Strategies that simplify the control of quadrupedal stance. I. Forces at the ground

Role of forelimb morphology in muscle sensorimotor functions during locomotion in the cat

Previous studies established strong links between morphological characteristics of mammalian hindlimb muscles and their sensorimotor functions during locomotion. Less is known about the role of forelimb morphology in motor outputs and generation of sensory signals. Here, we measured morphological characteristics of 46 forelimb muscles from six cats. These characteristics included muscle attachments, physiological cross-sectional area (PCSA) and fascicle length. We also recorded full-body mechanics and EMG activity of forelimb muscles during level overground and treadmill locomotion in seven and 16 adult cats of either sex, respectively. We computed forelimb muscle forces along with force- and length-dependent sensory signals mapped onto corresponding cervical spinal segments. We found that patterns of computed muscle forces and afferent activities were strongly affected by the muscle's moment arm, PCSA and fascicle length. Morphology of the shoulder muscles suggests distinct roles of the forelimbs in lateral force production and movements. Patterns of length-dependent sensory activity of muscles with long fibres (brachioradialis, extensor carpi radialis) closely matched patterns of overall forelimb length, whereas the activity pattern of biceps brachii length afferents matched forelimb orientation. We conclude that cat forelimb muscle morphology contributes substantially to locomotor function, particularly to control lateral stability and turning, rather than propulsion. KEY POINTS: Little is known about the role of forelimb muscle morphology in producing motor outputs and generating somatosensory signals. This information is needed to understand the contributions of forelimbs in locomotor control. We measured morphological characteristics of 46 muscles from cat forelimbs, recorded cat walking mechanics and electromyographic activity, and computed patterns of moment arms, length, velocity, activation, and force of forelimb muscles, as well as length- and force-dependent afferent activity during walking. We demonstrated that moment arms, physiological cross-sectional area and fascicle length of forelimb muscles contribute substantially to muscle force production and proprioceptive activity, to the regulation of locomotor cycle phase transitions and to control of lateral stability. The obtained information can guide the development of biologically accurate neuromechanical models of quadrupedal locomotion for exploring and testing novel methods of treatments of central nervous system pathologies by modulating activities in neural pathways controlling forelimbs/arms.

Neuromechanical Circuits of the Spinal Motor Apparatus

The evolution of mechanisms for terrestrial locomotion has resulted in multi-segmented limbs that allow navigation on irregular terrains, changing of direction, manipulation of external objects, and control over the mechanical properties of limbs important for interaction with the environment, with corresponding changes in neural pathways in the spinal cord. This article is focused on the organization of these pathways, their interactions with the musculoskeletal system, and the integration of these neuromechanical circuits with supraspinal mechanisms to control limb impedance. It is argued that neural pathways from muscle spindles and Golgi tendon organs form a distributive impedance controller in the spinal cord that controls limb impedance and coordination during responses to external disturbances. These pathways include both monosynaptic and polysynaptic components. Autogenic, monosynaptic pathways serve to control the spring-like properties of muscles preserving the nonlinear relationship between stiffness and force. Intermuscular monosynaptic pathways compensate for inertial disparities between the inertial properties of limb segments and help to control inertial coupling between joints and axes of rotation. Reciprocal inhibition controls joint stiffness in conjunction with feedforward cocontraction commands. Excitatory force feedback becomes operational during locomotion and increases muscular stiffness to accommodate the higher inertial loads. Inhibitory force feedback is widely distributed among muscles. It is integrated with excitatory pathways from muscle spindles and Golgi tendon organs to determine limb stiffness and interjoint coordination during interactions with the environment. The intermuscular distribution of force feedback is variable and serves to modulate limb stiffness to meet the physical demands of different motor tasks. © 2024 American Physiological Society. Compr Physiol 14:5789-5838, 2024.

