Center Of Mass Motion Research Articles

The Influence of Elite Race Walkers’ Year-Long Training on Changes in Total Energy and Energy Cost While Walking at Different Speeds

The aim of the study was to assess the influence of year-long training of race walkers on physiological cost and total energy center of mass (CoM). The assessment performed was based on indicating the differences between the resulting energy cost in a group of elite race walkers walking at technical, threshold, and racing speeds calculated by physiological and biomechanical methods before beginning and after finishing a year-long training cycle. The study involved 12 competitive race walkers who had achieved champion or international champion level. Their aerobic endurance was determined by means of a direct method, applying an incremental exercise test on the treadmill. The gait of the participants was recorded using the 3D Vicon analysis system. Changes in mechanical energy amounted to the value of the total external work of the muscles needed to accelerate and lift the center of mass during a normalized gait cycle. The highest influence on the total external work increase for increasing speeds of gait in both examinations was attributed to the changes in the kinetic energy resulting from the center of mass movement. A statistically significant decrease of the mean value of total external work for racing speed was observed in the second examination (p < 0.001). An approx. 8% decrease (NS) of EE energy cost, standardized by body mass and distance covered, was found between the first and second examinations. The energy cost and total external work were significantly differentiated by the walkers’ gait speeds (p < 0.05–0.001). The energy cost significantly differed from the total external work at p < 0.001.

Dynamic balance of circles with one support point on pommel horse exercise

Double-leg circles on pommel horse exercises require a high degree of dynamic balance. However, theoretical conditions for maintaining dynamic balance are unclear. The purpose of this paper is to propose a simple theoretical model of the dynamic balance of the circles, and to illustrate its qualitative properties. To this end, the body of a gymnast is simply modeled as one rigid body with one support point, and symmetric and constant-velocity circles are assumed in most analyses. The condition that the torques of wrist and shoulder joints are zero is assumed as a dynamic balance condition with minimum strength. A control law is proposed to demonstrate the motion. Various properties of the dynamic balance condition are analyzed as follows. In the symmetric circles, (1) as the period of a circle decrease, the heights and the radius of the toes increase, and (2) the rotation of the body around its longitudinal axis in the double-leg circle has the effect of lifting the toes. (3) The shoulder and wrist torques can change the pose around the dynamic balance condition. In asymmetric circles, (4) the radius of the center of mass (CoM) increases as the angular velocity of the CoM around the support point decreases, and (5) the body angle with respect to the horizontal plane increases as the upward acceleration of the CoM increases. Moreover, mechanical principles of the circles are discussed as follows. (6) The CoM motion during the symmetric circles of the simple model can be related to the conical pendulum. In the simple model, (7) the pommel reaction force should be parallel to the arm segment when the wrist torque and the shoulder torque are equal or zero, and (8) to change the vertical component of the angular momentum of the body around the support point, the wrist torque around the vertical axis is needed. These results provide theoretical and qualitative insights into understanding and improving pommel horse exercise.

Viscous Current-Induced Forces.

We study the motion (translational, vibrational, and rotational) of a diatomic impurity immersed in an electron liquid and exposed to electronic current. An approach based on the linear response time-dependent density functional theory combined with the Ehrenfest dynamics leads to a system of linear algebraic equations, which account for the competing and counteracting effects of the current-induced force (electron wind) and the electronic friction. We find and emphasize the coupling between the center of mass motion and that of the nuclei relative to each other, the feature due to the mediation of the two-body interaction by the environment. The current-induced forces, by means of the dynamic exchange-correlation (xc) kernel f_{xc}(r,r^{'},ω), include the electronic viscosity contribution. Starting from the ground state at the equilibrium internuclear distance and applying a current pulse, we observe three phases of the motion: (i)acceleration due to the prevalence of the current-induced force, (ii)stabilization upon balancing of the two forces, and (iii)deceleration due to the friction after the end of the pulse. At lower, but still metallic, electron densities, the dynamic xc contribution to the force significantly affects the acceleration (deceleration) at the first (third) phase of the process. For the Cs density (r_{s}≈6 a.u.), this correction amounts up to 40% in the rotation regime.

