Does the “eyes lead the hand” principle apply to reach-to-grasp movements evoked by unexpected balance perturbations?

The Effect of Handrail Cross-Sectional Design and Age on the Speed and Quality of Reach-To-Grasp Reactions to Recover Balance

OBJECTIVE

To determine the effect of handrail cross-section on the speed and quality of reach-to-grasp movements following balance loss in younger and older adults.

BACKGROUND

Grasping a handrail is a common strategy for balance recovery. For handrails to be effective, the design must enable fast and accurate reactive grasping. Little is known about the effect of handrail cross-section on the timing or quality of the reach-to-grasp movement following balance loss.

METHODS

Twenty-four younger and 16 older adults experienced incrementally increasing magnitudes of perturbations in the forward and backward direction until they were no longer able to recover balance. We analyzed the last trial where the participant could recover using only the handrail, without stepping or relying on the harness, the maximum withstood perturbation (MWP). Seven handrail cross-sections were tested.

RESULTS

Handrail cross-section did not affect the speed or timing of the reach-to-grasp reaction for younger or older adults. However, handrail cross-section affected the MWP, the grip types used, and the likelihood of making an error or adjustment when grasping. The greatest MWP and fewest errors occurred with 1.5" round handrails.

CONCLUSION

The absence of common strategies for accurately grasping complex shapes (reaching more slowly), combined with the higher frequency of errors with larger handrails, suggests that both older and younger adults prioritized quickly reaching the handrail over prehension during reach-to-grasp balance reactions.

APPLICATION

This work provides new insights on the effect of age and handrail cross-sectional design on reach-to-grasp reactions to recover balance, which can inform safer handrail design standards.

Effect of Handrail Height and Age on Trunk and Shoulder Kinematics Following Perturbation-Evoked Grasping Reactions During Gait

OBJECTIVE

To characterize the effect of handrail height and age on trunk and shoulder kinematics, and concomitant handrail forces, on balance recovery reactions during gait.

BACKGROUND

Falls are the leading cause of unintentional injury in adults in North America. Handrails can significantly enhance balance recovery and help individuals to avoid falls, provided that their design allows users across the lifespan to reach and grasp the rail after balance loss, and control their trunk by applying hand-contact forces to the rail. However, the effect of handrail height and age on trunk and shoulder kinematics when recovering from perturbations during gait is unknown.

METHOD

Fourteen younger and 13 older adults experienced balance loss (sudden platform translations) while walking beside a height-adjustable handrail. Handrail height was varied from 30 to 44 inches (76 to 112 cm). Trunk and shoulder kinematics were measured via 3D motion capture; applied handrail forces were collected from load cells mounted to the rail.

RESULTS

As handrail height increased (up to 42 inches/107 cm), peak trunk angular displacement and velocity generally decreased, while shoulder elevation angles during reaching and peak handrail forces did not differ significantly between 36 and 42 inches (91 and 107 cm). Age was associated with reduced peak trunk angular displacements, but did not affect applied handrail forces.

CONCLUSION

Higher handrails (up to 42 inches) may be advantageous for trunk control when recovering from destabilizations during gait.

APPLICATION

Our results can inform building codes, workplace safety standards, and accessibility standards, for safer handrail design.

The effect of handrail cross-sectional design and age on applied handrail forces during reach-to-grasp balance reactions

Handrails have been shown to reduce the likelihood of falls. Despite common use, little is known about how handrail shape and size affect the forces that people can apply after balance loss, and how these forces and the corresponding ability to recover balance depend on age. Following rapid platform translations, 16 older adults and 16 sex-matched younger adults recovered their balance using seven handrail cross-sections varying in shape and size. Younger adults were able to withstand higher perturbations, but did not apply higher forces, than older adults. However, younger adults achieved their peak resultant force more quickly, which may reflect slower rates of force generation with older adults. Considering handrail design, the 38 mm round handrails allowed participants to successfully recover from the largest perturbations and enabled the highest force generation. Conversely, tapered handrails had the poorest performance, resulting in the lowest force generation and withstood perturbation magnitudes. Our findings suggest that the handrail cross-sectional design affects the magnitude of force generation and may impact the success of recovery. Our findings can inform handrail design recommendations that support effective handrail use in demanding, balance recovery scenarios.