Simultaneous control of forward and backward locomotion by spinal sensorimotor circuits

Mammals walk in different directions, such as forward and backward. In human infants/adults and decerebrate cats, one leg can walk forward and the other backward simultaneously on a split-belt treadmill, termed hybrid or bidirectional locomotion. The purpose of the present study was to determine if spinal sensorimotor circuits generate hybrid locomotion and if so, how the limbs remain coordinated. We tested hybrid locomotion in 11 intact cats and in five following complete spinal thoracic transection (spinal cats) at three treadmill speeds with the hindlimbs moving forward, backward or bidirectionally. All intact cats generated hybrid locomotion with the forelimbs on a stationary platform. Four of five spinal cats generated hybrid locomotion, also with the forelimbs on a stationary platform, but required perineal stimulation. During hybrid locomotion, intact and spinal cats positioned their forward and backward moving hindlimbs caudal and rostral to the hip, respectively. The hindlimbs maintained consistent left-right out-of-phase alternation in the different stepping directions. Our results suggest that spinal locomotor networks generate hybrid locomotion by following certain rules at phase transitions. We also found that stance duration determined cycle duration in the different locomotor directions/conditions, consistent with a common rhythm-generating mechanism for different locomotor directions. Our findings provide additional insight on how left-right spinal networks and sensory feedback from the limbs interact to coordinate the hindlimbs and provide stability during locomotion in different directions. KEY POINTS: Terrestrial mammals can walk forward and backward, which is controlled in part by spinal sensorimotor circuits. Humans and cats also perform bidirectional or hybrid locomotion on a split-belt treadmill with one leg going forward and the other going backward. We show that cats with a spinal transection can perform hybrid locomotion and maintain left-right out-of-phase coordination, indicating that spinal sensorimotor circuits can perform simultaneous forward and backward locomotion. We also show that the regulation of cycle duration and phase duration is conserved across stepping direction, consistent with a common rhythm-generating mechanism for different stepping directions. The results help us better understand how spinal networks controlling the left and right legs enable locomotion in different directions.

Alteration of Postural Reactions in Rats with Different Levels of Dopamine Depletion

Dopamine (DA) is the critical neurotransmitter involved in the unconscious control of muscle tone and body posture. We evaluated the general motor capacities and muscle responses to postural disturbance in three conditions: normal DA level (wild-type rats, WT), mild DA deficiency (WT after administration of α-methyl-p-tyrosine-AMPT, that blocks DA synthesis), and severe DA depletion (DAT-KO rats after AMPT). The horizontal displacements in WT rats elicited a multi-component EMG corrective response in the flexor and extensor muscles. Similar to the gradual progression of DA-related diseases, we observed different degrees of bradykinesia, rigidity, and postural instability after AMPT. The mild DA deficiency impaired the initiation pattern of corrective responses, specifically delaying the extensor muscles' activity ipsilaterally to displacement direction and earlier extensor activity from the opposite side. DA depletion in DAT-KO rats after AMPT elicited tremors, general stiffness, and akinesia, and caused earlier response to horizontal displacements in the coactivated flexor and extensor muscles bilaterally. The data obtained show the specific role of DA in postural reactions and suggest that this experimental approach can be used to investigate sensorimotor control in different dopamine-deficient states and to model DA-related diseases.

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Effect of hindlimb unloading on recruitment of gastrocnemius medialis muscle during treadmill locomotion in rats

After hindlimb unloading (HU), the adaptive changing of the rat step cycle duration, kinematics of the ankle and knee joints, and duration of one-joint ankle extensor m. soleus (SOL) activity are detected. However, how the activity of their synergist gastrocnemius medialis muscle (GM) changes in locomotion after HU remains unknown. GM is a two-joint muscle that produces both extension and flexion torques at the ankle and knee, respectively, regardless of the step cycle phase. The aim of our study was to assess changes in the flexor and extensor activity of GM and their influence on hindlimb kinematics after HU. The hindlimb kinematics, activity of GM, and SOL were evaluated, and semitendinosus muscle (ST) activity was registered in six Wistar rats in treadmill locomotion before and after HU. The mean EMG of the GM activity, which was co-active with ST burst activity, significantly increased after HU. The mean EMG of the GM activity, which was co-active with SOL activity, was unchanged after HU, but both SOL and GM bursts had a tendency to increase in duration. Hyperextension of the knee joint and the tendency to overextension of the ankle joint in the late of the stance phase were revealed after HU. The results show that the absence of weight bearing leads to an increase only in the flexor activity of GM and does not affect the extensor GM activity. Possible mechanisms of changes in GM activity and joint kinematics after HU are discussed.

