Involvement of the head and trunk during gaze reorientation during standing and treadmill walking

As individuals stand or walk in an environment their gaze may be reoriented from one location to another in response to auditory or visual stimuli. In order to reorient gaze, the eyes and/or the head and trunk must rotate. However, what determines the exact degree of rotation of each segment while standing or walking is not fully understood. In the current study we show that when participants were asked to reorient their gaze towards light cues positioned at eccentric locations of up to 90° while standing or walking on a treadmill their eyes and head mainly facilitated the action. Rotations of the head-in-space were similar for both tasks, but the rotation of the shoulders- and hips-in-space were lower for the treadmill walking condition. It is argued that this difference in the level of head-on-trunk rotation during the two tasks is controlled by the vestibular feedback loop. The regulation of this feedback loop is performed by the cerebellum in response to the level of threat to postural stability.     

Visual information from the lower visual field is important for walking across multi-surface terrain

Visual information concerning characteristics of the environment is critical for safe navigation. The purpose of this study was to determine the importance of vision from the lower visual field for negotiating multi-surface terrain. Ten healthy young adults and ten healthy older adults walked across a walkway where the middle portion consisted of solid, rock, slippery, compliant, tilt, and irregular surfaces (i.e. multi-surface terrain). Participants performed the walking trials with and without special glasses that blocked the lower visual field. Head pitch angle along with step parameters were measured. Young and older adults demonstrated increased mean and maximum head pitch angle downward when the lower visual field was blocked suggesting the importance of vision from this area when stepping on multi-surface terrain. In addition, young and older adults altered their gait pattern by reducing gait speed and step length when the lower visual field was blocked. These results suggest that information from the lower visual field is normally used when walking across multi-surface terrain. The results have implications for those individuals who wear multi-focal glasses and who use them while walking in complex environments, which may challenge balance.     

Aging reduces the ability to change grip force and balance control simultaneously

The purpose of this study was to investigate the change in the fingertip forces and balance control of young adults and older adults. The subjects lifted an object of constant weight (i.e., 1500 g) using their right hand, first in a seated position and then in a standing position. We quantified the ability of the participants to adjust their fingertip forces across trials by comparing the percentage of change in the peak grip force, peak load force and the ratio between peak grip force and peak load force. Moreover, we quantified their ability to stabilize their balance following the lifting of the object in the standing condition. The results showed that in both conditions young adults reduced their peak grip force much more than older adults across trials. In the seated condition, young adults increased slightly their peak load force, across trials, while older adults reduced it. In the standing condition, both groups showed similar change in peak load force across trials. Remarkably, older adults improved their balance stability similarly to young adults in the standing condition. This observation suggests that the ability of the older adults to modulate grip force applied to an object while standing is diminished probably to dedicate more attention to the balance control task rather than fine-tuning the grip force. Reducing balance instability following repetitive lifting is certainly more beneficial as the consequences of a fall could be more dramatic than dropping a cup of coffee.     

Performance of the human vestibuloocular reflex during locomotion

1. The stability of gaze was measured in nine normal subjects during 30-s epochs of standing, walking in place, and running in place. The angle of gaze and head rotations in horizontal and vertical planes were measured using the magnetic search coil technique. Subjects visually fixed on a stationary object located at a distance of 100 m; thus measurements of gaze indicated the stability of images on the retina. 2. During standing, walking, or running in place, the standard deviation of the angle of gaze was less than 0.4 degrees, both horizontally and vertically. During standing and walking in place, peak gaze velocity (Gp) was less than 3.0 degrees/s. During running in place, Gp was less than 3.0 degrees/s horizontally but ranged up to 9.3 degrees/s vertically. 3. Visual acuity was measured during standing, walking, and running in place. During walking in place, five of nine subjects showed a small but significant (P = 0.03) decline in visual acuity compared with standing. During running in place, all nine subjects showed a small but significant (P = 0.002) decline in visual acuity compared with standing. 4. Stability of gaze was also measured during vigorous, voluntary head rotations in the horizontal (yaw) or vertical (pitch) planes, for 15-s epochs. Gp ranged as high as 70 degrees/s horizontally and 41 degrees/s vertically. All subjects reported illusory movement of the seen environment during these head rotations. 5. The suitability of linear systems techniques for analysis of the horizontal and vertical vestibuloocular reflex (VOR) during walking and running in place was assessed using coherence spectral analysis.(ABSTRACT TRUNCATED AT 250 WORDS)     

Age-related changes in prefrontal activity during walking in dual-task situations: A fNIRS study

Background

Previous studies suggest that the human gait is under control of higher-order cognitive processes, located in the frontal lobes, such that an age-related degradation of cognitive capabilities has a negative impact on gait.

