Multifocal Spectacles Increase Variability in Toe Clearance and Risk of Tripping in the Elderly

Abstract

purpose. Epidemiologic studies have indicated that elderly people who wear multifocal spectacles have an increased risk of tripping, particularly on stairs. Yet no studies have experimentally examined how wearing multifocal spectacles affects stair and step negotiation. The purpose of this study was to determine the effects of wearing multifocal compared with single-distance vision spectacles on minimum toe clearance and risk of tripping during step negotiation in the elderly.

methods. Nineteen healthy subjects (mean age, 71.4 years) performed a single step up to a new level (heights, 7.5, 15, and 22 cm) while wearing multifocal (bifocals and progressive addition lenses) or single-distance vision spectacles. Minimum horizontal and vertical toe clearance were assessed by analyzing data collected with a five-camera, three-dimensional motion-analysis system.

results. There was no difference in mean minimum toe clearance in subjects when wearing multifocal compared with single-distance vision spectacles. However, there was greater within-subject variability in vertical toe clearance when wearing multifocal spectacles (variance ratio, 1.53; P = 0.0004). Subjects were also significantly more likely to trip when wearing multifocal compared with single-vision spectacles (one-sided Fisher’s exact test P = 0.025).

conclusions. Because of increased within-subject variability in vertical toe clearance when wearing multifocal spectacles, elderly individuals may be at greater risk of falling when negotiating steps and stairs if they do not also consistently increase margins of safety (mean vertical toe clearance). This suggests that some elderly who are at high risk of falling may benefit from wearing single-distance vision rather than multifocal spectacles when walking.

Presbyopia is the normal, age-related, slowly progressive loss of the focusing power of the crystalline lens of the eye that causes difficulty in reading. Presbyopia first becomes noticeable around the age of the mid-40s and is easily corrected with reading spectacles. Individuals who also require visual correction for distant work may be prescribed two pairs of spectacles. Alternatively, a more common solution is to have only one pair of multifocal spectacles, usually bifocals, which correct both near and distance vision, or progressive addition lenses (PALs), which also provide clear vision at intermediate distances. Although convenient in terms of only needing one pair of glasses, multifocal spectacles are not without their disadvantages, which are due to optical imperfections inherent in their design. For example, the lower reading (near) segment of the lens (Figs. 1a 1b)impairs contrast sensitivity and depth perception when viewing beyond the typical reading distance of approximately 40 cm ¹ and may hinder the accurate detection of stair locations and/or obstacles in the lower part of the visual fields, thus increasing the risk of falling. In addition, PALs have a relatively narrow field of clear vision in the intermediate section of the lens, and the periphery of the lens is subject to optical distortions. There is a wide range of available PAL designs that vary in the relative widths of transition and near zones to meet specific occupational needs. First generation PALs used a “hard” design, with wide distance and reading zones and a narrow intermediate zone. More modern lenses use a “softer” design with reduced distance and reading zones, but a wider and longer intermediate zone. In addition, the intermediate and reading zones decrease as the reading addition is increased, which occurs with increasing age. Furthermore bifocals are subject to a sudden prismatic jump as fixation moves up and down across the top of the reading segment, which causes an apparent vertical displacement of objects (Fig. 1a) . 

Lord et al. ¹ recently reported epidemiologic data that showed that multifocal spectacle wearers are more than twice as likely to fall as nonmultifocal spectacle wearers and that this risk was increased during stair negotiation. This greater risk was attributed to impaired contrast sensitivity and depth perception at critical distances for detecting environmental hazards. ¹ Accident data have also shown that wearing multifocal spectacles is associated with an increase in “missed edge-of-step accidents.” ² These two epidemiologic studies highlighted the potential safety problems associated with wearing multifocal spectacles, but, to our knowledge, the present study is the first to determine experimentally whether and how stair and step negotiation is adapted when wearing multifocal spectacles compared with single-distance vision spectacles. We were unsure of what adaptations, if any, multifocal lens wearers would make during stair ascent. It is possible that the optical blur and distortions associated with multifocal spectacles lead the elderly to employ a more cautious stepping strategy similar to that used by elderly subjects when vision is acutely blurred, ^{3 4 5} to reduce the risk of falling. However, the epidemiologic studies ^{1 2} found that wearing multifocal spectacles increases the risk of tripping and falling, suggesting that safety strategies were not used or were not used consistently. The objective of this study was therefore to investigate whether and how the use of multifocal compared with single-distance vision spectacles affects body center-of-mass (CM) dynamics, temporal parameters, and minimum lead-limb toe clearance in healthy elderly subjects, ^{4 5 6} when they step up to a new level (three heights), and to determine whether there was subsequently an increased risk of tripping. 

