Effects of simulated hypo-gravity on lower limb kinematic and electromyographic variables during anti-gravitational treadmill walking.

Introduction: This study investigated kinematic and EMG changes in gait across simulated gravitational unloading levels between 100% and 20% of normal body weight. This study sought to identify if each level of unloading elicited consistent changes-particular to that percentage of normal body weight-or if the changes seen with unloading could be influenced by the previous level(s) of unloading. Methods: 15 healthy adult participants (26.3 ± 2.5 years; 53% female) walked in an Alter-G anti-gravity treadmill unloading system (mean speed: 1.49 ±0.37 mph) for 1 min each at 100%, 80%, 60%, 40% and 20% of normal body weight, before loading back to 100% in reverse order. Lower-body kinematic data were captured by inertial measurement units, and EMG data were collected from the rectus femoris, biceps femoris, medial gastrocnemius, and anterior tibialis. Data were compared across like levels of load using repeated measures ANOVA and statistical parametric mapping. Difference waveforms for adjacent levels were created to examine the rate of change between different unloading levels. Results: This study found hip, knee, and ankle kinematics as well as activity in the rectus femoris, and medial gastrocnemius were significantly different at the same level of unloading, having arrived from a higher, or lower level of unloading. There were no significant changes in the kinematic difference waveforms, however the waveform representing the change in EMG between 100% and 80% load was significantly different from all other levels. Discussion: This study found that body weight unloading from 100% to 20% elicited distinct responses in the medial gastrocnemius, as well as partly in the rectus femoris. Hip, knee, and ankle kinematics were also affected differentially by loading and unloading, especially at 40% of normal body weight. These findings suggest the previous level of gravitational load is an important factor to consider in determining kinematic and EMG responses to the current level during loading and unloading below standard g. Similarly, the rate of change in kinematics from 100% to 20% appears to be linear, while the rate of change in EMG was non-linear. This is of particular interest, as it suggests that kinematic and EMG measures decouple with unloading and may react to unloading uniquely.

Sunspot activity and birth defects among Texas births (1999-2016).

BACKGROUND: Building on findings that linked higher levels of sunspot (SS) activity with a range of health and adverse birth outcomes, we sought to understand how SS activity over a 17-year time period may be correlated with the occurrence of birth defects. METHODS: Data from the Texas Birth Defects Registry, vital events from the Texas Center for Health Statistics, and mean monthly numbers of sunspots from the National Oceanic and Atmospheric Administration were utilized. Poisson regression was used to calculate crude/adjusted prevalence ratios (cPRs/aPRs) and 95% confidence intervals for three quartiles (Q) of increasing SS activity (compared to a referent of low activity) and 44 birth defects (31 non-cardiac; 13 cardiac) with estimated dates of conception from 1998 to 2016. RESULTS: We found moderately protective aPRs (range: 0.60-0.89) in a little over half of the case groups examined in our quartiles of higher SS activity (19 non-cardiac; 6 cardiac), after adjusting for maternal age, race/ethnicity, and education. Particularly protective aPRs in the highest SS quartiles (Q3-4) were noted for: anophthalmia, cataract, gastroschisis, trisomy 18, ventricular septal defects, atrial septal defects, and pulmonary valve atresia or stenosis. Conversely, modestly elevated aPRs were noted for two defect groups (agenesis, aplasia, and hypoplasia of the lung and microcephaly [Q2-3]). Following an additional adjustment of year of conception, results remained similar although many of the estimates were attenuated. CONCLUSION: The seemingly protective associations between increasing SS activity may be an artifact of increasing spontaneous abortions that occur following conception during these periods of heightened SS activity.

Distinct Kinematic and Neuromuscular Activation Strategies During Quiet Stance and in Response to Postural Perturbations in Healthy Individuals Fitted With and Without a Lower-Limb Exoskeleton.

