July 2004
Volume 45, Issue 7
Free
Clinical and Epidemiologic Research  |   July 2004
Stepping Up to a New Level: Effects of Blurring Vision in the Elderly
Author Affiliations
  • Karen Heasley
    From the Department of Optometry,
  • John G. Buckley
    From the Department of Optometry,
  • Andy Scally
    Institute of Health Research, School of Health, and
  • Pete Twigg
    School of Engineering, Design and Technology, University of Bradford, Bradford, United Kingdom.
  • David B. Elliott
    From the Department of Optometry,
Investigative Ophthalmology & Visual Science July 2004, Vol.45, 2122-2128. doi:10.1167/iovs.03-1199
  • Views
  • PDF
  • Share
  • Tools
    • Alerts
      ×
      This feature is available to Subscribers Only
      Sign In or Create an Account ×
    • Get Citation

      Karen Heasley, John G. Buckley, Andy Scally, Pete Twigg, David B. Elliott; Stepping Up to a New Level: Effects of Blurring Vision in the Elderly. Invest. Ophthalmol. Vis. Sci. 2004;45(7):2122-2128. doi: 10.1167/iovs.03-1199.

      Download citation file:


      © 2015 Association for Research in Vision and Ophthalmology.

      ×
  • Supplements

purpose. To determine the effects of blurring vision on whole-body center-of-mass (CM) dynamics and foot-clearance parameters in elderly individuals performing a single step up to a new level.

methods. Twelve healthy subjects (mean age, 72.3 ±4.17 years) performed a single step up to a new level (heights of 73 and 146 mm). Trials were undertaken with vision optimally corrected and with vision diffusively blurred by light-scattering lenses (cataract simulation). CM and foot-clearance parameter data were assessed by analyzing data collected by a five-camera, three-dimensional (3-D) motion analysis system.

results. When vision was blurred, subjects took 11% longer to execute the stepping task (P < 0.05), mediolateral displacement of the point of application of the ground reaction force vector (i.e., weighted average of all pressures over the area in contact with the ground; the so called center of pressure, CP) decreased from 37.6% of stance width to 28.3% (P < 0.01), maximum distance between the mediolateral position of the CM and CP decreased by 9.8 mm (P < 0.01), and toe clearance (distance between tip of shoe and edge of step) increased in both the horizontal (28%) and vertical (19%) direction (P < 0.05).

conclusions. These findings suggest that when vision was blurred, subjects used a twofold safety-driven adaptation: First, to increase dynamic stability they ensured that the horizontal position of their CM was kept close to the center of the base of support and second, they increased horizontal and vertical toe clearance while swinging their lead limb forward to reduce the risk of tripping.