Previous research has reported that standing postural stability ⁷ and gait speed ⁸ decrease in older adults when their attention is divided between a motor and cognitive task. Others have found an increased risk of foot contact with a suddenly appearing obstacle under dual-task conditions in older compared with younger subjects. ⁹ Furthermore, older adults who stop walking to converse are at increased risk of falling. ¹⁰ Consequently, a secondary objective of the present study was to determine whether stepping characteristics under the different spectacle conditions are affected by performing a concurrent cognitive task. 

A single- rather than multiple-step paradigm was chosen because it is during the transition from level walking to stair ascent when more “mistakes” are made, rendering a trip more likely. ^{11 12} A single-step paradigm also minimized the likelihood of fatigue from undertaking repeated trials and allowed comparisons with stepping adaptations resulting from acute visual blur reported previously. ^{4 5 6} In addition, stair negotiation in elderly individuals with declining function is sometimes undertaken one step at a time. ¹³ The step heights equated to those of a curb (7.5 cm), a stair riser (15.0 cm), and a bus entry step (22.0 cm), obstacles frequently encountered in daily life. 

Methods

Subjects

Nineteen community-dwelling subjects (12 women and 7 men; mean age, 71.4 ± 4.5 years; range, 62–80; height, 1.66 ± 0.08 m; BMI, 26.05, ± 4.41) were recruited from volunteer patients who regularly attend the eye clinic at the University of Bradford for teaching purposes. All were independently mobile, able to follow simple instructions and, according to self-report, had no significant neurologic, musculoskeletal or cardiovascular disorders that could interfere with balance control or stepping. Those with vestibular disturbances, diabetes, or a history of falling in the previous year were excluded as were those taking medications that could affect balance or vision. Subjects had normal healthy eyes, determined by a full eye examination including ocular screening using slit lamp biomicroscopy, tonometry, indirect ophthalmoscopy, central visual field screening, and binocular vision assessment. Corrected visual acuity was equal to or better than the logarithm of the minimum angle of resolution (logMAR) 0.1 (Snellen 20/25) in either eye. Physical activity levels were determined by self-report using the activity scale of the Allied Dunbar National Fitness Survey 1992. ¹⁴ All subjects engaged in light to moderate physical activities including gardening, light housework, and dancing for at least 30 minutes, 5 days a week. Subjects had worn multifocal spectacles for at least 2 years (median, 12 years; range 2–30). Seven subjects habitually wore PALs and 12 wore bifocal spectacles. Mean (±1SD) distance spherical equivalent power was +2.83 ± 4.84 DS and the mean reading addition was + 2.35 ± 0.28 DS. The tenets of the Declaration of Helsinki were observed and the experiment gained approval from the University of Bradford ethics committee. All subjects gave written informed consent and were asked to refrain from alcohol intake during the evening before testing. 

Clinical Evaluation

To assess how vision was affected by the different portions of the multifocal lenses, we measured binocular visual function while the subjects wore full-aperture trial lenses in a trial frame fitted with their (1) near, (2) intermediate, and (3) distance refractive corrections. Contrast sensitivity ¹⁵ was measured with a letter-by-letter scoring system and chart luminance of 200 cd/m^{2 16} ; Regan logMAR high- and low-contrast visual acuity, ¹⁷ with a letter-by-letter scoring system and chart luminance of 160 cd/m^{2 18} ; and depth perception, with the Howard-Dohlman apparatus (mean of three trials). We wanted to determine visual function at a distance that would be encountered when negotiating steps and curbs in the “real world.” ¹⁹ Visual assessments were therefore undertaken at a distance that was equivalent to the distance between each subject’s eye in standing and the superior surface of the medium step (average, 1.4 m). LogMAR and depth perception (stereoacuity) scores were then derived by incorporating a correction factor for each subject’s working distance. To ensure that all subjects could perceive sensory cues from the soles of their feet, plantar cutaneous sensation was assessed by determining the ability to detect a 10-g force applied to five key sites (hallux, 1st, 3rd, and 5th metatarsal heads and heel) using a monofilament (Bailey instruments Ltd., Manchester, UK). ³ Fifteen subjects had normal sensation, and four had reduced sensation at one or two sites tested on the forefoot. The inability to detect pressure appeared to be due to callus formation. In all cases when the skin was tested immediately adjacent to the callused area sensation to pressure was present. Physical performance was assessed using the timed up and go test. ²⁰ Subjects took 8.5 ± 1.0 seconds to complete this test classifying them as functionally independent and nonfallers. ²¹  

