before the onset of locomotion, whereas the gravity constraint perceived between P10 and P30 conditioned the tuning of quadruped locomotion with long term consequences. These results support the existence of a critical period in the acquisition of locomotion in mice.

A robotic platform to assess, guide and perturb rat forelimb movements

Animal models are widely used to explore the mechanisms underlying sensorimotor control and learning. However, current experimental paradigms allow only limited control over task difficulty and cannot provide detailed information on forelimb kinematics and dynamics. Here we propose a novel robotic device for use in motor learning investigations with rats. The compact, highly transparent, three degree-of-freedom manipulandum is capable of rendering nominal forces of 2 N to guide or perturb rat forelimb movements, while providing objective and quantitative assessments of endpoint motor performance in a 50×30 mm(2) planar workspace. Preliminary experiments with six healthy rats show that the animals can be familiarized with the experimental setup and are able to grasp and manipulate the end-effector of the robot. Further, dynamic perturbations and guiding force fields (i.e., haptic tunnels) rendered by the device had significant influence on rat motor behavior (ANOVA, ). This approach opens up new research avenues for future characterizations of motor learning stages, both in healthy and in stroke models.

Characterization of recovered walking patterns and motor control after contusive spinal cord injury in rats

Currently, complete recovery is unattainable for most individuals with spinal cord injury (SCI). Instead, recovery is typically accompanied by persistent sensory and motor deficits. Restoration of preinjury function will likely depend on improving plasticity and integration of these impaired systems. Eccentric muscle actions require precise integration of sensorimotor signals and are predominant during the yield (E2) phase of locomotion. Motor neuron activation and control during eccentric contractions is impaired across a number of central nervous system (CNS) disorders, but remains unexamined after SCI. Therefore, we characterized locomotor recovery after contusive SCI using hindlimb (HL) kinematics and electromyographic (EMG) recordings with specific consideration of eccentric phases of treadmill (TM) walking. Deficits in E2 and a caudal shift of locomotor subphases persisted throughout the 3-week recovery period. EMG records showed notable deficits in the semitendinosus (ST) during yield. Unlike other HL muscles, recruitment of ST changed with recovery. At 7 days, the typical dual-burst pattern of ST was lost and the second burst (ST2) was indistinct. By 21 days, the dual-burst pattern returned, but latencies remained impaired. We show that ST2 burst duration is highly predictive of open field Basso, Beattie, Bresnahan (BBB) scores. Moreover, we found that simple changes in locomotor specificity which enhance eccentric actions result in new motor patterns after SCI. Our findings identify a caudal shift in stepping kinematics, irregularities in E2, and aberrant ST2 bursting as markers of incomplete recovery. These residual impairments may provide opportunities for targeted rehabilitation.

A small-scale robotic manipulandum for motor training in stroke rats

The investigation and characterization of sensori-motor learning and execution represents a key objective for the design of optimal rehabilitation therapies following stroke. By supplying new tools to investigate sensorimotor learning and objectively assess recovery, robot assisted techniques have opened new lines of research in neurorehabilitation aiming to complement current clinical strategies. Human studies, however, are limited by the complex logistics, heterogeneous patient populations and large dropout rates. Rat models may provide a substitute to explore the mechanisms underlying these processes in humans with larger and more homogeneous populations. This paper describes the development and evaluation of a three-degrees-of-freedom robotic manipulandum to train and assess precision forelimb movement in rats before and after stroke. The mechanical design is presented based on the requirements of interaction with rat kinematics and kinetics. The characterization of the robot exhibits a compact, low friction device, with a sufficient bandwidth suitable for motor training studies with rodents. The manipulandum was integrated with an existing training environment for rodent experiments and a first study is currently underway.

Enabling techniques for in vitro studies on mammalian spinal locomotor mechanisms

The neonatal rodent spinal cord maintained in vitro is a powerful model system to understand the central properties of spinal circuits generating mammalian locomotion. We describe three enabling approaches that incorporate afferent input and attached hindlimbs. (i) Sacral dorsal column stimulation recruits and strengthens ongoing locomotor-like activity, and implementation of a closed positive-feedback paradigm is shown to support its stimulation as an untapped therapeutic site for locomotor modulation. (ii) The spinal cord hindlimbs-restrained preparation allows suction electrode electromyographic recordings from many muscles. Inducible complex motor patterns resemble natural locomotion, and insights into circuit organization are demonstrated during spontaneous motor burst 'deletions', or following sensory stimuli such as tail and paw pinch. (iii) The spinal cord hindlimbs-pendant preparation produces unrestrained hindlimb stepping. It incorporates mechanical limb perturbations, kinematic analyses, ground reaction force monitoring, and the use of treadmills to study spinal circuit operation with movement-related patterns of sensory feedback while providing for stable whole-cell recordings from spinal neurons. Such techniques promise to provide important additional insights into locomotor circuit organization.