Within the elderly population, falls are the major cause of injury-related admissions to emergency departments worldwide. For example, in the United Kingdom, falls accounted for 57% of all injury-related hospitalization in 1995–1996 in the age group 65 years and over, 1 whereas in the United States from 1992 to 1995 and in Australia during 1997 to 1998, the incidence was 24% 2 and 54%, 3 respectively. 
It is widely accepted that stairs are a common risk factor for falls in the elderly. 4 5 6 7 In addition, falls can frequently occur on transitions between levels, such as when negotiating steps or curbs. 8 Visual impairment (the incidence of which increases with age) has been strongly associated with an elevated risk of falling. 9 10 11 12 13 14 15 16 17 For example, in a study assessing the causes of admissions to an acute geriatric clinic, a high prevalence of visual impairment was found in those patients admitted after a fall. 18 Furthermore, it was reported that 79% of the visual impairment was correctable, either through updated spectacles (40%) or cataract surgery (37%). 18 Cataract has also been associated with increased incidence of falls in community-dwelling elderly. 12 14 Moreover, reduced contrast sensitivity, which is the main visual function affected by cataract, 19 20 has been found to be associated with falls. 16 21 22 23 It thus seems highly pertinent to investigate how cataract can affect how the elderly ascend stairs or steps. 
There is a paucity of literature indicating how vision impairment affects step and stair negotiation by the elderly, and the few studies undertaken concentrate on stair descent. 24 25 Findings indicate that blurring an individual’s vision results in an increase in foot clearance and a foot placement position that is farther back on the step. It has been suggested that these adaptations are to compensate for the difficulty and errors in perceiving the edge of the step. 25 It has also been reported that a reduction in luminance has little affect on the contact forces between the foot and the ground in elderly or young subjects. 24  
In the present study we investigated how blurring vision by using a simulated cataract affects whole-body center-of-mass (CM) dynamics and foot-clearance parameters in healthy elderly individuals performing a single step onto a new level, the height of which was equivalent to a curb or a household stair riser. We chose the experimental paradigm of a single step rather than, for example, analyzing stair ascent, because the likelihood of elderly subjects fatiguing was minimized, the parameters linked to the execution of a single step have been shown to be the same as those involved in performing a multistep gait task, 26 and stair ascent in elderly individuals with declining function is sometimes undertaken one step at a time. 27  
A single step onto a new level involves moving from a stable static situation to a dynamic unstable one, and then, once the step is complete, the transfer of body weight from one limb to the other. This can be assessed by determining the relationship between the trajectories of the (fore and aft, and side-to-side) components of the CM and point of application of the ground-reaction force (GRF) vector (i.e., the weighted average of all pressures over the area in contact with the ground; the so-called center of pressure, CP). 28 29 30 To maintain balance, the CM must be kept within the base of support (which can be approximated as the surface area under and between the feet). Displacements in the CP during standing are often evaluated because they reflect the central nervous system’s (CNS) response to movements of the CM. For example, to ensure the CM is kept safely within the base of support when standing, the CP continuously moves either more posterior or more anterior of the CM as the individual continually sways back and forth. Thus, the CM and CP, over time, converge to the same average location and have displacements that are generally in-phase but of different magnitudes. 31  
For a step to occur, the CM must be moved toward the stance limb, allowing the swing limb to be lifted from the floor. For this to occur, the CP is initially moved backward and toward the swing limb so that the CM moves forward and laterally toward the stance limb. This motion is brought about primarily through muscular activity across the ankle and hip joints. This divergence of the CP and CM is required for a step to be taken. However, if the divergence is too large, instability occurs. 32 After foot contact of the swing limb, the CM is again supported by both limbs, and the CM and CP thus reconverge. Performing a gait initiation for a step up also requires appropriate swing limb kinematics to ensure that the foot has adequate horizontal and vertical clearance, so that it safely clears the edge/apex of the step as it is swung forward. The foot must then be optimally placed on the step’s surface to allow body weight to be transferred from one limb to the other in a controlled and thus safe manner. 
By evaluating the relationship between the horizontal trajectories of the CM and CP and the foot-clearance parameters, we sought in the present study to gain insight into how diffusively blurring vision, using goggles to simulate the effects of cataract, would affect the key biomechanical factors required to ascend stairs and steps safely. 
Methods
Subjects
Twelve subjects (mean age, 72.3 ± 4.17 years; height, 162.8 ± 8.4 cm; and mass, 73 ± 11.8 kg) volunteered to take part in the study. All were regular attendees at the University of Bradford Teaching Eye Clinic. The subjects (seven men, five women) were screened using an in-house self-report health status questionnaire. Subjects with a history of falling and any mobility impairment such as joint replacement, severe arthritis, or previous injury affecting gait were excluded, as were subjects with two or more comorbidities and/or a history of amblyopia, strabismus, eye disease, ocular surgery, corrected binocular visual acuity (VA) worse than 0.0 logarithm of the minimum angle of resolution (logMAR; Snellen equivalent 20/20) and/or visible ocular disease as determined by ocular screening using slit-lamp biomicroscopy, tonometry, indirect opthalmoscopy and central visual field screening. The 12 subjects self-reported they regularly partook in light to moderate activities, as described in the activity level scale of the Allied Dunbar National fitness survey 1992. 33 Examples include gardening, social dancing, and heavy housework. 
The tenets of the Declaration of Helsinki were followed and the study gained ethical approval from the University’s Research Ethics Committee. Written informed consent was obtained from all subjects. 
Assessment of Visual Function
A subjective refraction was performed to obtain the subjects’ optimal refractive correction at 4.0 m. Each patient’s vision was subsequently corrected using appropriate trial case lenses placed in a trial frame. We chose to use the 4.0-m correction (rather than one that provided optimal focus of the step derived from each subject’s height) because in the “real” world subjects would use their distance vision spectacles when walking. Binocular VA was measured with the Early Treatment Diabetic Retinopathy Study (ETDRS) logMAR chart, using a by-letter scoring system, chart luminance of 160 cd/m2 and a 4.0-m working distance. 34 Binocular contrast sensitivity (CS) was measured using the Pelli-Robson chart at 1.0 m using a by-letter scoring system and a chart luminance of 200 cd/m2. 35 Binocular VA and CS were remeasured with the subjects’ vision blurred by placing cataract simulation lenses (Vistech Consultants Inc., Dayton, OH) over the trial frame. 36 37  