Combined translational and rotational perturbations of standing balance reveal contributions of reduced reciprocal inhibition to balance impairments in children with cerebral palsy.

Balance impairments are common in cerebral palsy. When balance is perturbed by backward support surface translations, children with cerebral palsy have increased co-activation of the plantar flexors and tibialis anterior muscle as compared to typically developing children. However, it is unclear whether increased muscle co-activation is a compensation strategy to improve balance control or is a consequence of reduced reciprocal inhibition. During translational perturbations, increased joint stiffness due to co-activation might aid balance control by resisting movement of the body with respect to the feet. In contrast, during rotational perturbations, increased joint stiffness will hinder balance control as it couples body to platform rotation. Therefore, we expect increased muscle co-activation in response to rotational perturbations if co-activation is caused by reduced reciprocal inhibition but not if it is merely a compensation strategy. We perturbed standing balance by combined backward translational and toe-up rotational perturbations in 20 children with cerebral palsy and 20 typically developing children. Perturbations induced forward followed by backward movement of the center of mass. We evaluated reactive muscle activity and the relation between center of mass movement and reactive muscle activity using a linear feedback model based on center of mass kinematics. In typically developing children, perturbations induced plantar flexor balance correcting muscle activity followed by tibialis anterior balance correcting muscle activity, which was driven by center of mass movement. In children with cerebral palsy, the switch from plantar flexor to tibialis anterior activity was less pronounced than in typically developing children due to increased muscle co-activation of the plantar flexors and tibialis anterior throughout the response. Our results thus suggest that a reduction in reciprocal inhibition causes muscle co-activation in reactive standing balance in children with cerebral palsy.

Relationship among the COM Motion, the Lower Extremity and the Trunk during the Squat.

Squatting is a common motion in activities of daily living and is frequently used in training programs. Squatting requires a shift of the body in both vertical and anterior-posterior directions. Postural control during squatting is considered a mixed strategy; however, details and roles of the trunk and lower limb joints are unclear. The purpose of this study was to investigate the relationship among the kinematics of the lower limb, the trunk and the center of mass (COM) descent during squatting. Twenty-six healthy young adults performed repeated parallel squats. Lower limb joint and trunk angles and the COM were analyzed using a 3D motion analysis system. We evaluated the relationship between the kinematics and the squat depth by performing correlation analysis and multiple linear regression analysis. The ankle was the first to reach its maximum angle, and the remaining joints reached their maximum angles at the maximum squat depth. The knee joint motion and the squat depth were significantly correlated and there was a correlation between the hip and the ankle joint motion and the anteroposterior displacement of the COM during squatting. Multivariate linear regression analysis indicated that squat depth was predicted by both the knee and ankle motion and that anteroposterior displacement of the COM was predicted by the hip, ankle, and knee joint motion. The knees contributed to the vertical COM motion during squatting, while the hips contributed to the COM motion in the anteroposterior direction. On the other hand, the ankles contributed to COM motions in both the vertical and anteroposterior directions during squatting.

Diffusion enhancement and autoparametric resonance.

The possibility of an autoparametric resonance in an isolated many-particle system induces a specific behavior of the particles in the presence of thermal noise. In particular, the variance associated with a resonant mode, and consequently that of the associated particles, is strongly increased compared to what it would have in the absence of parametric resonance. In this paper we consider a dimer submitted to a periodic potential for which there are only two modes, the center of mass motion and the internal vibration mode. This is the simplest system which is dynamically rich enough to exhibit an autoparametric excitation of the internal vibrations by the center of mass motion. The consequences of this autoparametric excitation on the particles diffusion will be discussed according to the stiffness of the interaction and to the initial energy of the dimer, the relevant parameters which characterize this dynamics.

Mechanical Constraints during Vertical Climbing Reveals Limited Deviation from Theoretical Minima.