Sheep Quickstep while the Floor Rock and Rolls: Visuomotor Lateralization during Simulated Sea Travel

Unpredictable floor motions during transport disturbs animals' balance, requiring stepping to move the centre of gravity in the direction of body movement. When repeated regularly, this may be stressful, requiring involvement of the right brain hemisphere, hence we investigated the existence of behavioral laterality in sheep during prolonged floor motions. Six sheep were restrained in pairs on a programmable rocking platform, in which they were unable to turn around. They were exposed to three continuous rocking motion treatments (roll, pitch or both) in a regular or irregular pattern for 1 h periods in a changeover design. Right forelimb and left hindlimb diagonal stepping was more frequent in response to the motion treatment of irregular roll and pitch, which previous research has suggested to be the most stressful from heart rate measurements. An overall strategy to maintain balance appeared to be the use of the right hindlimb as a stabilizer, which was repositioned least often of all limbs until towards the end of the hour of experimental treatment. Of each tested pair, sheep restrained on the left side of the rocking floor stepped significantly often than its partner restrained on the right side, and we postulate the existence of visuomotor lateralization as left restrained sheep were unable to view their partner within the field of view of their left eye. We also investigated which side sheep lie down on, which if left lateralized could explain our observed bipedal diagonal control of sheep balance under stress. From the observation of 412 web-based images of sheep, there was an overall left-sided laterality to their lying, as has been observed in cattle. We conclude that stepping activity in sheep in response to a motion stressor is lateralized, providing evidence that floor motion experienced in transport may induce stress responses.

Coordination of lower extremity multi-joint control strategies during the golf swing

This study aimed to understand how players coordinate the multi-joint control strategies of the rear and target legs to satisfy the lower extremity and whole-body mechanical objectives during the golf swing when hitting shots with different clubs. Highly skilled golf players (n = 10) performed golf swings with a 6-iron and a driver. Joint kinetics were calculated using ground reaction forces and segment kinematics to determine net joint moments (NJMs) during the interval of interest within the downswing. Between club difference in NJMs and 3D support moments were compared across the group and within a player. Although player-specific multi-joint control strategies arose, players generally increased target leg ankle, knee, and hip NJMs when hitting with the driver while maintaining the relative contribution to the 3D support moment. Multi-joint control strategies used to control the target and rear legs were found to be different, yet the majority of the 3D support moment was produced by NJMs about an axis perpendicular to the leg planes. These results emphasize the importance of recognizing how an individual player coordinates multi-joint control from each leg, and highlights the need to design interventions that are player and leg specific to aid in improving player performance.

Transplants of Neurotrophin-Producing Autologous Fibroblasts Promote Recovery of Treadmill Stepping in the Acute, Sub-Chronic, and Chronic Spinal Cat

