Eye Movements Affect Postural Control in Young and Older Females

People with Long Covid and ME/CFS Exhibit Similarly Impaired Dexterity and Bimanual Coordination: A Case-Case-Control Study

PURPOSE

Dexterity and bimanual coordination had not previously been compared between people with long COVID and people with Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS). Therefore, this study determined dexterity and bimanual coordination in people with long COVID (∼16 month illness duration; n=21) and ME/CFS (∼16 year illness duration; n=20), versus age-matched healthy controls (n=20).

METHODS

Dexterity, and bimanual coordination was determined using the Purdue pegboard test.

RESULTS

The main findings of the present investigation were that people with ME/CFS and people with long COVID were generally comparable for Purdue pegboard tests (p>0.556 and d<0.36 for pairwise comparisons). It is worth noting however, that both these patient groups performed poorer in the Perdue pegboard test than healthy controls (p<0.169 and d>0.40 for pairwise comparisons).

CONCLUSIONS

These data suggest that both people with long COVID and people with ME/CFS have similarly impaired dexterity, and bimanual coordination. Therefore, there is an urgent need for interventions to target dexterity and bimanual coordination in people with ME/CFS, and given the current pandemic, people with long COVID.

Neck muscle activation in response to eye movement depends on sitting posture and is modified in whiplash associated disorders: Cross-sectional study

BACKGROUND

Activity of specific neck muscles is modulated by eye movement. This activity modulation is exaggerated in people with whiplash associated disorders (WAD), but it is unknown whether it is impacted by sitting posture.

OBJECTIVE

This study investigated; (i) whether activity of cervical muscles differs with spinal posture; (ii) whether the effect of eye gaze direction (horizontal/vertical) on neck muscle activity differs between postures, and (iii) whether these effects differ between individuals with and without WAD.

METHODS

In three seated postures (normal relaxed, head forward, sit tall) electromyography (EMG) was recorded right obliquus capitis inferior (OI), multifidus (MF), splenius capitis (SP) and left sternocleidomastoid (SCM) with fine-wire and surface electrodes in ten healthy controls and nine with WAD. Electro-oculography recorded eye movements.

RESULTS

In controls, EMG was less for extensor muscles in Sit Tall than Head Forward, but higher in SCM. Only SC EMG modulated with eye movement. When WAD participants adopted similar sitting postures several responses were different; compared to Normal Relaxed posture OI EMG was less in Head Forward; MF EMG was less in Sit Tall; and SC was less in Head Forward and Sit Tall. Neck muscles in WAD were generally more sensitive to eye movement, except SC which did not modulate.

CONCLUSIONS

These finding support the hypothesis that neck muscle activity is influenced by spinal posture and eye movement. In WAD, this relationship is distorted and the response to eye movement is increased.

SIGNIFICANCE

These observations have potential implications for clinical management of individuals with WAD.

People with Long Covid and ME/CFS Exhibit Similarly Impaired Balance and Physical Capacity: A Case-Case-Control Study

PURPOSE

Postural sway and physical capacity had not previously been compared between people with long COVID and people with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Therefore, this study determined postural sway and physical capacity in people with long COVID (∼16-month illness duration; n = 21) and ME/CFS (∼16-year illness duration; n = 20), vs age-matched healthy controls (n = 20).

METHODS

Postural sway was during a 30-s static stand test. Physical capacity was determined using the Timed Up and Go test and 5 Times Sit to Stand test. Throughout, participants wore isoinertial measurement units.

RESULTS

Postural sway was worse (ie, greater) in people with long COVID and ME/CFS than controls, but not different between long COVID and ME/CFS. Performance of the Timed Up and Go test and 5 Times Sit to Stand test were worse in long COVID and ME/CFS than controls, but not different between long COVID and ME/CFS. Of long COVID and ME/CFS participants, 87% and 13% exceeded the threshold for muscle weakness in the 5 Times Sit to Stand test and Timed Up and Go test, respectively.

CONCLUSIONS

These data suggest that both people with long COVID and people with ME/CFS have similarly impaired balance and physical capacity. Therefore, there is an urgent need for interventions to target postural sway and physical capacity in people with ME/CFS, and given the current pandemic, people with long COVID.

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.

The Effects of Different Kinds of Smooth Pursuit Exercises on Center of Pressure and Muscle Activities during One Leg Standing

This study examined the effects of gaze fixation and different kinds of smooth-pursuit eye movements on the trunk and lower extremity muscle activities and center of pressure.

METHODS

Twenty-four subjects were selected for the study. The activity of trunk and lower limb muscles (tibialis anterior, lateral gastrocnemius, medial gastrocnemius, vastus midialis obliques, vastus lateralis, biceps femoris, rectus abdominis, and erector spinae) and the COP (center of pressure) (surface area ellipse, length, and average speed) were measured to observe the effects of gaze fixation and different kinds of smooth-pursuit eye movements on the center of pressure and muscle activities during one leg standing. Before the experiment, a Gaze point GP3 HD Eye Tracker (Gazept, Vancouver, BC, Canada) was used to train eye movement so that the subjects would be familiar with smooth eye movement. Repeated each exercise 3 times at random. In order to avoid the sequence deviation caused by fatigue, the movement sequence is randomly selected.

