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.

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.

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.

Cognitive task modality influences postural control during quiet standing in healthy older adults.

BACKGROUND: The interstimulus interval of a cognitive task was found to have a limited effect on postural control in young adults, while visual cognitive tasks were found to improve stability compared to auditory tasks. It is of interest to investigate whether postural control in healthy older adults is sensitive to these types of cognitive task manipulations. AIMS: The objectives of the present experiment were to evaluate the impact of interstimulus interval and modality of a continuous cognitive task on postural control in healthy older adults. METHODS: Fifteen healthy older adults (70 ± 3.2 years, 3 male) were asked to stand with feet together on a force platform while performing auditory and visual cognitive tasks performed with interstimulus intervals of 2 and 5 s. RESULTS: Visual tasks led to reductions in sway area and sway variability in the anterior-posterior direction compared to auditory tasks (ps ≤ 0.05). The interstimulus interval did not lead to a change in sway, except for a small change in the medial-lateral direction for the 2-s interval compared to the 5-s interval (p = 0.05). DISCUSSION AND CONCLUSIONS: Results suggest that the interstimulus interval had a very limited effect on postural sway. The modality of the cognitive task had a greater effect on postural sway, as visual cognitive tasks yielded smaller sway area and anterior-posterior sway variability than auditory conditions. Visual stimuli may have acted as an anchor, yielding reduced sway.

Age-related differences in Fukuda stepping and Babinski-Weil tests, within-day variability and test-retest reliability.

BACKGROUND: The Fukuda stepping and Babinski-Weil tests are associated with unperceived body rotation and linear displacements in young adults, but performance in older adults on these two tests has yet to be determined. AIM: The main purpose was to compare the performance and reliability of the Fukuda stepping and Babinski-Weil tests in young and older adults. METHODS: Fifty older and 50 young adults executed three trials of each test on day 1 (test) and day 2 (retest). Lateral and longitudinal displacements and body rotation relative to the starting position were measured. Means and standard deviations (SD) were compared between the two groups with Mann-Whitney tests. Test-retest reliability was assessed with intra-class correlation coefficients (ICC). Foot preference was determined from the score on the Waterloo Footedness Questionnaire and correlated with test scores. RESULTS: Lateral and longitudinal displacements were significantly larger in older than young participants on the Fukuda stepping test (p < .01) and significantly smaller on the Babinski-Weil test (p < .001). Older participants displayed a significantly smaller SD on the three Babinski-Weil test trials (p < .001). Displacement and rotation measures ICC ranged between 0.25 and 0.77 in older and between 0.58 and 0.80 in young participants. Foot preference correlated with rotation on the Fukuda stepping test in young (p < .05), but not in older participants (p > .05). DISCUSSION AND CONCLUSION: Linear displacements, but not body rotation, were different between older and young adults. There was no clear age-related differences in test-retest reliability, but the moderate reliability indicates that performance can vary from day to day in both age groups.

Balance and mobility training with or without concurrent cognitive training improves the timed up and go (TUG), TUG cognitive, and TUG manual in healthy older adults: an exploratory study.

BACKGROUND AND AIMS: The purpose was to explore the impact of balance and mobility training (BMT), balance and mobility plus cognitive training (BMT + C) and no training on the timed up and go (TUG), TUG cognitive (TUGcog), and TUG manual (TUGman) in older adults. A preliminary experiment examined the stability of these TUG measures over a 5-week period in older adults. METHODS: Fifteen participants in the BMT group (70.2 ± 3.2 years) and 14 participants in the BMT + C group (68.7 ± 5.5 years) trained one-on-one, 3×/week for 12 weeks on a balance obstacle course. The BMT group and the BMT + C group completed two or three tasks simultaneously, respectively. Fifteen participants in the control group received no training (66.7 ± 4.2 years). The TUG, TUGcog, and TUGman were measured in seconds at baseline, after the 12-week training, and after the 12-week follow-up. During the preliminary experiment, ten participants (67.0 ± 6.9 years) completed the three TUG measures 1/week for 5 weeks. RESULTS: Both the BMT and BMT + C groups, but not the control group, exhibited significantly faster TUG, TUGcog, and TUGman after the intervention and maintained these improvements at the 12-week follow-up. No differences between the BMT and BMT + C groups emerged. The preliminary experiment showed that the three TUG measures were stable across five testing sessions. DISCUSSION AND CONCLUSION: Both training groups improved functional mobility after the interventions and sustained these improvements over 12 weeks. This is likely not a function of repeating the TUG, TUGcog, and TUGman tests since no repeated exposure effect was shown.

Continuous Cognitive Task Promotes Greater Postural Stability than an Internal or External Focus of Attention in Older Adults.