Methods

Using functional Near-Infrared-Spectroscopy (fNIRS) we investigate the frontocortical hemodynamic correlates of dual-task walking in two conditions. 15 young and 10 older individuals walked on a treadmill while completing concurrent tasks that had either visual (checking) or verbal-memory (alphabet recall) demands. We compared subjects' motor performance, as well as their prefrontal activity in single- and dual-task walking.

Results

Our behavioral data partly confirm previous accounts on higher dual-task costs in stepping parameters (i.e., decreased step duration) in old age, particularly with a visual task and negative dual-task cost (i.e., improved performance) during the verbal task in young adults. Functional imaging data revealed little change of prefrontal activation from single- to dual-task walking in young individuals. In the elderly, however, prefrontal activation substantially decreased during dual-task walking with a complex visual task.

Conclusion

We interpret these findings as evidence for a shift of processing resources from the prefrontal cortex to other brain regions when seniors face the challenge of walking and concurrently executing a visually demanding task.     

Changes in postural sway as a function of prolonged walking

For optimal balance, the postural system needs to quickly detect and respond to perturbations. The aim of this study was to investigate the immediate and long-term impact of walking at different speeds on standing balance and postural stability. Center of pressure (COP) motion was measured from 14 young individuals at discrete time intervals after they walked on a treadmill at three speeds (preferred walking speed (PWS), 120 %-PWS, 140 %-PWS). Results revealed that walking at a faster speed had the greatest impact on postural stability. This was reflected by increases in the amount (path length, range, 95 % ellipse), variability (standard deviation, SD), and structure (approximate entropy, ApEn) of COP motion and were most evident when compared to pre-walking assessments. In subsequent trials following pre-walking assessments there was a leveling-off for specific COP variables (range, variability, and ApEn) and a decline in path length. This plateau effect was observed even though measures of physical exertion (HR, RPE) continued to increase over the entire walking trial. Together, these results indicate that, despite the constant task demands induced by fast walking, the postural system was able to rapidly compensate and adjust appropriately.     

Neuromuscular responses of elderly women to tasks of increasing complexity imposed during walking

Summary The purpose of this electromyographic study was to examine the effects of age on lower limb muscle response characteristics during reaction tasks of varying complexity when standing and walking. Ten young (mean age 22 years) and ten elderly (mean age 68 years) women participated in the investigation. No differences between groups were recorded in muscle activation times (MAT) in response to a visual light directional stimulus (LDS) during all standing reaction tasks. All the women required a longer period of time to respond to LDS when walking than standing. Completion of complex walking reaction tasks produced significantly longer (P<0.001) MAT in all subjects than the more simple walking reaction tasks. Delays in neuromuscular response were significantly greater (P<0.05) in the older women as the complexity of the walking reaction task increased. Movement behaviour of subjects during the walking reaction trials suggested that relatively greater demands were placed on the neuromuscular systems of the elderly during those tasks of greatest complexity. It was postulated that these movements were directed more at compensating for a reduced neuromuscular ability to control efficiently balance homeostasis. The results of this study provide further support for the hypothesis that delays in the central processing of information during reaction tasks may occur with ageing.     

Age-related changes in head and eye coordination

The effect of ageing upon head movements during gaze shifts is unknown. We have investigated age-related changes in head and eye coordination in a group of healthy volunteers.

Horizontal head and eye movements were recorded in 53 subjects, aged between 20 and 83 years, during the performance of saccades, antisaccades, smooth pursuit and a reading task. The subjects were divided into three groups, young subjects (20–40 years), middle-aged subjects (41–60 years) and older subjects (over 60 years).