Center of mass (COM) mechanics, often used as an energetic proxy during locomotion, has primarily focused on level movement and hardly explores climbing scenarios. This study examines three-dimensional COM movements across five phylogenetically distinct species to test theoretical expectations of climbing costs, explore how interspecific variation (i.e., different limb numbers, adhesion mechanisms, body masses [0.008-84 kg], and limb postures) affects COM mechanics, and determine the impact of out-of-plane COM movements on climbing costs. A parallel experiment with rosy-faced lovebirds explores how inclination angle affects COM mechanical energy and how these empirical data align with theoretical expectations. Results indicate that, irrespective of anatomical differences, total mechanical costs of climbing are primarily driven by potential energy, outweighing contributions from kinetic energy. Despite species exhibiting significant out-of-plane kinematics, these movements have minimal impact on overall locomotor costs. Inclination angle changes have minimal effects, as potential energy accumulation dominates quickly as steepness increases, suggesting climbing occurs even on acutely angled substrates from a COM perspective. The study challenges prior assumptions about factors influencing climbing costs, such as body mass, speed, or posture, indicating a lack of evident anatomical or behavioral adaptations for climbing efficiency across species. The research sheds light on the universal challenges posed by the mechanical demands of scaling vertical substrates, offering valuable insights for functional morphologists studying climbing behaviors in extant and fossilized species.

Center of mass position does not drive energetic costs during climbing.

Climbing animals theoretically should optimize the energetic costs of vertical climbing while also maintaining stability. Many modifications to climbing behaviors have been proposed as methods of satisfying these criteria, focusing on controlling the center of mass (COM) during ascent. However, the link between COM movements and metabolic energy costs has yet to be evaluated empirically. In this study, we manipulated climbing conditions across three experimental setups to elicit changes in COM position, and measured the impact of these changes upon metabolic costs across a sample of 14 humans. Metabolic energy was assessed via open flow respirometry, while COM movements were tracked both automatically and manually. Our findings demonstrate that, despite inducing variation in COM position, the energetic costs of climbing remained consistent across all three setups. Differences in energetic costs were similarly not affected by body mass; however, velocity had a significant impact upon both cost of transport and cost of locomotion, but such a relationship disappeared when accounting for metabolic costs per stride. These findings suggest that climbing has inescapable metabolic demands driven by gaining height, and that attempts to mitigate such a cost, with perhaps the exception of increasing speed, have only minimal impacts. We also demonstrate that metabolic and mechanical energy costs are largely uncorrelated. Collectively, we argue that these data refute the idea that efficient locomotion is the primary aim during climbing. Instead, adaptations towards effective climbing should focus on stability and reducing the risk of falling, as opposed to enhancing the metabolic efficiency of locomotion.

Biomechanical Balance Measures During Timed Up and Go Test Improve Prediction of Prospective Falls in Older Adults

ObjectiveTo assess the feasibility of using biomechanical gait balance measures, the frontal and sagittal plane center of mass (COM)-Ankle angles, to prospectively predict recurrent falls in community-dwelling older adults. DesignA cohort study with a 1-year longitudinal follow-up. Logistic regression was used to test the ability of the COM-Ankle angles to predict prospective falls. SettingGeneral community. ParticipantsSixty older adults over the age of 70 years were recruited using a volunteer sample. InterventionsNot applicable. Main Outcome Measure(s)Biomechanical balance parameters: the sagittal and frontal plane COM-Ankle angles during the sit-to-walk and turning phases of the timed Up and Go test. The COM-Ankle angles are the inclination angles of the line formed by the COM and lateral ankle (malleolus) marker of the stance foot in the sagittal and frontal planes. We also included the following clinical balance tests in the analysis: Activity-Specific Balance Confidence, Berg Balance Scale, Fullerton Advanced Balance scale, and timed Up and Go test. Their abilities to predict falls served as a reference for the biomechanical balance parameters. ResultsWhen the biomechanical gait balance measures were added to all the confounders, the explained variance was increased from 25.3% to 50.2%. Older adults who have a smaller sagittal plane COM-Ankle angle at seat-off, a greater frontal plane COM range of motion during STW and a smaller frontal plane angle during turning were more likely to become recurrent fallers. Conclusion(s)Our results indicated that dynamic biomechanical balance parameters could provide valuable information about a participant's future fall risks beyond what can be explained by demographics, cognition, depression, strength, and past fall history. Among all biomechanical parameters investigated, frontal plane COM motion measures during STW and turning appear to be the most significant predictors for future falls.