Adult cats show limited spontaneous locomotor capabilities following spinal transection, but recover treadmill stepping with body-weight-supported training. Delivery of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) and neurotrophic factor 3 (NT-3) can substitute for body-weight-supported training, and promotes a similar recovery in a shorter period of time. Autologous cell grafts would negate the need for the immunosuppressive agents currently used with most grafts, but have not shown functional benefits in incomplete spinal cord injury models and have never been tested in complete transection or chronic injury models. In this study, we explored the effects of autologous fibroblasts, prepared from the individual cats and modified to produce BDNF and NT-3, on the recovery of locomotion in acute, sub-chronic and chronic full-transection models of spinal injury. Fourteen female cats underwent complete spinal transection at T11/T12. Cats were separated into four groups: sham graft at the time of injury, and BDNF and NT-3 producing autologous fibroblasts grafted at the time of injury, 2 weeks after injury, or 6 weeks after injury. Kinematics were recorded 3 and 5 weeks after cell graft. Additional kinematic recordings were taken for some cats until 12 weeks post-graft. Eleven of 12 cats with neurotrophin-producing grafts recovered plantar weight-bearing stepping at treadmill speeds from 0.3 to 0.8 m/sec within 5 weeks of grafting, whereas control cats recovered poor quality stepping at low speeds only (≤ 0.4 m/sec). Further, kinematic measures in cats with grafts were closer to pre-transection values than those for controls, and recovery was maintained up to 12 weeks post-grafting. Our results show that not only are autologous neurotrophin-producing grafts effective at promoting recovery of locomotion, but that delayed delivery of neurotrophins does not diminish the therapeutic effect, and may improve outcome.

Characterization and validation of a split belt treadmill for measuring hindlimb ground-reaction forces in able-bodied and spinalized felines

BACKGROUND

The measurement of ground reaction forces (GRFs) in animals trained to locomote on a treadmill after spinal cord injury (SCI) could prove valuable for evaluating training outcomes; however, quantitative measures of the GRFs in spinal felines are limited.

NEW METHOD

A split belt treadmill was designed and constructed to measure the GRFs of feline hindlimbs during stepping. The treadmill consists of two independent treadmill assemblies, each mounted on a force plate. The design allows measurements of the vertical (Fz), fore-aft (Fy) and mediolateral (Fx) ground-reaction forces for both hindlimbs while the forelimbs are resting on a platform.

RESULTS

Static and dynamic noise tests revealed little to no noise at frequencies below 6Hz. Validation of the force plate measurements with a hand-held force sensor force showed good agreement between the two force readings. Peak normalized (to body mass) vertical GRFs for intact cats were 4.89±0.85N/kg for the left hindlimb and 4.79±0.97N/kg for the right. In comparison, trained spinalized cats peak normalized vertical GRFs were 2.20±0.94N/kg for the left hindlimb and 2.85±0.99N/kg for the right.

COMPARISON WITH OTHER EXISTING METHODS

Previous methods of measuring GRFs used stationary single force plates or treadmill mounted to single force plate. Using independent treadmills for each hindlimb allows measurement of the individual hindlimb's GRFs in spinalized cats following body-weight supported treadmill training.

CONCLUSIONS

The split belt force treadmill enables the simultaneous recording of ground-reaction forces for both hindlimbs in cats prior to spinalization, and following spinalization and body-weight-supported treadmill training (BWST).

Lower extremity control during turns initiated with and without hip external rotation

The pirouette turn is often initiated in neutral and externally rotated hip positions by dancers. This provides an opportunity to investigate how dancers satisfy the same mechanical objectives at the whole-body level when using different leg kinematics. The purpose of this study was to compare lower extremity control strategies during the turn initiation phase of pirouettes performed with and without hip external rotation. Skilled dancers (n=5) performed pirouette turns with and without hip external rotation. Joint kinetics during turn initiation were determined for both legs using ground reaction forces (GRFs) and segment kinematics. Hip muscle activations were monitored using electromyography. Using probability-based statistical methods, variables were compared across turn conditions as a group and within-dancer. Despite differences in GRFs and impulse generation between turn conditions, at least 90% of each GRF was aligned with the respective leg plane. A majority of the net joint moments at the ankle, knee, and hip acted about an axis perpendicular to the leg plane. However, differences in shank alignment relative to the leg plane affected the distribution of the knee net joint moment when represented with respect to the shank versus the thigh. During the initiation of both turns, most participants used ankle plantar flexor moments, knee extensor moments, flexor and abductor moments at the push leg׳s hip, and extensor and abductor moments at the turn leg׳s hip. Representation of joint kinetics using multiple reference systems assisted in understanding control priorities.