Many individuals with disabling conditions have difficulty with gait and balance control that may result in a fall. Exoskeletons are becoming an increasingly popular technology to aid in walking. Despite being a significant aid in increasing mobility, little attention has been paid to exoskeleton features to mitigate falls. To develop improved exoskeleton stability, quantitative information regarding how a user reacts to postural challenges while wearing the exoskeleton is needed. Assessing the unique responses of individuals to postural perturbations while wearing an exoskeleton provides critical information necessary to effectively accommodate a variety of individual response patterns. This report provides kinematic and neuromuscular data obtained from seven healthy, college-aged individuals during posterior support surface translations with and without wearing a lower limb exoskeleton. A 2-min, static baseline standing trial was also obtained. Outcome measures included a variety of 0 dimensional (OD) measures such as center of pressure (COP) RMS, peak amplitude, velocities, pathlength, and electromyographic (EMG) RMS, and peak amplitudes. These measures were obtained during epochs associated with the response to the perturbations: baseline, response, and recovery. T-tests were used to explore potential statistical differences between the exoskeleton and no exoskeleton conditions. Time series waveforms (1D) of the COP and EMG data were also analyzed. Statistical parametric mapping (SPM) was used to evaluate the 1D COP and EMG waveforms obtained during the epochs with and without wearing the exoskeleton. The results indicated that during quiet stance, COP velocity was increased while wearing the exoskeleton, but the magnitude of sway was unchanged. The OD COP measures revealed that wearing the exoskeleton significantly reduced the sway magnitude and velocity in response to the perturbations. There were no systematic effects of wearing the exoskeleton on EMG. SPM analysis revealed that there was a range of individual responses; both behaviorally (COP) and among neuromuscular activation patterns (EMG). Using both the OD and 1D measures provided a more comprehensive representation of how wearing the exoskeleton impacts the responses to posterior perturbations. This study supports a growing body of evidence that exoskeletons must be personalized to meet the specific capabilities and needs of each individual end-user.

Comparison of Treadmill Gait Between a Pediatric-Aged Individual With SYNGAP1-Related Intellectual Disability and a Fraternal Twin.

SYNGAP1-related Intellectual Disability (SYNGAP1-ID) is a rare neurodevelopmental condition characterized by profound intellectual disability, gross motor delays, and behavioral issues. Ataxia and gait difficulties are often observed but have not yet been characterized by laboratory-based kinematic analyses. This investigation identified gait characteristics of an individual with SYNGAP1-ID and compared these with a neurotypical fraternal twin. Lower limb kinematics were collected with a 12-camera motion capture system while both participants walked on a motorized treadmill. Kinematic data were separated into strides, and stride times calculated. Sagittal plane hip, knee, and ankle joints were filtered and temporally normalized to 100 samples. Minimum and maximum joint angles, range of motion (ROM) and angular velocities were obtained for each joint by stride and averaged for each participant. ROM symmetry between left and right joints was also calculated. Discrete relative phase (DRP) was used to assess coordination and variability between joints within a single limb and compared across limbs. Phase portraits were calculated by joint, and their areas were computed with a MATLAB script. Statistical parametric mapping (SPM) was used to assess differences in joint angle waveforms between participants. P1, the individual with SYNGAP1-ID, displayed significantly reduced stride times relative to the fraternal twin, i.e., P2. A majority of minimum, maximum angles, ROMs, and angular velocities were significantly different between P1 and P2. Phase portrait areas were consistently less in P1 relative to P2 and there were differences in knee and ankle symmetries. DRP showed no differences between individuals, suggesting that P1's coordinative events remained similar to those observed during neurotypical gait (P2). SPM revealed significant differences between the left and right legs at the knee and ankle joints of P1 while P2 joint left and right waveforms were nearly identical for all joints. Additionally, SPM revealed there were significant differences between P1 and P2 for all joints. This investigation identified several major gait features of an individual with SYNGAP1-ID and provided a comprehensive characterization of these features by utilizing both linear and non-linear analyses. While limited in generalizability, this report provides a strong quantitative appraisal of gait in an individual with SYNGAP1-ID as well as an analysis pathway for future investigations.

Characteristic behaviors associated with gait of individuals with Rett syndrome.