RESULT

The center of pressure and muscle activities were increased significantly when the smooth-pursuit eye movement with one leg standing compared with gaze fixation with one leg standing. In smooth-pursuit eye movements, the changes in the center of pressure and muscle activities were increased significantly with eye and head movement. When the head and eyes moved in opposite directions, the center of pressure and muscle activities were increased more than with any other exercises.

CONCLUSION

Smooth-pursuit eye movement with one leg movement affects balance. In particular, in the smooth-pursuit eye movement with one leg standing, there were higher requirements for balance when the eyes and head move in the opposite direction. Therefore, this movement can be recommended to people who need to enhance their balance ability.

Deterioration of postural control due to the increase of similarity between center of pressure and smooth-pursuit eye movements during standing on one leg

Upright postural control is regulated by afferent and efferent/reafferent visual mechanisms. There are two types of efferent and conjugate eye movements: saccades and smooth pursuits. Although postural control is improved by saccades, the effects of smooth pursuits on postural control are still debated, because the difficulties of postural and visual tasks differ in the previous research. Additionally, the mechanisms that interfere with postural control and smooth pursuit are not fully understood. To address these issues, we examined the effects of different patterns of smooth-pursuit eye movement on the path length of the center of pressure (COP) displacement under bipedal and unipedal standing conditions. The relative frequency and amplitude of the COP displacement were remarkably increased when uniform linear visual targets were presented during unipedal standing. In addition, dynamic time warping analysis demonstrated that the similarity between the displacement of the COP and eye movements was increased by the presentation of uniform linear visual targets with orientation selectivity during unipedal standing but not during bipedal standing. In contrast, the attenuation of similarity between the displacement of the COP and eye movements significantly decreased the path length, relative frequency, and amplitude of the COP displacement. Our results indicate that postural stability is deteriorated by the increase of similarity between the displacement of the COP and smooth-pursuit eye movements under unstable conditions.

Does Physical Activity Moderate the Relationship between Myopia and Functional Status in Children 9-11 Years of Age?

Although previous studies have reported an association between physical activity (PA) and myopia in school-aged children, little is known about the link between myopia and children’s functional status. The purpose of this study was to investigate dynamic balance control in relation to the daily PA levels of myopic schoolchildren aged 9−11 years (n = 52) versus a non-myopic control group (n = 53). A single leg stance test performed on the instability platform of the Biodex Balance System was used to assess balance control. The overall stability index (OSI), anterior-posterior stability index (APSI) and medial-lateral stability index (MLSI) were analyzed. PA levels were calculated using the World Health Organization European Childhood Obesity Surveillance Initiative family record form. Myopes and non-myopes were separated into three subgroups based on PA level (low, moderate and high). Myopia significantly affected OSI (F = 40.46, p < 0.001), APSI (F = 33.93, p < 0.001) and MLSI (F = 49.51, p < 0.001). There were significant differences (p < 0.001) between myopic and non-myopic children with low and moderate levels of PA, whilst there were no differences between compared children with high levels of PA. High PA levels had a positive impact on balance control in myopes. Our results showed that PA levels moderate the relationship between myopia and children’s functional status.

Fast Saccadic Eye Movements Contribute to the Worsened Postural Sway in Older Adults Who Have Experienced Falls

The purpose of this study was to examine changes and between-group differences in postural sway during saccadic eye movement in older adults (n = 152). The participants were stratified into older adults who have experienced a fall (n = 58) (faller group) and those who have not (n = 94) (non-faller group). We measured postural sway during saccadic eye movement. Saccadic eye movement speed was such that the target was displayed at 0.5 Hz, 2 Hz, and 3 Hz. Postural sway was measured based on path length, velocity, and length between the maximal and minimal position of center of pressure in mediolateral and anteroposterior direction. In the faller group, path length, velocity, and mediolateral displacement of the center of pressure increased significantly during 3 Hz saccadic eye movement stimulation. However, in the non-faller group, there was no significant change in the center of pressure parameters during saccadic eye movement stimulation. Mediolateral displacement of the center of pressure increased significantly in both groups during saccadic eye movement, especially at 3 Hz. Therefore, rapid saccadic eye movement stimulation can contribute to the worsened postural sway in older adults who have experienced falls, and rapid external environmental stimuli may contribute to the deterioration of the upright standing stability in older adults.

Validation of a LiDAR-based player tracking system during football-specific tasks

Tracking players’ movements in sports is important to monitor and optimise exercise volume, avoid injuries, and enhance game performance. A new LiDAR-based system (Sportlight®) purports to provide accurate velocity and acceleration metrics derived from player movements. This study examined the validity of the LiDAR-based system against a 3D motion analysis system. Two competitive football players (age: 18 years, height: 1.74 ± 0.01 m, mass: 66.5 ± 7.8 kg; playing experience at this level: 3 years) completed nine trials each of six sport-specific movements, consisting of straight-line sprints, cuts, and curved runs. Trials were recorded concurrently by a four-unit LiDAR system and a 64-camera 3D motion analysis system. Instantaneous velocity and acceleration, and time spent within key performance indicator bands (defined by velocity and acceleration thresholds) were compared between systems. Agreement between the systems was evaluated by root mean square error. Differences in time spent within each key performance indicator band between systems were assessed with t tests and standardised effect sizes. Velocity root mean square error values ranged from 0.04 to 0.14 m·s−1 across all movements and acceleration root mean square error values ranged from 0.16 to 0.7 m·s−2. Differences between systems for time spent within each key performance indicator band were mostly trivial. These results show that the LiDAR-based system can provide valid measures of velocity and acceleration in football-specific tasks, thus providing accurate tracking of players and calculation of relevant key performance indicators.

