Increasing Visual Biofeedback Scale Changes Postural Control Complexity.

Visual biofeedback (vFB) during quiet stance has been shown to improve postural control. While this improvement has been quantified by a reduction in the center of pressure (COP) sway, the effect on COP complexity remains unexplored. As such, 20 young adults (12 females; aged 23.63 ± 3.17 years) were asked to remain in a static upright posture under different visual biofeedback magnitude (no feedback [NoFB], magnified by 1 [vFB1], magnified by 5 [vBF5] and magnified by 10 [vBF10]). In addition to confirming, through traditional COP variables (i.e. standard deviation, mean velocity, sway area), that vFB scaling improved postural control, results also suggested changes in COP complexity. Specifically, sample entropy and wavelet analysis showed that increasing the vFB scale from 1:1 to 1:5 and 1:10 led to a more irregular COP and a shift toward higher frequency. Together, and particularly from a complexity standpoint, these findings provided additional understandings of how vFB and vFB scaling improved postural control.

Investigating motor preparation in synchronous hand and foot movements under reactive vs. predictive control.

Synchronizing hand and foot movements under reactive versus predictive control results in differential timing structures between the responses. Under reactive control, where the movement is externally triggered, the electromyographic (EMG) responses are synchronized, resulting in the hand displacement preceding the foot. Under predictive control, where the movement is self-paced, the motor commands are organized such that the displacement onset occurs relatively synchronously, requiring the EMG onset of the foot to precede that of the hand. The current study used a startling acoustic stimulus (SAS), which can involuntarily trigger a prepared response, to investigate whether these results are due to differences in a pre-programmed timing structure of the responses. Participants performed synchronous movements of the right heel and right hand under both reactive and predictive modes of control. The reactive condition involved a simple reaction time (RT) task, whereas the predictive condition involved an anticipation-timing task. On selected trials, a SAS (114 dB) was presented 150 ms prior to the imperative stimulus. Results from the SAS trials revealed that while the differential timing structures between the responses was maintained under both reactive and predictive control, the EMG onset asynchrony under predictive control was significantly smaller following the SAS. These results suggest that the timing between the responses, which differs between the two control modes, is pre-programmed; however, under predictive control, the SAS may accelerate the internal timekeeper, resulting in a shortened between-limb delay.

Involvement of the prefrontal cortex in motor sequence learning: A functional near-infrared spectroscopy (fNIRS) study.

Our previous functional near-infrared spectroscopy (fNIRS) study on motor sequence learning (Polskaia et al., 2020) did not detect the same decrease in activity in the left dorsolateral prefrontal cortex (DLPFC) associated with movement automaticity, as reported by Wu et al. (2004). This was partly attributed to insufficient practice time to reach neural efficiency. Therefore, we sought to expand on our previous work to better understand the contribution of the prefrontal cortex (PFC) to motor sequence learning by examining learning across a longer period of time. Participants were randomly assigned to one of two groups: control or trained. fNIRS was acquired at three time points: pre-test, post-test, and retention. Participants performed four sequences (S1, S2, S3, and S4) of right-hand finger tapping. The trained group also underwent four days of practice of S1 and S2. No group differences in the left DLPFC and ventrolateral (VLPFC) were found between sessions for S1 and S2. Our findings revealed increased contribution from the right VLPFC in post-test for the trained group, which may reflect the active retrieval of explicit information from long-term memory. Our results suggest that despite additional practice time, explicit motor sequence learning requires the continued involvement of the PFC.

The impact of eye movement on postural control depends on the type of oculomotor behavior and the visual task.

Recent evidence suggests that performing a task inducing saccades will improve stability when compared to static fixation. However, they assume the linearity of postural control by only interpreting the area of displacement and/or the velocity of sway. Conversely, non-linear measures could bring a complementary understanding of postural control. The aim is to examine the effect of eye movements on different linear and non-linear measures of stability. 21 healthy adults (24.0 ± 3.3 years) were asked to stand on a force platform with their feet together and look at the monitor in front of them. Five conditions were tested: eyes closed, random looking, fixed static point, saccade, and visual pursuit (gaze shift angle of 15°). Five 60-second trials per condition were performed. An ANOVA with repeated measures was completed for each postural control variables in each direction: antero-posterior (AP) and medio-lateral (ML). The absence of vision had a negative impact on sway, as seen by an increase in sway area and variability as well as reduced contributions from the ultra-low band. The saccade led to greater stability than the random looking, as evidenced by a smaller area. However, the visual pursuit led to decreased stability compared to random looking, as evidenced by a larger area, as well as increased variability. Of note, the energy contained in the very-low band, which indicates the contribution of the vestibular system, was highest in the visual pursuit compared to the fixed static point. The findings support that the visual system is an important, but complex contributor to stability as different eye movements result in distinct postural responses with saccade and visual pursuit causing a decrease and an increase in sway, respectively.