BACKGROUND: Individuals with Rett syndrome (RTT) exhibit impaired motor performance and gait performance, leading to decreased quality of life. Currently, there is no robust observational instrument to identify gait characteristics in RTT. Current scales are limited as individuals with intellectual disorders may be unable to understand instructions. Our primary purpose was to utilize video analysis to characterize the behaviors associated with walking in individuals with RTT and explore the relationship between behaviors during overground and during treadmill walking. METHODS: Fourteen independently ambulatory females with RTT were video-taped and observed during overground and treadmill walking. Their gait was codified into an observational checklist to reveal prominent features associated with gait in this population. RESULTS: Participants exhibited similar rates of freezing, veering, and hand stereotypies between overground and treadmill walking; however, freeze duration was shortened during treadmill walking. Toe walking was prominently exhibited during overground, but not treadmill walking. During both walking modes, participants required extensive external motivation to maintain their walking patterns. CONCLUSIONS: Results identify several gait characteristics observable during overground and treadmill walking. In general, participants behaved similarly during overground and treadmill walking. We conclude that both overground and treadmill walking are appropriate tools to evaluate gait in this population.Implications for rehabilitationLocomotor rehabilitation may increase the quantity of walking performed by the patients, which can alleviate negative effects of the sedentary lifestyle commonly observed in patients with Rett syndrome (RTT).Video analysis of natural walking can be an effective tool to characterize gait in patients with RTT which does not require particular instructions which may not be fully understood.Both overground and treadmill walking are appropriate means of evaluating gait in individuals with RTT.

Fear of Falling: Significant Barrier in Fall Prevention Approaches.

Fear of falling is a critical component in fall prevention approaches; however, it is often overlooked in the majority of fall prevention exercises. Alternative fall prevention approaches that take fear of falling into account are necessary. This article discusses fall prevention activities that are feasible for individuals with limited mobility who have an increased fear of falling. Health care providers should consider the degree to which a patient has a fear of falling and recommend activities that fit most to their patient's comfort level.

Self-Monitoring of Balance Performance can Reduce the Rate of Falls Among Older Adults.

Background: 29% of older adults fall annually, resulting in the leading cause of accidental death. Fall prevention programs typically include exercise training and self-monitoring of physical activity has a positive effect on the self-efficacy and self-regulation of exercise behaviors. We assessed if self-monitoring of fall risk, without an intervention, impacts fall rates. Methods: Fifty-three older adults had open access to a balance measuring platform which allowed them to self-monitor their postural stability and fall risk using a simple 1-min standing balance test. 12-month retrospective fall history was collected and a monthly/bimonthly fall log captured prospective falls. Participants had access to self-monitoring for up to 2.2 years. Fall history and fall incidence rate ratios and their confidence intervals were compared between the periods of time with and without access to self-monitoring. Results: A 54% reduction in the number of people who fell and a 74% reduction in the number of falls was observed when participants were able to self-monitor their postural stability and fall risk, after normalizing for participation length. Further, 42.9% of individuals identified as having high fall risk at baseline shifted to a lower risk category at a median 34 days and voluntarily measured themselves for a longer period of time. Discussion: We attribute this reduction in falls to changes in health behaviors achieved through empowerment from improved self-efficacy and self-regulation. Providing older adults with the ability to self-monitor their postural stability and intuit their risk of falling appears to have modified their health behaviors to successfully reduce fall rates.

Kinematics associated with treadmill walking in Rett syndrome.

BACKGROUND AND PURPOSE: Individuals with Rett syndrome suffer from severely impaired cognitive and motor performance. Current movement-related therapeutic programs often include traditional physical therapy activities and assisted treadmill walking routines for those individuals who are ambulatory. However, there are no quantitative reports of kinematic gait parameters obtained during treadmill walking. The purpose of this research was to characterize the kinematic patterns of the lower limbs during treadmill walking as speed was slowly increased. METHODS: Seventeen independently ambulatory females diagnosed with a methyl-CpG-binding protein 2 gene mutation walked on a motorized treadmill while joint kinematics were obtained by a camera-based motion capture system and analysis software. RESULTS: Stride times progressively decreased as treadmill speeds increased. There were significant main effects of speed on sagittal knee and hip ranges of motion and hip velocity. There were large joint asymmetries and variance values relative to other ambulatory patient populations, although variance values decreased as walking speed increased. CONCLUSIONS: The results indicate that individuals with Rett syndrome can adapt their kinematic gait patterns in response to increasing treadmill speed, but only within a narrow range of speeds. We suggest that treadmill training for ambulatory individuals with Rett syndrome may promote improved walking kinematics and possibly provide overall health benefits.Implications for rehabilitationWalking is an activity that can counter the negative impacts of the sedentary lifestyle of many individuals with disabilities, including those individuals with Rett syndrome.Documentation of the lower limb kinematic patterns displayed during walking by ambulatory females with Rett syndrome can be used by clinicians to evaluate their patients' gait performance in response to therapeutic and pharmacological interventions designed to promote walking.The ability to adapt to increases in treadmill speed suggests that a training program of treadmill walking may be effective in promoting improved gait performance in individuals with Rett syndrome.