Protocol
A five-camera, three-dimensional (3-D) motion analysis system (Vicon 250; Oxford Metrics Ltd., Oxford, UK) was used to record (at 50 Hz) subjects as they stepped up to a new level. The subjects started from a stationary standing position with feet placed separately on two adjacent force platforms embedded in the floor (Fig. 1) . The force platforms (AMTI OR6-7–1000; Advanced Mechanical Technology Inc., Boston, MA) were used to determine (at 100 Hz) the contact forces between the feet and the ground, from which CP displacements could be computed. The cameras were wall mounted at approximately 2.3 m above the floor and were positioned around the laboratory to enable viewing the performer from all directions (i.e., from approximately every 72°). Reflective markers (25 mm diameter) were attached to each subject, either directly to skin, onto clothing, or on elasticized bands over clothing, to the following body locations: fifth metatarsal heads, second metatarsal heads, lateral malleoli, calcanei, lateral aspect of each shank and thigh, lateral femoral condyles, anterior superior iliac spines, sacrum, medial and lateral sides of wrists, lateral epicondyles, acromions and xiphoid processes, jugular notch, vertebrae T10 and C7, and the anterior-lateral and posterior-lateral aspect of the head. Markers were also placed on the top surface and front edge of the step. These markers were used to define its position (particularly the step’s apex) relative to the subject (Fig. 1 , inset). 
Subjects completed steps onto heights of 73 and 146 mm, which correspond to typical dimensions of an average roadside curb and indoor stair rise, respectively. The steps were made of medium density fiberboard (MDF) of 18-mm thickness, which were bonded together to create a solid block with a standing surface area of 928 × 508 mm. No covering was used on the steps, but the force platforms were covered, like the surrounding floor, with a matt green foam-backed vinyl floor covering (≈2 mm thick). The laboratory was illuminated with fluorescent strip lighting mounted in the ceiling (∼2.8 m above the floor). The illuminance over the platforms and step area was approximately 400 lux, and the luminance of the floor and the top surface of the step were 15 and 36 cd/m2 respectively, measured using a photometer (CS-100; Minolta Co. Ltd., Osaka, Japan). Contrast between the floor and the top surface of the step was 71%. 
Subjects started with their toes behind a line that marked half their foot length away from the leading edge of the step. They were given the instruction “step up,” at which point they took a single step onto the new level and came to a stationary standing position with both feet on top of the step. Subjects were instructed to ensure that the whole of the leading foot was placed beyond the edge of the step, and trials when this did not occur were discarded and repeated. Subjects were free to choose which foot to lead with, and once chosen, all subjects repeatedly lead with the same foot. Data capture was stopped 5 seconds after the step had been completed. Before data collection, each subject performed (with optimally corrected vision) a practice trial at both of the step heights. 
Data were collected during a single testing session for each subject. Each step height was performed twice with the subject’s optimal correction provided by trial case lenses in a trial frame and twice with cataract simulators, which were also placed in the trial frame. Using trial case lenses avoided having differences in stepping strategy occur as a consequence of some subjects’ wearing multifocal spectacles and others’ wearing single vision distance spectacles. 38 Throughout the stepping trials, subjects wore their usual, everyday shoes. The order in which each step height and visual condition were performed was randomized. An observer was present during data collection to ensure that if the subjects tripped, they did not fall and hurt themselves. Subjects were given a short, seated rest each time the next step height and/or vision condition was prepared. 
Using a computer program (Plug-in Gait software, Vicon; Oxford Metrics Systems Ltd.), coordinate data of each marker were filtered using the cross-validating, quintic, spline-smoothing routine 39 and were then processed to define a 3-D linked segment model of the subject, whereby markers attached to each body segment (e.g., forearm, thigh, head) were used to define its length and/or girth. To avoid hindering limb movements, limb segment markers were positioned only on lateral aspects of segments. Thus, limb segment girths were further defined by determining, where applicable (e.g., knee and ankle), proximal and distal joint widths in the frontal plane using an anthropometer. These values, along with those determined from the markers, were used to determine segmental volume, and by multiplying by a reference density value 40 provided segmental mass. CM of each segment was then calculated with standard regression equations 40 and whole-body CM was calculated as the weighted sum of all segment CM locations. 41 42 The anthropometer was also used to measure the position of the shoe tip sole relative to the second and fifth metatarsal heads, to allow accurate assessment of the shoe tip position (Fig. 1 , inset). Joint angles, determined as the resultant angle between two adjacent segments, were also calculated. The following data, computed for each time point of the stepping movement, were exported (at 50 Hz) in ASCII format for further analysis (see Fig. 1 for coordinate reference system): whole body CM coordinates (x, y), GRFs from each force platform (Fx, Fy, Fz), center of pressure coordinates (x,y) from each force platform (CPl and CPr), swing limb heel marker coordinates (x, y, z), swinging limb’s shoe tip (Fig. 1 , inset) coordinates (x, y, z), and swing limb ankle flexion and extension joint angle (θy). 
Data Analysis
Using the following formula, 31 force and center of pressure data from each platform were combined to provide global center of pressure coordinates (CP)  
where CPl and CPr are the CPs under the left and right foot, respectively. Fz l and Fz r are the vertical reaction forces under the left and right foot, respectively. 
The stepping movement was divided into four subphases (Fig. 2) . 32 43 (A) Anticipatory: from the instant when the CP first moved beyond 10 mm laterally, up to the instant of heel-off of the leading limb, defined as the point at which the height of the heel marker exceeded 1 mm for three consecutive frames. (B) Initial Swing: from heel-off to maximum heel height, determined as the instant when the heel marker reached its maximum height. (C) Terminal Swing: from maximum heel height to the instant of foot contact, established as the point at which the swing limb heel trajectory ceased to travel vertically downward. This coincided with the vertical GRF on the stance limb dropping below one body weight during single support. (D) Weight Transfer: from the instant of foot contact (swing limb) to the instant of toe-off of the contralateral support limb, defined as the instant the vertical GRF dropped below 20 N. 