Intraspinal microstimulation produces over-ground walking in anesthetized cats

OBJECTIVE

Spinal cord injury causes a drastic loss of motor, sensory and autonomic function. The goal of this project was to investigate the use of intraspinal microstimulation (ISMS) for producing long distances of walking over ground. ISMS is an electrical stimulation method developed for restoring motor function by activating spinal networks below the level of an injury. It produces movements of the legs by stimulating the ventral horn of the lumbar enlargement using fine penetrating electrodes (≤50 μm diameter).

APPROACH

In each of five adult cats (4.2-5.5 kg), ISMS was applied through 16 electrodes implanted with tips targeting lamina IX in the ventral horn bilaterally. A desktop system implemented a physiologically-based control strategy that delivered different stimulation patterns through groups of electrodes to evoke walking movements with appropriate limb kinematics and forces corresponding to swing and stance. Each cat walked over an instrumented 2.9 m walkway and limb kinematics and forces were recorded.

MAIN RESULTS

Both propulsive and supportive forces were required for over-ground walking. Cumulative walking distances ranging from 609 to 835 m (longest tested) were achieved in three animals. In these three cats, the mean peak supportive force was 3.5 ± 0.6 N corresponding to full-weight-support of the hind legs, while the angular range of the hip, knee, and ankle joints were 23.1 ± 2.0°, 29.1 ± 0.2°, and 60.3 ± 5.2°, respectively. To further demonstrate the viability of ISMS for future clinical use, a prototype implantable module was successfully implemented in a subset of trials and produced comparable walking performance.

SIGNIFICANCE

By activating inherent locomotor networks within the lumbosacral spinal cord, ISMS was capable of producing bilaterally coordinated and functional over-ground walking with current amplitudes <100 μA. These exciting results suggest that ISMS may be an effective intervention for restoring functional walking after spinal cord injury.

Modification of impulse generation during piqué turns with increased rotational demands

During initiation of a piqué turn, a dancer generates impulse to achieve the desired lateral translation and whole-body rotation. The goal of this study was to determine how individuals regulate impulse generation when initiating piqué turns with increased rotational demands. Skilled dancers (n=10) performed single (∼360°) and double (∼720°) piqué turns from a stationary position. Linear and angular impulse generated by the push and turn legs were quantified using ground reaction forces and compared across turn conditions as a group and within a dancer using probability-based statistical methods. The results indicate that as the rotation demands of the piqué turn increased, the net angular impulse generated increased whereas net lateral impulse decreased. Early during turn initiation, the free moment contributed to angular impulse generation. Later during turn initiation, horizontal reaction forces were controlled to generate angular impulse. As rotational demands increased, the moment applied increased primarily from redirection of the horizontal reaction force (RFh) at the push leg and a combination of RFh magnitude and moment arm increases at the turn leg. RFh at each leg were coordinated to limit unwanted net linear impulse. Knowledge of observed subject-specific mechanisms is important to inform the design of turning performance training tools.

An Engineering Model of Human Balance Control-Part I: Biomechanical Model

We developed a balance measurement tool (the balanced reach test (BRT)) to assess standing balance while reaching and pointing to a target moving in three-dimensional space according to a sum-of-sines function. We also developed a three-dimensional, 13-segment biomechanical model to analyze performance in this task. Using kinematic and ground reaction force (GRF) data from the BRT, we performed an inverse dynamics analysis to compute the forces and torques applied at each of the joints during the course of a 90 s test. We also performed spectral analyses of each joint's force activations. We found that the joints act in a different but highly coordinated manner to accomplish the tracking task-with individual joints responding congruently to different portions of the target disk's frequency spectrum. The test and the model also identified clear differences between a young healthy subject (YHS), an older high fall risk (HFR) subject before participating in a balance training intervention; and in the older subject's performance after training (which improved to the point that his performance approached that of the young subject). This is the first phase of an effort to model the balance control system with sufficient physiological detail and complexity to accurately simulate the multisegmental control of balance during functional reach across the spectra of aging, medical, and neurological conditions that affect performance. Such a model would provide insight into the function and interaction of the biomechanical and neurophysiological elements making up this system; and system adaptations to changes in these elements' performance and capabilities.