Effects of Shank Vibration on Lean After-Effect.

Postural adaptability is related to central sensory integration and reweighting efficiency. Incline-interventions lead to lean after-effect (LAE), but it is not fully known how sensory reweighting may affect the magnitude and duration of LAE. We tasked fifteen young and healthy subjects with performing incline-interventions under conditions designed to perturb proprioception during or after the incline-intervention. We found that support surface configuration affected responses to tendon vibration. Additionally, vibration during an incline-intervention did not inhibit LAE, but vibration during an after-effect significantly affected LAE. Results reinforce claims that postural adaptation is based on modifications of central mechanisms of perception, not peripheral shank proprioceptors and improve our understanding of the role of sensory reweighting and sensory integration into postural adaptability.

The Posterior Parietal Cortex Is Involved in Gait Adaptation: A Bilateral Transcranial Direct Current Stimulation Study.

Gait is one of the fundamental behaviors we use to interact with the world. The functionality of the locomotor system is thus related to enriching interactions with our environment. The posterior parietal cortex (PPC) has been found to contribute to motor adaptation during both visuomotor and postural adaptation tasks. Additionally, structural or functional deficits of the PPC lead to impairments in gaits such as shortened steps and increased step width. Based on the aforementioned roles of the PPC, and the importance of gait adaptability, the current investigation sought to identify the role of the PPC in gait adaptation. To achieve this, we performed transcranial direct current stimulation (tDCS) over the bilateral PPC before performing a split-belt treadmill gait adaptation paradigm. We used three stimulation conditions in a within-subject design. tDCS was administered in a randomized and double-blinded order. Following each stimulation session, subjects first performed baseline walking with both belts running at the same speed. Then, subjects walked for 15 min on an uncoupled treadmill, with the belts being driven at a 3:1 speed ratio. Last, they returned to normal (i.e., tied-belt) walking for 5 min. Results from 15 young and healthy subjects identified that subjects required more steps to adapt to split-belt walking following the suppression of the left hemisphere PPC, contralateral to the fast belt. Furthermore, while suppression of the left hemisphere PPC did not increase the number of steps required to re-adapt to tied-belt walking, this condition did lead to increased magnitude of after-effects. Together, these findings indicate that the PPC is involved in locomotor adaptation. These results support previous literature regarding the upper body or postural adaptation and extend these findings to the realm of gait. Results highlight the PPC as a potential target for neurorehabilitation designed to improve gait adaptability.

Non-invasive Brain Stimulation of the Posterior Parietal Cortex Alters Postural Adaptation.

Effective central sensory integration of visual, vestibular, and proprioceptive information is required to promote adaptability in response to changes in the environment during postural control. Patients with a lesion in the posterior parietal cortex (PPC) have an impaired ability to form an internal representation of body position, an important factor for postural control and adaptation. Suppression of PPC excitability has also been shown to decrease postural stability in some contexts. As of yet, it is unknown whether stimulation of the PPC may influence postural adaptation. This investigation aimed to identify whether transcranial direct current stimulation (tDCS) of the bilateral PPC could modulate postural adaptation in response to a bipedal incline postural adaptation task. Using young, healthy subjects, we delivered tDCS over bilateral PPC followed by bouts of inclined stance (incline-interventions). Analysis of postural after-effects identified differences between stimulation conditions for maximum lean after-effect (LAE; p = 0.005) as well as a significant interaction between condition and measurement period for the average position (p = 0.03). We identified impaired postural adaptability following both active stimulation conditions. Results reinforce the notion that the PPC is involved in motor adaptation and extend this line of research to the realm of standing posture. The results further highlight the role of the bilateral PPC in utilizing sensory feedback to update one's internal representation of verticality and demonstrates the diffuse regions of the brain that are involved in postural control and adaptation. This information improves our understanding of the role of the cortex in postural control, highlighting the potential for the PPC as a target for sensorimotor rehabilitation.

