Locomotor Stability Research Articles

From Behavior to Bio-Inspiration: Aerial Reorientation and Multi-Plane Stability in Kangaroo Rats, Computational Models, and Robots.

Tails play essential roles in functions related to locomotor stability and maneuverability among terrestrial and arboreal animals. In kangaroo rats, bipedal hopping rodents, tails are used as effective inertial appendages for stability in hopping, but also facilitate stability and maneuverability during predator escape leaps. The complexity of tail functionality shows great potential for bio-inspiration and robotic device design, as maneuvering is accomplished by a long and light-weight inertial appendage. To (1) further understand the mechanics of how kangaroo rats use their tails during aerial maneuvers and (2) explore if we can achieve this behavior with a simplified tail-like appendage (i.e., template), we combined quantified animal observations, computational simulations, and experiments with a two degrees of freedom (2-DoF) tailed robot. We used video data from free-ranging kangaroo rats escaping from a simulated predator and analyzed body and tail motion for the airborne phase. To explain tail contributions to body orientation (i.e., spatial reorientation), we built a mid-air kangaroo rat computational model and demonstrated that the three-dimensional body orientation of the model can be controlled by a simplified 2-DoF tail with a nonlinear control strategy. Resulting simulated trajectories show movement patterns similar to those observed in kangaroo rats. Our robot experiments show that a lightweight tail can generate a large yaw displacement and stabilize pitch and roll angles to zero simultaneously. Our work contributes to better understanding of the form-function relationship of the kangaroo rat tail and lays out an important foundation for bio-inspiration in robotic devices that have lightweight tail-like appendages for mid-air maneuvering.

Evaluating Motor Dysfunction and Oxidative Stress Induced by Trichloroethylene in Wistar Rats.

Trichloroethylene, a chlorinated solvent widely used as a degreasing agent, is a common environmental contaminant. Emerging evidence suggests that chronic exposure to trichloroethylene (TCE) contributes to the development of Parkinson's disease (PD). TCE induced LRRK2 kinase activity in the rat brain and produced a significant dopaminergic lesion in the nigrostriatal tract with elevated oxidative stress. Here we have utilized TCE-induced PD model for the assessment of test drug. Oral gavage administration of TCE at a dose of 1000mg/kg/day for 6weeks was utilized to induced PD. Muscle grip strength was estimated by rotarod and grid performance test. Motor activity by actophotometer and locomotor stability were assessed by forelimb locomotor scale (FLS) and forelimb step alternation test (FSAT). However, the postural stability was assessed by postural stability test (PST). Biochemical estimation consists of determination of malondialdehyde (MDA), superoxide dismutase (SOD), catalase (CAT), GSH level (reduced glutathione), and nitrite concentration.

Age- and sex-dependent alterations in primary somatosensory cortex neuronal calcium network dynamics during locomotion.

Over the past 30 years, the calcium (Ca2+ ) hypothesis of brain aging has provided clear evidence that hippocampal neuronal Ca2+ dysregulation is a key biomarker of aging. Age-dependent Ca2+ -mediated changes in intrinsic excitability, synaptic plasticity, and activity have helped identify some of the mechanisms engaged in memory and cognitive decline based on work done mostly at the single-cell level and in the slice preparation. Recently, our lab identified age- and Ca2+ -related neuronal network dysregulation in the cortex of the anesthetized animal. Still, investigations in the awake animal are needed to test the generalizability of the Ca2+ hypothesis of brain aging. Here, we used in vigilo two-photon imaging in ambulating mice, to image GCaMP8f in the primary somatosensory cortex (S1), during ambulation and at rest. We investigated aging- and sex-related changes in neuronal networks in the C56BL/6J mouse. Following imaging, gait behavior was characterized to test for changes in locomotor stability. During ambulation, in both young adult and aged mice, an increase in network connectivity and synchronicity was noted. An age-dependent increase in synchronicity was seen in ambulating aged males only. Additionally, females displayed increases in the number of active neurons, Ca2+ transients, and neuronal activity compared to males, particularly during ambulation. These results suggest S1 Ca2+ dynamics and network synchronicity are likely contributors of locomotor stability. We believe this work raises awareness of age- and sex-dependent alterations in S1 neuronal networks, perhaps underlying the increase in falls with age.