Background/Study Context: Recent evidence suggests that removing attention from postural control using either an external focus or a cognitive task will improve stability in healthy young adults. Due to increases in attentional requirements of upright stance in older adults, it is unclear if similar benefits would be observed in this population. The aim of the present study was to examine the effect of attentional focus and of a continuous cognitive task on postural control in older adults. METHODS: Sixteen healthy older adults (71.9 ± 4.32 years) were asked to stand quietly on a force platform with feet together in three different conditions: internal focus (minimizing movement of the hips), external focus (minimizing movement of markers placed on the hips), and cognitive task (silently counting the occurrence of a single digit in a 3-digit number sequence). A one-way analysis of variance with repeated measures on condition was performed for each postural control measure. RESULTS: Hypotheses were partially supported because the cognitive task led to greater stability than both focus conditions, as evidenced by a smaller sway area (p < .01, ηp2 = .41), reduced sway variability (anterior-posterior: p = .001, ηp2 = .37; medial-lateral: p < .0001, ηp2 = .49), and higher mean power frequency in the anterior-posterior direction (p = .01, ηp2 = .78). However, no difference was observed between internal and external focus conditions. CONCLUSIONS: A continuous, attention-demanding cognitive task significantly improved stability in older adults compared with an internal or external focus of attention. This suggests that older adults were able to effectively allocate their attention away from postural control, allowing a more automatic type of control to operate. Future studies should investigate a variety of cognitive tasks to determine the degree of postural improvement that can be observed in older adults.

Examining the stability of dual-task posture and reaction time measures in older adults over five sessions: a pilot study.

BACKGROUND AND AIM: Improved performance may be inherent due to repeated exposure to a testing protocol. However, limited research has examined this phenomenon in postural control. The aim was to determine the influence of repeated administration of a dual-task testing protocol once per week for 5 weeks on postural sway and reaction time. METHODS: Ten healthy older adults (67.0 ± 6.9 years) stood on a force plate for 30 s in feet apart and semi-tandem positions while completing simple reaction time (SRT) and choice reaction time (CRT) tasks. They were instructed to stand as still as possible while verbally responding as fast as possible to the stimuli. RESULTS: No significant differences in postural sway were shown over time (p > 0.05). A plateau in average CRT emerged as the time effect revealed longer CRT during session 1 compared to sessions 3-5 (p < 0.05). Furthermore, the time effect for within-subject variability of CRT uncovered no plateaus as it was less variable in session 5 than sessions 1-4 (p < 0.05). DISCUSSION: The lack of a plateau in variability of CRT may have emerged as older adults may require longer to reach optimal performance potential in a dual-task context. CONCLUSION: Postural sway and SRT were stable over the 5 testing sessions, but variability of CRT continued to improve over time. These findings form a basis for future studies to examine performance-related improvements due to repeated exposure to a testing protocol in a dual-task setting.

Magnifying the Scale of Visual Biofeedback Improves Posture.

Biofeedback has been shown to minimize body sway during quiet standing. However, limited research has reported the optimal sensitivity parameters of visual biofeedback related to the center of pressure (COP) sway. Accordingly, 19 young adults (6 males; 13 females; aged 21.3 ± 2.5) stood with feet together and performed three visual biofeedback intensities [unmodified biofeedback (UMBF), BF magnified by 5 (BF5), BF magnified by 10 (BF10)], along with control trials with no biofeedback (NBF). The participants were instructed to stand as still as possible while minimizing the movements of the visual target. The findings revealed that UMBF produced significantly greater COP displacement in both the anterior-posterior (AP) and medial-lateral directions, as well as greater standard deviation of the COP in the AP direction (p < 0.05). Additionally, NBF showed significantly greater 95 % area ellipse than the UMBF, BF5, and BF10 intensities (p < 0.001). Therefore, the most sensitive COP scales generated the least amount of postural sway. However, there were no significant differences on any of the COP measures between BF5 and BF10. This research provides insight with respect to the proper scale on which biofeedback should be given in order to improve postural control (i.e., BF5 or BF10).

Prioritizing attention on a reaction time task improves postural control and reaction time.

Flexible and appropriate allocation of attention resources is important during dual-tasking to achieve task goals while maintaining postural safety. This pilot study aimed to examine the influence of explicit prioritization of attention on the dual-task paradigm by employing two levels of difficulty for the postural tasks and reaction time (RT) tasks in healthy young adults. The task entailed standing on a force platform on two feet or on one foot, attending to posture or RT, and completing a simple or choice RT task. Participants verbally responded "top" as soon as the light cue illuminated. In general, attending to RT produced faster RTs (F(1,19) = 30.9, p < 0.001) and improved center of pressure (COP) Displacement (F(1,19) = 5.1, p < 0.05) and 95% Area Ellipse (F(1,19) = 7.1, p < 0.05). These findings suggest that prioritizing attention away from posture may be beneficial for postural performance when completing a second task.