The anticipatory phase is when postural adjustments are made to move the body’s CM from a stable stationary state to an unstable dynamic one. 43 Thus, the effects of blurring vision were examined for this phase by determining the divergence between the CM and CP and the peak displacement of the CP in the anterior-posterior (A/P) and medial-lateral (M/L) directions. To gain an idea of how blurring vision affects foot-clearance parameters, the minimum horizontal and vertical toe clearance (i.e., minimum distance between shoe tip and apex of step; Fig. 1 , inset) and the ankle angle at minimum vertical toe clearance and foot contact were examined. Finally, to identify which phase (if any) would be most affected by blurred vision, duration and peak CM velocities (in A/P and M/L directions) for each phase were examined. 
Data were analyzed on computer with a random-effects population averaged model (Stata ver.7.0; Stat Corp., College Station, TX). This multivariate model was obtained using the generalized least squares (GLS) random-effects estimator, which produces a matrix-weighted average of between-subjects and within-subjects results. An exchangeable correlation structure was judged to be appropriate, given the experimental design, and due to the exploratory nature of the study no type I error adjustment of the α level was deemed unnecessary. Thus, level of significance was set at P < 0.05. Factors of interest were incorporated sequentially and their statistical significance was tested using a likelihood ratio test. Factors with P < 0.1 were provisionally retained, whereas those more than 0.1 were dropped. The final model adopted was the most parsimonious one that was thought to explain the data (adequately). The probabilities quoted herein are those associated with the specific terms in the final regression model, which were:
  1.  
    Vision: a fixed factor with two levels, normal (optimal correction) and blurred (cataract simulation)
  2.  
    Repetition: a fixed factor with two levels (trial one and trial two)
  3.  
    Step height: a fixed factor with two levels (low and high step)
  4.  
    Phase: a fixed factor with four levels (Anticipatory, Initial Swing, Terminal Swing, and Weight Transfer).
Results
Visual Assessments
With vision optimally corrected, group mean VA was −0.08 ± 0.03 logMAR (Snellen equivalent, 20/16) and CS was 1.70 ± 0.08 logCS. When vision was blurred by the light-scattering lenses these values were 0.12 ± 0.07 logMAR (Snellen equivalent 20/26) and 0.97 ± 0.09 logCS, respectively. 
CM Dynamics and Foot-Clearance Parameters
Mean (±SD) toe clearance, CM and CP parameters, and phase durations are shown in Table 1 for the normal and blurred vision conditions. When vision was blurred, mean duration across both step heights and across all subphases increased significantly (P = 0.026), and subjects took, on average, 11% longer to complete the entire movement. The interactions between the effect of the cataract simulation on duration with step height, phase, or repetition were not significant (P > 0.1). The backward displacement of the CP during the anticipatory and initial swing phases was unchanged across vision conditions and step heights (P > 0.1), whereas blurring vision caused the peak sideways displacement of the CP to decrease from 37.6% of stance width to 28.3% (Table 1 , P = 0.0001). Peak CM–CP divergence reflected the findings in CP displacement—that is, when vision was blurred only the divergence in the M/L direction was found to significantly decrease (by 9.8 mm, Table 1 , P < 0.0001). Despite the significant difference between vision conditions in CM–CP M/L divergence, differences in M/L CM peak velocity were not significant (P > 0.1), nor were differences in A/P CM peak velocity (P > 0.1). The interactions of subphase or repetition with vision condition were nonsignificant for M/L CP displacement or CM-CP divergence (P > 0.1). 
With vision blurred, minimum toe clearance across both step heights increased significantly (mean vertical increase of +19% [11.1 mm], P = 0.019, mean horizontal increase of +28% [22.0 mm], P = 0.0004; Fig. 3 ). Vertical clearance was found to decrease with repetition—that is, there was a mean decrease in clearance of 11.4 mm (P = 0.009) from the first to the second attempt—whereas, horizontal clearance was not significantly different (P = 0.11). Toe-clearance values were greater for the higher step trials, with horizontal and vertical clearances 10.5 (P = 0.03) and 7.5 (P = 0.05) mm larger than for the low step, respectively. The interactions of vision and step height were nonsignificant (P > 0.1) for both horizontal and vertical toe clearance. 
The ankle angle at minimum vertical toe clearance and at the instant of foot contact was similar for both visual conditions (Table 1 , P = 0.75), and the ankle was significantly more dorsiflexed (by 4.0°) in the higher step trials (P = 0.0005). From the instant of minimum vertical toe clearance to the instant of foot contact, there was a significant decrease in ankle dorsiflexion (by 13.3°, P = 0.0001). The interaction or repetition terms yielded no relationships of significance. 
Discussion
The light-scattering lenses reduced contrast sensitivity to a level similar to that of a dense cataract. 44 It has been shown that reductions in CS (rather than VA) are related to postural instability 16 21 22 and falls in the elderly. 16 22 23 Thus, the results presented herein provide some insight into how stepping up to a new level (and thus stair ascent) may be affected by cataract in elderly individuals. 
It has been shown that the CM–CP divergence during gait initiation, sometimes referred to as the CM–CP moment arm, 45 can be used as a tool to assess balance function in elderly people. It has also been suggested that the instant of maximum divergence signifies positional instability. 32 In the present study we found that with blurred vision, subjects decreased their M/L CM–CP divergence across the anticipatory and initial swing phases, whereas A/P divergence remained unchanged. The findings relating to M/L divergence were mainly a consequence of the decreased M/L displacement in the CP with blurred vision (P < 0.0001). These findings suggest that subjects were particularly cautious regarding M/L instability and used a safety-driven adaptation with blurred vision that ensured the CM was kept well within the safety margins of the base of support. When stepping on to a new level, M/L instability would be difficult to correct, particularly during the single limb support phase where any displacement of the CM that is lateral of the support limb would be likely to result in a sideways loss of balance. 46 47  