Quantifying changes in individual-specific template-based representations of center-of-mass dynamics during walking with ankle exoskeletons using Hybrid-SINDy

Ankle exoskeletons alter whole-body walking mechanics, energetics, and stability by altering center-of-mass (CoM) motion. Controlling the dynamics governing CoM motion is, therefore, critical for maintaining efficient and stable gait. However, how CoM dynamics change with ankle exoskeletons is unknown, and how to optimally model individual-specific CoM dynamics, especially in individuals with neurological injuries, remains a challenge. Here, we evaluated individual-specific changes in CoM dynamics in unimpaired adults and one individual with post-stroke hemiparesis while walking in shoes-only and with zero-stiffness and high-stiffness passive ankle exoskeletons. To identify optimal sets of physically interpretable mechanisms describing CoM dynamics, termed template signatures, we leveraged hybrid sparse identification of nonlinear dynamics (Hybrid-SINDy), an equation-free data-driven method for inferring sparse hybrid dynamics from a library of candidate functional forms. In unimpaired adults, Hybrid-SINDy automatically identified spring-loaded inverted pendulum-like template signatures, which did not change with exoskeletons (p > 0.16), except for small changes in leg resting length (p < 0.001). Conversely, post-stroke paretic-leg rotary stiffness mechanisms increased by 37–50% with zero-stiffness exoskeletons. While unimpaired CoM dynamics appear robust to passive ankle exoskeletons, how neurological injuries alter exoskeleton impacts on CoM dynamics merits further investigation. Our findings support Hybrid-SINDy’s potential to discover mechanisms describing individual-specific CoM dynamics with assistive devices.

Center of mass motion and plantar pressure distribution during walking in overweight individuals

BackgroundGait imbalance has been reported in overweight individuals and could further impair their mobility and quality of life. As the feet are the most distal part of the body and sensitively interface with external surroundings, evaluating the plantar pressure distribution can provide critical insights into their roles in regulating gait balance control. Therefore, the purpose of this study was to evaluate the effect of body weight and different gait speeds on the plantar pressure distribution and whole-body center of mass (COM) motion during walking. MethodsEleven overweight individuals (OB) and 13 non-overweight individuals (NB) walked on a 10-meter walkway at three speed conditions (preferred, 80% and 120% of preferred speed). Gait balance was quantified by the mediolateral COM sway. Plantar pressure data were obtained using wireless pressure-sensing insoles that were inserted into a pair of running shoes. Analysis of variance models were used to examine the effect of body size, gait speeds, or their interactions on peak mediolateral COM and peak plantar pressure during walking. ResultsSignificant group effects of peak plantar pressure under the lateral forefoot (P = 0.03), lateral midfoot (P = 0.02), and medial heel (P = 0.02) were observed. However, the mediolateral COM motion and spatiotemporal gait parameters only revealed significant speed effects. SignificanceFindings from this study indicated that overweight individuals exhibited increased plantar pressure under the lateral aspect of the foot, particularly during the late stance phase of walking, in an effort to maintain a comparable mediolateral COM motion to that of non-overweight individuals. Such elevated pressure in overweight individuals may potentially increase the risk of musculoskeletal pathology in the long term. The identified patterns are noteworthy as they have practical implications for designing targeted interventions and improving the overall health of individuals with a high BMI.

Short term use of balance pads on postural sway and musculus multifidus cross sectional area in horses

Horses with musculoskeletal injuries have poor postural stability with larger center of mass (COM) movements and asymmetry/atrophy of the musculus (m.) multifidus. The objective of this study was to determine the effect of balance pad use in horses on both COM motions and m. multifidus cross sectional area (CSA). We hypothesized that use of balance pads would decrease COM motions and increase m. multifidus CSA and symmetry. Six healthy adult horses stood on balance pads for 5 min twice daily, 5 days per week for 4 weeks. At baseline and study completion, COM motion was determined using a portable media device, with horse standing on and off balance pads. Ultrasound of m. multifidus was performed bilaterally at select thoracic (T) and lumbar (L) vertebra: T12, T14, T16, T18, L2, and L5. Data were analyzed using mixed-model ANOVA. At baseline, there was significantly more medial-lateral and cranio-caudal motion when horses stood on pads compared to square stance (P < 0.006). When standing on pads, there was a significant decrease in motion at 4 weeks compared to baseline (P < 0.0004). There was a significant increase in m. multifidus CSA at T14 at 4 weeks compared to baseline (P = 0.021). When comparing baseline and 4 weeks CSA, there was no significant difference in m. multifidus asymmetry. These results suggest that implementation of balance pads for 4 weeks can improve postural stability in horses and increase CSA at T14. Longer duration of use may be needed for additional changes to paraspinal musculature.

