Variability in the Length and Frequency of Steps of Sighted and Visually Impaired Walkers Sarah J

Variability in the Length and Frequency of Steps of Sighted and Visually Impaired Walkers Sarah J. Mason, Gordon E. Legge, and Christopher S. Kallie Abstract: The variability of the length and frequency of steps was measured in sighted and visually impaired walkers at three different paces. The variability was low, especially at the preferred pace, and similar for both groups. A model incorporating step counts and step frequency provides good estimates of the distance traveled. Applications to wayfinding technology are discussed. Sighted people often estimate distances estimates of distance should take into ac- by counting their steps (for example, to count changes in walking speed. Third, it approximate the dimensions of a large is often impractical to count steps accu- room). Such estimates implicitly rely on rately, given the concurrent cognitive de- the consistency of the length of one’s mands of other activities. Ramsey, steps. In wayfinding situations, where es- Blasch, Kita, and Johnson (1999) found timates of distance may be useful, people that visually impaired cane users de- who are visually impaired (that is, those creased the length of their stride when who are blind or have low vision) may not they had to concurrently complete an au- make use of step counting for several rea- ditory task or anticipate a simulated sons: First, it is possible that the length of drop-off. Finally, the practical value of steps is less consistent for visually im- estimates of distance relies on a map that paired people because of the use of mo- converts these distances into useful infor- bility devices. Second, since walking mation about the layout of the environ- speed affects the length of steps, accurate ment. In an unfamiliar environment, a person who is visually impaired rarely has access to a suitable map. In a familiar Many thanks to members of the Minnesota environment, he or she may conceivably Laboratory for Low Vision Research for their memorize a cognitive map on the basis of assistance, especially Rudrava Roy for writing the software program interfaced to the pe- step counts (for example, Dr. Walker’s of- dometer system and MiYoung Kwon for help fice door is on the left, 30 steps down the in preparing the manuscript. We also thank corridor from the main entrance). Scott Kalpin for his helpful discussion and Currently, there is wide interest in the for supplying the computer-readable pedome- development of assistive technology for ter. This research was funded by Grant wayfinding by people who are visually EY02857 from the National Institutes of Health and Grant H133A011903 from the impaired. It has been demonstrated that National Institute of Disability and Rehabili- signals from global positioning satellites tation Research. (GPS) can be incorporated into wayfind- ©2005 AFB, All Rights Reserved Journal of Visual Impairment & Blindness, December 2005 741 ing technology for use outdoors (Loomis, In principle, such technology would Golledge, & Klatzky, 1998; Loomis, take care of two of the problems just Golledge, Klatzky, Speigle, & Tietz, listed regarding step counting for the 1994). For instance, GPS-based software nonvisual estimation of distance: The has been developed by the Sendero technology, rather than the pedestrian, Group and runs on the BrailleNote re- would count the steps and would convert freshable braille notetaker (by Pulse Data the step counts into an estimated position HumanWare). This technology updates a on a map of the building. This article re- position continuously, so there is no need ports on the results related to the two re- to estimate distances by counting steps. maining issues: How accurately can dis- Because GPS signals are usually not tances be estimated from step counts, and available indoors, another method for how is this accuracy affected by a per- tracking a pedestrian’s position is neces- son’s walking pace? sary. Stable landmarks, such as the loca- Prior research has found that the tion of the elevators, or special-purpose length of the stride of healthy, sighted markers, such as the Talking Signs In- adults remains fairly consistent over frared Communications System (by Talk- time, especially at an individual’s pre- ing Signs), may provide information on ferred walking pace. The participants in one’s position at key points in a building. Sekiya, Nagasaki, Ito, and Furuna’s Technology for estimating distance and (1997) study walked on a flat walkway direction from these landmarks could with a freely chosen step rate at five dif- then be useful. As part of a larger project ferent self-regulated speeds. The length on indoor wayfinding technology at the of their strides was consistent (variable Minnesota Laboratory for Low Vision error < 4 centimeters, or 1.6 inches), par- Research at the University of Minnesota, ticularly at the preferred walking speed we have been exploring technologies for (variable error < 2.5 centimeters, or 0.9 estimating the distance and direction