Effects of simulated trunk flexion contracture on the margin of stability during obstacle crossing in elderly individuals

BackgroundTrunk flexion contracture is an abnormal posture in elderly individuals with lumbar kyphosis. It is unclear whether this posture affects locomotor stability (margin of stability [MoS]) during obstacle crossing, which is a common trigger for falls in elderly people. Research questionDoes trunk flexion contracture negatively affect MoS during obstacle crossing in elderly people? MethodsTen healthy elderly individuals performed five trials of obstacle crossing using a comfortable speed under two experimental conditions, namely, with (FLEX) or without (NORMAL) a hard lumbar brace to simulate trunk flexion contracture. The obstacle-crossing motion was captured using an optical motion analysis system in order to calculate the MoS in the anteroposterior direction. The MoS at initial contact (IC) and that when the swing foot was above the obstacle (Obs) was compared between FLEX and NORMAL. A greater MoS suggests greater risk of a forward fall. The trunk and lower limb joint angles were measured at Obs. ResultsFLEX significantly increased the MoS at IC, whereas the MoS at Obs did not differ between the two conditions. FLEX demonstrated a crouch posture characterized by an increased flexion angle of stance-side hip and knee joints at the Obs instant. SignificanceForward fall chance might be increased at IC in obstacle crossing with trunk flexion contracture. Meanwhile, the MoS at Obs might be controlled by increasing the crouch posture to offset a forward shift in the CoM position due to the trunk flexion. Because the risk of a stumble on an obstacle and of forward falls should be higher at Obs than at IC, the crouch posture seems to be an effective adaptation that enables elderly people with trunk flexion contracture to safely cross obstacles.

The role of sensory feedback from carpal sinus hairs in locomotor kinematics of rats (Rattus norvegicus, Rodentia) during walking on narrow substrates

Carpal sinus hairs on the forearms are assumed to have evolved within the stem lineage of Theria. The presence and similar position of these specialized tactile hairs in scansorial and terrestrial species as well as earlier studies on rats indicate a biological role in sensing substrate irregularities in high structured environments to ensure the dynamic stability of the body during locomotion. While these sensors were considered as one functional unit until so far, the present study deals with the biological role of the single tactile hairs of the trident, assuming a role in sensing substrate diameters and adapting limb coordination and body posture to different arboreal inclinations. To investigate the influence of each hair, we studied the locomotion of rats on poles of two different diameters whereby we selectively removed individual carpal sinus hairs. The rats walked at speeds ranging from 0.12 m/s to 0.58 m/s. Normal-light high-speed cameras and x-ray fluoroscopy visualized the hairs and body dynamics during locomotion. The time lag between first contact of the hairs to the branch until contact of the forepaw was 56–108 ms. Within this time window the pronation/supination of the paw and anterior body posture are adjusted to the substrate diameter. We presume that the most proximal sinus hair (located between the medial and lateral one) senses the paw-substrate distance through the increasing bend from its first branch-contact until the contact of the paw. The medial and the lateral hairs touch the pole sides and thereby, may collect information about the properties of the small-diameter substrate. The removal of single hairs from the group results in minor changes of kinematic parameters, but locomotor stability is seriously impaired when more than one hair is cut. The kinematic responses span from a more crouched body posture and higher forearm pronation to paw slipping, muscle tremor or complete refusal to walk on the narrow substrate.

Sensitivity of the S1 neuronal calcium network to insulin and Bay-K 8644 in vivo: Relationship to gait, motivation, and aging processes.