Sit-to-walk strategy classification in healthy adults using hip and knee joint angles at gait initiation

Forward continuation, balance, and sit-to-stand-and-walk (STSW) are three common movement strategies during sit-to-walk (STW) executions. Literature identifies these strategies through biomechanical parameters using gold standard laboratory equipment, which is expensive, bulky, and requires significant post-processing. STW strategy becomes apparent at gait-initiation (GI) and the hip/knee are primary contributors in STW, therefore, this study proposes to use the hip/knee joint angles at GI as an alternate method of strategy classification. To achieve this, K-means clustering was implemented using three clusters corresponding to the three STW strategies; and two feature sets corresponding to the hip/knee angles (derived from motion capture data); from an open access online database (age: 21–80 years; n = 10). The results identified forward continuation with the lowest hip/knee extension, followed by balance and then STSW, at GI. Using this classification, strategy biomechanics were investigated by deriving the established biomechanical quantities from literature. The biomechanical parameters that significantly varied between strategies (P < 0.05) were time, horizontal centre of mass (COM) momentum, braking impulse, centre of pressure (COP) range and velocities, COP–COM separation, hip/knee torque and movement fluency. This alternate method of strategy classification forms a generalized framework for describing STW executions and is consistent with literature, thus validating the joint angle classification method.

Tactile Features of Human Finger Contact Motor Primitives.

The human hand interacts with the environment via physical contact, and tactile information is closely associated with finger movement patterns. Studying the relationship between motor primitives of the finger and the corresponding tactile feedback provides valuable insight into the nature of touch and informs the simulation of humanoid tactile. This research decomposed finger contact into three fundamental motor primitives: contact-on, stick-to-slip, and full slip, then examined the tactile features associated with each motor primitive, including the center of mass (COM) and the centroid of the contact pressure distribution matrix and the total contact area. The change in fingertip contact area during contact-on was in accordance with a first-order kinetic model. In the stick-to-slip, there was a generalized linear relationship between the fingertip skin stretch and the magnitude of the tangential force. Moreover, the skin stretch of the fingertip mirrored the direction of the motion. During the full slip, the COM's movement effectively represented the direction of the tangential force, with an error margin of no more than five degrees. Experiments showed that certain fingertip motions can be portrayed, transmitted, and replicated using tactile information. This research opens potential avenues for remote immersive physical communication in robotics and other related fields.

Hierarchical jumping optimization for hydraulic biped wheel-legged robots

This paper addresses the challenging task of implementing dynamic jumping motions for biped wheel-legged robots, a motion mode that significantly enhances their mobility and adaptability to complex terrains. However, due to the under-actuation and dynamic instability nature of this type of robots, executing high-dynamic motions remains difficult. To tackle this issue, this paper proposes a novel Hierarchical Jumping Optimization (HJO) framework specifically designed for hydraulic biped wheel-legged robots. The framework involves an upper layer center of mass (CoM) trajectory optimization based on the dynamic properties of a dual-mass spring loaded inverted pendulum (DM-SLIP) model, along with the CoM dynamic coupling relationship introduced by the under-actuated nature of two-wheel self-balancing robots. This leads to a reduction in the dimensionality of optimization variables, facilitating effective CoM trajectory generation. Subsequently, the CoM motions of the robot in the Cartesian space are mapped into the joint space in real-time by using the middle layer hierarchical operational space control (HOSC). By incorporating centroid angular momentum as a task objective, the stability of the robot in highly dynamic motions can be enhanced. Finally, the proposed HJO framework is experimentally validated through jumping experiments on an actual robot. The experimental results demonstrate that the framework optimally utilizes the robot’s under-actuated characteristics, significantly reducing computational costs associated with planning and tracking, while simultaneously preserving motion accuracy and robust.