The effects of muscle vibration on gait control: a review.

Background: The purpose of the review is to summarize the literature surrounding the use of muscle vibration as it relates to modifying human gait. Methods: After a brief introduction concerning historical uses and early research identifying the effect of vibration on muscle activation, we reviewed 32 articles that used muscle vibration during walking. The review is structured to address the literature within four broad categories: the effect of vibration to 'trigger' gait-like lower limb motions, the effect of vibration on gait control of healthy individuals and individuals with clinical conditions in which gait disorders are a prominent feature, and the effect of vibration training protocols on gait. Results: The acute effects of vibration during gait involving healthy participants is varied. Some authors reported differences in segmental kinematic and spatiotemporal measures while other authors reported no differences in these outcome measures. The literature involving participants with clinical conditions revealed that vibration consistently had a significant impact on gait, suggesting vibration may be an effective rehabilitation tool. All of the studies that used vibration therapy over time reported significant improvement in gait performance. Conclusions: This review highlights the difficulties in drawing definitive conclusions as to the impact of vibration on gait control, partly because of differences in walking protocols, site of vibration application, and outcome measures used across different investigative teams. It is suggested that the development of common investigative methodologies and outcome measures would accelerate the identification of techniques that may provide optimal rehabilitation protocols for individuals experiencing disordered gait control.

Perception of threshold-level whole-body motion during mechanical mastoid vibration.

BACKGROUND: Vibration applied on the mastoid has been shown to be an excitatory stimulus to the vestibular receptors, but its effect on vestibular perception is unknown. OBJECTIVE: Determine whether mastoid vibration affects yaw rotation perception using a self-motion perceptual direction-recognition task. METHODS: We used continuous, bilateral, mechanical mastoid vibration using a stimulus with frequency content between 1 and 500 Hz. Vestibular perception of 10 healthy adults (M±S.D. = 34.3±12 years old) was tested with and without vibration. Subjects repeatedly reported the perceived direction of threshold-level yaw rotations administered at 1 Hz by a motorized platform. A cumulative Gaussian distribution function was fit to subjects' responses, which was described by two parameters: bias and threshold. Bias was defined as the mean of the Gaussian distribution, and equal to the motion perceived on average when exposed to null stimuli. Threshold was defined as the standard deviation of the distribution and corresponded to the stimulus the subject could reliably perceive. RESULTS: The results show that mastoid vibration may reduce bias, although two statistical tests yield different conclusions. There was no evidence that yaw rotation thresholds were affected. CONCLUSIONS: Bilateral mastoid vibration may reduce left-right asymmetry in motion perception.

Efficacy of Stochastic Vestibular Stimulation to Improve Locomotor Performance During Adaptation to Visuomotor and Somatosensory Distortion.