Subjects took on average 11% longer (P = 0.026) to complete the stepping action when vision was blurred. By looking at the duration of the individual subphases of the stepping movement, we had hoped to determine where in the stepping action the main adaptations due to blurring vision would occur. However, increases in duration were similar across the four subphases (P > 0.05). The anticipatory phase is when the CNS integrates sensory signals and relays the information to the neuromuscular system to convert postural adjustments into intended movement. 48 Findings suggest that with cataract simulation the visual system was not able to provide efficiently the appropriate exteroceptive information regarding the environment, in this case the step height dimensions, and thus more time was required in preparation for movement. The increase in duration in the other phases can be attributed to a combination of disrupted exproprioceptive information regarding accurate foot placement and impaired exteroceptive information regarding step dimension. The resultant increase in toe clearance (both horizontal and vertical) indicates the swing limb foot had a higher trajectory and thus moved over a greater distance, which would have required more time. In addition, the reduction in M/L CM–CP divergence when vision was blurred, would have had the effect of reducing the speed at which body weight was initially transferred from being supported by both limbs onto one limb, to allow the other to be lifted from the ground before being swung forward. The diminished exproprioceptive and exteroceptive information would have also meant subjects would have had to rely more on somatosensory feedback to indicate that the foot had landed in an appropriate and safe position, which would have delayed the transfer of body weight from the trailing to the lead limb until subjects were confident this had occurred. 
The increase in vertical toe clearance when stepping up with blurred vision is similar to the findings of increased vertical toe clearance when stepping over obstacles in patients with cataracts 44 and AMD. 49 Previous work investigating toe clearance parameters in the elderly when stepping over obstacles has commonly reported the vertical component of the toe trajectory vector as the foot crosses the obstacle. 50 In the present study, the horizontal and vertical toe clearance, as the foot crossed the edge of the step during its forward swing, were both found to increase significantly when vision was blurred. The increased horizontal toe clearance occurred despite the fact that the starting distance away from the step was controlled. This implies that, when vision was blurred, subjects had difficulty clearly defining the edge of the step or perceiving the height of the step’s surface and increased both horizontal and vertical clearance to provide a greater margin of safety. 
The increase in vertical toe clearance did not result from an increase in ankle dorsiflexion—that is, ankle angle changes between the visual conditions were nonsignificant (P > 0.1). Thus, the increased elevation of the toe must have been the product of adaptations at the hip or knee. However, a limitation of the study was that we did not look at knee or hip kinematics; thus, we cannot confirm or deny this presumption. Spaulding et al. 49 found that compared with control subjects, the ankle in subjects with AMD is significantly more dorsiflexed (lifting the toe above the heel) than in control subjects, as it moves over the edge of a raised surface while walking. They postulated that subjects used this adaptation because there was less chance of tripping if the heel rather than the toe hit the obstacle. In the present study, we hypothesized that a similar adaptation would occur when vision was blurred, but this was not found. This could suggest that a change in ankle angle when stepping may be an adaptation learned over a period of time by individuals with chronic conditions such as ARM or cataract, or that such an adaptation would be warranted during walking but would have limited value when stepping up to a new level. 
Another important finding regarding vertical toe clearance was that it was seen to significantly decrease from the first to the second trial, by an average of 11.4 mm (P = 0.009) across both visual conditions. This suggests that the desire to conserve energy (increasing the height of the foot above the step involves more physiological work) is at conflict with the desire to increase margins of safety. This is similar to the findings of Simoneau et al., 25 who found that a cyclic feedback response mechanism allowed their subjects to decrease step clearance while going down a flight of stairs—that is, it was less going over the middle steps than it was when negotiating the upper steps. Although in the present study a single-step protocol was used, it appears as if a similar learning and adaptation mechanism was present in the form of a feedback pathway. The range of minimum toe clearance on the second trial was 25.2 to 120.3 mm (mean, 68.6 mm) and 18.2 to 103.8 mm (mean, 52.9 mm) with and without blur, respectively. This highlights that foot clearance and placement may be a critical factor in the etiology of falls, because with such small margins of safety any misjudgments are likely to cause tripping. Furthermore, with such a small margin of error, the possible role of poorly fitting shoes or slippers or ill-fitting carpets in tripping is also highlighted. 
Why vertical clearance reduced with repetition but horizontal clearance remained unchanged is cause for speculation. During stepping, there is a period when body weight is supported between the limb on the floor and the limb on the step’s surface, and somatosensory information from the stepping limb would provide knowledge about the height of the step’s surface that could be used in the subsequent trial. However, no such information is provided with regard to the location of the edge of the step, because the toes do not make contact with the edge of the step, as to do so would result in a trip. In addition, subjects were instructed to ensure that the whole of the leading foot was placed beyond the edge of the step. Thus, the only sensory feedback subjects received regarding the horizontal position of the edge of the step was through the visual system, and it follows that horizontal toe clearance would not reduce with trial repetition when vision was blurred. 
In summary, the findings of the present study indicate that diffusively blurring vision resulted in subjects taking longer to complete the step up to a new level, and this was completed with less divergence between the M/L position of the CM and CP, and with greater vertical and horizontal toe clearance (P < 0.05). These findings suggest that when vision was blurred subjects used a twofold safety driven adaptation. First to increase dynamic stability they ensured the horizontal position of their CM was kept close to the center of the base of support, and second, to reduce the risk of tripping, they altered swing limb kinematics to increase toe clearance and provide greater room for error. This highlights the importance of accurate visual feedback in the precise control of body CM dynamics and swing limb kinematics when stepping. 
Differences between adaptations produced by visual blur from simulations and chronic eye disease are important, because they indicate adaptations that are learned over time and therefore indicate adaptations that could be taught. However, the temporary effects of blurring vision by wearing cataract simulation goggles may be different from the effects of cataractous blur, which probably develop over time. Thus, further work is needed to determine whether the chronic effects of cataractous blur would result in adaptations similar to those found in the present study. In addition, as subjects wore their distance refractive correction using single-vision lenses, individuals who habitually wear multifocal lenses might have used a different stepping strategy to the one they would normally use. Thus, more work is also needed to determine whether stepping strategy is affected by spectacle lens type. 
Figure 1.
 