Neuronal hippocampal Ca2+ dysregulation is a critical component of cognitive decline in brain aging and Alzheimer's disease and is suggested to impact communication and excitability through the activation of a larger after hyperpolarization. However, few studies have tested for the presence of Ca2+ dysregulation in vivo, how it manifests, and whether it impacts network function across hundreds of neurons. Here, we tested for neuronal Ca2+ network dysregulation in vivo in the primary somatosensory cortex (S1) of anesthetized young and aged male Fisher 344 rats using single‐cell resolution techniques. Because S1 is involved in sensory discrimination and proprioception, we tested for alterations in ambulatory performance in the aged animal and investigated two potential pathways underlying these central aging‐ and Ca2+‐dependent changes. Compared to young, aged animals displayed increased overall activity and connectivity of the network as well as decreased ambulatory speed. In aged animals, intranasal insulin (INI) increased network synchronicity and ambulatory speed. Importantly, in young animals, delivery of the L‐type voltage‐gated Ca2+ channel modifier Bay‐K 8644 altered network properties, replicating some of the changes seen in the older animal. These results suggest that hippocampal Ca2+ dysregulation may be generalizable to other areas, such as S1, and might engage modalities that are associated with locomotor stability and motivation to ambulate. Further, given the safety profile of INI in the clinic and the evidence presented here showing that this central dysregulation is sensitive to insulin, we suggest that these processes can be targeted to potentially increase motivation and coordination while also reducing fall frequency with age.

The spring-mass model and other reductionist models of bipedal locomotion on inclines.

The spring-mass model is a model of locomotion aimed at giving the essential mathematical laws of the trajectory of the center of mass of an animal during bouncing gaits, such as hopping (one-dimensional) and running (two-dimensional). This reductionist mechanical system has been extensively investigated for locomotion over horizontal surfaces, whereas it has been largely neglected on other ecologically relevant surfaces, including inclines. For example, how the degree of inclination impacts the dynamics of the center of mass of the spring-mass model has not been investigated thoroughly. In this work, we derive a mathematical model which extends the spring-mass model to inclined surfaces. Among our results, we derive an approximate solution of the system, assuming a small angular sweep of the limb and a small spring compression during stance, and show that this approximation is very accurate, especially for small inclinations of the ground. Furthermore, we derive theoretical bounds on the difference between the Lagrangian and Lagrange equationsof the true and approximate system, and discuss locomotor stability questions of the approximate solutions. We test our models through a sensitivity analysis using parameters relevant to the locomotion of bipedal animals (quail, pheasant, guinea fowl, turkey, ostrich, and humans) and compare our approximate solution to the numerically derived solution of the exact system. We compare the two-dimensional spring-mass model on inclines with the one-dimensional spring-mass model to which it reduces under the limit of no horizontal velocity; we compare the two-dimensional spring-mass model on inclines with the inverted-pendulum model on inclines towards which it converges in the case of high stiffness-to-mass ratio. We include comparisons with historically prevalent no-gravity approximations of these models, as well. The insights we have gleaned through all these comparisons and the ability of our approximation to replicate some of the kinematic changes observed in animals moving on different inclines (e.g. reduction in vertical oscillation of the center of mass and decreased stride length) underlines the valuable and reasonable contributions that very simple, reductionist models, like the spring-mass model, can provide.

Changes to margins of stability from walking to obstacle crossing in older adults while walking fast and with a dual-task.

It is not well understood how older adults meet the combined locomotor demands of obstacle avoidance at fast speeds as compared to obstacle avoidance under cognitive loads. The purpose of this study was to quantify changes in locomotor stability (margin of stability, MOS) from walking to crossing obstacles at fast speeds versus with added cognitive demands in older adults. Community-dwelling older adults walked on an unobstructed and obstructed path at their preferred speed (preferred); during a dualtask (verbal fluency); and at their 'fastest comfortable' speed (fast). We used motion capture to calculate MOS in the anteroposterior direction, and compared minimum MOS between crossing foot and support phase (lead single support, lead double support, trail single support, trail double support) and tested for within subject changes using a linear mixed effect regression model [Condition (preferred, fluency, fast) x Walkway (unobstructed, obstructed) x Phase (single support, double support) x Foot (lead, trail)]. We examined crossing kinematics (approach distance, toe clearance, and recovery distance) between conditions. A significant omnibus effect partially supported our predictions. A Condition x Walkway x Phase interaction supported that older adults increased stability under a cognitive load and prioritized stability, demonstrated by not changing MOS from walking to obstacle crossing. During fast obstacle crossing they decreased stability during double support and exhibit more stability in single support, when vulnerable to external perturbations (contacting the obstacle). During a dual-task, older adults took shorter and higher steps over the obstacle to ensure they cleared it safely, but at fast speeds they increased the length of their crossing step without higher toe clearance. The results suggest older adults attempt to preserve stability when crossing obstacles under both cognitive and speed demands, but may be unable to ensure a safer limb elevation to avoid obstacles at fast speeds as they do under cognitive demands.