Protocol

Each subject was prescribed three pairs of spectacles, one bifocal, one PAL and one single-distance vision. Individual subjects were provided with slightly different frames and sizes to ensure optimal fit. The three frames used by each subject were identical in frame style and size and were fitted to ensure the same back vertex distance and pantoscopic angle. The bifocal type was a 28-mm diameter D-segment and the PALs were Norville NCF5 (The Norville Group, Ltd., Gloucester, UK), a commonly used PAL in the United Kingdom that uses a compromise hard–soft design. An isocylinder plot of a plano distance and +2.00-D addition NCF5 PAL lens is shown in Figure 2 . All PALs were positioned with the fitting cross-alignment at the center of the pupil in primary gaze and the top of the bifocal segment was aligned with the patient’s lower lid. 

For each subject, data were collected during a single 2-hour session. A five-camera, three-dimensional motion-analysis system (Vicon 250; Oxford Metric Ltd., Oxford, UK) recorded (at 50 Hz) subjects stepping up from a force platform onto a step that was placed directly over an adjacent force platform (OR6-7; AMTI, Boston, MA). Force data were collected at 100 Hz. Subjects wore their own shorts, t-shirt, and low-healed shoes. Reflective markers (25-mm diameter) were attached either directly onto skin or clothing or on elasticized bands at the following locations: superior aspect of the 2nd and 5th metatarsal heads, lateral malleoli, posterior aspect of the calcanei, lateral aspects of each shank and thigh, lateral femoral condyles, anterior superior iliac spines, sacrum, medial and lateral aspects of the wrists, lateral humeral epicondyles, acromions, xiphoid process, jugular notch, spinous processes of the 7th cervical and 10th thoracic vertebrae and the anterolateral and posterolateral aspects of the head. Markers were not placed on the tip of the shoe, as it was believed that this could create a disruption to “normal” toe clearance. Instead the shoe tip was retrospectively defined by determining its position relative to the second and fifth metatarsal head markers. ^{4 5} Markers were also placed on the step to define the front edge and surface of the step relative to the foot. Each step was constructed from medium-density fiberboard (46.4 × 50.8 cm). The steps were covered with a matt green lino identical with that covering the surrounding floor. Room illuminance, measured at head height, was approximately 300 lux, and the luminance of the floor and top surface of the step was 30 cd/m² measured using a photometer (CS-100; Minolta Co. Ltd., Osaka, Japan). 

Subjects started from a stationary standing position with the tips of their shoes positioned a comfortable width apart immediately behind a line that was half their foot length away from the front surface of the step. After approximately 5 seconds in this position (looking straight ahead), subjects were instructed to “step up,” at which point they took a single step up onto the new level at their own comfortable speed and then came to a stationary position with their feet side by side. Direction of gaze was not controlled, and subjects were free to choose where they looked when stepping. Further trials were also conducted with a concurrent cognitive task (medium step height only). During these trials, subjects were instructed, once they had aligned their feet behind the line, to count backward in 2s from a number greater than 100 (randomly selected by the investigator) and to continue counting until they had completed the step. For each step condition, trials were repeated wearing single-vision, PAL, and bifocal spectacles. Subjects were not informed of which pair of spectacles they had been given. All trials were repeated three times with the order of visual–cognitive condition and step height randomized. Wearing their own spectacles, subjects were given a familiarization trial at each step height and while performing the cognitive dual task. Subjects led with the same self-selected limb in all trials. Any trial that was not completed according to these instructions was discarded and repeated (except trials in which subjects tripped, which were saved for further analysis). An assistant stood close by to ensure that subjects did not fall if they should trip. Subjects rested each time the step height was changed. 