A system for the determination of planar force vectors from spontaneously active chicken embryos

Generally, a combination of kinematic, electromyographic (EMG), and force measurements are used to understand how an organism generates and controls movement. The chicken embryo has been a very useful model system for understanding the early stages of embryonic motility in vertebrates. Unfortunately, the size and delicate nature of embryos makes studies of motility during embryogenesis very challenging. Both kinematic and EMG recordings have been achieved in embryonic chickens, but two-dimensional force vector recordings have not. Here, we describe a dual-axis system for measuring force generated by the leg of embryonic chickens. The system employs two strain gauges to measure planar forces oriented with the plane of motion of the leg. This system responds to forces according to the principles of Pythagorean geometry, which allows a simple computational program to determine the force vector (magnitude and direction) generated during spontaneous motor activity. The system is able to determine force vectors for forces >0.5 mN accurately and allows for simultaneous kinematic and EMG recordings. This sensitivity is sufficient for force vector measurements encompassing most embryonic leg movements in midstage chicken embryos allowing for a more complete understanding of embryonic motility. Variations on this system are discussed to enable nonideal or alternative sensor arrangements and to allow for translation of this approach to other delicate model systems.

Individuals with transtibial limb loss use interlimb force asymmetries to maintain multi-directional reactive balance control

BACKGROUND

Deficits in balance control are one of the most common and serious mobility challenges facing individuals with lower limb loss. Yet, dynamic postural balance control among individuals with lower limb loss remains poorly understood. Here we examined the kinematics and kinetics of dynamic balance in individuals with unilateral transtibial limb loss.

METHODS

Five individuals with unilateral transtibial limb loss, and five age- and gender-matched controls completed a series of randomly applied multi-directional support surface translations. Whole-body metrics, e.g. peak center-of-mass displacement and net center-of-pressure displacement were compared across cohorts. Stability margin was computed as the difference between peak center-of-pressure and center-of-mass displacement. Additionally, center-of-pressure and ground reaction force magnitude and direction were compared between the prosthetic, intact, and control legs.

FINDINGS

Peak center-of-mass displacement and stability margin did not differ between individuals with transtibial limb loss and controls for all perturbation directions except those loading only the prosthetic leg; in such cases the stability margin was actually larger than controls. Despite similar center-of-mass displacement, greater center-of-pressure displacement was observed in the intact leg during anterior-posterior perturbations, and under the prosthetic leg in medial-lateral perturbations. Further, in the prosthetic leg, ground reaction forces were smaller and spanned fewer directions.

INTERPRETATION

Deficits in balance control among individuals with transtibial limb loss may be due to their inability to use their prosthetic leg to generate forces that are equal in magnitude and direction to those of unimpaired adults. Targeting this force-generating deficit through technological or rehabilitation innovations may improve balance control.

Contribution of supraspinal systems to generation of automatic postural responses

Different species maintain a particular body orientation in space due to activity of the closed-loop postural control system. In this review we discuss the role of neurons of descending pathways in operation of this system as revealed in animal models of differing complexity: lower vertebrate (lamprey) and higher vertebrates (rabbit and cat). In the lamprey and quadruped mammals, the role of spinal and supraspinal mechanisms in the control of posture is different. In the lamprey, the system contains one closed-loop mechanism consisting of supraspino-spinal networks. Reticulospinal (RS) neurons play a key role in generation of postural corrections. Due to vestibular input, any deviation from the stabilized body orientation leads to activation of a specific population of RS neurons. Each of the neurons activates a specific motor synergy. Collectively, these neurons evoke the motor output necessary for the postural correction. In contrast to lampreys, postural corrections in quadrupeds are primarily based not on the vestibular input but on the somatosensory input from limb mechanoreceptors. The system contains two closed-loop mechanisms - spinal and spino-supraspinal networks, which supplement each other. Spinal networks receive somatosensory input from the limb signaling postural perturbations, and generate spinal postural limb reflexes. These reflexes are relatively weak, but in intact animals they are enhanced due to both tonic supraspinal drive and phasic supraspinal commands. Recent studies of these supraspinal influences are considered in this review. A hypothesis suggesting common principles of operation of the postural systems stabilizing body orientation in a particular plane in the lamprey and quadrupeds, that is interaction of antagonistic postural reflexes, is discussed.