Examining the influence of mental stress on balance perturbation responses in older adults

BACKGROUND

Reach-to-grasp responses following balance perturbations are important to fall prevention but are often ineffective in older adults. The ability to shift attention from an ongoing cognitive task to balance related processes has been shown to influence reach-to-grasp effectiveness in older adults. However, the added influence of stress and anxiety - known to negatively affect attention shifting ability - has not yet been explored in relation to recovery from balance perturbations. Given that fear and anxiety over falling is a key fall risk factor, an understanding of how such a negative mental state may affect postural reactions is important. This study aimed to investigate the effect of varied induced emotional states on reach-to-grasp balance responses in older adults.

METHODS

Healthy older adults (mean age 70.5 ± 5.38 years) stood laterally between 2 handrails with contact sensors. A safety harness with an integrated loadcell was worn to prevent falls and measure the amount of harness assistance (expressed as percent body weight). With instructions to grasp one rail to restore balance, participants' balance was laterally disturbed using surface translations under three randomized conditions: no cognitive task, neutral (verb generation) task, and mental stress task with negative prompts (paced auditory serial addition). The primary outcome was frequency of protective grasps. Secondary outcomes included frequency of harness assistance during trials with grasp errors as well as wrist movement time, trajectory distance, and peak velocity.

RESULTS

Perceived level of distress was highest for the mental stress task compared to no task (p < 0.001) and neutral task conditions (p = 0.008). The mental stress task resulted in the lowest percentage of protective grasps (p < 0.001) in response to balance perturbations. Closer examination of trials that resulted in grasp errors (i.e., collisions or overshoots), revealed increased harness assistance and reduced peak velocity of wrist movement (p < 0.001) under the mental stress condition compared to grasp errors that occurred under the no task or neutral task condition.

DISCUSSION AND CONCLUSION

Distressing mental thoughts immediately prior to a balance perturbation lead to reduced effectiveness in reach-to-grasp balance responses compared to no or neutral cognitive tasks and should be considered as a possible fall risk factor.

A kinematic analysis of balance recovery following an unexpected forward balance loss during stair descent

Falls during stair descent pose a major health concern. A stronger understanding of recovery from balance loss during stair descent is needed to guide fall prevention strategies and environmental design. We characterized balance recovery strategies, trunk and center-of-mass (COM) kinematics, and handrail use following unexpected forward balance loss during stair descent, and the effect of perturbation magnitude on these outcomes. Eighteen young adults experienced a rapid platform translation during stair descent to disrupt balance. Deception was used to reduce anticipation. All participants used compensatory stepping to recover balance, and most applied forces to the handrail in multiple directions. Higher perturbation magnitude resulted in higher COM velocity and handrail forces, more frequent incomplete steps, and quicker step contact time. Our findings provide a foundation for understanding balance recovery on stairs. The findings emphasize the importance of designing stairways that enable compensatory stepping, and handrails that permit adequate force generation in multiple directions to facilitate balance recovery on stairs.

Characterizing the demands of backward balance loss and fall recovery during stair descent to prevent injury

Understanding the demands of balance recovery on stairs is important for developing strategies to prevent falls on stairs. This study characterized recovery strategies and whole-body movement following unexpected backward balance loss during stair descent in twelve young adults. Following balance loss, peak downward COM velocity was approximately double that experienced during non-perturbation stair descent. Participants used several balance recovery strategies: harness reliance (n = 1), no grasping reaction (n = 3), and grasping some environmental feature (n = 8). Of the five participants who used the handrail, four demonstrated grasping errors. Peak resultant handrail forces ranged from 24.2N to 238.3N. The results highlight the challenge of balance recovery during stair descent, showing that some people will use any available surface to arrest a fall. Our findings serve as a benchmark to understand the impact of stair-related interventions on fall recovery.

Effect of handrail height and age on the timing and speed of reach-to-grasp balance reactions during slope descent

We investigated the effect of handrail height on the timing and speed of reach-to-grasp balance reactions during slope descent, in fourteen younger and thirteen older adults. Participants walked along an 8° slope mounted to a robotic platform. Platform perturbations evoked reach-to-grasp reactions. Handrail height did not significantly affect handrail contact time (i.e., time from perturbation onset to handrail contact) or movement time (i.e., time from EMG latency to handrail contact). Participants appeared to compensate for the increased hand-handrail distance with higher rails via increased peak upward hand speed, and decreased vertical handrail overshoot. Aging was associated with slower EMG latency, reduced hand acceleration time, and increased hand deceleration time. Our findings suggest that participants were not disadvantaged by higher handrails from reach-to-grasp timing or speed perspectives, and that other metrics (e.g., center-of-mass control after grasping) may be more important when evaluating handrail designs for balance recovery.