Data Analysis

Using commercial software (Plug-In Gait; Oxford Metrics Ltd., Oxford, UK) the 3-D coordinate trajectory data of each marker were filtered and processed to define a 3-D linked-segment model of the subject. ⁴ Coordinate data of the body CM, point of application of the ground reaction force vector (the center of pressure; COP), step edge, and the lead limb’s heel and shoe tip, along with sagittal plane head angle and ground reaction force (GRF) data were exported (at 50 Hz) in ASCII format for further analysis. Minimum horizontal and vertical toe clearance were defined as the minimum distance between the shoe tip and the apex of the step as it crossed the vertical and horizontal position of the apex of the step, respectively. A trip incident was deemed to have occurred if the foot made contact with the upright of the step during the lead limb swing phase. Such incidents were recorded during data collection by simple observation and were subsequently confirmed by determining when horizontal toe clearance was zero at the same time as vertical toe clearance was negative (below the top surface of the step). Movement time was recorded from the initiation of movement (the instant when the COP first moved laterally by 10 mm) up to step completion (the instant of toe-off of the trail limb). Whole-body CM dynamics were evaluated by determining the peak divergence between the CM and COP in the lateral and anterior directions at lead limb heel-off and during single-limb support, ⁴ and the peak CM velocity during stepping. In an attempt to determine whether there were any differences in which portion of the lens subjects viewed through when wearing each type of spectacle, the amount of head flexion occurring during the movement and the time that peak head flexion occurred relative to movement initiation, were assessed. 

Statistical Analysis

Data were analyzed with a generalized estimating equation, random-effects, population-averaged model (Stata ver. 8.0; Stat Corp., College Station, TX). This multivariate statistical model was obtained by using the xtreg command that uses the generalized least squares (GLS) random-effects estimator, which produces a matrix-weighted average of the between-subjects and within-subject results. Level of significance was set at P = 0.05. Factors of interest were incorporated sequentially and the significance of the three- and four-level factor was tested by using a likelihood ratio (χ²) test after dropping individual factors from the model. Significance between two particular conditions (e.g., dual tasking versus normal) was determined with Wald χ² tests. All appropriate interaction terms were included in the model. Factors with P < 0.1 were provisionally retained, whereas those >0.1 were dropped. The probabilities quoted are those associated with the specific terms in the final regression model which were:

1.  
   Trial spectacles: a fixed factor with three levels (single-distance vision, bifocal, and PAL).
2.  
   Repetition: a fixed factor with three levels (trials 1, 2, and 3).
3.  
   Step height: a fixed factor with four levels (low, medium, medium-dual task, and high).

The effect of trial spectacles on within-subject variability in minimum toe clearance was examined by using the variance ratio test. The effect of prescription (distance, intermediate and near) on visual function (high/low contrast visual acuity, contrast sensitivity and depth perception) was examined using a two-way repeated-measures ANOVA. 

Results

Refractive Error and Visual Assessments

Visual acuity, contrast sensitivity, and depth perception scores for the three refractive prescriptions are presented in Table 1 . 

Spectacle prescription significantly affected all the visual test scores (Table 1) . Post hoc analysis indicated that this was due to significantly worse vision when viewing through the near compared with both the distance and intermediate prescriptions. 

Lead-Limb Toe Clearance

Mean lead-limb minimum vertical toe clearance significantly decreased as step height increased (low, 75.4 mm; medium, 68.6 mm; medium-dual task, 65.7 mm; and high, 57.2 mm; χ² ₃ = 32.57, P < 0.001), as did horizontal toe clearance (low, 119.5 mm; intermediate, 109.0 mm; medium-dual task, 102.2 mm; and high, 94.8 mm; χ² ₃ = 41.73, P < 0.001). The type of spectacles worn (P > 0.40), trial repetition (P > 0.10), and dual cognitive task (P > 0.10) had no effect on mean vertical or horizontal toe clearance, and there were no significant interactions between minimum toe clearance, step height, and type of spectacles worn (P > 0.10). 

There was approximately 20% (≈5 mm) greater within-subject variability in minimum vertical toe clearance across step heights when subjects wore PAL or bifocal compared with single-vision spectacles (Table 2) . This difference was significant (F = 1.61, P = 0.0004; F = 1.40, P = 0.013 for PALs and bifocals, respectively). Within-subject variability in vertical toe clearance did not differ when they wore PAL compared with bifocal spectacles (F = 1.15, P = 0.30), or under the medium compared with the medium-dual task (f = 1.04, P = 0.81). Spectacle type had no effect on within-subject variability in minimum horizontal toe clearance (P > 0.10). 

Figure 3illustrates the dispersion in vertical toe clearance scores at each step height. Because there were no significant differences between PALs and bifocals, clearance data were pooled as “multifocals.” 