Step dimensions and placement alongside force platforms (P1, P2). Subjects started with the toes positioned behind a line half a foot-length (their own) away from the edge of the step. Inset: diagram showing how the minimum vertical and horizontal toe clearance from the apex of the step was defined. Minimum horizontal and vertical toe clearance did not necessarily occur at the same instant.
Figure 1.
 
Step dimensions and placement alongside force platforms (P1, P2). Subjects started with the toes positioned behind a line half a foot-length (their own) away from the edge of the step. Inset: diagram showing how the minimum vertical and horizontal toe clearance from the apex of the step was defined. Minimum horizontal and vertical toe clearance did not necessarily occur at the same instant.
Figure 2.
 
Subphases of stepping: A, Anticipatory (beginning of movement to heel-off); B, Initial Swing (heel-off to maximum heel height); C, Terminal Swing (max heel height to foot contact); D, Weight Transfer (foot contact to toe-off support limb).
Figure 2.
 
Subphases of stepping: A, Anticipatory (beginning of movement to heel-off); B, Initial Swing (heel-off to maximum heel height); C, Terminal Swing (max heel height to foot contact); D, Weight Transfer (foot contact to toe-off support limb).
Table 1.
 
Parameters for Blurred and Normal Vision
Table 1.
 
Parameters for Blurred and Normal Vision
Duration (s) Toe Clearance CM-CP Divergence Peak CP Displacement Peak CM Velocity Ankle Angle (deg)
Z (mm) X (mm) M/L (mm) A/P (mm) M/L (% Stance Width) A/P (mm) M/L (mm/s) A/P (mm/s)
Normal 0.44 ± 0.20 58.24 ± 33.98 79.07 ± 43.41 82.91 ± 30.34 34.17 ± 19.42 37.62 ± 17.99 10.79 ± 6.97 254.5 ± 75.2 289.9 ± 186.9 9.26 ± 8.64
Blurred 0.49 ± 0.27* 69.36 ± 25.41* 101.05 ± 32.99* 73.07 ± 29.34* 34.71 ± 18.71 28.28 ± 16.94* 10.94 ± 9.20 247.0 ± 86.1 289.0 ± 196.0 9.02 ± 9.31
Figure 3.
 
The effects of visual blur on the mean (± SD) minimum horizontal and vertical toe clearance from the leading edge of the step, averaged across step heights and repetitions.
Figure 3.
 