Effects of speed and direction of perturbation on electroencephalographic and balance responses

The modulation of perturbation-evoked potential (PEP) N1 as a function of different biomechanical characteristics of perturbation has been investigated before. However, it remains unknown whether the PEP N1 modulation contributes to the shaping of the functional postural response. To improve this understanding, we examined the modulation of functional postural response in relation to the PEP N1 response in ten healthy young subjects during unpredictable perturbations to their upright stance-translations of the support surface in a forward or backward direction at two different amplitudes of constant speed. Using independent components from the fronto-central region, obtained from subject-specific head models created from the MRI, our results show that the latency of onset of the functional postural response after the PEP N1 response was faster for forward than backward perturbations at a constant speed but was not affected by the speed of perturbation. Further, our results reinforce some of the previous findings that suggested that the N1 peak amplitude and peak latency are both modulated by the speed of perturbation but not by the direction of the perturbation. Our results improve the understanding of the relation between characteristics of perturbation and the neurophysiology of reactive balance control and may have implications for the design of brain-machine interfaces for populations with a higher risk of falls.

Arm reactions in response to an unexpected slip-Impact of aging

Slips and falls represent a serious public safety concern in older adults, with the segment of the United States population over the age of 65 accounting for about three quarters of all fall related deaths. The majority of falls in older adults are due to trips and slips. The objective of this study was to investigate how age affects arm reactions generated in response to unexpected slips. Thirty-three participants divided into two age groups (16 young, 17 old) participated in this study. Participants were exposed to two conditions: known dry walking (baseline) and an unexpected slip initiated when stepping onto a glycerol-contaminated floor. The upper extremity parameters of interest included the timing and amplitude of the shoulder flexion moment generated in response to the slip as well as the resulting angular kinematics (trajectories). The analysis of the kinetic data revealed a delayed shoulder flexion reaction to slips in older adults compared to their young counterparts, as well as a greater flexion moment magnitude. Knowledge of such upper body reaction mechanisms to unexpected slips may help to improve balance recovery training in older adults, as well as aid in the implementation of environmental modifications, e.g. handrails, to reduce falls-related injuries.

A novel method for synchronizing motion capture with other data sources for millisecond-level precision

Synchronization of multiple data collection systems is necessary for accurate temporal alignment of data, and is particularly important when considering rapid movements which occur in less than one second. This paper describes a novel method for synchronizing multiple data collection instruments including load cells and a motion capture system, using a common analog signal. An application of the synchronization method is demonstrated using biomechanical data collected during a rapid reach-to-grasp reaction, where data from motion capture and load cells are collected. Results are provided to validate and demonstrate the accuracy of the synchronization of motion capture with other data collection systems. During the reach-to-grasp trials, delays between the data collection systems ranged from 4ms to 235ms. The large range and variability in delay times between trials highlights the need for synchronization on a continual basis, rather than application of an average or constant value to correct for time delays between systems.

The role of the cerebral cortex in postural responses to externally induced perturbations

The ease with which we avoid falling down belies a highly sophisticated and distributed neural network for controlling reactions to maintain upright balance. Although historically these reactions were considered within the sub cortical domain, mounting evidence reveals a distributed network for postural control including a potentially important role for the cerebral cortex. Support for this cortical role comes from direct measurement associated with moments of induced instability as well as indirect links between cognitive task performance and balance recovery. The cerebral cortex appears to be directly involved in the control of rapid balance reactions but also setting the central nervous system in advance to optimize balance recovery reactions even when a future threat to stability is unexpected. In this review the growing body of evidence that now firmly supports a cortical role in the postural responses to externally induced perturbations is presented. Moreover, an updated framework is advanced to help understand how cortical contributions may influence our resistance to falls and on what timescale. The implications for future studies into the neural control of balance are discussed.

Prismatic displacement effect of progressive multifocal glasses on reaction time and accuracy in elderly people

BACKGROUND

Multifocal glasses (bifocals, trifocals, and progressives) increase the risk of falling in elderly people, but how they do so is unclear. To explain why glasses with progressive addition lenses increase the risk of falls and whether this can be attributed to false projection, this study aimed to 1) map the prismatic displacement of a progressive lens, and 2) test whether this displacement impaired reaction time and accuracy.

METHODS

The reaction times of healthy ≥75-year-olds (31 participants) were measured when grasping for a bar and touching a black line. Participants performed each test twice, wearing their progressives and new, matched single vision (distance) glasses in random order. The line and bar targets were positioned according to the maximum and minimum prismatic displacement effect through the progressive lens, mapped using a focimeter.