Neural Correlates of Dual-Task Processing following Motor Sequence Learning: A Functional Near-Infrared Spectroscopy (fNIRS) Study.

The current study used functional near-infrared spectroscopy (fNIRS) to examine cerebral oxygenation changes in the prefrontal cortex (PFC) associated with dual-task processing before and after motor sequence learning. Participants performed self-initiated sequential finger movements that were 4 and 12 units in length with a visual letter-counting task. After practice, dual-task sequence-4 performance revealed decreased activity in the right dorsolateral PFC, medial PFC, and orbitofrontal cortex. However, dual-task sequence-12 performance revealed increased activity in the right ventrolateral PFC when compared to the left hemisphere. The findings suggest that dual-task interference was reduced following practice for dual-task sequence-4. The results also suggest that increased right hemisphere activation is needed to maintain performance when the primary sequential task (e.g., dual-task sequence-12) has a high level of difficulty.

Effect of Center of Mass Immobilization on Center of Pressure Displacement in Single and Dual-Task.

Recent research showed that artificially immobilizing the center of mass (COM) of participants in a standing position increased the center of pressure (COP) variability. This increase has been interpreted as an exploratory behavior. The objectives of this study are to investigate if this exploratory behavior is (1) reflected in other COP variables and (2) automatically controlled using a dual-task paradigm. Sixteen young adults were strapped to an apparatus which allowed them to sway freely ("unlocked") or to be immobilized ("locked") without their knowledge. Participants undertook the two phases (unlocked and locked) in a single-task and dual-task condition. Results suggested the presence of an exploratory behavior through different COP variables. Results also suggested this exploratory behavior to be automatic by nature.

Hemodynamic responses of quiet standing simultaneously performed with different cognitive loads in older adults.

Postural control decrements with advancing age have been largely identified in the literature. Dual-task paradigms have been utilized to increase older adults' stability in order to direct the attention towards the completion of a secondary task, leaving the automatic motor control processes to modulate posture unconstrained. To the extent that older adults allocate greater attentional resources to maintain an upright posture, the present study aimed at replicating St-Amant et al. (2020) protocol to investigate automatic postural control and prefrontal cortex activation in older adults when simultaneously performing quiet standing wtih different attention-demanding cognitive tasks. Nineteen healthy older adults (71.47 ± 6.01 years) were recruited and self-reported no hearing, musculoskeletal and neurological problems. Older adults were required to perform three different cognitive loads while seated (SC), quietly standing on a force platform (SM), and performing both tasks simultaneously (DT). Static center-of-pressure measurements and wavelet discrete transform did not reveal postural automaticity in dual-task conditions. Conversely, sample entropy values were significantly greater when performing n-back compared to all other tasks in the medial-lateral direction, and significantly greater than SM in the anterior-posterior direction. The relative concentration of oxygenated hemoglobin (HbO) of the left hemisphere was significantly greater than the right when performing n-back, and significantly greater within the left hemisphere when performing n-back compared to double-number sequence. Collectively, our results do not support the presence of automatic postural control in dual-task conditions for older adults. The present study highlights the importance of using numerous variables when investigating posture in order to capture its complexity.

Effect of Dual Task on Step Variability during Stepping in Place without Vision.

Stepping in place without vision is a spatial orientation task that is associated with unperceived foot displacements. This study was aimed at determining whether foot displacement variability is modified by a concurrent cognitive task. Fourteen young adults stepped for 50 steps with their vision blocked and performed a continuous mental counting task. 3-D Kinematic data from both feet (heel and big toe) was recorded. The variability of foot displacements was either unchanged or slightly lower in dual task, while the foot displacements were significantly shorter (p < 0.05) in dual task than without the cognitive task. The results suggest that the concurrent cognitive task might have allowed a better control of the repetitive lower limb movements.