Transfer of Anolis locomotor behavior across environments and species.

Arboreal animals must learn to modulate their movements to overcome the challenges posed by the complexity of their heterogeneous environment, reduce performance failure, and survive. Anolis lizards are remarkable in the apparent ease with which they conquer this heterogeneity, demonstrating an impressive ability to modulate their locomotor behavior to maintain stable locomotion on widely disparate surfaces. Significant progress has been made towards understanding the impact of substrate structure on the behavioral plasticity of arboreal species, but it is unclear whether the same strategies employed to shift between substrates in one context can be employed to shift between those same substrates in a new context. Is the kinematic shift between broad and narrow perches achieved in a similar way on different inclines? Do all species within an ecomorph make similar adjustments? Here, we analyze the limb movements of two trunk-crown Anolis ecomorphs, A. carolinensis and A. evermanni, running on 6 different surfaces (3 inclinations × 2 perch diameters), from the perspective of Transfer Learning. Transfer learning is that field of machine learning which aims at exploiting the knowledge gained from one task to improve generalization about another, related task. In our setting, we use transfer learning to show that the strategies employed to improve locomotor stability on narrow perches are transferred across environments with different inclines. Further, behaviors used on vertical inclines are shared, and thus transfer well, across perch diameters whereas the relationship between horizontal and intermediate inclines change on different perch diameters, leading to lower transfer learning of shallow inclines across perch diameters. Interestingly, the best incline for transfer of behavior differs between limbs: forelimb models learn best from the vertical incline and hind limb models learn best from horizontal and intermediate inclines. Finally, our results suggest both that subtle differences exist in how A. carolinensis and A. evermanni adjust their behaviors in typical trunk-crown environments and that they may have converged on similar strategies for modulating forelimb behavior on vertical surfaces and hind limb behavior on shallow surfaces. The transfer of behavior is analogous to phenotypic plasticity, which likely plays a key role in the rapid adaptive evolution characteristic of Anolis lizards. This work is an example of how modern statistical methodology can provide an interesting perspective on new biological questions, such as on the role and nuances of behavioral plasticity and the key behaviors that help shape the versatility and rapid evolution of Anolis lizards.