The mechanical actions of muscles predict the direction of muscle activation during postural perturbations in the cat hindlimb

Humans and cats respond to balance challenges, delivered via horizontal support surface perturbations, with directionally selective muscle recruitment and constrained ground reaction forces. It has been suggested that this postural strategy arises from an interaction of limb biomechanics and proprioceptive networks in the spinal cord. A critical experimental validation of this hypothesis is to test the prediction that the principal directions of muscular activation oppose the directions responding muscles exert their forces on the environment. Therefore, our objective was to quantify the endpoint forces of a diverse set of cat hindlimb muscles and compare them with the directionally sensitive muscle activation patterns generated in the intact and decerebrate cat. We hypothesized that muscles are activated based on their mechanical advantage. Our primary expectation was that the principal direction of muscle activation during postural perturbations will be directed oppositely (180°) from the muscle endpoint ground reaction force. We found that muscle activation during postural perturbations was indeed directed oppositely to the endpoint reaction forces of that muscle. These observations indicate that muscle recruitment during balance challenges is driven, at least in part, by limb architecture. This suggests that sensory sources that provide feedback about the mechanical environment of the limb are likely important to appropriate and effective responses during balance challenges. Finally, we extended the analysis to three dimensions and different stance widths, laying the groundwork for a more comprehensive study of postural regulation than was possible with measurements confined to the horizontal plane and a single stance configuration.

Absence of postural muscle synergies for balance after spinal cord transection

Although cats that have been spinalized can also be trained to stand and step with full weight support, directionally appropriate long-latency responses to perturbations are impaired, suggesting that these behaviors are mediated by distinct neural mechanisms. However, it remains unclear whether these responses reflect an attenuated postural response using the appropriate muscular coordination patterns for balance or are due to fundamentally different neural mechanisms such as increased muscular cocontraction or short-latency stretch responses. Here we used muscle synergy analysis on previously collected data to identify whether there are changes in the spatial organization of muscle activity for balance within an animal after spinalization. We hypothesized that the modular organization of muscle activity for balance control is disrupted by spinal cord transection. In each of four animals, muscle synergies were extracted from postural muscle activity both before and after spinalization with nonnegative matrix factorization. Muscle synergy number was reduced after spinalization in three animals and increased in one animal. However, muscle synergy structure was greatly altered after spinalization with reduced direction tuning, suggesting little consistent organization of muscle activity. Furthermore, muscle synergy recruitment was correlated to subsequent force production in the intact but not spinalized condition. Our results demonstrate that the modular structure of sensorimotor feedback responses for balance control is severely disrupted after spinalization, suggesting that the muscle synergies for balance control are not accessible by spinal circuits alone. Moreover, we demonstrate that spinal mechanisms underlying weight support are distinct from brain stem mechanisms underlying directional balance control.

Optimization of muscle activity for task-level goals predicts complex changes in limb forces across biomechanical contexts

Optimality principles have been proposed as a general framework for understanding motor control in animals and humans largely based on their ability to predict general features movement in idealized motor tasks. However, generalizing these concepts past proof-of-principle to understand the neuromechanical transformation from task-level control to detailed execution-level muscle activity and forces during behaviorally-relevant motor tasks has proved difficult. In an unrestrained balance task in cats, we demonstrate that achieving task-level constraints center of mass forces and moments while minimizing control effort predicts detailed patterns of muscle activity and ground reaction forces in an anatomically-realistic musculoskeletal model. Whereas optimization is typically used to resolve redundancy at a single level of the motor hierarchy, we simultaneously resolved redundancy across both muscles and limbs and directly compared predictions to experimental measures across multiple perturbation directions that elicit different intra- and interlimb coordination patterns. Further, although some candidate task-level variables and cost functions generated indistinguishable predictions in a single biomechanical context, we identified a common optimization framework that could predict up to 48 experimental conditions per animal (n = 3) across both perturbation directions and different biomechanical contexts created by altering animals' postural configuration. Predictions were further improved by imposing experimentally-derived muscle synergy constraints, suggesting additional task variables or costs that may be relevant to the neural control of balance. These results suggested that reduced-dimension neural control mechanisms such as muscle synergies can achieve similar kinetics to the optimal solution, but with increased control effort (≈2×) compared to individual muscle control. Our results are consistent with the idea that hierarchical, task-level neural control mechanisms previously associated with voluntary tasks may also be used in automatic brainstem-mediated pathways for balance.