The effects of visual blur on the mean (± SD) minimum horizontal and vertical toe clearance from the leading edge of the step, averaged across step heights and repetitions.
 
Cripps R, Carman J. Falls by the elderly in Australia: trends and data for 1998. 2001; Australian Institute of Health and Welfare Canberra.
Fuller GF. Falls in the elderly. Am Fam Physician. 2000;61:2159–2168. [PubMed]
Dowsell T, Towner E, Cryer C, Jarvis S, Edwards P, Lowe P. Accidental Falls: Fatalities and Injuries—An Examination of the Data Sources and Review of the Literature on Preventive Strategies. 1999; Department of Trade and Industry London, UK.
Roys MS. Serious stair injuries can be prevented by improved stair design. Appl Ergon. 2001;32:135–139. [CrossRef] [PubMed]
Wyatt JP, Beard D, Busuttil A. Fatal falls down stairs. Injury. 1999;30:31–34. [CrossRef] [PubMed]
Startzell JK, Owens DA, Mulfinger LM, Cavanagh PR. Stair negotiation in older people: a review. J Am Geriatr Soc. 2000;48:567–580. [PubMed]
Tinetti ME, Doucette JT, Claus EB. The contribution of predisposing and situational risk factors to serious fall injuries. J Am Geriatr Soc. 1995;43:1207–1213. [PubMed]
Gallagher EM, Scott VJ. The STEPS Project: participatory action research to reduce falls in public places among seniors and persons with disabilities. Can J Public Health. 1997;88:129–133. [PubMed]
Tinetti ME, Williams CS. The effect of falls and fall injuries on functioning in community-dwelling older persons. J Gerontol. 1998;53:M112–M119.
Rubenstein LZ, Josephson KR. The epidemiology of falls and syncope. Clin Geriatr Med. 2002;18:141–158. [CrossRef] [PubMed]
Glynn RJ, Seddon JM, Krug JH, Jr, Sahagian CR, Chiavelli ME, Campion EW. Falls in elderly patients with glaucoma. Arch Ophthalmol. 1991;109:205–210. [CrossRef] [PubMed]
Ivers RQ, Cumming RG, Mitchell P, Attebo K. Visual impairment and falls in older adults: the Blue Mountains Eye Study. J Am Geriatr Soc. 1998;46:58–64. [PubMed]
Ivers RQ, Norton R, Cumming RG, Butler M, Campbell AJ. Visual impairment and risk of hip fracture. Am J Epidemiol. 2000;152:633–639. [CrossRef] [PubMed]
McCarty CA, Fu CL, Taylor HR. Predictors of falls in the Melbourne visual impairment project. Aust NZ J Public Health. 2002;26:116–119.
Felson DT, Anderson JJ, Hannan MT, Milton RC, Wilson PW, Kiel DP. Impaired vision and hip fracture. The Framingham Study. J Am Geriatr Soc. 1989;37:495–500. [PubMed]
Lord SR, Clark RD, Webster IW. Visual acuity and contrast sensitivity in relation to falls in an elderly population. Age Ageing. 1991;20:175–181. [CrossRef] [PubMed]
Lord SR, Dayhew J. Visual risk factors for falls in older people. J Am Geriatr Soc. 2001;49:508–515. [CrossRef] [PubMed]
Jack CI, Smith T, Neoh C, Lye M, McGalliard JN. Prevalence of low vision in elderly patients admitted to an acute geriatric unit in Liverpool: elderly people who fall are more likely to have low vision. Gerontology. 1995;41:280–285. [CrossRef] [PubMed]
Elliott DB, Situ P. Visual acuity versus letter contrast sensitivity in early cataract. Vision Res. 1998;38:2047–2052. [CrossRef] [PubMed]
Adamsons I, Rubin GS, Vitale S, Taylor HR, Stark WJ. The effect of early cataracts on glare and contrast sensitivity: a pilot study. Arch Ophthalmol. 1992;110:1081–1086. [CrossRef] [PubMed]
Turano K, Rubin GS, Herdman SJ, Chee E, Fried LP. Visual stabilization of posture in the elderly: fallers vs. nonfallers. Optom Vis Sci. 1994;71:761–769. [CrossRef] [PubMed]
Lord S, Menz H. Visual contributions to postural stability in older adults. Gerontology. 2000;46:306–310. [CrossRef] [PubMed]
Grisso J, Kelsey J, Strom B, et al. Risk-factors for falls as a cause of hip fracture in women. N Engl J Med. 1991;324:1326–1331. [CrossRef] [PubMed]
Christina K, Cavanagh PR. Ground reaction forces and frictional demands during stair descent: effects of age and illumination. Gait Posture. 2002;15:153–158. [CrossRef] [PubMed]
Simoneau GG, Cavanagh PR, Ulbrecht JS, Leibowitz HW, Tyrrell RA. The influence of visual factors on fall-related kinematic variables during stair descent by older women. J Gerontol. 1991;46:M188–M195. [CrossRef] [PubMed]
Dietrich G, Breniere Y, Do MC. Organization of local anticipatory movements in single step initiation. Hum Move Sci. 1994;13:195–210. [CrossRef]