An fNIRS Investigation of Discrete and Continuous Cognitive Demands During Dual-Task Walking in Young Adults.

Introduction: Dual-task studies have demonstrated that walking is attention-demanding for younger adults. However, numerous studies have attributed this to task type rather than the amount of required to accomplish the task. This study examined four tasks: two discrete (i.e., short intervals of attention) and two continuous (i.e., sustained attention) to determine whether greater attentional demands result in greater dual-task costs due to an overloaded processing capacity. Methods: Nineteen young adults (21.5 ± 3.6 years, 13 females) completed simple reaction time (SRT) and go/no-go (GNG) discrete cognitive tasks and n-back (NBK) and double number sequence (DNS) continuous cognitive tasks with or without self-paced walking. Prefrontal cerebral hemodynamics were measured using functional near-infrared spectroscopy (fNIRS) and performance was measured using response time, accuracy, and gait speed. Results: Repeated measures ANOVAs revealed decreased accuracy with increasing cognitive demands (p = 0.001) and increased dual-task accuracy costs (p < 0.001). Response times were faster during the single compared to dual-tasks during the SRT (p = 0.005) and NBK (p = 0.004). DNS gait speed was also slower in the dual compared to single task (p < 0.001). Neural findings revealed marginally significant interactions between dual-task walking and walking alone in the DNS (p = 0.06) and dual -task walking compared to the NBK cognitive task alone (p = 0.05). Conclusion: Neural findings suggest a trend towards increased PFC activation during continuous tasks. Cognitive and motor measures revealed worse performance during the discrete compared to continuous tasks. Future studies should consider examining different attentional demands of motor tasks.

The Fukuda Stepping Test Is Influenced by a Concurrent Cognitive Task and Step Height in Healthy Young Adults: A Descriptive Study.

Purpose: The Fukuda stepping test assesses spatial orientation in people with vestibular disorders. To standardize the test, it is important to know which factors influence the outcome. This study investigated the impact of two factors, a concurrent cognitive task and step height, on the Fukuda stepping test in healthy individuals. Method: A total of 16 young adults participated and completed 20 trials of the 50-step Fukuda stepping test in four conditions: comfortable and high step height and with and without a cognitive task. Body kinematics were collected using the three-dimensional motion analysis Vicon system. The cognitive task was to listen to a sequence of three-digit numbers and identify the total number of times that one pre-determined digit was presented. Results: All participants slowly turned and drifted forward during the test. The concurrent cognitive task yielded significantly shorter forward displacement and lateral deviation and lower stepping height, and high stepping produced significantly greater body rotation and lateral deviation. Conclusions: Performance on the Fukuda stepping test in healthy young individuals is influenced by a concurrent cognitive task and by step height. Clinicians and researchers must instruct people to use a comfortable step height during the test, and they must be aware that a concurrent cognitive task may improve test performance, at least among young adults.

Objectif : le test de Fukuda évalue l'orientation spatiale chez les personnes ayant une pathologie vestibulaire. Pour le standardiser, il est important de connaître les facteurs qui influent sur le résultat. La présente étude portait sur l'effet de deux facteurs, une tâche cognitive conjuguée à la hauteur des pas sur le test de Fukuda chez des personnes en bonne santé. Méthodolgie : au total, 16 jeunes adultes ont terminé les 20 essais du test de Fukuda de 50 pas dans quatre conditions : pas d'une hauteur confortable ou élevée, accompagnés ou non d'une tâche cognitive. Les chercheurs ont enregistré la cinématique à l'aide du système d'analyse du mouvement tridimensionnel de Vicon. La tâche cognitive consistait à écouter une série de nombres à trois chiffres puis à établir le nombre de répétitions d'un chiffre prédéterminé. Résultats : tous les participants ont tourné et avancé lentement pendant le test. L'ajout de la tâche cognitive était lié à un déplacement et à une déviation latérale moins marqués et à un pas moins élevé. Une hauteur de pas élevée était associée à une rotation du corps et une déviation latérale plus importantes. Conclusion : la performance au test de Fukuda est influencée par une tâche cognitive simultanée et par la hauteur du pas chez de jeunes adultes en bonne santé. Les cliniciens et les chercheurs doivent expliquer l'importance d'adopter une hauteur de pas confortable pendant le test et doivent savoir qu'une tâche cognitive simultanée peut améliorer la performance, du moins chez les jeunes adultes.