Animal robots: Research foundation, key technologies and development forecasts

<p indent="0mm">Animal robots are a class of bio-syncretic robots with the visual, auditory and tactile perception capabilities of animals. They make use of the animal’s inherent motor system and act directly on specific brain regions (nucleus) or nervous systems of animal carriers through simulated neural signals to induce neuronal activity in the relevant functional brain regions in order to achieve motor behavior control of the carriers. This type of special robots has natural and significant advantages in many aspects such as locomotor stability, flexibility, environmental adaptability and energy supply for its locomotion, overcoming the difficulties of traditional bionic robots in terms of distribution drive, environmental perception and energy supply. It has significant theoretical and application values and is one of the important research directions in the development of robotics. The research was highly integrated with animal intelligence and machine intelligence, involving animal behavior, neuroscience, microelectromechanical technology, mechanics and communication technology, and is a frontier area of multidisciplinary integration. These robots are expected to perform tasks that are not possible for humans or traditional bionic robots. It can be used in the field of public security to perform tasks such as counter-terrorism, search and rescue, and temporal awareness. In the civilian field, it can be widely used in environmental surveys and detection of confined spaces. In scientific research, it can be used as a research tool to explore the structure of motor neural networks and unknown areas such as neural information interaction patterns, neuroplasticity and functional repair. In this paper, we review the development of animal locomotor behavior regulation in the past <sc>20 years,</sc> systematically sort out the relationship between the animal locomotor nervous system and the locomotor behavior regulation, and briefly summarize the neural mechanisms of animal locomotor regulation, the generation and transmission of animal locomotor regulation signals, the composition and characteristics of signals regulating animal locomotor behaviors, and microelectrode fabrication and implantation techniques. According to the motion scenarios of different animal robots (in water, on land and in the air), the stimulation modulation methods, the brain areas (nucleus) that are stimulated and the corresponding movement modulation behaviors of the animal robots studied are summarized. The important research results of several typical animal robots are categorized and reviewed in terms of methods for locomotor modulation behaviors, optimization of remote control stimulation systems, and research on automatic navigation systems, and the research problems and key technical challenges that need to be broken through for future animal robots are summarized and discussed. Based on this, to effectively play the role of animal robots and meet the needs of practical applications, the paper outlines a framework for future research on animal robots based on the bottlenecks in animal behavioral control technology and summarizes and analyzes four aspects of animal robots, including research on motor neural control mechanisms, miniaturization of stimulation and control systems, application of communication and navigation technologies, and design strategies for self-powered systems. The future development trends and application prospects of animal robots are predicted.

Head-Mounted and Hand-Held Displays Diminish the Effectiveness of Fall-Resisting Skills.

Use of head-mounted displays (HMDs) and hand-held displays (HHDs) may affect the effectiveness of stability control mechanisms and impair resistance to falls. This study aimed to examine whether the ability to control stability during locomotion is diminished while using HMDs and HHDs. Fourteen healthy adults (21–46 years) were assessed under single-task (no display) and dual-task (spatial 2-n-back presented on the HMD or the HHD) conditions while performing various locomotor tasks. An optical motion capture system and two force plates were used to assess locomotor stability using an inverted pendulum model. For perturbed standing, 57% of the participants were not able to maintain stability by counter-rotation actions when using either display, compared to the single-task condition. Furthermore, around 80% of participants (dual-task) compared to 50% (single-task) showed a negative margin of stability (i.e., an unstable body configuration) during recovery for perturbed walking due to a diminished ability to increase their base of support effectively. However, no evidence was found for HMDs or HHDs affecting stability during unperturbed locomotion. In conclusion, additional cognitive resources required for dual-tasking, using either display, are suggested to result in delayed response execution for perturbed standing and walking, consequently diminishing participants’ ability to use stability control mechanisms effectively and increasing the risk of falls.

Kinematic adjustments to arboreal locomotion in Japanese macaques (Macaca fuscata).

Although biomechanical adaptations to arboreal locomotion have been well investigated in primates and other mammals in laboratory settings, the results are not consistent, and more species need to be studied to build a comprehensive picture of this. Here, we used three-dimensional videography to quantify kinematic parameters thought to be associated with locomotor stability while two Japanese macaques walked on terrestrial and simulated arboreal substrates (a horizontal pole, which was narrow relative to the animal's body width). The parameters investigated included temporal-spatial gait variables, those associated with compliant walking, the height of the shoulder and hip, and hand and foot clearance during the swing phase. We found that there were many individual differences in kinematic adjustments made by the monkeys during arboreal locomotion. More importantly, the results were consistent between the monkeys for three parameters: maximum hand clearance, maximum hip height, and maximum foot clearance. The monkeys showed lower maximum hand and foot clearances during arboreal locomotion than during terrestrial locomotion, indicating that the hands and feet were kept close to the substrate surface during the swing phase. They also showed lower maximum hip heights during arboreal locomotion, suggesting that their whole-body centers of mass were lowered. These consistent kinematic adjustments can be interpreted as strategies for enhancing stability and reducing the risk of falling from a height. Overall, these results show that Japanese macaques make significant biomechanical adaptations to arboreal locomotion that are not fully consistent with existing data for other animals.