Eye Position Shifts Body Sway Under Foot Dominance Bias in the Absence of Visual Feedback

Purpose

The purpose of this study was to investigate whether information on extraocular muscle proprioception without visual information affects postural control.

Methods

Thirty-five healthy young volunteers participated in the study. Postural control outcomes included the center of pressure (CoP) for static standing, the total length of the sway of the CoP (LNG), and the sway area (SA), as well as the mean CoP in the mediolateral and anteroposterior directions. The following five eye-fixing positions were used: eye-up (E-Up), eye-down (E-Down), eye-right (E-Right), eye-left (E-Left), and eye-center (Center eye position). One-way ANOVA and Bonferroni correction was performed for statistical processing. Electrooculograms were recorded to detect eye orientation errors, measured with the eyes closed.

Results

The results of this study showed no significant difference between the LNG and SA results when comparing respective eye positions (E-up, E-down, E-right, E-left) relative to E-Center (control). However, the average CoP was shifted to the right at E-Up, E-Down, and E-Left.

Conclusion

These findings indicate that postural control may be affected by eye-body coordination depending on the position of the eyes, even without visual information.

The Differentiation of Self-Motion From External Motion Is a Prerequisite for Postural Control: A Narrative Review of Visual-Vestibular Interaction

The visual system is a source of sensory information that perceives environmental stimuli and interacts with other sensory systems to generate visual and postural responses to maintain postural stability. Although the three sensory systems; the visual, vestibular, and somatosensory systems work concurrently to maintain postural control, the visual and vestibular system interaction is vital to differentiate self-motion from external motion to maintain postural stability. The visual system influences postural control playing a key role in perceiving information required for this differentiation. The visual system’s main afferent information consists of optic flow and retinal slip that lead to the generation of visual and postural responses. Visual fixations generated by the visual system interact with the afferent information and the vestibular system to maintain visual and postural stability. This review synthesizes the roles of the visual system and their interaction with the vestibular system, to maintain postural stability.

Oculomotor stimulation without visual input has no impact on postural control

It has been well established that eye movements have an impact on balance, and it has been hypothesized that extraocular oculomotor signals could play a significant role in this effect. Unfortunately, this hypothesis could not be confirmed as the previous methodology did not allow for the independent assessment of the differential effects of visual and oculomotor stimulation. The objective of the present study is to examine the impact of motor movements of the eyes without visual stimulation on balance. Static postural control, a prerequisite for balance, was assessed using a force platform in 20 participants. They were asked either to remain still without moving or to make movements of the tongue or eyes at a rate of two cycles per second. Movements were monitored using electrophysiological recordings. Each of the conditions was performed with eyes open and with eyes closed. Significant changes in postural control were observed due to eye movements when the eyes were open, but no significant differences were found between the conditions when the eyes were closed. The results confirm that the visual system provides important spatial cues for balance, allowing the body to be better positioned in space, and reject the possibility that extraocular signals are directly involved in postural stability.

Trunk and head displacements stabilized to perform both horizontal and vertical saccadic eye movements

Vision is crucial for humans to interact with their surrounding environment, and postural sway is reduced to allow short eye movements. However, the extent of subtle changes in postural control for horizontal and vertical eye movements remains unclear. The goal of this study was to investigate the effects of vertical and horizontal eye movements on head and trunk control in young adults. Fifteen healthy adults (23.4 ± 4.7 years) stood upright in three conditions for 60 s: fixation, horizontal, and vertical guided eye movements. In fixation, participants had to fixate on a stationary target. In both the horizontal and vertical eye movements, the target was presented with a frequency of 0.5 Hz and a visual angle of 11°. Eye displacement was monitored using a SMI eye tracker (ETG2.0) and trunk and head sway were monitored using infrared markers (Optotrak 3020, NDI). The mean sway amplitude was lower in both directions for eye movements and lowest in the vertical direction compared to the fixation condition. The sway area was also lower in vertical eye movement than in the fixation condition. We also found that the sway reduction was greater at head than at trunk level. The median frequency sway in the anterior-posterior direction was higher in both eye movements than in fixation. Based upon these results, we suggest that to perform short eye movements, postural sway is more strongly controlled at the head level than at the trunk and in vertical eye movements than in horizontal movements.