Absent loading response knee flexion: The impact on gait kinetics and centre of mass motion in individuals with unilateral transfemoral amputation, and the effect of microprocessor controlled knee provision

BackgroundIndividuals with unilateral transfemoral amputation walk with increased levels of asymmetry, and this is associated with reduced gait efficiency, back pain and overuse of the intact limb. This study investigated the effect of walking with a unilateral absence of loading response knee flexion on the symmetry of anterior-posterior kinetics and centre of mass accelerations. MethodsA retrospective cohort study design was used, assessing three-dimensional gait data from individuals with unilateral transfemoral amputation (n = 56). The anterior-posterior gait variables analysed included; peak ground reaction forces, impulse, centre of mass acceleration, as well as rate of vertical ground reaction force increase in early stance. With respect to these variables, this study assessed the symmetry between intact and prosthetic limbs, compared intact limbs against a healthy unimpaired control group, and evaluated effect on symmetry of microprocessor controlled knee provision. FindingsSignificant between-limb asymmetries were found between intact and prosthetic limbs across all variables (p < 0.0001). Intact limbs showed excessive loading when compared with control group limbs after speed normalisation across all variables (p < 0.0001). No improvement in kinetic symmetry following microprocessor controlled knee provision was found. InterpretationThe gait asymmetries for individuals with transfemoral amputation identified in this study suggest that more should be done by developers to address the resultant overloading of the intact limb, as this is thought to have negative long-term effects. The provision of microprocessor controlled knees did not appear to improve the asymmetries faced by individuals with transfemoral amputation, and clinicians should be aware of this when managing patient expectations.

The Asymptotic Structure of the Centred Hyperbolic 2-Monopole Moduli Space

We construct an asymptotic metric on the moduli space of two centred hyperbolic monopoles by working in the point particle approximation, that is treating well-separated monopoles as point particles with an electric, magnetic and scalar charge and re-interpreting the dynamics of the 2-particle system as geodesic motion with respect to some metric. The corresponding analysis in the Euclidean case famously yields the negative mass Taub-NUT metric, which asymptotically approximates the L&lt;sup&gt;2&lt;/sup&gt; metric on the moduli space of two Euclidean monopoles, the Atiyah-Hitchin metric. An important difference with the Euclidean case is that, due to the absence of Galilean symmetry, in the hyperbolic case it is not possible to factor out the centre of mass motion. Nevertheless we show that we can consistently restrict to a 3-dimensional configuration space by considering antipodal configurations. In complete parallel with the Euclidean case, the metric that we obtain is then the hyperbolic analogue of negative mass Taub-NUT. We also show how the metric obtained is related to the asymptotic form of a hyperbolic analogue of the Atiyah-Hitchin metric constructed by Hitchin.

Control of center of mass motion during walking correlates with gait and balance in people with incomplete spinal cord injury.

There is evidence that ambulatory people with incomplete spinal cord injury (iSCI) have an impaired ability to control lateral motion of their whole-body center of mass (COM) during walking. This impairment is believed to contribute to functional deficits in gait and balance, however that relationship is unclear. Thus, this cross-sectional study examines the relationship between the ability to control lateral COM motion during walking and functional measures of gait and balance in people with iSCI. We assessed the ability to control lateral COM motion during walking and conducted clinical gait and balance outcome measures on 20 ambulatory adults with chronic iSCI (C1-T10 injury, American Spinal Injury Association Impairment Scale C or D). To assess their ability to control lateral COM motion, participants performed three treadmill walking trials. During each trial, real-time lateral COM position and a target lane were projected on the treadmill. Participants were instructed to keep their lateral COM position within the lane. If successful, an automated control algorithm progressively reduced the lane width, making the task more challenging. If unsuccessful, the lane width increased. The adaptive lane width was designed to challenge each participant's maximum capacity to control lateral COM motion during walking. To quantify control of lateral COM motion, we calculated lateral COM excursion during each gait cycle and then identified the minimum lateral COM excursion occurring during five consecutive gait cycles. Our clinical outcome measures were Berg Balance Scale (BBS), Timed Up and Go test (TUG), 10-Meter Walk Test (10MWT) and Functional Gait Assessment (FGA). We used a Spearman correlation analysis (ρ) to examine the relationship between minimum lateral COM excursion and clinical measures. Minimum lateral COM excursion had significant moderate correlations with BBS (ρ = -0.54, p = 0.014), TUG (ρ = 0.59, p = 0.007), FGA (ρ = -0.59, p = 0.007), 10MWT-preferred (ρ = -0.59, p = 0.006) and 10MWT-fast (ρ = -0.68, p = 0.001). Control of lateral COM motion during walking is associated with a wide range of clinical gait and balance measures in people with iSCI. This finding suggests the ability to control lateral COM motion during walking could be a contributing factor to gait and balance in people with iSCI.