Astronauts exposed to microgravity face sensorimotor challenges affecting balance control when readapting to Earth's gravity upon return from spaceflight. Small amounts of electrical noise applied to the vestibular system have been shown to improve balance control during standing and walking under discordant sensory conditions in healthy subjects, likely by enhancing information transfer through the phenomenon of stochastic resonance. The purpose of this study was to test the hypothesis that imperceptible levels of stochastic vestibular stimulation (SVS) could improve short-term adaptation to a locomotor task in a novel sensory discordant environment. Healthy subjects (14 males, 10 females, age = 28.7 ± 5.3 years, height = 167.2 ± 9.6 cm, weight = 71.0 ± 12.8 kg) were tested for perceptual thresholds to sinusoidal currents applied across the mastoids. Subjects were then randomly and blindly assigned to an SVS group receiving a 0-30 Hz Gaussian white noise electrical stimulus at 50% of their perceptual threshold (stim) or a control group receiving zero stimulation during Functional Mobility Tests (FMTs), nine trials of which were done under conditions of visual discordance (wearing up/down vision reversing goggles). Time to complete the course (TCC) was used to test the effect of SVS between the two groups across the trials. Adaptation rates from the normalized TCCs were also compared utilizing exponent values of power fit trendline equations. A one-tailed independent-samples t-test indicated these adaptation rates were significantly faster in the stim group (n = 12) than the control (n = 12) group [t(16.18) = 2.00, p = 0.031]. When a secondary analysis was performed comparing "responders" (subjects who showed faster adaptation rates) of the stim (n = 7) group to the control group (n = 12), independent-samples t-tests revealed significantly faster trial times for the last five trials with goggles in the stim group "responders" than the controls. The data suggests that SVS may be capable of improving short-term adaptation to a locomotion task done under sensory discordance in a group of responsive subjects.

Assessing multiple muscle activation during squat movements with different loading conditions - an EMG study.

Surface electromyography (EMG) is a valuable tool in clinical diagnostics and research related to human neuromotor control. Non-linear analysis of EMG data can help with detection of subtle changes of control due to changes of external or internal constraints during motor tasks. However, non-linear analysis is complex and results may be difficult to interpret, particularly in clinical environments. We developed a non-linear analysis tool (SYNERGOS) that evaluates multiple muscle activation (MMA) features and provides a single value for description of activation characteristics. To investigate the responsiveness of SYNERGOS to kinetic changes during cyclic movements, 13 healthy young adults performed squat movements under different loading conditions (100%-120% of body weight). We processed EMG data to generate SYNERGOS indices and used two-way repeated measures ANOVA to determine changes of MMA in response to loading conditions during movement. SYNERGOS values were significantly different for each loading condition. We concluded that the algorithm is sensitive to kinetic changes during cyclic movements, which may have implications for applications in a variety of experimental and diagnostic settings.

A new fall-inducing technology platform: Development and assessment of a programmable split-belt treadmill.

Typical technologies for fall reduction/prevention training incorporate mechanical obstacles or cables/pulleys to induce trip or slip perturbations. This paper proposes a technology platform that uses a split-belt treadmill equipped with one force plate underneath each belt and a real-time gait phase detection algorithm. A proof-of-concept study validates the method for inducing trip perturbations in healthy young adults (n=10) by using kinematic measures from a full body motion capture system to characterize the effects of the perturbations. Preliminary results show that the proposed method successfully induces a trip and its congruent postural responses. The major findings have implications for designing intervention programs to reduce or prevent falls by individuals with a high risk of falls.

The Effect of Vibrotactile Cuing on Recovery Strategies From a Treadmill-Induced Trip.

Effective fall prevention technologies need to detect and transmit the key information that will alert an individual in advance about a potential fall. This study investigated advanced vibrotactile cuing that may facilitate trip recovery for balance-impaired individuals who are prone to falling. A split-belt treadmill that simulated unpredictable trip perturbations was developed to compare balance recovery without and with cuing. Kinetic and kinematic measures from force plates and full body motion capture system were used to characterize the recovery responses. Experiment I evaluated recovery adaptation resulting from repeated trip exposure without vibrotactile cuing. Experiment II investigated the effects of vibrotactile cuing as a function of cuing location (upper arm, trunk, lower leg) and lead time prior to a trip (250, 500 ms). Experiment I showed that trip recovery improved progressively from the fourth to the eighth trial. Experiment II showed that trip recovery was almost the same as the eighth trial in Experiment I, regardless of the location of the cuing stimulus and lead time. The results suggest that a combination of vibrotactile cuing and hazard detection technology could reduce the risk of trips and falls.

Alterations in over-ground walking patterns in obese and overweight adults.