Hemodynamic and behavioral changes in older adults during cognitively demanding dual tasks.

INTRODUCTION: Executive functions play a fundamental role in walking by integrating information from cognitive-motor pathways. Subtle changes in brain and behavior may help identify older adults who are more susceptible to executive function deficits with advancing age due to prefrontal cortex deterioration. This study aims to examine how older adults mitigate executive demands while walking during cognitively demanding tasks. METHODS: Twenty healthy older adults (M = 71.8 years, SD = 6.4) performed simple reaction time (SRT), go/no-go (GNG), n-back (NBK), and double number sequence (DNS) cognitive tasks of increasing difficulty while walking (i.e., dual task). Functional near infra-red spectroscopy (fNIRS) was used to measure the hemodynamic response (i.e., oxy- [HbO2] and deoxyhemoglobin [HbR]) changes in the prefrontal cortex (PFC) during dual and single tasks (i.e., walking alone). In addition, performance was measured using gait speed (m/s), response time (s), and accuracy (% correct). RESULTS: Using repeated measures ANOVAs, neural findings demonstrated a main effect of task such that ∆HbO2 (p = .047) and ∆HbR (p = .040) decreased between single and dual tasks. An interaction between task and cognitive difficulty (p = .014) revealed that gait speed decreased in the DNS between single and dual tasks. A main effect of task in response time indicated that the SRT response time was faster than all other difficulty levels (p < .001). Accuracy performance declined between single and dual tasks (p = .028) and across difficulty levels (p < .001) but was not significantly different between the NBK and DNS. CONCLUSION: Findings suggest that a healthy older adult sample might mitigate executive demands using an automatic locomotor control strategy such that shifting conscious attention away from walking during the dual tasks resulted in decreased ∆HbO2 and ∆HbR. However, decreased prefrontal activation was inefficient at maintaining response time and accuracy performance and may be differently affected by increasing cognitive demands.

The effects of Kinesiology Tape on static postural control in individuals with functional ankle instability.

OBJECTIVE: To investigate if applying Kinesiology Tape (KT) on the unstable ankle may improve static postural control in individuals with Functional ankle instability. DESIGN: A repeated measured study. Participants performed a series of static quiet bipedal and unipedal stances on a force platform. Measurements were taken at three different times: baseline or no tape, immediately and 24 h after the taping application with the tape remaining on the ankle. SETTING: A university's psychomotor laboratory. PARTICIPANTS: Twenty young adults with Functional ankle instability aged from 18 to 30 years old. OUTCOME MEASURES: Postural control was assessed by four measures derived from the centre of pressure data: Area of 95% Confidence ellipse, standard deviation of displacements, mean velocity and mean power frequency. The analysis of variance (ANOVA) was performed to determine any significant improvement in postural control over time due to KT. RESULTS: Only minor changes in mean velocity and MPF in unipedal stances were observed immediately after KT application. However, the overall results indicated statistically insignificant improvements in postural control neither immediately after KT application nor after 24 h. CONCLUSION: Results suggest that the use of KT did not affect bipedal and unipedal stances of individuals with functional ankle instability.

Number of Trials Needed to Assess Postural Control of Young Adults in Single and Dual-Task.

Despite its popularity, there is a lack of standardization when assessing postural control. This study aimed to suggest how many trials should be used when assessing young adults' postural control with a specific single-task and dual-task quiet stance protocol. Two groups of 15 participants performed 20 trials of 60 s (feet together, eyes open) with or without a dual-task. The number of trials needed to obtain two consecutive intra-class correlation coefficients (ICC(2,k)) ≥0.900 was then assessed for seven center of pressure variables. Although inconsistency was observed between variables and tasks, five trials seems to be a good compromise.

Unveilling the cerebral and sensory contributions to automatic postural control during dual-task standing.