Exploring possible ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (upper Campanian) of Alberta, Canada

ABSTRACT Fossil tracks should theoretically capture differences in pedal anatomy between growth stages of the same taxon, particularly those related to the soft tissue of the foot, providing a more realistic view of pedal ontogeny than skeletal material alone. However, recognizing these ontogenetic trajectories is complicated by the influence of preservation and kinematics on track morphology, as well as the inherent difficulty of referring different tracks to a single taxon. Here, we explore differences in track morphology from a collection of tracks attributed to tyrannosaurids from Unit 4 of the Wapiti Formation (upper Campanian) in western Canada. Along with morphology, close geographic and stratigraphic associations suggest that the tracks pertain to similar tyrannosaurid trackmakers. A geometric morphometric analysis of the track outlines reveals size-dependent increase in relative track robusticity, driven primarily by an increase in ‘heel’ breadth and surface area. This relationship is lost when the dataset is expanded to include tyrannosaurid tracks globally, which we attribute to increased stratigraphic and taxonomic ‘noise’ within the global dataset that masks the tightly constrained patterns obtained from the Wapiti Formation tracks. Although there is some substrate and kinematic influence on certain aspects of track morphology, we hypothesize that the observed size-dependent relationship reflects genuine expansion in the breadth of the heel soft tissues and probably their overall surface area associated with growth. Increased pedal robusticity likely assisted with weight bearing and locomotor stability as body mass increased over ontogeny, supporting previous hypotheses that some tyrannosaurids underwent a growth-related reduction in relative agility and/or cursorial performance.

Minimal shoes improve stability and mobility in persons with a history of falls

Postural and walking instabilities contribute to falls in older adults. Given that shoes affect human locomotor stability and that visual, cognitive and somatosensory systems deteriorate during aging, we aimed to: (1) compare the effects of footwear type on stability and mobility in persons with a history of falls, and (2) determine whether the effect of footwear type on stability is altered by the absence of visual input or by an additional cognitive load. Thirty participants performed standing and walking trials in three footwear conditions, i.e. conventional shoes, minimal shoes, and barefoot. The outcomes were: (1) postural stability (movement of the center of pressure during eyes open/closed), (2) walking stability (Margin of Stability during normal/dual-task walking), (3) mobility (the Timed Up and Go test and the Star Excursion Balance test), and (4) perceptions of the shoes (Monitor Orthopaedic Shoes questionnaire). Participants were more stable during standing and walking in minimal shoes than in conventional shoes, independent of visual or walking condition. Minimal shoes were more beneficial for mobility than conventional shoes and barefoot. This study supports the need for longitudinal studies investigating whether minimal footwear is more beneficial for fall prevention in older people than conventional footwear.

Multiple strategies to correct errors in foot placement and control speed in human walking.

Neural feedback plays a key role in maintaining locomotor stability in the face of perturbations. In this study, we systematically identified properties of neural feedback that contribute to stabilizing human walking by examining how the nervous system responds to small kinematic deviations away from the desired gait pattern. We collected data from 20 participants (9 men and 11 women). We simultaneously applied (1) small continuous mechanical perturbations, forces at the ankles that affected foot placement, and (2) small continuous sensory perturbations, movement of a virtual visual scene that produced the illusion of change in walking speed, to compare how neural feedback responds to actual and illusory kinematic deviations. We computed phase-dependent impulse response functions that describe kinematic and muscular responses to small brief perturbations to identify critical phases of the gait cycle when the nervous system modulates muscle activity. In addition to the known foot-placement strategies that counteract kinematic displacement, such as the modulation of the hamstring muscle group during swing, we identified phase-specific muscle modulations that compensated for the perturbations. In particular, our results suggested that an early-stance modulation of anterior leg muscles (i.e., dorsiflexors and quadriceps) is a general control mechanism that serves to control forward body propulsion and compensates for errors in foot placement. Another detected general compensatory strategy was the late-stance modulation of the rectus femoris and gastrocnemius muscles, which controls walking speed in the next cycle.