Logarithmic transformations of the head gain were significantly greater in the older subjects compared to the young subjects during the saccadic task (P=0.001), antisaccadic task (P=0.0004), smooth pursuit at 20°/s (P=0.001) and 40°/s (P=0.005), but not reading. For saccadic and antisaccadic tasks, the increase in transformed head gain was non-linear with significant differences between older and middle-aged subjects but not middle-aged and young subjects. Head movement tendencies were highly consistent for related tasks.

Head movement gain during gaze shifts significantly increases with age, which may contribute to dizziness and balance problems experienced by the elderly.     

Age-related kinematic changes in late visual-cueing during obstacle circumvention

On a daily basis, we are challenged by common environmental obstacles (e.g. street posts) that require simple and often rapid modifications to our gait patterns to avoid collisions. Poor vision appears to be responsible for important reductions in postural stability during gait; and therefore, individuals with impaired vision, such as the elderly, may be at a greater risk of falling, especially under conditions where stepping avoidance strategies may be constrained by the environment. The purpose of the current study was to examine the body segment and eye-gaze reorientation strategy, role of base of support, as well as visual areas of interest attended to by healthy young (YA) and older adults (OA) when only given limited time, one stride, to prepare for an obstacle circumvention task. Six YA and six OA were asked to perform ten walking trials which required them to circumvent an obstacle in their travel path. Participants used one of two avoidance strategies, either lead leg crossing-over trail leg (narrow base of support) or lead leg stepping-out (wide base of support). Results indicate that base of support constraints did not affect segment reorientation sequence in either age group. The general segment reorientation sequence in YA was initiated by trunk yaw and head yaw, followed by gaze and finally, by M-L foot deviation. No trunk roll deviations were observed. In OA, the general segment reorientation sequence was the following: trunk yaw and trunk roll, gaze and finally, M-L foot deviation. No head yaw deviations were observed. Our findings suggest that YA utilized a foot placement strategy to perform the transient change in travel direction while OA relied on a hip strategy. In addition, YA spent more time gazing straight ahead at the obstacle and the wall, while OA spent more time looking at the ground. This strategy indicates that OA use a more cautious strategy to safely avoid the obstacle. Findings from the present work contribute further knowledge regarding locomotor adjustments during a common, and complex, everyday task in young and older adults.     

Evidence for the use of rotational optic flow cues for locomotor steering in healthy older adults

Optic flow is a powerful visual cue for the control of locomotion. Considerable research has focused on how healthy young people use and perceive optic flow. However, little is known on how older adults use this type of visual motion to control walking. The purpose of this study is to investigate the ability of young and older adults to adjust their physical walking trajectory in response to a rotation of the optic flow presented in a virtual environment. Ten healthy young adults (mean age 23.49 ± 4.72 yr) and 10 healthy older adults (mean age 76.22 ± 3.11 yr) participated in the study. Subjects were instructed to walk straight in a virtual environment viewed within a head-mounted display unit as they walked overground for 5 m, while the focus of expansion was gradually rotated to the left or the right by 40°. All subjects responded with a similar strategy by rotating their head and body in the direction away from the orientation of the perturbation. The younger subjects achieved almost complete corrections and had very small net heading errors. In contrast, the older adults had delayed and smaller reorientations, particularly in the head, thus showing significantly larger heading errors compared with younger subjects. We conclude that older adults retain the ability to use optic flow to control their walking trajectory, although smaller, delayed head rotations and larger heading errors may indicate an age-dependent effect on sensorimotor coordination.     