Development of less invasive neuromuscular electrical stimulation model for motor therapy in rodents

BACKGROUND

Combination therapy is essential for functional repairs of the spinal cord. Rehabilitative therapy can be considered as the key for reorganizing the nervous system after spinal cord regeneration therapy. Functional electrical stimulation has been used as a neuroprosthesis in quadriplegia and can be used for providing rehabilitative therapy to tap the capability for central nervous system reorganization after spinal cord regeneration therapy.

OBJECTIVE

To develop a less invasive muscular electrical stimulation model capable of being combined with spinal cord regeneration therapy especially for motor therapy in the acute stage after spinal cord injury.

METHODS

The tibialis anterior and gastrocnemius motor points were identified in intact anesthetized adult female Fischer rats, and stimulation needle electrodes were percutaneously inserted into these points. Threshold currents for visual twitches were obtained upon stimulation using pulses of 75 or 8 kHz for 200 ms. Biphasic pulse widths of 20, 40, 80, 100, 300, and 500 µs per phase were used to determine strength-duration curves. Using these parameters and previously obtained locomotor electromyogram data, stimulations were performed on bilateral joint muscle pairs to produce reciprocal flexion/extension movements of the ankle for 15 minutes while three-dimensional joint kinematics were assessed.

RESULTS

Rhythmic muscular electrical stimulation with needle electrodes was successfully done, but decreased range of motion (ROM) over time. High-frequency and high-amplitude stimulation was also shown to be effective in alleviating decreases in ROM due to muscle fatigue.

CONCLUSIONS

This model will be useful for investigating the ability of rhythmic muscular electrical stimulation therapy to promote motor recovery, in addition to the efficacy of combining treatments with spinal cord regeneration therapy after spinal cord injuries.

Passive elastic properties of the rat ankle

Passive properties of muscles and tendons, including their elasticity, have been suggested to influence motor control. We examine here the potential role of passive elastic muscle properties at the rat ankle joint, focusing on their potential to specify an equilibrium position of the ankle. We measured the position-dependent passive torques at the rat ankle before and after sequential cuts of flexor (a.k.a. dorsiflexor) and extensor (a.k.a. plantarflexor) ankle muscles. We found that there was a passive equilibrium position of the ankle that shifted systematically with the cuts, demonstrating that the passive torques produced by ankle flexor and extensor muscles work in opposition in order to maintain a stable equilibrium. The mean equilibrium position of the intact rat ankle ranged from 9.3° to 15.7° in extension relative to the orthogonal position, depending on the torque metric. The mean shift in equilibrium position due to severing extensors ranged from 4.4° to 7.7°, and the mean shift due to severing flexors was smaller, ranging from 0.9° to 2.5°. The restoring torques generated by passive elasticity are large enough (approximately 1.5-5 mNm for displacements of 18° from equilibrium) to affect ankle movement during the swing phase of locomotion, and the asymmetry of larger extension vs. flexion torques is consistent with weight support, demonstrating the importance of accounting for passive muscle properties when considering the neural control of movement.

Stance and swing phase detection during level and slope walking in the cat: effects of slope, injury, subject and kinematic detection method

In quadrupeds, there have been limited comparisons of gait timing events detection (e.g., paw contact, PC and paw-off, PO) determined from kinematics and forceplates. The goal of this study was to investigate the effect of different slopes (0, -27, +27°), recovery times after ankle extensor nerve injury and repair (2, 6, 12 weeks), subjects and detection methods on accuracy of kinematically derived PC and PO timings during feline walking. Right hindlimb kinematics and ground reaction forces (GRF) of 4 cats walking along a sloped walkway with embedded forceplates were recorded. A total of 963 walking cycles were analyzed. Gait timings were determined from five kinematic methods based on displacements, velocities or accelerations of hindlimb markers. GRF based 'gold standard' timings for PC and PO were used to determine the systematic and random error of kinematic timing. Systematic errors between the kinematic methods differed significantly (p<0.05). Methods based on vertical paw peak acceleration and velocity gave the smallest systematic errors for PC and PO, respectively. The smallest random errors (standard deviations) for PC and PO were demonstrated by method based on paw horizontal displacement relative to greater trochanter: 13.4ms and 6.6ms, respectively. Effects of slope and subject on systematic errors of kinematic methods were significant, whereas effects of recovery time after nerve injury were not. It was concluded that timing of gait events can be determined consistently using kinematics, although adjustments must be made to account for the systematic error which varies according to subject and slope condition.