Effects of upper limb loss and prosthesis use on proactive mechanisms of locomotor stability

Persons with upper limb loss (ULL) experience a high prevalence of falls, with the majority of falls occurring when walking. This issue may be related to altered arm dynamics, which play an important role in proactive mechanisms of locomotor stability. This study investigated effects of ULL and prosthesis use on proactive stability mechanisms, particularly if matching the mass and inertia of the impaired limb to the sound limb would enhance locomotor stability. Gait data were collected on adults with unilateral ULL during level walking while: (1) not wearing a prosthesis, (2) wearing their customary prosthesis, (3) wearing a mock prosthesis that matched the sound limb mass and inertia. Main and interaction effects of limb side and condition on trunk rotations, arm swing, step width, free vertical moment, and margin-of-stability were analyzed. Across conditions, arm swing, free vertical moment, and margin-of-stability were 2.27, 1.13, and 1.20 times greater, respectively, on the sound limb side than the impaired limb side. Persons with ULL display asymmetry in proactive mechanisms of locomotor stability with potentially greater medial-lateral stability on the sound limb side irrespective of prosthesis use, but heavier prostheses reduced the walking base of support. This bias may enhance fall risk on the impaired side if the prosthetic limb is used inappropriately to regain balance following a disturbance. Research is warranted to explore the consequences of this asymmetry on perturbation response.

Instability Resistance Training Decreases Motor Noise During Challenging Walking Tasks in Older Adults: A 10-Week Double-Blinded RCT.

Locomotor stability is challenged by internal perturbations, e.g., motor noise, and external perturbations, e.g., changes in surface compliance. One means to compensate for such perturbations is to employ motor synergies, defined here as co-variation among a set of elements that acts to stabilize, or provide similar trial-to-trial (or step-to-step) output, even in the presence of small variations in initial conditions. Whereas evidence exists that synergies related to the upper extremities can be trained, the extent to which lower limb synergies, such as those which may be needed to successfully locomote in complex environments, remains unknown. The purpose of this study was to evaluate if resistance training (RT) in unstable environments could promote coordination patterns associated with stronger synergies during gait. Sixty-eight participants between the age of 65 and 80 were randomly assigned to one of three different RT modalities: stable whole-limb machine-based RT (S-MRT), instability free-weight RT (I-FRT), and stable machine-based adductor/abductor RT (S-MRTHIP). Before and after RT, participants walked across an even lab floor and a more challenging uneven surface with and without holding a weighted bag. The uncontrolled manifold control analysis (UCM) was used to calculate the synergy index (i.e., strength of the kinematic synergy) related to stabilization of our performance variable, the mediolateral trajectory of the swing foot, under each condition. Regardless of RT group, there was no effect of RT on the synergy index when walking across the even lab floor. However, the synergy index during the two uneven surface conditions was stronger after I-FRT but was not affected by the other RT modalities. The stronger synergy index for the I-FRT group was due to improved coordination as quantified by an overall increase in variability in elemental variable space but a decrease in the variability that negatively affects performance. The unstable environment offered by I-FRT allows for exploration of motor solutions in a manner that appears to transfer to challenging locomotor tasks. Introducing tasks that promote, rather than limit, exploration of motor solutions seems to be a valuable exercise modality to strengthen kinematic synergies that cannot be achieved with traditional strengthening paradigms (e.g., S-MRT).Clinical Trial Registration: www.ClinicalTrials.gov, identifier NCT03017365.

Fall Prevalence and Contributors to the Likelihood of Falling in Persons With Upper Limb Loss.