Stability of gait and interlimb coordination in older adults

Most falls in older adults occur when walking, specifically following a trip. This study investigated the short- and longer term responses of young (n = 24, 27.6 ± 4.5 yr) and older adults (n = 18, 69.1 ± 4.2 yr) to a trip during gait at comfortable speed and the role of interlimb coordination in recovery from tripping. Subjects walked on a self-paced treadmill when forward movement of their dominant leg was unexpectedly arrested for 250 ms. Recovery of center of mass (COM) movements and of double-support duration following perturbation was determined. In addition, the disruption and recovery of interlimb coordination of the arms and legs was evaluated. Although young and older subjects used similar lower limb strategies in response to the trip, older adults had less stable COM movement patterns before perturbation, had longer transient destabilization (>25%) after perturbation, required more gait cycles to recover double-support duration (older, 3.48 ± 0.7 cycles; young, 2.88 ± 0.4 cycles), and had larger phase shifts that persisted after perturbation (older, -83° to -90°; young, -39° to -42°). Older adults also had larger disruptions to interlimb coordination of the arms and legs. The timing of the initial disruption in coordination was correlated with the disturbance in gait stability only in young adults. In older adults, greater initial COM instability was related to greater longer term arm incoordination. These results suggest a relationship between interlimb coordination and gait stability, which may be associated with fall risk in older adults. Reduced coordination and gait stability suggest a need for stability-related functional training even in high-functioning older adults.     

Where do we look when we walk on stairs? Gaze behaviour on stairs,transitions, and handrails

Stair walking is a challenging locomotor task, and visual information about the steps is considered critical to safely walk up and down. Despite the importance of such visual inputs, there remains relatively little information on where gaze is directed during stair walking. The present study investigated the role of vision during stair walking with a specific focus on gaze behaviour relative to (1) detection of transition steps between ground level and stairs, (2) detection of handrails, and (3) the first attempt to climb an unfamiliar set of stairs. Healthy young adults (n = 11) walked up and down a set of stairs with 7 steps (transitions were defined as the two top and bottom steps). Gaze behaviour was recorded using an eye tracker. Although participants spent most part of the time looking at the steps, gaze fixations on stair features covered less than 20% of the stair walking time. There was no difference in the overall number of fixations and fixation time directed towards transitions compared to the middle steps of the stairs. However, as participants approached and walked on the stairs, gaze was within 4 steps ahead of their location. The handrail was rarely the target of gaze fixation. It is noteworthy that these observations were similar even in the very first attempt to walk on the stairs. These results revealed the specific role of gaze behaviour in guiding immediate action and that stair transitions did not demand increased gaze behaviour in comparison with middle steps. These findings may also indicate that individuals may rely on a spatial representation built from previous experience and/or visual information other than gaze fixations (e.g. dynamic gaze sampling, peripheral visual field) to extract information from the surrounding environment.     

A standing posture is associated with increased susceptibility to the sound-induced flash illusion in fall-prone older adults

Recent research has provided evidence suggesting a link between inefficient processing of multisensory information and incidence of falling in older adults. Specifically, Setti et al. (Exp Brain Res 209:375–384, 2011) reported that older adults with a history of falling were more susceptible than their healthy, age-matched counterparts to the sound-induced flash illusion. Here, we investigated whether balance control in fall-prone older adults was directly associated with multisensory integration by testing susceptibility to the illusion under two postural conditions: sitting and standing. Whilst standing, fall-prone older adults had a greater body sway than the age-matched healthy older adults and their body sway increased when presented with the audio–visual illusory but not the audio–visual congruent conditions. We also found an increase in susceptibility to the sound-induced flash illusion during standing relative to sitting for fall-prone older adults only. Importantly, no performance differences were found across groups in either the unisensory or non-illusory multisensory conditions across the two postures. These results suggest an important link between multisensory integration and balance control in older adults and have important implications for understanding why some older adults are prone to falling.     

The hallucinogen 1-[2,5-dimethoxy-4-iodophenyl]-2-aminopropane (DOI) increases cortical extracellular glutamate levels in rats

We examined how young and older adults adapt their posture to static balance tasks of increasing difficulty. Participants stood barefoot on a force platform in normal quiet, Romberg-sharpened and one-legged stance. Center of pressure (CoP) variations, electromyographic (EMG) activity of ankle and hip muscles and kinematic data were recorded. Both groups increased postural sway as a result of narrowing the base of support. Greater CoP excursions, EMG activity and joint displacements were noted in old compared to younger adults. Older adults displayed increased hip movement accompanied by higher hip EMG activity, whereas no similar increase was noted in the younger group. It is concluded that older adults rely more on their hip muscles when responding to self induced perturbations introduced by increased task constraints during quiet standing.     