Obesity has been associated with negative effects on postural control, including falls. Previous studies revealed different outcomes regarding the effects of obesity on gait features, and the use of BMI may lead to bias in assessing the true effects of obesity on gait. To better understand the effects of obesity on gait, it is important to examine gait features and associated body composition measures. The purpose of this study was: (1) to assess gait features of normal weight, overweight and obese adults, and (2) to assess the relationship between body composition measures and gait features. Thirty participants were assigned to one of three groups based upon their BMI at the onset of the study: healthy weight (BMI: 18.5-24.9kg/m2), overweight (BMI: 25-29.9kg/m2), or obese (BMI: 30-40kg/m2). Participants performed straight-line over-ground walking through a 200m hallway at their natural preferred speed while wearing their own shoes. The angular displacements, range of motion (ROM), and approximate entropy of kinematic data of the bilateral hips, knees, and ankles in the sagittal plane were computed. Walking speed, step length, stride length, single leg support phase, double leg support phase, swing phase and bilateral stance phase times were extracted from the GaitRite data. Overall, body mass and BMI were associated with peak flexion and ROM in the knees as well as single support, double support, stance, and swing phases. Body fat percentage did not exhibit correlations with measured gait features. Gait variables were more highly correlated with BMI and body mass instead of percent body fat, suggesting that absolute mass is more influential on gait features rather than amount of fat tissue.

Balancing sensory inputs: Sensory reweighting of ankle proprioception and vision during a bipedal posture task.

During multisensory integration, it has been proposed that the central nervous system (CNS) assigns a weight to each sensory input through a process called sensory reweighting. The outcome of this integration process is a single percept that is used to control posture. The main objective of this study was to determine the interaction between ankle proprioception and vision during sensory integration when the two inputs provide conflicting sensory information pertaining to direction of body sway. Sensory conflict was created by using bilateral Achilles tendon vibration and contracting visual flow and produced body sway in opposing directions when applied independently. Vibration was applied at 80Hz, 1mm amplitude and the visual flow consisted of a virtual reality scene with concentric rings retreating at 3m/s. Body sway elicited by the stimuli individually and in combination was evaluated in 10 healthy young adults by analyzing center of pressure (COP) displacement and lower limb kinematics. The magnitude of COP displacement produced when vibration and visual flow were combined was found to be lesser than the algebraic sum of COP displacement produced by the stimuli when applied individually. This suggests that multisensory integration is not merely an algebraic summation of individual cues. Instead the observed response might be a result of a weighted combination process with the weight attached to each cue being directly proportional to the relative reliability of the cues. The moderating effect of visual flow on postural instability produced by vibration points to the potential use of controlled visual flow for balance training.

Vibrotactile cuing revisited to reveal a possible challenge to sensorimotor adaptation.

Motor responses to unexpected external perturbations require the adjustment of the motor commands driving the ongoing activity. Strategies can be learned with practice to compensate for these unpredictable perturbations (e.g., externally induced slips and trips). It has been hypothesized that response improvements reflect the adaptation of motor commands through updates of an internal model. This hypothesis may be nuanced when a pre-existing motor response could be used. In that case, since a relatively adequate response is known, only the timing of the command needs to be determined. If so, then it could be inferred that the timing of movement initiation and the specific sequence of motor commands can be dissociated. Previously, we quantified the benefits of cuing vs. learning on recovery motor responses resulting from a trip induced by the abrupt stop of one side of a split belt treadmill. Trip occurrence was randomized within a series of strides. Two groups of young adults participated to two distinct experiments (learning, cuing). In the learning experiment, trip recovery improved progressively from the 4th to the 8th trial to reach an "adapted response". In the cuing experiment, trip recovery was immediate (from 1st trial). Expanding from these results, the aim of the present work was to differentiate the processes underlying the generation of motor compensation strategies in response to an external perturbation under time uncertainty. A supplementary analysis revealed that "cued" responses were kinematically similar to the "adapted response" and remained invariant regardless of cue lead time (250, 500 ms before trip) and application location of the cue (arm, trunk, lower leg). It is posited that all responses (cued and non-cued) are the expression of a pre-existing motor program derived from life experiences. Here, the cue significantly reduces time uncertainty and adaptation consists primarily in resolving time uncertainty based on the trial-by-trial learning of the stochastic property of trip occurrence in order to reduce the response delay. Hence, response time delay and motor program parameters appear to stem from two distinct processes.