Arms are important for locomotor stability and preventing falls by controlling whole-body angular momentum, redirecting the body's center of mass, and providing support to arrest descent. Hence, upper limb loss (ULL) can increase fall risk. However, the prevalence of falls and factors that influence fall risk have not previously been reported for people with ULL. This study quantified fall prevalence in persons with ULL at or proximal to the wrist and identified clinical factors that contributed to the likelihood of falling. This was a cross-sectional study. Factors including body and health characteristics, activity level, fall history, prosthesis use, and balance confidence were determined for persons with ULL proximal to the wrist using an online survey. Logistic regression analyses assessed the contribution of these factors to the classification of fallers (≥2 falls in previous year) and nonfallers. A percentage (28.6%) of participants (n=105) reported experiencing 2 or more falls in the past year. The regression model (R2=0.473) correctly classified 84.5% of cases and indicated that increased likelihood of falling was significantly influenced by reduced balance confidence, use of upper limb prostheses, and reduced physical capabilities. Data were collected online from a convenience sample, and fall classification was based on retrospective data. Falls in persons with ULL are prevalent, suggesting that clinicians should use screening methods to identify at-risk individuals. Balance confidence, use of upper limb prostheses, and perceived physical capabilities could be useful screening metrics. Research is warranted to better understand the factors that underlie fall risk in persons with ULL and the efficacy of therapeutic interventions capable of mitigating fall risk.

Locomotor stability in able-bodied trunk-flexed gait across uneven ground

This study aimed to explore the control of dynamic stability of the imposed trunk-flexed gaits across uneven ground. For ten young healthy participants, we compared the anteroposterior margin of stability (MoS) and lower limb joint kinematics at foot-contact during accommodating a consecutive stepdown and step-up (10-cm visible drop) to that of level steps while maintaining four postures: regular erect, ∼30°, ∼50° and maximal trunk flexion from the vertical. Two-way repeated measures ANOVAs revealed no significant step × posture interactions for the MoS (p = .187) and for the parameters that contributed to the MoS calculation (p > .05), whereas significant interactions were found for the hip flexion, hip position (relative to the posterior boundary of the base of support) and the knee flexion. The main effect of step (p = .0001), but not posture (p = .061), on the MoS was significant. Post hoc tests, compared with the level step, showed that the decreased magnitude of the MoS during stepping down (p = .011)—mainly due to a further forward displacement of the center of mass position (p = .006)—significantly increased in the immediate following step-up (p = .002) as a consequence of a substantial increase in the base of support (p = .003). In the stepdown versus level step, the hip and knee flexions as well as the hip position did not significantly change in the trunk-flexed gaits (p > .05). In the step-up, the knee flexion increased (except for the gaits with the maximum trunk flexion), whereas other kinematic variables remained unchanged. Quantifying the step-to-step control of dynamic stability in a perturbed walking reflected continuous control adaptations through the interaction between gait and posture. In fact, the able-bodied participants were able to safely control the motion of the body’s CoM with the combination of compensatory kinematic adjustments in lower-limb and adaptations in stepping pattern.

Proactive Locomotor Adjustments Are Specific to Perturbation Uncertainty in Below-Knee Prosthesis Users

Sensory-motor deficits associated with below-knee amputation impair reactions to external perturbations. As such, below-knee prosthesis users rely on proactive control strategies to maintain locomotor stability. However, there are trade-offs (metabolic, comfort, etc.) associated with proactive strategies. We hypothesize that because proactive control strategies are costly, prosthesis users and non-impaired participants will use a priori knowledge (timing, direction) of an impending lateral perturbation to make specific gait adaptations only when the timing of the perturbation is known and the adaptation can be temporally-limited. This hypothesis was partially supported. When the perturbation timing was predictable, only prosthesis users, and only on their impaired side, increased their lateral margin of stability during the steps immediately preceding the perturbation when perturbation direction was either unknown or known to be directed towards their impaired side. This strategy should reduce the likelihood of requiring a corrective step to maintain stability. However, neither group exhibited substantial proactive adaptations compared to baseline walking when perturbation timing was unpredictable, independent of perturbation direction knowledge. The absence of further proactive stabilization behaviors observed in prosthesis users in anticipation of a certain but temporally unpredictable perturbation may be partially responsible for impaired balance control.