The contribution of postural control and bilateral coordination to the impact of dual tasking on gait

The simultaneous performance of a cognitive task while walking typically alters the gait pattern. In some populations, these alterations have been associated with an increased risk of falls, motivating study of this response from the clinical perspective. The mechanisms responsible for these effects are not fully understood. The concurrent requirement to control upright posture and stepping, a bilaterally coordinated rhythmic task, may be the cause of this so-called dual-tasking effect. To evaluate this possibility, the present study was designed to isolate the individual contribution of these two demands by assessing the effects of cognitive loading on standing (i.e., postural control without bilateral coordination of stepping), cycling (i.e., bilateral coordination similar to stepping, but with minimal postural demands), and walking. We also investigated the effects of aging and parkinsonism on the performance of these three tasks in response to cognitive loading, also referred to as a dual task. Twenty-one healthy young adults, 15 healthy older adults, and 18 patients with Parkinson’s disease were assessed while walking, standing, and cycling, with and without an additional cognitive load. In the young adults, the performance on the two motor tasks that involved bilateral coordination deteriorated significantly in response to the dual task, while standing was not impacted. Similar results, although less robust, were observed among the healthy older adults. In contrast, among the patients with Parkinson’s disease, the dual-task costs, i.e., the impact of the simultaneously performed cognitive task on the gait pattern, were high in all motor tasks. These findings suggest that walking is especially vulnerable to cognitive loading, in part, because of the unique sensitivity of bilateral coordination of limb movements to the effects of dual tasking.     

Dynamic stability control in forward falls: postural corrections after muscle fatigue in young and older adults

Many studies report that muscle strength loss may alter the human system’s capacity to generate rapid force for balance corrections after perturbations, leading to deficient recovery behaviours. Yet little is known regarding the effect of modifications in the neuromuscular system induced by fatigue on dynamic stability control during postural perturbations. This study investigates the effect of muscle strength decline induced by fatiguing contractions on the dynamic stability control of young and older adults during forward falls. Eleven young and eleven older male adults had to regain balance after sudden falls before and after submaximal fatiguing knee extension–flexion contractions. Young subjects had a higher margin of stability than older ones before and after the fatiguing task. This reflects their enhanced ability in using mechanisms for maintaining dynamic stability (i.e. a greater base of support). The margin of stability, the boundary of the base of support and the position of the extrapolated centre of mass, remained unaffected by the reduction in muscle strength induced by the fatiguing contractions, indicating an appropriate adjustment of the motor commands to compensate the deficit in muscle strength. Both young and older adults were able to counteract the decreased horizontal ground reaction forces after the fatiguing task by flexing their knee to a greater extent, leading to similar decreases in the horizontal velocity of centre of mass as in the pre fatigue condition. The results demonstrate the ability of the central nervous system to rapidly modify the execution of postural corrections including mechanisms for maintaining dynamic stability.     

Behavioral and neural adaptations in response to five weeks of balance training in older adults: a randomized controlled trial

Background

While the positive effect of balance training on age-related impairments in postural stability is well-documented, the neural correlates of such training adaptations in older adults remain poorly understood. This study therefore aimed to shed more light on neural adaptations in response to balance training in older adults.

Methods

Postural stability as well as spinal reflex and cortical excitability was measured in older adults (65–80 years) before and after 5 weeks of balance training (n = 15) or habitual activity (n = 13). Postural stability was assessed during one- and two-legged quiet standing on a force plate (static task) and a free-swinging platform (dynamic task). The total sway path was calculated for all tasks. Additionally, the number of errors was counted for the one-legged tasks. To investigate changes in spinal reflex excitability, the H-reflex was assessed in the soleus muscle during quiet upright stance. Cortical excitability was assessed during an antero-posterior perturbation by conditioning the H-reflex with single-pulse transcranial magnetic stimulation.

Results

A significant training effect in favor of the training group was found for the number of errors conducted during one-legged standing (p = .050 for the static and p = .042 for the dynamic task) but not for the sway parameters in any task. In contrast, no significant effect was found for cortical excitability (p = 0.703). For spinal excitability, an effect of session (p < .001) as well as an interaction of session and group (p = .009) was found; however, these effects were mainly due to a reduced excitability in the control group.