Saccadic eye movement performance reduces visual manipulation influence and center of pressure displacements in older fallers

This study examined changes in postural control and gaze performance of faller and non-faller older adults under conditions of visual tasks and optical flow manipulations. Fifteen older non-fallers (69.8 years, ± 3.2) and fifteen older fallers (71.1 years, ± 6.4) stood on a force platform inside a moving room wearing an eye tracker. Four tasks were performed: gaze fixation; predictable saccades; unpredictable saccades; and free-viewing. The stimuli appeared at a frequency of 1.1 Hz during conditions of predictable and unpredictable saccades. Sixteen trials were divided into two blocks. In the first block, the room remained stationary. In the second block, the room oscillated, without the participant's awareness, with a 0.6 cm amplitude and 0.2 Hz frequency. Results showed postural sway attenuation in older fallers during the saccadic tasks compared to gaze fixation and free-viewing tasks, in both stationary and moving room conditions. Both groups showed increased center of pressure (CoP) magnitude during the moving room condition and CoP displacements strongly coupled to the room's movement. The influence of the moving room on the postural sway was reduced during the saccadic tasks for both older groups. Older fallers exhibited higher variability compared to older non-fallers. Gaze behavior differences between groups were dependent on the goals of the visual tasks. Therefore, CoP displacements of older adults are reduced during saccadic tasks regardless of their falling history. However, postural and gaze performance of older fallers suggests increased deterioration of postural and oculomotor control which may be used as a predictor of fall risk.

Visuomotor control dynamics of quiet standing under single and dual task conditions in younger and older adults

Visual input facilitates stable postural control; however, ageing alters visual gaze strategies and visual input processing times. Understanding the complex interaction between visual gaze behaviour and the effects of age may inform future interventions to improve postural control in older adults. The purpose of this study was to determine effects of age and dual task on gaze and postural sway dynamics, and the sway-gaze complexity coupling to explore the coupling between sensory input and motor output. Ten older and 10 younger adults performed single and dual task quiet standing while gaze behaviour and centre of mass motion were recorded. The complexity and stability of postural sway, saccade characteristics, visual input duration and complexity of gaze were calculated in addition to sway-gaze coupling quantified by cross-sample entropy. Dual tasking increased complexity and decreased stability of sway with increased gaze complexity and visual input duration, suggesting greater automaticity of sway with greater exploration of the visual field but with longer visual inputs to maintain postural stability in dual task conditions. In addition, older adults had lower complexity and stability of sway than younger adults indicating less automated and stable postural control. Older adults also demonstrated lower gaze complexity, longer visual input durations and greater sway-gaze coupling. These findings suggest older adults adopted a strategy to increase the capacity for visual information input, whilst exploring less of the visual field than younger adults.

Movement Time of Lower Trunk Muscles during Dynamic Postural Control in Response to a Sudden Visual Stimulus during Walking: A Pilot Study

Postural control during walking is maintained by the combination of various factors. Among these factors, adjustment of trunk movement is essential for maintaining postural control, and the response of muscles to unpredictable stimuli affects postural control. Loss of balance while walking increases the risk of accidents, the frequency of which depends on age and sex. In this study, we investigated whether there was a difference in the movement time of trunk muscles to sudden stimulation while walking according to age and sex. Fourteen healthy individuals aged 20-30 years (6 men, 8 women) and 12 individuals aged 50-70 years (4 men, 8 women) were included in the study. Movement time of bilateral erector spinae and rectus abdominis muscles in response to visual stimulation during walking was examined using surface electromyography. Movement time was calculated as the total muscle activation time excluding the reaction time. This study revealed no significant differences in movement time of the erector spinae muscles according to sex or age. The role of the rectus abdominis muscles in maintaining posture during walking was insignificant. In conclusion, the movement time of trunk muscles in response to sudden visual stimulation during walking did not differ by age or sex, and the difference in accident frequency may be associated with deterioration of other factors required to maintain posture.

Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity

Understanding the modulations of motor control in the presence of perturbations in task conditions of varying complexity is a key element towards the design of effective perturbation-based balance exercise programs. In this study we investigated the effect of mechanical perturbations, induced by an unstable surface, on muscle activation and visuo-postural coupling, when actively tracking target motion cues of different complexity. Four postural tasks following a visual oscillating target of varying target complexity (periodic-sinusoidal vs. chaotic-Lorenz) and surface (stable-floor vs. unstable-foam) were performed. The electromyographic activity of the main plantarflexor and dorsiflexor muscles was captured. The coupling between sway and target was assessed through spectral analysis and the system's local dynamic stability through the short-term maximum Lyapunov exponent. We found that external perturbations increased local instability and deteriorated visuo-motor coupling. Visuo-motor deterioration was greater for the chaotic target, implying that the effect of the induced perturbations depends on target complexity. There was a modulation of the neuromotor system towards amplification of muscle activity and coactivation to compensate surface-related perturbations and to ensure robust motor control. Our findings provide evidence that, in the presence of perturbations, target complexity induces specific modulations in the neuromotor system while controlling balance and posture.

Postural balance and oculomotor control are influenced by neck kinaesthetic functions in elite ice hockey players

BACKGROUND

Research indicates that neck kinaesthetic awareness plays an important role in oculomotor and balance control, however the relationship has not been studied in athletes. As performance in ice hockey demands visual perception acuity during skating, while constantly shifting between unilateral and bilateral stances in sports specific posture more in-depth relationship should be studied.

RESEARCH QUESTION

What is the relationship between neck kinaesthetic awareness, postural balance and eye movement control in professional ice hockey players and non-trained individuals?