Effect of ankle weight on blind navigation.

This study tested the hypotheses that loading the ankle with a 2.3 kg weight would modify deviation (unilateral loading) and distance (unilateral and bilateral loading) during three blind navigation tasks. Ankle loading increased the distance traveled while navigating toward a previously seen target at an 8 m distance and reduced the undetected fore-aft displacement while stepping in place for 100 steps. Unilateral ankle loading had no effect on deviation during these tasks, nor in walking back and forth on an imaginary straight line. The results suggest that somatosensory cues associated with ankle loading and the increased effort to walk and step interacted with motor and cognitive functions involved in blind navigation and influenced the control of anterior-posterior body displacement.

The influence of age and surface compliance on changes in postural control and attention due to ankle neuromuscular fatigue.

The reduction in the quality and integration of sensory information with aging could increase the alterations in postural control associated with muscle fatigue observed in younger adults. This study aimed to compare changes in postural control and attentional demands due to ankle muscle fatigue, with intact and reduced proprioceptive information at the ankle, between young and older adults. Eleven young (24 ± 4 years) and 13 older (65 ± 4 years) men stood quietly on a force platform (blindfolded) under four experimental conditions (combinations of firm (FS)/compliant (CS) surfaces and single/dual tasks), before and immediately after a fatiguing exercise. The fatiguing exercise, performed on a dynamometer, consisted of maintaining an isometric contraction of the plantarflexors at 50 % of maximum until exhaustion. Both COP sway area and COP sway velocity were greater on the CS compared to FS and increased with fatigue for both groups in all conditions. COP sway area showed a greater increase with fatigue in older adults when standing on the CS. Reaction time (secondary task) increased significantly after fatigue, but only for older adults when standing on the CS. The effects of fatigue on postural control are more important when proprioceptive information at the ankle is altered. In particular, older adults had more difficulty and may have needed more attention to stand quietly, compared with young adults.

Attentional focus influences postural control and reaction time performances only during challenging dual-task conditions in healthy young adults.

The dual-task paradigm has previously been used to investigate the attentional demands associated with postural control. Previous studies have identified both the focus of attention and the difficulty of a postural task as potential factors influencing dual-task performance. The aim of this study was to examine how the instructed focus of attention influences dual-task performance during quiet standing tasks of various levels of difficulty. Thirteen young adults participated in two testing sessions consisting of standing as still as possible on a force platform in different postural conditions, while simultaneously performing a simple reaction time (RT) task. Postural task difficulty was manipulated by various combinations of three bases of support (feet together, tandem and single leg) and two visual conditions (eyes opened and closed). Participants were instructed to focus on either their balance or their RT performance, depending on the testing session. When comparing postural control with respect to session focus, anterior-posterior sway velocity decreased with the addition of the simple RT task when the focus was on balance, but only during the more difficult dual-task conditions. In contrast, sway area and medial-lateral sway velocity did not change between the two instructed focus sessions. Participants responded faster in all dual-task conditions when focusing on RT performance than on balance. The modified attention allocation index indicated that participants' ability to modulate their allocation of attentional resources to respond positively to instruction was more pronounced in the most challenging postural condition. The present findings could have important implications for the interpretation of dual-task performance in both clinical and research settings.

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.

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.

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.

Effects of fatiguing isometric and isokinetic ankle exercises on postural control while standing on firm and compliant surfaces.

BACKGROUND: Fatiguing exercises used across studies to induce alterations in postural control are diverse and may explain the different findings reported. This study aimed to compare the effects of two types of fatiguing plantarflexion exercises on postural control on a firm and a compliant surface. Ten healthy young men (29 ± 4 years) were asked to stand as steadily as possible for 30 s, blindfolded with feet together, on a firm and a compliant surface before and immediately after an isometric and an isokinetic fatiguing exercise. RESULTS: Maximal force reduction due to fatigue was found significant but similar between exercises. No significant difference was found between the fatiguing exercises on all Center of Pressure (CoP) parameters. Both fatiguing exercises induced increases in CoP excursion area, CoP variability and CoP velocity in both planes (antero-posterior, mediolateral) on the compliant surface. On the firm surface, both fatiguing exercises only induced increases in CoP variability and CoP velocity in the fatigued plane (antero-posterior). CONCLUSIONS: Isometric and isokinetic fatiguing exercises, when producing a similar level of force reduction, induce similar decreases in postural control. The effects of fatigue on postural control in healthy young men are more pronounced when standing on a compliant surface, i.e. when proprioceptive information at the ankle is altered.