Conclusions

In line with previous results, older adults’ postural stability was improved after balance training. However, these improvements were not accompanied by significant neural adaptations. Since almost identical studies in young adults found significant behavioral and neural adaptations after four weeks of training, we assume that age has an influence on the time course of such adaptations to balance training and/or the ability to transfer them from a trained to an untrained task.

     

Can telling older adults where to look reduce falls? Evidence for a causal link between inappropriate visual sampling and suboptimal stepping performance

Older adults at high risk of falling look away prematurely from targets they are stepping on in order to fixate future constraints in their walking path. This gaze behaviour is associated with decreased stepping accuracy and precision. The aims of the present study were to determine whether this apparently maladaptive gaze behaviour can be altered through intervention and to measure any corresponding improvements in stepping performance. Sixteen older adults, randomly placed into a control or intervention group, walked a 10-m path placing their feet into targets while their gaze direction and lower limb kinematics were measured. On average, both groups looked away from a stepping target around 100 ms prior to foot contact and the extent of early gaze transfer correlated with stepping errors. The participants returned on a separate day and repeated the experiment; however, the intervention group was instructed to maintain gaze on each target until heel contact. Following intervention, on average participants delayed gaze transfer from the first target until after heel contact and this change in behaviour resulted in a significant reduction in stepping errors. We propose that suboptimal visual sampling strategies contribute to the incidence of falls in the elderly.     

A direct comparison of local dynamic stability during unperturbed standing and walking

Standing and walking are very different tasks. It might be reasonable, therefore, to assume that the mechanisms used to maintain the stability of standing and walking should be quite different. However, many studies have shown that postural stability measures can generally predict risk of falls, even though most falls occur during locomotor tasks and not during postural tasks. This suggests that there is at least some commonality among the mechanisms governing the control of both standing and walking. The present study was conducted to determine whether the postural stability either is or is not directly related to locomotor stability. Twenty healthy adults, age 18–73 years, walked on a motorized treadmill at their preferred walking speed for three trials of 5 min. They also stood on a force plate for three trials of 5 min. Both tasks were performed without imposing any additional external perturbations. The motion of each subject’s trunk segment was recorded and described using a multi-dimensional state space defined in the same manner for both tasks. Local dynamic stability was quantified from the mean divergence over time of locally perturbed trajectories in state space, which was parameterized as a double exponential process. Divergence parameters were compared to determine the relationship between local dynamic stability during standing and walking. Standing and walking exhibited local dynamic stability properties that were significantly different (P<0.001) and not correlated (P>0.1). Divergence parameters were also compared to traditional center of pressure (COP) measures obtained from standing trials. COP measures were significantly correlated to local divergence parameters for standing, but not to those for walking. This study provides direct evidence that the mechanisms governing standing and walking stability are significantly different.     

Gaze behavior governing balance recovery in an unfamiliar and complex environment

Visuospatial information regarding obstacles and other environmental constraints on limb movement is essential for the successful planning and execution of stepping movements. Visuospatial control strategies used during gait and volitional stepping have been studied extensively; however, the visuospatial strategies that are used when stepping rapidly to recover balance in response to sudden postural perturbation are not well established. To study this, rapid forward stepping reactions were evoked by unpredictable support-surface acceleration while subjects stood amid multiple obstacles that moved intermittently and unpredictably prior to perturbation onset (PO). To prevent predictive control, subjects performed only one trial (their very first exposure to the perturbation and environment). Visual scanning of the obstacles and surroundings occurred prior to PO in all subjects; however, gaze was never redirected at the obstacles, step foot or landing site in response to the perturbation. Surprisingly, the point of gaze at time of foot-contact was consistently and substantially anterior to the step-landing site. Despite the apparent absence of ‘online’ visual feedback related to the foot movement, the compensatory step avoided obstacle contact in 10 of 12 young adults and 9 of 10 older subjects. The results indicate that the balance-recovery reaction was typically modulated on the basis of visuospatial environmental information that was acquired and continually updated prior to perturbation, as opposed to a strategy based on ‘online’ visual control. The capacity to do this was not adversely affected by aging, despite a tendency for older subjects to look downward less frequently than young adults.