inch). traveled by pedestrians who are visually Although Danion, Varraine, Bonnard, impaired. A straightforward method for and Pailhous (2003) did not find that the estimating distance would be to count preferred walking speed necessarily mini- steps with a computer-readable pedome- mized variation in the length of strides, ter. If the step counts could be reliably they found that human gait was consis- converted into distance estimates and if tent. Their participants walked on a the pedometer was interfaced to a treadmill at 25 combinations of fre- computer-readable digital map of a quency and step length (imposed by the building, then a pedestrian’s current po- experimenter). The variability in the sition in the building (computed as an length of strides (expressed as a coeffi- estimated distance from a known land- cient of variation, SD/M)was generally mark) could be determined. (More re- under 3%. Stride length was found to be fined technology would be necessary to most consistent at the frequency of 1Hz estimate the direction of travel. In build- (60 strides per minute), more variable for ings, however, direction is often highly shorter strides, and more consistent for constrained by the layout of corridors.) longer strides. 742 Journal of Visual Impairment & Blindness, December 2005 ©2005 AFB, All Rights Reserved To our knowledge, no study on vari- tween the placements of one foot), we ability in the length of steps has been car- chose to count steps (the distance in the ried out using participants who are visu- forward direction between alternate feet). ally impaired. One could hypothesize that We chose this measure to maintain con- the lack of visual cues may increase vari- sistency with those parts of the experi- ation in a visually impaired individual’s ment that used the pedometer, which walking pattern. Nakamura (1997) found counted steps, not strides. that the length of the stride of adults who Experiment 2 was conducted two are blind who walked at their preferred months later, with four of the participants pace was significantly shorter than that from Experiment 1. The participants were of sighted adults, possibly because of the again asked to walk at a slow, preferred, need to integrate nonvisual input to walk and fast pace, but this time on walkways safely. Considering that Danion et al. that were 20, 40, and 80 feet long. Experi- (2003) found sighted walkers’ shorter ment 2 had two aims. The first was to de- strides to be more variable in length, termine how reproducible each partici- there is a possibility that the short strides pant’s mean step length was (especially at found in people who are visually im- the preferred pace, at which a pedometer paired may also show increased variabil- would be calibrated). The second was to ity. Studies on veering (for example, Guth compare the accuracy of two methods of & LaDuke, 1995) have concluded that estimating the distance traveled. Previous people who are visually impaired often research has found that the relationship have difficulty maintaining a straight- between length and frequency of steps is ahead direction when walking; perhaps linear (Inman, Ralston, & Todd, 1981; attaining a consistent step length may Danion et al., 2003). Instead of calibrat- also prove difficult for them. ing a pedometer with an individual’s pre- Experiment 1 consisted of 18 partici- ferred step length at the preferred pace, a pants (6 sighted and 12 visually im- more accurate way of estimating the dis- paired) walking on a flat walkway 80 feet tance traveled may be to calibrate by mea- long. Eight trials were performed at each suring the linear relationship between the of three paces (slow, preferred, and fast), length and frequency of steps for an indi- from which variability in the length and vidual. Distance would then be estimated frequency of steps was calculated. Each from a linear equation that takes step fre- participant then walked an additional quency into account in estimating step eight trials at his or her preferred pace length. For the four participants who re- wearing a computer-readable pedometer, turned for Experiment 2, we calculated to test the pedometer’s accuracy. Com- their mean preferred-pace step length bining each participant’s individual vari- from Experiment 1 and the linear equa- ability in step length with the pedometer’s tion that best represented the relationship error rate gave us an estimation of the ac- between the length and frequency of steps curacy of a pedometer-based system for for each trial from Experiment 1. We then estimating walking distances. used both methods to estimate the dis- Although prior studies, cited earlier, tance traveled in Experiment 2, to com- have measured strides (the distance be- pare the accuracy of each method. ©2005 AFB, All Rights Reserved Journal of Visual Impairment & Blindness, December 2005 743 Our overall aim was to compare the ally impaired (with Snellen acuities rang- variability in the length and frequency of ing from 20/180 to total blindness).

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