Voluntary muscle coactivation in quiet standing elicits reciprocal rather than coactive agonist-antagonist control of reactive balance.

Muscle coactivation increases in challenging balance conditions as well as with advanced age and mobility impairments. Increased muscle coactivation can occur both in anticipation of (feedforward) and in reaction to (feedback) perturbations, however, the causal relationship between feedforward and feedback muscle coactivation remains elusive. Here, we hypothesized that feedforward muscle coactivation would increase both the body's initial mechanical resistance due to muscle intrinsic properties and the later feedback-mediated muscle coactivation in response to postural perturbations. Young adults voluntarily increased leg muscle coactivation using visual biofeedback before support-surface perturbations. In contrast to our hypothesis, feedforward muscle coactivation did not increase the body's initial intrinsic resistance to perturbations, nor did it increase feedback muscle coactivation. Rather, perturbations with feedforward muscle coactivation elicited a medium- to long-latency increase of feedback-mediated agonist activity but a decrease of feedback-mediated antagonist activity. This reciprocal rather than coactivation effect on ankle agonist and antagonist muscles enabled faster reactive ankle torque generation, reduced ankle dorsiflexion, and reduced center of mass (CoM) motion. We conclude that in young adults, voluntary feedforward muscle coactivation can be independently modulated with respect to feedback-mediated muscle coactivation. Furthermore, our findings suggest feedforward muscle coactivation may be useful for enabling quicker joint torque generation through reciprocal, rather than coactivated, agonist-antagonist feedback muscle activity. As such our results suggest that behavioral context is critical to whether muscle coactivation functions to increase agility versus stability.NEW & NOTEWORTHY Feedforward and feedback muscle coactivation are commonly observed in older and mobility impaired adults and are considered strategies to improve stability by increasing body stiffness prior to and in response to perturbations. In young adults, voluntary feedforward coactivation does not necessarily increase feedback coactivation in response to perturbations. Instead, feedforward coactivation enabled faster ankle torques through reciprocal agonist-antagonist muscle activity. As such, coactivation may promote either agility or stability depending on the behavioral context.

Cognitive and visual task effects on gaze behaviour and gait of younger and older adults

Cognitive dual tasks alter gait of younger and older adults and recent research has demonstrated that they also influence gaze behaviour and standing postural control. These findings suggest that age-related changes in cognitive and gaze function might increase fall risk in older adults. The purpose of this study was to determine the effect cognitive and visual dual tasks on the gait and gaze behaviour of younger and older adults. Ten older and ten younger adults walked for 3 min on a treadmill at preferred walking speed under three conditions, single task, cognitive and visual dual task conditions. Gait dynamics were measured using accelerometry and gaze behaviour was measured using wearable eye-trackers. Stride time variability and centre of mass (COM) motion complexity increased in dual-task conditions in older adults but had no difference for younger adults. Dual tasks had limited effect on gaze behaviour; however, visual input duration was greater, and visual input frequency and saccade frequency were lower in older than younger adults. The gaze adaptations in older adults may be the result of slower visual processing or represent a compensatory strategy to suppress postural movement. The increase in gait COM motion complexity in older adults suggests the dual tasks led to more automatic gait control resulting from both cognitive and visual tasks.