RESULTS

Progressive spectacle lenses have large areas of prismatic displacement in the central visual axis and edges. Reaction time was faster for progressives compared with single vision glasses with a centrally-placed horizontal grab bar (mean difference 101 ms, P=0.011 [repeated measures analysis]) and a horizontal black line placed 300 mm below center (mean difference 80 ms, P=0.007). There was no difference in accuracy between the two types of glasses.

CONCLUSION

Older people appear to adapt to the false projection of progressives in the central visual axis. This adaptation means that swapping to new glasses or a large change in prescription may lead to a fall. Frequently updating glasses may be more beneficial.

Do aging and dual-tasking impair the capacity to store and retrieve visuospatial information needed to guide perturbation-evoked reach-to-grasp reactions?

A recent study involving young adults showed that rapid perturbation-evoked reach-to-grasp balance-recovery reactions can be guided successfully with visuospatial-information (VSI) retained in memory despite: 1) a reduction in endpoint accuracy due to recall-delay (time between visual occlusion and perturbation-onset, PO) and 2) slowing of the reaction when performing a concurrent cognitive task during the recall-delay interval. The present study aimed to determine whether this capacity is compromised by effects of aging. Ten healthy older adults were tested with the previous protocol and compared with the previously-tested young adults. Reactions to recover balance by grasping a small handhold were evoked by unpredictable antero-posterior platform-translation (barriers deterred stepping reactions), while using liquid-crystal goggles to occlude vision post-PO and for varying recall-delay times (0-10s) prior to PO (the handhold was moved unpredictably to one of four locations 2s prior to vision-occlusion). Subjects also performed a spatial- or non-spatial-memory cognitive task during the delay-time in a subset of trials. Results showed that older adults had slower reactions than the young across all experimental conditions. Both age groups showed similar reduction in medio-lateral end-point accuracy when recall-delay was longest (10s), but differed in the effect of recall delay on vertical hand elevation. For both age groups, engaging in either the non-spatial or spatial-memory task had similar (slowing) effects on the arm reactions; however, the older adults also showed a dual-task interference effect (poorer cognitive-task performance) that was specific to the spatial-memory task. This provides new evidence that spatial working memory plays a role in the control of perturbation-evoked balance-recovery reactions. The delays in completing the reaction that occurred when performing either cognitive task suggest that such dual-task situations in daily life could increase risk of falling in seniors, particularly when combined with the general age-related slowing that was observed across all experimental conditions.

Effects of uni- and multimodal cueing on handrail grasping and associated gaze behavior in older adults

INTRODUCTION

It appears that age-related changes in visual attention may impair ability to acquire the visuospatial information needed to grasp a handrail effectively in response to sudden loss of balance. This, in turn, may increase risk of falling. To counter this problem, we developed a proximity-triggered cueing system that provides a visual cue (flashing lights) and/or verbal cue ("attention use the handrail") to attract attention to the handrail. This study examined the effect of handrail cueing on grasping of the rail and associated gaze behavior in a large cohort (n=160) of independent and ambulatory older adults (age 64-80).

METHODS

The handrail and cueing system was mounted on a large (2 m×6 m) motion platform configured to simulate a real-life environment. Subjects performed a daily-life task that required walking to the end of the platform, which was triggered to perturb balance by moving suddenly when they were adjacent to the rail. To prevent adaptation, each subject performed only one trial, and a deception was used to ensure that the perturbation was truly unexpected. Each subject was assigned to one of four cue conditions: visual, verbal, multimodal (visual-plus-verbal) or no cue.

RESULTS

Verbal cueing attracted overt visual attention to the handrail and markedly increased proactive grasping (prior to the onset of the balance perturbation) particularly when delivered unimodally. Subjects were otherwise much more likely to grasp the rail in reaction to the perturbation. A possible trend for visual cueing to improve the accuracy of these reactions was offset by adverse effects on reaction speed and on frequency of proactive grasping.

CONCLUSIONS

The results support the viability of using unimodal verbal cueing to reduce fall risk by increasing proactive handrail use. Conversely, they do not strongly support use of visual cueing (either alone or in combination with verbal cueing) and suggest that it may even have adverse effects. Further study is needed to evaluate effects of handrail cueing in a wide range of populations and real-life settings.