Somatosensory control of balance during locomotion in decerebrated cat

Postmammillary decerebrated cats can generate stepping on a moving treadmill belt when the brain stem or spinal cord is stimulated tonically and the hindquarters are supported both vertically and laterally. While adequate propulsion seems to be generated by the hindlimbs under these conditions, the ability to sustain equilibrium during locomotion has not been examined extensively. We found that tonic epidural spinal cord stimulation (5 Hz at L5) of decerebrated cats initiated and sustained unrestrained weight-bearing hindlimb stepping for extended periods. Detailed analyses of the relationships among hindlimb muscle EMG activity and trunk and limb kinematics and kinetics indicated that the motor circuitries in decerebrated cats actively maintain equilibrium during walking, similar to that observed in intact animals. Because of the suppression of vestibular, visual, and head-neck-trunk sensory input, balance-related adjustments relied entirely on the integration of somatosensory information arising from the moving hindquarters. In addition to dynamic balance control during unperturbed locomotion, sustained stepping could be reestablished rapidly after a collapse or stumble when the hindquarters switched from a restrained to an unrestrained condition. Deflecting the body by pulling the tail laterally induced adaptive modulations in the EMG activity, step cycle features, and left-right ground reaction forces that were sufficient to maintain lateral stability. Thus the brain stem-spinal cord circuitry of decerebrated cats in response to tonic spinal cord stimulation can control dynamic balance during locomotion using only somatosensory input.

Physiological and circuit mechanisms of postural control

The postural system maintains a specific body orientation and equilibrium during standing and during locomotion in the presence of many destabilizing factors (external and internal). Numerous studies in humans have revealed essential features of the functional organization of this system. Recent studies on different animal models have significantly supplemented human studies. They have greatly expanded our knowledge of how the control system operates, how the postural functions are distributed within different parts of CNS, and how these parts interact with each other to produce postural corrections and adjustments. This review outlines recent advances in the studies of postural control in quadrupeds, with special attention given the neuronal postural mechanisms.

Early stages in a sensorimotor transformation

We present a model for several early stages of the sensorimotor transformations involved in targeted arm movement. In psychophysical experiments, human subjects pointed to the remembered locations of randomly placed targets in three-dimensional space. They made consistent errors in distance, and from these errors stages in the sensorimotor transformation were deduced. When subjects attempted to move the right index finger to a virtual target they consistently undershot the distance of the more distal targets. Other experiments indicated that the error was in the sensorimotor transformation rather than in the perception of distance. The error was most consistent when evaluated using a spherical coordinate system based at the right shoulder, indicating that the neural representation of target parameters is transformed from a retinocentric representation to a shoulder-centered representation. According to the model, the error in distance results from the neural implementation of a linear approximation in the algorithm to transform shoulder-centered target parameters into a set of arm orientations appropriate for placing the finger on the target. The transformation to final arm orientations places visually derived information into a frame of reference where it can readily be combined with kinesthetically derived information about initial arm orientations. The combination of these representations of initial and final arm orientations could give rise to the representation of movement direction recorded in the motor cortex by Georgopoulos and his colleagues. Later stages, such as the transformation from kinematic (position) to dynamic (force) parameters, or to levels of muscle activation, are beyond the scope of the present model.