Tripping Incidents

There were nine trials (in eight subjects) in which the subject accidentally hit the front face of the step with the leading foot. In all these cases, the subject regained balance without falling. No trip incidents occurred at the low step height or when subjects wore single-distance vision spectacles (Table 3) . There was no significant difference in the likelihood of tripping with PAL compared with bifocal spectacles (one-sided Fisher’s exact test, P = 0.51), or when performing the medium–dual compared with the medium step task (one-sided Fisher’s exact test P = 0.50). Tripping data for PALs and bifocals were therefore pooled as “multifocals.” Subjects were more likely to trip when wearing multifocal than single-distance vision spectacles (one-sided Fisher’s exact test P = 0.025). 

CM Dynamics, Movement Time, and Head Angle

Trial spectacles had no effect on CM-COP divergence or step duration (P > 0.1). Spectacles also had no effect on the amount of head flexion that occurred during the movement (mean 24°, P ≫ 0.1), or the time when peak head flexion occurred (mean 0.69 seconds after movement initiation, which was 0.27 seconds before lead limb foot contact on the step, P ≫ 0.1). There were no significant interactions between step height and spectacle type for CM dynamics, movement time, or head angle (P > 0.1). Under the medium-dual compared with medium step condition, there was significantly less divergence between the CM and COP at lead limb heel-off and during single-limb support in both anteroposterior (32% and 9% less, P < 0.001) and mediolateral (25% and 8% less; P < 0.001) directions. This resulted in a reduced-peak CM forward velocity and an increased movement time (P < 0.001) for the dual-task condition. 

Discussion

The decreased mean vertical toe clearance as step height increased confirms previous findings, ⁴ and this could be related to conservation of energy and/or reduced lower limb strength or flexibility in the elderly. Mean toe clearance did not significantly increase when wearing multifocal compared with single-vision spectacles. This, coupled with a significantly greater variability in vertical toe clearance when wearing multifocal lenses resulted in an increased number of occasions when the vertical clearance was insufficient and tripping occurred. This also explains why no trips occurred at the low step where the mean vertical toe clearance was significantly greater than at the medium and high steps, irrespective of the spectacles worn. These results may explain why epidemiologic studies found that regular multifocal wearers were more likely to fall than nonmultifocal wearers and, most important, were more likely to fall on stairs and steps. ^{1 2}  

The higher incidence of tripping and greater within-subject variability in vertical toe clearance when wearing multifocal compared with single-distance vision spectacles most likely occurred because of inaccurate visual information regarding the exact location of the step edge when wearing multifocals, perhaps due to blurring of the step edge when looking through the reading portion of the lens (as highlighted by the significant reductions in visual acuity, contrast sensitivity, and depth perception through the near vision portion of the lenses; Table 1 ), the prismatic jump associated with bifocals, and/or the peripheral distortion associated with PALs. The optical defects associated with the lower portions of multifocal lenses when looking beyond 40 cm (Fig. 1)could be avoided by flexing the head to look through the distance vision portion of the lens. Despite this, the amount of head flexion and when it occurred was similar for each spectacle type, and subjects made no precautionary increase in toe clearance or reduction in CM-COP divergence to increase stability. The lack of precautionary measures is inconsistent with the stepping adaptations made by elderly subjects in response to acutely blurred vision ^{3 4 5} and is probably because the subjects in the present study had adapted to the moderate blur provided by multifocals over many years. 

Lord et al. ¹ suggested that wearing any type of spectacles including single-distance vision could constitute a fall risk due to the limited field of view permitted by spectacle frames. The findings of the present study (which used identical frames for each spectacle type) do not support this hypothesis, because none of the subjects hit the step when wearing single-vision spectacles. Similarly Davies et al. ² also found that wearing single-distance vision spectacles did not significantly increase the incidence of injurious falls. 

The increased step execution time when performing the concurrent cognitive task is in support of previous findings indicating dual tasking leads to a decrease in walking speed ⁸ and limb velocity when crossing obstacles. ²³ The reduced CM-COP divergence and CM velocity was probably an attempt to increase stability by keeping the CM close to the base of support. ^{4 24} These adaptations can be attributed to the increased attentional demands of performing a concurrent cognitive and motor task. ²⁵ Despite the greater attentional load, there was no significant difference in vertical toe clearance or tripping incidence under the dual-task condition, which agrees with previous reports. ²⁶ Others have found a deleterious affect on obstacle avoidance during dual tasking, but this was determined when an obstacle suddenly appeared in the travel path. ²³  

The study had certain limitations. First, it was underpowered to detect any differences between PAL and bifocal spectacles. Second, it was not possible to track the subjects’ eye movements when stepping, and thus future work is needed to determine which portion of a multifocal lens individuals view through when negotiating steps and other environmental hazards. Third, the PAL data were obtained using Norville NCF5 lenses (The Norville Group, Ltd.,), which have a compromise hard–soft design and may not apply to other PAL lenses, particularly those that have a softer design with a wider intermediate zone. 