The influence of handrail predictability on compensatory arm reactions in response to a loss of balance

The current study examined whether compensatory arm reactions are influenced by the participant's knowledge of the handrail location prior to losing their balance. Thirteen young adults stood on a motor driven platform that could translate in the forward or backward directions. A handrail was positioned in a location that was either predictable (i.e., always on the participant's right) or unpredictable (i.e., on either the participant's right or left) to the participant. Unpredictability of the handrail location was ensured by using liquid crystal goggles to occlude the participant's vision until the onset of each translation. In response to each surface translation, participants were instructed to reach for and grasp the handrail as fast as possible. EMG activity from the posterior and anterior deltoids of the left and right arms as well as kinematic data of the wrist were recorded to quantify the resulting arm responses. It was found that in response to a loss of balance, participants activated the reaching arm 7 ms earlier (p = 0.020) and with a 21-30% greater amplitude (p = 0.010-0.029) during the predictable compared to unpredictable handrail condition. The earlier and larger EMG activity resulted in a 19% earlier initiation of arm movement (p = 0.016) and a 24% earlier handrail contact (p = 0.002) when the handrail was in a predictable compared to unpredictable location. These findings indicate that when a handrail is predictably located, individuals will pre-select their upcoming compensatory arm reactions prior to losing their balance and may be more effective in re-gaining stability.

Interactions between cold ambient temperature and older age on haptic acuity and manual performance

The impact of exposure to cold on individuals' motor skills demands a deeper understanding of the ways in which cold weather influences psychomotor and haptic performance. In this study, various facets of psychomotor performance were evaluated in order to determine the impacts of ambient cold exposure on older persons. Healthy younger and older persons performed a battery of haptic psychomotor tests at room (23° C) and cold (1° C) ambient temperatures. The results indicate that older individuals do not perform as well as younger persons across the battery of tests, with cold temperature further degrading their performance in dexterity tasks (in, for example, Minnesota Manual Dexterity test placing: F [1, 16] = 10.23, p < .01) and peak precision grip force generation (F [1, 16] = 18.97, p < .01). The results suggest that cold weather may have an impact on the occupations older persons are able to perform during the winter months.

Arm reactions evoked by the initial exposure to a small balance perturbation: a pilot study

Perturbation of whole-body stability often evokes rapid arm reactions. It has been suggested that the earliest arm activation is a generic (e.g. startle-like) response to which a later stabilizing (e.g. counterweight or reach-to-grasp) or impact-protection component can be appended. To examine whether the initial part of the reaction is generic, we examined arm reactions evoked by small balance perturbations in 12 healthy young adults while varying perturbation direction (rightward or forward platform translation) and environmental conditions (handrail present or absent). The perturbation magnitude was selected to be sufficiently small to obviate the need to use the arms for stabilization. To avoid adaptation or habituation, analysis focused on each subject's very first exposure to the perturbation. Most subjects exhibited active movement of both arms in reaction to the perturbation, but there was large (non-stereotypical) inter-subject variation in muscle-onset latency and arm kinematics. Furthermore, the velocity and direction of the initial arm movement were affected by perturbation direction, in a manner consistent with functional strategies (counterweight strategy in backward falls, hybrid counterweight/protective strategy in leftward falls). Although subjects never contacted the handrail, responses were slower when it was present. These results are not consistent with a generic stereotyped response, but suggest instead that even the earliest component of first-trial arm reactions was functionally modulated.

The use of peripheral vision to guide perturbation-evoked reach-to-grasp balance-recovery reactions

For a reach-to-grasp reaction to prevent a fall, it must be executed very rapidly, but with sufficient accuracy to achieve a functional grip. Recent findings suggest that the CNS may avoid potential time delays associated with saccade-guided arm movements by instead relying on peripheral vision (PV). However, studies of volitional arm movements have shown that reaching is slower and/or less accurate when guided by PV, rather than central vision (CV). The present study investigated how the CNS resolves speed-accuracy trade-offs when forced to use PV to guide perturbation-evoked reach-to-grasp balance-recovery reactions. These reactions were evoked, in 12 healthy young adults, via sudden unpredictable antero-posterior platform translation (barriers deterred stepping reactions). In PV trials, subjects were required to look straight-ahead at a visual target while a small cylindrical handhold (length 25%> hand-width) moved intermittently and unpredictably along a transverse axis before stopping at a visual angle of 20°, 30°, or 40°. The perturbation was then delivered after a random delay. In CV trials, subjects fixated on the handhold throughout the trial. A concurrent visuo-cognitive task was performed in 50% of PV trials but had little impact on reach-to-grasp timing or accuracy. Forced reliance on PV did not significantly affect response initiation times, but did lead to longer movement times, longer time-after-peak-velocity and less direct trajectories (compared to CV trials) at the larger visual angles. Despite these effects, forced reliance on PV did not compromise ability to achieve a functional grasp and recover equilibrium, for the moderately large perturbations and healthy young adults tested in this initial study.