METHODS

In this observational study, centre-of-pressure was measured using force plate in twenty-eight hockey players and thirty non-trained participants during different stances in eyes-opened and eyes-closed conditions. Butterfly test and Head-to-Neutral Relocation test were performed to assess neck kinaesthesia. Horizontal smooth pursuit eye movements were measured using video-oculography. Linear regression was used to determine the relationship between neck kinaesthesia, body sway and oculomotor control.

RESULTS

Time-on-target in Butterfly test was able to predict low to medium proportions of variance in amplitude and velocity parameters for single leg stances in hockey players (R² = .220-.698). Head-to-Neutral Relocation test was able to predict low to medium proportion of variance in average eye movement velocity during first 100 milliseconds of smooth pursuit initiation for both groups (R² = .262-.541).

SIGNIFICANCE

Findings from our study suggest that cervical spine afferent input plays an important role in maintaining unilateral postural balance in hockey players, with less evidence presented in controls. Sports specific posture or upper body loading could lead to adaptations in neck proprioception, not frequently considered when searching for balance related injury risk factors or performance deficits. Our study suggests, that in addition to balance control, neck kinaesthesia can also affect oculomotor performance which was present in both groups. This is especially evident when initiating changes in eye movement direction.

Impaired eye tracking is associated with symptom severity but not dynamic postural control in adolescents following concussion

PURPOSE

The purpose of the study was to (1) examine the relationship between self-reported symptoms and concussion-related eye tracking impairments, and (2) compare gait performance between (a) adolescents with a concussion who have normal eye tracking, (b) adolescents with a concussion who have abnormal eye tracking, and (c) healthy controls.

METHODS

A total of 30 concussed participants (age: 14.4 ± 2.2 years, mean ± SD, 50% female) and 30 controls (age: 14.2 ± 2.2 years, 47% female) completed eye tracking and gait assessments. The BOX score is a metric of pupillary disconjugacy, with scores <10 classified as normal and ≥10 abnormal. Symptoms were collected using the Post-Concussion Symptom Scale (PCSS), and gait speed was measured with triaxial inertial measurement units. We conducted a linear regression to examine the relationship between PCSS and BOX scores and a two-way mixed effects analysis of variance to examine the effect of group (abnormal BOX, normal BOX, and healthy control) on single- and dual-task gait speed.

RESULTS

There was a significant association between total PCSS score and BOX score in the concussion group (β = 0.16, p = 0.004, 95% confidence interval (95%CI): 0.06‒0.27), but not in the control group (β = 0.21, p = 0.08, 95%CI: -0.03 to 0.45). There were no significant associations between PCSS symptom profiles and BOX scores in the concussion or control groups. There were also no significant differences in single-task (Abnormal: 1.00 ± 0.14 m/s; Normal: 1.11 ± 0.21 m/s; Healthy: 1.14 ± 0.18 m/s; p = 0.08) or dual-task (Abnormal: 0.77 ± 0.15 m/s; Normal: 0.84 ± 0.21 m/s; Healthy: 0.90 ± 0.18 m/s; p = 0.16) gait speed.

CONCLUSION

The concussed group with impaired eye tracking reported higher total symptom severity, as well as worse symptom severity across the 5 PCSS symptom domain profiles. However, eye tracking deficits did not appear to be driven by any particular symptom domain. While not statistically significant, the slower gait speeds in those with abnormal BOX scores may still be clinically relevant since gait-related impairments may persist beyond clinical recovery.

Comparison of Postural Sway, Plantar Cutaneous Sensation According to Saccadic Eye Movement Frequency in Young Adults

The crossover trial study aimed to identify the saccadic eye movement (SEM) frequency to improve postural sway (PS) and plantar cutaneous sensation (PUS) in young adults. The 17 participants randomly performed 0.5-, 2-, and 3-Hz SEM. The SEM frequency was determined to allow the target to appear once per 2 s (0.5 Hz), twice per second (2 Hz), or thrice per second (3 Hz). SEM performance time was 3 min with a washout period of 5 min. PS and PUS were measured at baseline and during 0.5-Hz, 2-Hz, and 3-Hz SEMs using a Zebris FDM 1.5 force plate. PS was determined by measuring the sway area, path length, and speed of center of pressure (COP) displacement, and PUS was determined via the plantar surface area (PSA). In PS parameters, there was a significant difference among the SEM frequencies in the COP_{sway area} PSA_{left foot} and PSA_{right foot}. Compared to that at baseline, COP_{sway area} decreased at 0.5 Hz and 2 Hz, while PSA_{left foot} and PSA_{right foot} increased at 2 Hz. These results suggest that 2 Hz SEM may improve PS and PSA.