OBJECTIVES: The postural control dual-task literature has demonstrated greater postural stability during dual-task in comparison to single task (i.e., standing balance alone through the examination of multiple kinetic and kinematic measures. This improve stability is thought to reflect an automatic mode of postural control during dual-task. Recently, sample entropy (SampEn) and wavelet discrete transform have supported the claim of automaticity, as higher SampEn values and a shift toward increased contributions from automatic sensory systems have been demonstrated in dual-task settings. In order to understand the cortical component of postural control, functional near-infrared spectroscopy has been used to measure cortical activation during postural control conditions. However, the neural correlates of automatic postural behaviour have yet to be fully investigated. Therefore, the purpose of this study is to confirm the presence of automatic postural control through static and dynamic balance measures, and to investigate the prefrontal cortex activation when concurrently performing quiet standing and the auditory cognitive tasks of varied difficulty. METHOD: Eighteen healthy young adults (21.4 ± 3.96 yo), 12 females and 6 males, with no balance deficits were recruited. Participants were instructed to either quietly stand on a force platform (SM), perform three cognitive tasks while seated (SC) or perform both aforementioned tasks concurrently (DT). RESULTS: Results supported automatic postural control with lower area and standard deviation of center-of-pressure in DT conditions compared to SM. As for SampEn and the wavelet analysis, DT conditions demonstrated greater values than SM, and a shift from vision to a cerebellar contribution. For the most difficult cognitive task, the DNS task, a trend toward significantly lower right hemisphere prefrontal cortex activation compared to left hemisphere activation in DT was found, potentially representing a decrease in cognitive control, and the presence of automaticity. CONCLUSION: These findings suggest that the simultaneous performance of a difficult cognitive task and posture yields automatic postural behaviour, and provides insight into the neural correlates of automaticity.

A functional near-infrared spectroscopy (fNIRS) examination of how self-initiated sequential movements become automatic.

The neural mechanisms underlying movement automaticity have been investigated using PET and fMRI and more recently functional near-infrared spectroscopy (fNIRS). As fNIRS is an emerging technique, the objective of the present study was to replicate the functional magnetic resonance imaging-related motor sequence findings as reported by Wu et al. (J Neurophysiol 91:1690-1698, https://doi.org/10.1152/jn.01052.2003, 2004) using fNIRS. Seventeen right-handed participants practiced self-initiated sequential finger movements of two lengths (4 and 12) until a level of automaticity was achieved. Automaticity was evaluated by performing a visual-letter-counting task concurrently with the sequential finger movements. Our data were unable to replicate the pre-to-post-practice decrease in cortical activity in the left dorsolateral prefrontal cortex for both motor sequence tasks. The findings did reveal increased contribution from the right hemisphere following learning. The observed lateralization is suggestive of explicit learning and the involvement of working memory in motor sequence production.

Effect of Bilateral and Unilateral Plantarflexor Muscle Fatigue on Blind Navigation Precision and Gait Parameters.

The objective was to evaluate the impact of bilateral and unilateral fatigue of the plantarflexor muscles on blind navigation. Thirty-eight young adults walked 8-m without vision before fatigue (pre-fatigue), then fatigued either one or both of their plantarflexor muscles by performing isometric contractions. After each fatigue, two blind navigation trials were performed (post-fatigue trials 1 and 2). Results revealed no effect of bilateral muscle fatigue on navigation precision and gait parameters. Unilateral muscle fatigue led to longer linear distance travelled during post-fatigue trial 2 compared to pre-fatigue and to a change in angular deviation between pre- and post-fatigue. In general, results suggest that participants were able to make adaptive changes to counter muscle fatigue during blind navigation.

Absence of Ankle Stiffening While Standing in Focus and Cognitive Task Conditions in Older Adults.

Research suggests that an external focus or cognitive task may improve postural control. Removing attention from movement production may promote automaticity, or the tasks may promote ankle stiffening. To investigate these two theories, twenty older adults stood while performing baseline standing, internal focus, external focus, and two cognitive tasks. Changes in postural control occurred in external focus and cognitive task conditions compared to baseline and internal focus, while no change occurred in cocontraction indices. This suggests that an external focus and cognitive task can improve postural control in older adults. Since no change occurred in cocontraction indices across conditions, this suggests that stiffening cannot explain these changes. Instead, changes could be due to automaticity of sway.