Myles C. Stairs. Durward BR Baer GD Rowe PJ eds. Functional Human Movement. 2002;107–120. Butterworth Heinemann Edinburgh, UK.
Breniere Y, Do M, Sanchez J. A biomechanical study of gait initiation process. J Biophys Med Nucl. 1981;5:197–205.
Breniere Y, Do MC. When and how does steady state gait movement induced from upright posture begin?. J Biomech. 1986;19:1035–1040. [CrossRef] [PubMed]
Jian Y, Winter D, Gilchrist L. Trajectory of the body COG and COP during initiation and termination of gait. Gait Posture. 1993;1:9–22. [CrossRef]
Winter D. Human balance and posture control during standing and walking. Gait Posture. 1995;3:193–214. [CrossRef]
Zachazewski JE, Riley PO, Krebs DE. Biomechanical analysis of body mass transfer during stair ascent and descent of healthy subjects. J Rehabil Res Dev. 1993;30:412–422. [PubMed]
Allied Dunbar National Fitness Survey. A report on activity patterns and fitness levels: report commissioned by the Sports Council and Health Education Authority. 1992; Sports Council and Health Education Authority London, UK.
Ferris F, Bailey I. Standardizing the measurement of visual acuity for clinical research studies: Guidelines from the Eye Care Technology Forum. Ophthalmology. 1996;103:181–182. [CrossRef] [PubMed]
Elliott DB, Bullimore MA, Bailey IL. Improving the reliability of the Pelli-Robson contrast sensitivity test. Clin Vis Sci. 1991;6:471–475.
Elliott DB, Bullimore MA, Patla AE, Whitaker D. Effect of a cataract simulation on clinical and real world vision. Br J Ophthalmol. 1996;80:799–804. [CrossRef] [PubMed]
Anand V, Buckley JG, Scally A, Elliott DB. Postural stability changes in the elderly with cataract simulation and refractive blur. Invest Ophthalmol Vis Sci. 2003;44:4670–4675. [CrossRef] [PubMed]
Lord SR, Dayhew J, Howland A. Multifocal glasses impair edge-contrast sensitivity and depth perception and increase the risk of falls in older people. J Am Geriatr Soc. 2002;50:1760–1766. [CrossRef] [PubMed]
Woltring HJ. A fortran package for generalised cross-validatory spline smoothing and differentiation. Adv Eng Software. 1986;8:104–113. [CrossRef]
Dempster WT. Space requirements of the seated operator: technical Report 55-159. 1955; Wright-Patterson Air Force Base Dayton, OH.
Eames M, Cosgrove A, Baker R. Comparing methods of estimating the total body centre of mass in three-dimensions in normal and pathological gaits. Hum Move Sci. 1999;18:637–646. [CrossRef]
Gutierrez EM, Bartonek A, Haglund-Akerlind Y, Saraste H. Centre of mass motion during gait in persons with myelomeningocele. Gait Posture. 2003;18:37–46. [CrossRef] [PubMed]
Gelat T, Breniere Y. Adaptation of the gait initiation process for stepping on to a new level using a single step. Exp Brain Res. 2000;133:538–546. [CrossRef] [PubMed]
Elliott DB, Patla AE, Furniss M, Adkin A. Improvements in clinical and functional vision and quality of life after second eye cataract surgery. Optom Vis Sci. 2000;77:13–24. [CrossRef] [PubMed]
Chang H, Krebs DE. Dynamic balance control in elders: gait initiation assessment as a screening tool. Arch Phys Med Rehabil. 1999;80:490–494. [CrossRef] [PubMed]
Maki BE, Holliday PJ, Topper AK. A prospective study of postural balance and risk of falling in an ambulatory and independent elderly population. J Gerontol. 1994;49:M72–M84. [CrossRef] [PubMed]
Sims KJ, Brauer SG. A rapid upward step challenges medio-lateral postural stability. Gait Posture. 2000;12:217–224. [CrossRef] [PubMed]
Riley PO, Mann RW, Hodge WA. Modelling of the biomechanics of posture and balance. J Biomech. 1990;23:503–506. [CrossRef] [PubMed]
Spaulding SJ, Patla AE, Elliott DB, Flanagan J, Rietdyk S, Brown S. Waterloo Vision and Mobility Study: gait adaptations to altered surfaces in individuals with age-related maculopathy. Optom Vis Sci. 1994;71:770–777. [CrossRef] [PubMed]
Krell J, Patla AE. The influence of multiple obstacles in the travel path on avoidance strategy. Gait Posture. 2002;16:15–19. [CrossRef] [PubMed]
×
×

This PDF is available to Subscribers Only

Sign in or purchase a subscription to access this content. ×

You must be signed into an individual account to use this feature.

×