Postural Control during Progressively Increased Balance-Task Difficulty in Athletes with Unilateral Transfemoral Amputation: Effect of Ocular Mobility and Visuomotor Processing

This study examined postural control during single leg stance test with progressively increased balance-task difficulty in soccer players with unilateral transfemoral amputation (n = 11) compared to able-bodied soccer players (n = 11). The overall stability index (OSI), the anterior/posterior stability index, and the medial/lateral stability index during three balance tasks with increasing surface instability were estimated. The oculomotor and visuomotor contribution to postural control in disabled athletes was analyzed. Oculomotor function, simple and choice reaction times, and peripheral perception were assessed in a series of visuomotor tests. The variation in OSI demonstrated significantly greater increases during postural tests with increased balance-task difficulty in the able-bodied soccer players compared to amputees (F_(2,40) = 3.336, p < 0.05). Ocular mobility index correlated (p < 0.05) with OSI in conditions of increasing balance-task difficulty. Moreover, speed of eye-foot reaction has positive influence (p < 0.05) on stability indexes in tasks with an unstable surface. Amputee soccer players displayed comparable postural stability to able-bodied soccer players. Disabled athletes had better adaptability in restoring a state of balance in conditions of increased balance-task difficulty than the controls. The speed of visuomotor processing, characterized mainly by speed of eye-foot reaction, significantly contributed to these results.

Increased visual distraction can impair landing biomechanics

Failed jump landings represent a key mechanism of musculoskeletal trauma. It has been speculated that cognitive dual-task loading during the flight phase may moderate the injury risk. This study aimed to explore whether increased visual distraction can compromise landing biomechanics. Twenty-one healthy, physically active participants (15 females, 25.8 ± 0.4 years) completed a series of 30 counter-movement jumps (CMJ) onto a capacitive pressure platform. In addition to safely landing on one leg, they were required to memorize either one, two or three jersey numbers shown during the flight phase (randomly selected and equally balanced over all jumps). Outcomes included the number of recall errors as well as landing errors and three variables of landing kinetics (time to stabilization/TTS, peak ground reaction force/pGRF, length of the centre of pressure trace/COPT). Differences between the conditions were calculated using the Friedman test and the post hoc Bonferroni-Holm corrected Wilcoxon test. Regardless of the condition, landing errors remained unchanged (p = .46). In contrast, increased visual distraction resulted in a higher number of recall errors (chi² = 13.3, p = .001). Higher cognitive loading, furthermore, appeared to negatively impact mediolateral COPT (p < .05). Time to stabilization (p = .84) and pGRF (p = .78) were unaffected. A simple visual distraction in a controlled experimental setting is sufficient to adversely affect landing stability and task-related short-term memory during CMJ. The ability to precisely perceive the environment during movement under time constraints may, hence, represent a new injury risk factor and should be investigated in a prospective trial.

Visual Fixations and Motion Sensitivity: Protocol for an Exploratory Study

Background

Motion sensitivity after vestibular disorders is associated with symptoms of nausea, dizziness, and imbalance in busy environments. Dizziness and imbalance are reported in places such as supermarkets and shopping malls which have unstable visual backgrounds; however, the mechanism of motion sensitivity is poorly understood.

Objective

The main aim of this exploratory observational study is to investigate visual fixations and postural sway in response to increasingly complex visual environments in healthy adults and adults with motion sensitivity.

Methods

A total of 20 healthy adults and 20 adults with motion sensitivity will be recruited for this study. Visual fixations, postural sway, and body kinematics will be measured with a mobile eye tracker device, force plate, and 3D motion capture system, respectively. Participants will be exposed to experimental tasks requiring visual fixation on letters, projected on a range of backgrounds on a large screen during quiet stance. Descriptive statistics (mean and standard deviation) will be calculated for each of the variables. One-way independent-measures analyses of variance will be performed to investigate the differences between groups for all variables.

Results

Data collection was started in May 2019 and was completed by February 2020. It was approved by Health and Disability Ethics Committees, Ministry of Health, New Zealand on November 2, 2018 (Ethics ref: 18/CEN/193). We are currently processing the data and will begin data analysis in July 2020. We expect the results to be available for publication by the end of 2020. The trial was funded by the Neurology Special Interest Group, Physiotherapy New Zealand, and the Eisdell Moore Centre in November 2018.

Conclusions

This study will provide a detailed investigation of visual fixations in response to increasingly complex visual environments. Investigating characteristics of visual fixations in healthy adults and those with motion sensitivity will provide insight into this disabling condition and may inform the development of new intervention strategies which explicitly cater to the needs of this population.

Trial Registration

Australian New Zealand Clinical Trials Registry, ACTRN12619000254190; https://tinyurl.com/yxbn7nks

International Registered Report Identifier (IRRID)

PRR1-10.2196/16805

Oculomotor Control in Amputee Soccer Players

The authors investigated the dynamics of saccadic parameters during a stationary oculomotor target task in amputee soccer players (n = 16), able-bodied soccer players (n = 16), and nonathletic control subjects (n = 16). Eye movements during the visual-search tasks were recorded binocularly using a mobile eye-tracking system, and the gaze parameters were analyzed (fixation duration, saccade duration, saccade amplitude, saccade average acceleration, saccade peak deceleration, saccade average velocity, and ocular mobility index). The average saccade acceleration in the amputee soccer players was significantly lower than in the able-bodied players (p = .021). Other saccade characteristics in disabled athletes were comparable to those of the able-bodied groups. Moreover, the able-bodied soccer players presented faster saccadic parameters than nonathletes in terms of saccade acceleration (p = .002), deceleration (p = .015), and velocity (p = .009). The modification of oculomotor functions may result from extensive practice and participation in ball games. The authors' hypothesis that oculomotor functions in amputee soccer players may be impaired was not fully confirmed.