Automaticity of Postural Control while Dual-tasking Revealed in Young and Older Adults.

Background: Postural control improvements in external focus and cognitive task conditions are thought to occur because directing attention away from postural control allows greater automaticity. We aimed to support this theory by using three dynamic measures of postural control that may reveal changes in the structure or composition of sway: the discrete wavelet transform, sample entropy, and rambling trembling analyses.Methods: We analyzed the center of pressure data from twenty-two healthy young adults (20.8 ± 2.82 years) and twenty healthy older adults (69.02 ± 3.47 years). Participants stood with feet together in five conditions: baseline standing, internal focus, external focus, easy cognitive task, and difficult cognitive task. Analyses of variance were used to examine the effect of condition and age on the three dynamic measures.Results: The wavelet transform revealed a shift toward greater contributions from higher frequency bands in cognitive task conditions, suggesting greater automaticity. Sample entropy was higher in cognitive task conditions, suggesting more complex sway and automatic control. The external focus and difficult cognitive tasks increased trembling in young adults, suggesting increased contributions from spinal reflex components.Conclusion: Results support the theory that stability improvements in cognitive task conditions were due to automaticity in young and older adults. They also suggest that tasks that are more difficult are better at promoting automaticity than tasks requiring less cognitive involvement.

The influence of carrying an anterior load on attention demand and obstacle clearance before, during, and after obstacle crossing.

Carrying an anterior load during obstacle negotiation increases attention demand, which may differ at various crossing stages. Less is known on the impact of lower visual field obstruction and the weight of the anterior load on obstacle negotiation and attention demand. The objectives of this study were to: (1) determine if carrying a weighted anterior load, lower visual field occlusion, or both, modify obstacle clearance and/or reaction time (RT); and (2) examine whether RT is modulated across obstacle crossing phases as measured by a probe RT protocol. Sixteen young adults crossed an obstacle while carrying no load, a clear 5 kg load, and an opaque 5 kg load, while performing a simple RT task. Auditory stimuli were presented at five locations: (1) two steps before the obstacle; (2) one step before the obstacle; (3) as the leading limb crossed the obstacle; (4) as the lead limb touched down after the obstacle; and (5) as the trail limb crossed the obstacle. The toe clearance height of the leading limb was greatest for the weighted opaque box load type followed by the weighted clear box type compared to the no box load type. Carrying an anterior load during obstacle crossing did not influence RT. RTs were longer at the pre-crossing and beginning of the crossing phases compared to after-crossing phases. Results suggest that carrying a weighted anterior load and lower visual field occlusion increase the risk for tripping. Attention demands differ across obstacle crossing phases during dual-tasking and should be considered in fall-risk assessments.

Older and young adults adopt different postural strategies during quiet bipedal stance after ankle plantarflexor fatigue.

BACKGROUND: The individual effects of fatigue and age on postural control have been extensively studied. However, there is a lack of knowledge about the combined effects of fatigue and age on postural control and posture. RESEARCH QUESTION: The purpose of this study was to investigate the influence of age and muscle fatigue on bipedal stance postural control and posture. METHODS: Twenty-three participants, fourteen young adults and nine older adults performed 30-s bipedal postural trials before and after fatigue. The fatigue protocol consisted in repeating heel raises until exhaustion. Back, hip, knee and ankle joint angles and centre of pressure (COP) displacements were recorded with a 3D motion capture system and a force plate. RESULTS: Fatigue induced similar increases in sway area and COP velocity for both populations. This was associated with a forward displacement of the COP position for both populations but with different postural adaptations. Similar increased flexions (+1.7 to 1.8°) for all joint angles (ankle, knee, hip and back) joints were observed after fatigue for older adults, whereas young adults mainly showed increased flexion for the ankle joint (+3.1°) and decreased flexion at the hip (-1.2°). SIGNIFICANCE: It can be argued that older adults act on all joints because they are too weak to sufficiently modify any joint individually, contrary to young adults. These posture adaptations may be strategies to cope with fatigue by limiting the reliance on fatigued plantarflexors, increasing ankle stiffness, increasing the sensitivity of muscle spindles, increasing the number of degrees of freedom of the task.