The biomechanics of locomotor compensation after peripheral nerve lesion in the rat

Functional recovery in animal models of nervous system disorders commonly involves behavioural compensation, in which animals alter the use of their limbs after injury, making it difficult to distinguish 'true' recovery from substitution of novel movements. The purpose of this study is to investigate how abnormal movements are produced by using biomechanical assessment of limb joint motion, an approach commonly used to diagnose human pathological gait. Rats were trained to cross a runway whilst kinetic (ground reaction forces) and kinematic (limb segment positions) data were synchronously recorded. Inverse dynamic analysis was used to calculate limb joint moments, or torques, and joint mechanical power throughout the stride for major joints of the forelimbs and hindlimbs, both before and after denervation of a major ankle extensor muscle. Before surgery, rats moved with joint moment and power profiles comparable to other quadrupeds, with differences attributable to species variation in limb posture. After surgery, rats trotted asymmetrically, with a near plantigrade stance of the left hindlimb. Surprisingly, ankle joint moments and power were largely preserved, with dramatic reductions in range of motion and joint moments at the proximal joints of the affected limb. Stiffening of the proximal limb compensated for increased compliance at the ankle but decreased the total mechanical work done by the injured limb. In turn, more work was done by the opposite, i.e. uninjured, hindlimb. This is the first study to quantify the biomechanical adjustments made within and between limbs in laboratory rodents after nervous system injury.

Gait modification during approach phase when stepping over an obstacle in rats

Stepping over obstacles to avoid tripping is an essential component in safe and smooth locomotion. Obstacle avoidance during locomotion is completed in several steps during the approach phase toward the obstacle and stepping over the obstacle. The purpose of this study was to investigate gait modification during the approach phase when stepping over obstacles of different heights in rats. In all four limbs, the toe height when the toe was just above the obstacle increased depending on the obstacle height, leaving a safe margin. However, the horizontal distance between toe and obstacle just prior to stepping over was not influenced by obstacle height. In the fore- and hindlimbs that served as trailing limbs, it was found that the stride length and its related swing phase duration in the final step were significantly shorter than those in both the penultimate step and overground locomotion. These results suggest that adjustment of trailing limb in the final step during the approach phase is important in preparation for the stepping movement over an obstacle.

Chondroitinase ABC reduces time to muscle reinnervation and improves functional recovery after sciatic nerve transection in rats

Application of chondroitinase ABC (ChABC) to injured peripheral nerves improves axon regeneration, but it is not known whether functional recovery is also improved. Recordings of EMG activity [soleus (Sol) M response and H reflexes] evoked by nerve stimulation and of Sol and tibialis anterior (TA) EMG activity and hindlimb and foot kinematics during slope walking were made to determine whether ChABC treatment of the sciatic nerve at the time of transection improves functional recovery. Recovery of evoked EMG responses began as multiple small responses with a wide range of latencies that eventually coalesced into one or two more distinctive and consistent responses (the putative M response and the putative H reflex) in both groups. Both the initial evoked responses and the time course of their maturation returned sooner in the ChABC group than in the untreated (UT) group. The reinnervated Sol and TA were coactivated during treadmill locomotion during downslope, level, and upslope walking throughout the study period in both UT and ChABC-treated rats. By 10 wk after nerve transection and repair, locomotor activity in Sol, but not TA, had returned to its pretransection pattern. There was an increased reliance on central control of Sol activation across slopes for both groups as interpreted from elevated prestance Sol EMG activity that was no longer modulated with slope. Limb length and orientation during locomotion were similar to those observed prior to nerve injury during upslope walking only in the ChABC-treated rats. Thus treatment of cut nerves with ChABC leads to improvements in functional recovery.