Positive Relations Between Vision and Posture in the Fixation Task Performed Upright

In an upright stance, individuals sway in unpredictable ways. Their eyes also move in unpredictable ways in fixation tasks. The objective of this study was to analyze visual functions, postural control, and cognitive involvement in stationary gaze. A total of 14 healthy young adults performed a fixation task and a free-viewing task (three trials per task, 45 s per trial). As expected, the results showed many (n = 32) significant positive Pearson correlation coefficients between the eye and center of pressure/body (head, neck, and lower back) movements in the fixation task. In the free-viewing task, the correlations were nonsignificant. Only 3 of the 32 significant correlations (9.4%) were significantly related to cognitive involvement (measured with a subjective questionnaire). These results indirectly strengthened the validity of the synergistic model of postural control.

LRRK2 Biology from structure to dysfunction: research progresses, but the themes remain the same

Since the discovery of leucine-rich repeat kinase 2 (LRRK2) as a protein that is likely central to the aetiology of Parkinson’s disease, a considerable amount of work has gone into uncovering its basic cellular function. This effort has led to the implication of LRRK2 in a bewildering range of cell biological processes and pathways, and probable roles in a number of seemingly unrelated medical conditions. In this review we summarise current knowledge of the basic biochemistry and cellular function of LRRK2. Topics covered include the identification of phosphorylation substrates of LRRK2 kinase activity, in particular Rab proteins, and advances in understanding the activation of LRRK2 kinase activity via dimerisation and association with membranes, especially via interaction with Rab29. We also discuss biochemical studies that shed light on the complex LRRK2 GTPase activity, evidence of roles for LRRK2 in a range of cell signalling pathways that are likely cell type specific, and studies linking LRRK2 to the cell biology of organelles. The latter includes the involvement of LRRK2 in autophagy, endocytosis, and processes at the trans-Golgi network, the endoplasmic reticulum and also key microtubule-based cellular structures. We further propose a mechanism linking LRRK2 dimerisation, GTPase function and membrane recruitment with LRRK2 kinase activation by Rab29. Together these data paint a picture of a research field that in many ways is moving forward with great momentum, but in other ways has not changed fundamentally. Many key advances have been made, but very often they seem to lead back to the same places.

Semi tandem base of support degrades both saccadic gaze control and postural stability particularly in older adults

Differences in the postural stabilization of older and young adults have been shown to be task-dependent on both visual and postural challenges; however, the gaze behavior during such tasks has rarely been examined. This study investigated the effects of horizontal and vertical saccades on gaze control, center of pressure (CoP) and head displacement of young and older adults on different bases of support. Ten young adults (20.7 ± 3.4 years) and ten older adults (71.6 ± 3.1 years) remained in an upright stance on a force platform wearing an eye-head tracker device. The participants performed 30-second trials according to two bases of support (feet apart and semi-tandem) and three gaze behavior (fixation, horizontal and vertical saccades) conditions. Older adults presented greater CoP amplitude (p < 0.002) and velocity (p < 0.001) (ML axis), and higher head amplitude (ML) (p < 0.002) than young adults during the semi tandem base. Head displacement of both groups presented higher velocity (ML axis) during horizontal (p < 0.001) and vertical saccades (p < 0.01) than the fixation task only on the semi tandem base. There was higher number of fixations (p < 0.001) and lower mean fixation duration (p < 0.001) on the semi-tandem base (p < 0.05). The results showed higher gaze latency variability in vertical saccades for older adults (p < 0.01). Challenging postural tasks may alter postural adjustments and gaze control during saccadic tasks. Particularly, the greater postural instability of older adults increased the gaze latency variability during saccadic tasks, suggesting some deterioration in the posture-gaze relation with aging.

How Eye Movements Stabilize Posture in Patients With Bilateral Vestibular Hypofunction

Chronic patients with bilateral vestibular hypofunction (BVH) complain of oscillopsia and great instability particularly when vision is excluded and on irregular surfaces. The real nature of the visual input substituting to the missing vestibular afferents and improving posture control remains however under debate. Is retinal slip involved? Do eye movements play a substantial role? The present study tends to answer this question in BVH patients by investigating their posture stability during quiet standing in four different visual conditions: total darkness, fixation of a stable space-fixed target, and pursuit of a visual target under goggles delivering visual input rate at flicker frequency inducing either slow eye movements (4.5 Hz) or saccades (1.2 Hz). Twenty one chronic BVH patients attested by both the caloric and head impulse test were examined by means of static posturography, and compared to a control group made of 21 sex-and age-matched healthy participants. The posturography data were analyzed using non-linear computation of the center of foot pressure (CoP) by means of the wavelet transform (Power Spectral Density in the visual frequency part, Postural Instability Index) and the fractional Brownian-motion analysis (stabilogram-diffusion analysis, Hausdorff fractal dimension). Results showed that posture stability was significantly deteriorated in darkness in the BVH patients compared to the healthy controls. Strong improvement of BVH patients' posture stability was observed during fixation of a visual target, pursuit with slow eye movements, and saccades, whereas the postural performance of the control group was less affected by the different visual conditions. It is concluded that BVH patients improve their posture stability by (1) using extraocular signals from eye movements (efference copy, muscle re-afferences) much more than the healthy participants, and (2) shifting more systematically than the controls to a more automatic mode of posture control when they are in dual-task conditions associating the postural task and a concomitant visuo- motor task.