Summary

This study is the first to determine experimentally the effects of wearing multifocal compared with single-distance vision spectacles on step negotiation in the elderly. Findings indicated that the use of multifocal spectacles was associated with no significant increase in vertical toe clearance, but an increased within-subject variability in vertical toe clearance and an increased likelihood of tripping. The increased risk of tripping occurred despite the relatively optimal test conditions of high illuminance levels and the requirement of performing only a single step. In unfamiliar or less-favorable environments, or when negotiating a change in level during walking, the risk may be even greater. These findings indicate that to reduce the risk of tripping, elderly individuals who are at high risk of falling will probably benefit from wearing single-vision instead of multifocal spectacles when walking. 

 Supported by a Nursing and AHP (Applied Health Psychology) Researcher Development Award R694 from the Department of Health and a National Health Service Research and Development grant (LJ). John Buckley is funded by Nursing and AHP Fellowship Grant 3991/3322 from the Health Foundation, United Kingdom.

 Submitted for publication May 31, 2006; revised September 22 and November 7, 2006; accepted January 12, 2007.

 Disclosure: L. Johnson, None; J.G. Buckley, None; A.J. Scally, None; D.B. Elliott, None

 The publication costs of this article were defrayed in part by page charge payment. This article must therefore be marked “advertisement” in accordance with 18 U.S.C. §1734 solely to indicate this fact.

 

Figure 1.

 

Sections of a bifocal (a) and progressive addition lens or PAL (b) that provide clear vision for distance and near objects. The PAL also includes a corridor of clear vision for objects at intermediate distances. The PAL design also produces areas of aberrational astigmatism or distortion. These are shown as contour plots of isocylindrical lines that join points with similar amounts of surface aberrational astigmatism.

Figure 2.

 

An isocylinder plot of a plano distance/+ 2.00-D addition Norville NCF5 PAL lens (The Norville, Group, Ltd., Gloucester, UK), ²² courtesy of Dr. Colin Fowler (Department of Vision Sciences, University of Aston, UK).

Table 1.

 

Visual Function Test Results with Distance, Intermediate and Near Refractive Prescriptions

Test	Distance	Intermediate	Near
High-contrast visual acuity (logMAR)	−0.06 (0.09)	−0.06 (0.11)	0.20 (0.15)*
Low-contrast visual acuity (logMAR)	0.06 (0.09)	0.04 (0.13)	0.33 (0.20)*
Contrast sensitivity (log)	1.93 (0.05)	1.91 (0.07)	1.79 (0.15)*
Depth perception (min arc)	17.40 (14.40)	20.37 (11.91)	29.28 (18.25), †

 Data are expressed as the mean ± SD.

*  Significant difference between distance and near and intermediate and near (P < 0.001).

†  Significant difference between distance and near (P < 0.05).

Table 2.

 

Within-Subject Variability (SD) in Minimum Toe Clearance (mm) for Each Spectacle Type Averaged across Step Height

Spectacles	Vertical Toe Clearance Variability (95% CI)	Horizontal Toe Clearance Variability (95% CI)
Single	24.8 (22.5–27.3)	35.3 (32.0–38.9)
PAL	31.4 (28.5–34.3)	39.2 (35.6–43.2)
Bifocal	29.3 (26.6–32.3)	37.9 (34.4–41.8)

Figure 3.

 

Box-and-whisker plot of vertical toe clearance by spectacle type and step condition. Toe clearance decreased as step height increased, and the dispersion was greater in subjects wearing multifocal compared with single-distance vision spectacles. Clearances of zero or less (indicating a trip incidence) only occurred when subjects wore multifocal spectacles (right).

Table 3.

 

Number of Tripping Incidents by Step Height and Spectacles

Spectacles	Step Height
		Low	Medium	Medium Dual Task	High	Total
Single vision	0	0	0	0	0
PAL	0	3	1	1	5
Bifocal	0	0	1	3	4