Femoral loading mechanics in the Virginia opossum, Didelphis virginiana: torsion and mediolateral bending in mammalian locomotion

Studies of limb bone loading in terrestrial mammals have typically found anteroposterior bending to be the primary loading regime, with torsion contributing minimally. However, previous studies have focused on large, cursorial eutherian species in which the limbs are held essentially upright. Recent in vivo strain data from the Virginia opossum (Didelphis virginiana), a marsupial that uses a crouched rather than an upright limb posture, have indicated that its femur experiences appreciable torsion during locomotion as well as strong mediolateral bending. The elevated femoral torsion and strong mediolateral bending observed in D. virginiana might result from external forces such as a medial inclination of the ground reaction force (GRF), internal forces deriving from a crouched limb posture, or a combination of these factors. To evaluate the mechanism underlying the loading regime of opossum femora, we filmed D. virginiana running over a force platform, allowing us to measure the magnitude of the GRF and its three-dimensional orientation relative to the limb, facilitating estimates of limb bone stresses. This three-dimensional analysis also allows evaluations of muscular forces, particularly those of hip adductor muscles, in the appropriate anatomical plane to a greater degree than previous two-dimensional analyses. At peak GRF and stress magnitudes, the GRF is oriented nearly vertically, inducing a strong abductor moment at the hip that is countered by adductor muscles on the medial aspect of the femur that place this surface in compression and induce mediolateral bending, corroborating and explaining loading patterns that were identified in strain analyses. The crouched orientation of the femur during stance in opossums also contributes to levels of femoral torsion as high as those seen in many reptilian taxa. Femoral safety factors were as high as those of non-avian reptiles and greater than those of upright, cursorial mammals, primarily because the load magnitudes experienced by opossums are lower than those of most mammals. Thus, the evolutionary transition from crouched to upright posture in mammalian ancestors may have been accompanied by an increase in limb bone load magnitudes.

Effect of slope and sciatic nerve injury on ankle muscle recruitment and hindlimb kinematics during walking in the rat

Slope-related differences in hindlimb movements and activation of the soleus and tibialis anterior muscles were studied during treadmill locomotion in intact rats and in rats 4 and 10 weeks following transection and surgical repair of the sciatic nerve. In intact rats, the tibialis anterior and soleus muscles were activated reciprocally at all slopes, and the overall intensity of activity in tibialis anterior and the mid-step activity in soleus increased with increasing slope. Based on the results of principal components analysis, the pattern of activation of soleus, but not of tibialis anterior, changed significantly with slope. Slope-related differences in hindlimb kinematics were found in intact rats, and these correlated well with the demands of walking up or down slopes. Following recovery from sciatic nerve injury, the soleus and tibialis anterior were co-activated throughout much of the step cycle and there was no difference in intensity or pattern of activation with slope for either muscle. Unlike intact rats, these animals walked with their feet flat on the treadmill belt through most of the stance phase. Even so, during downslope walking limb length and limb orientation throughout the step cycle were not significantly changed from values found in intact rats. This conservation of hindlimb kinematics was not observed during level or upslope walking. These findings are interpreted as evidence that the recovering animals adopt a novel locomotor strategy that involves stiffening of the ankle joint by antagonist co-activation and compensation at more proximal joints. Their movements are most suitable to the requirements of downslope walking but the recovering rats lack the ability to adapt to the demands of level or upslope walking.

TAT-μUtrophin mitigates the pathophysiology of dystrophin and utrophin double-knockout mice

Previously, we demonstrated functional substitution of dystrophin by TAT-μUtrophin (TAT-μUtr) in dystrophin-deficient mdx mice. Herein, we addressed whether TAT-μUtr could improve the phenotype of dystrophin and utrophin double-knockout (mdx:utr(-/-)) mice. Specifically, we quantitatively compared survival and quality of life assessments in mdx:utr(-/-) mice receiving TAT-μUtr protein administration against placebo-treated mdx:utr(-/-) mice (PBS). Additionally, skeletal muscles from TAT-μUtr and PBS mice were tested in vivo and ex vivo for strength and susceptibility to eccentric contraction-induced injury. We found the TAT-μUtr treatment extended life span 45% compared with mice administered PBS. This was attributed to significantly increased food consumption (3.1 vs. 1.8 g/24 h) due to improved ability to search for food as daily cage activities were greater in TAT-μUtr mice (e.g., 364 vs. 201 m ambulation/24 h). The extensor digitorum longus muscles of TAT-μUtr-treated double-knockout mice also displayed increased force-generating capacity ex vivo (8.3 vs. 6.4 N/cm(2)) and decreased susceptibility to injury ex vivo and in vivo. These data indicate that the functional benefits of TAT-μUtr replacement treatment extend to the mdx:utr(-/-) double-knockout mouse and support its development as a therapy to mitigate muscle weakness in patients with Duchenne muscular dystrophy.