Visually fixating or tracking another person decreases balance control in young and older females walking in a real-world scenario

Balance control during overground walking was assessed in 10 young (23.6 ± 3.4) and 10 older (71.0 ± 5.5 years) healthy females during free gaze, and when fixating or tracking another person in an everyday use waiting room. Balance control was characterised by medial/lateral sacrum acceleration dispersion, and gaze fixations were simultaneously assessed with eye tracking equipment. The results showed decreased balance control when fixating a stationary (p = 0.003, g_av = 0.19) and tracking a walking (p = 0.027, g_av = 0.16) person compared to free gaze. The older adults exhibited reduced baseline stability throughout, but the decrease caused by the visual tasks was not more profound than the younger adults. The decreased balance control when fixating on or tracking the observed person was likely due to more challenging conditions for interpreting retinal flow, which facilitated less reliable estimates of self-motion through vision. The older adults either processed retinal flow during the tasks as effectively as the young adults, or they adopted a more rigid posture to facilitate visual stability, which masked any ageing effect of the visual tasks. The decrease in balance control, the first to be shown in this context, may warrant further investigation in those with ocular or vestibular dysfunction.

The Drosophila hep pathway mediates Lrrk2-induced neurodegeneration

Although the pathogenesis of Parkinson's disease (PD) remains unclear, mutations in leucine-rich repeat kinase 2 (Lrrk2) are among the major causes of familial PD. Most of these mutations disrupt Lrrk2 kinase and (or) GTPase domain function, resulting in neuronal degeneration. However, the signal pathways underlying Lrrk2-induced neuronal degeneration are not fully understood. There is an expanding body of evidence that suggests a link between Lrrk2 function and MAP kinase (MAPK) cascades. To further investigate this link in vivo, genetic RNAi screens of the MAPK pathways were performed in a Drosophila model to identify genetic modifier(s) that can suppress G2019S-Lrrk2-induced PD-like phenotypes. The results revealed that the knockdown of hemipterous (hep, or JNKK) increased fly survival time, improved locomotor function, and reduced loss of dopaminergic neurons in G2019S-Lrrk2 transgenic flies. Expression of the dominant-negative allele of JNK (JNK-DN), a kinase that is downstream of hep in G2019S-Lrrk2 transgenic flies, elicited a similar effect. Moreover, treatment with the JNK inhibitor SP600125 partially reversed the G2019S-Lrrk2-induced loss of dopaminergic neurons. These results indicate that the hep pathway plays an important role in Lrrk2-linked Parkinsonism in flies. These studies provide new insights into the molecular mechanisms underlying Lrrk2-linked PD pathogenesis and aid in identifying potential therapeutic targets.

Head flexion and different walking speeds do not affect gait stability in older females

Head flexion is destabilizing in older individuals during quiet stance, yet the effect head flexion has on gait is not known. The study examined whether head flexion and gait parameters were altered when walking freely and fixed to a visual target, at different walking speeds. 15 young (23±4years) and 16 older (76±6years) healthy females walked at three different walking speeds (slow, comfortable, and fast) under two visual conditions (natural and fixed [focusing on a visual target set at eye level]). Head flexion was assessed using 2D video analysis, whilst gait parameters (step length, double support time, step time, and gait stability ratio) were recorded during a 9m flat walkway. A mixed design ANOVA was performed for each variable, with age as the between-subject factor and, visual condition and walking speed as within-subject factors. When walking freely, older displayed a greater need for head flexion between walking speeds (P<0.05) when compared to young. Walking under fixed condition reduced head flexion at all walking speeds in the older (P<0.05), but had no effect on the young (P>0.05). Walking at different speeds showed no difference in head flexion when walking under either visual condition and had no effect on gait stability for both groups. Despite older displaying differences in head flexion between visual conditions, there was no effect on gait parameters. Walking speed presented trivial difference in head flexion in older females, whilst overall gait stability was unaffected by different walking speeds.

Smooth pursuits decrease balance control during locomotion in young and older healthy females

Dynamic balance control-characterised as movement of the trunk and lower limbs-was assessed during fixation of a fixed target, smooth pursuits and saccadic eye movements in ten young (22.9 ± 1.5 years) and ten older (72.1 ± 8.2 years) healthy females walking overground. Participants were presented with visual stimuli to initiate eye movements, and posture and gaze were assessed with motion analysis and eye tracking equipment. The results showed an increase in medial/lateral (ML) trunk movement (C7: p = 0.012; sacrum: p = 0.009) and step-width variability (p = 0.052) during smooth pursuits compared to a fixed target, with no changes for saccades compared to a fixed target. The elders demonstrated greater ML trunk movement (sacrum: p = 0.037) and step-width variability (p = 0.037) than the younger adults throughout, although this did not interact with the eye movements. The findings showed that smooth pursuits decreased balance control in young and older adults similarly, which was likely a consequence of more complicated retinal flow. Since healthy elders are typically already at a postural disadvantage, further decreases in balance caused by smooth pursuits are undesirable.