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Gait Characteristics of Young and Older Individuals Negotiating a Raised Surface

Implications for the Prevention of Falls

^a Centre for Rehabilitation, Exercise and Sport Science and School of Human Movement, Recreation and Performance, Victoria University, Melbourne, Australia
^b School of Health Sciences, Deakin University, Burwood, Victoria, Australia

Rezaul K. Begg, Biomechanics Unit, Centre for Rehabilitation, Exercise and Sport Science, Victoria University, PO Box 14428, MCMC, Melbourne, Victoria 8001, Australia E-mail: rezaul.begg{at}vu.edu.au.

Decision Editor: William B. Ershler, MD

	Abstract

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Background. Falls in older individuals are a major public health issue because of the financial cost of surgery and rehabilitation and the human cost of associated pain and disability. Older individuals are most likely to fall when negotiating an obstacle or obstruction during locomotion. This research was aimed at investigating lower limb motion while a subject negotiated a raised surface.

Methods. The gait of six healthy young (Y) women (mean age 23.1 years) and six healthy older (O) women (mean age 67.6 years) were analyzed with a PEAK motion analyzer and a dual-force-platform system during unobstructed walking and when the subjects were stepping on and off a raised surface of 15 cm. The effect of age on foot clearance and force platform variables was analyzed.

Results. During stepping on, the young women cleared the step by the lead foot by a significantly greater margin than the older subjects did (Y = 10.6 cm, O = 9.1 cm; p < .05) but trail-foot clearance was not significantly different (Y = 9.4 cm, O = 8.8 cm). Foot clearance in stepping off was low compared with that of ascent, and the older individuals had a significantly higher lead (Y = 1.5 cm, O = 3.3 cm, p < .05) and trail (Y = 1.0 cm, O = 2.1 cm) vertical clearance. Older individuals positioned both the lead and the trail foot relatively farther from the step edge on ascending a raised surface, respectively, Y = 87% and O = 93% of the step cycle and Y = 29% and O = 34%. Foot placement in descent was qualitatively similar for the two groups. The force and the impulse data under the lead and the trail feet confirm modulations consistent with the foot clearance data.

Conclusion. In negotiating a raised surface older individuals appear to use a nonoptimal foot placement strategy in which, compared with that of young subjects, the trail foot is placed a long way from the edge of the step. The older subjects allowed very little correction time and little latitude in foot placement beyond the edge of the step, suggesting that the approach to the obstacle may be a critical determinant of safety.

FALLS in older individuals are, worldwide, a major public health issue because of the financial cost of treatment and rehabilitation and the human cost of associated pain and disability. Tinetti and Speechley ⁽¹⁾, for example, reported that 32% of a sample of community-dwelling older persons fell at least once a year, with 24% of those sustaining serious injuries. In Australia, the falls-related financial costs of medical treatment and rehabilitation have been estimated to be approximately $2.5 billion per year ⁽²⁾, and in response to this problem an extensive epidemiological literature has emerged that documents the incidence and cause of falls in older people. This work has identified associated risk factors such as visual and attention deficits, the effects of medication, environmental hazards, and age-related declines in neuromuscular function ^(3)(4)(5). Despite such findings, little is known about the direct cause of perceptual–motor errors associated with walking, particularly when a person is negotiating obstacles, that are likely to lead to a fall.

One category of falls-related incidents that has received little research attention is that associated with negotiating raised surfaces. In the everyday environment, people are continually required to accommodate changes in support surface elevation. In some cases such changes are small, as in stepping over the edge of a carpet, but in other situations the changes in elevation are large, as in climbing stairs or stepping on or off a roadside curb. A large proportion of falls in public places occurs on steps ⁽⁶⁾ and, more generally, in older individuals, falls are most likely to occur when they are negotiating an obstacle or obstruction.

Previous experimental studies of potentially hazardous modifications to lower limb trajectories have primarily concerned changes to the walking gait when a person is stepping over an obstacle such as a metal rod ⁽⁷⁾⁽⁸⁾ or a wooden block ⁽⁹⁾. There has also been limited research on gait adaptations to stair walking ^(10)(11) but there have been no previous attempts to show why negotiating a raised surface might pose a particular hazard to older people. This may be applicable to situations in which individuals confront a roadside curb, a porch step, or other contingencies demanding an abrupt change in limb elevation.

Previous research has suggested that foot placement before and after an obstacle may play a critical role in the success of obstacle negotiation. Chen and colleagues ⁽¹²⁾, for example, showed that older participants not only crossed obstacles more slowly than young controls but also that their foot placement was such that the obstacle was crossed farther forward (by 10% of step length) in the swing phase of the crossing cycle. They argued that this strategy could be interpreted as a safety mechanism in providing greater toe clearance over the obstacle. One drawback of this practice is that, in the event of contact with the obstacle, older individuals would have less time to recover their gait. Chen and colleagues ⁽¹²⁾ did not, however, reveal how individuals' foot placement would be affected in accommodating obstacles requiring a change in whole body elevation, such as stepping onto a raised surface.

Foot clearance over an obstacle, measured vertically from the obstacle to either the toe or heel, has been reported in the literature, with the assumption that a large clearance represents greater safety. Chen and colleagues ⁽¹²⁾ showed lead-foot clearance of up to 12 cm when subjects were stepping over a 15-cm obstacle, and Patla and Rietdyk ⁽⁹⁾ measured lead-toe clearance at 10 cm independent of obstacle height (4 cm to 26 cm). Less commonly, foot clearance when subjects were stepping across an obstacle has been investigated, with clearance measured from either the toe or the heel to the boundary of the area to be stepped across. Sparrow and colleagues ⁽¹¹⁾ measured heel clearance when subjects were stepping across obstacles, when the obstacle to be stepped across was formed by two parallel strips of tape on the floor, essentially an obstacle of zero height. Sparrow and colleagues ⁽¹¹⁾ found that heel clearance of the lead foot was approximately 7 cm beyond the tape defining the farther boundary of the area to step across. In the present study the heel was used as the anatomical marker for lead-foot clearance and the surface–toe distance was the clearance measure for the trail foot. This was done consistently with our earlier work, showing the toe to be most frequently the lowest point on the trail foot in stepping over an obstacle and, on most trials, the heel was shown to be the lowest point of the lead foot ⁽⁷⁾.

Collectively these results suggest that when a person is stepping over an obstacle that projects vertically, with effectively zero constraint on limb position horizontally, and is stepping across obstacles, young individuals invariably maintain clearance within safe bounds. It has not previously been shown, however, how foot clearance is affected in accommodating a raised surface. In this case there is a requirement to clear the obstacle vertically and horizontally, in order to position the foot safely beyond the edge of the step. In the experiment reported here, therefore, both foot placement horizontally and foot clearance vertically were investigated for stepping on and off a raised surface. The aim of the experiment was therefore to show the effects of age-related declines in gait on this type of potentially hazardous obstacle negotiation task.

Our related work on stepping over obstacles has revealed that the lead foot (which goes over the obstacle first) and the trail foot not only have different trajectories ⁽⁷⁾ but also different ground reaction-force (kinetic) characteristics ⁽⁸⁾. The kinetics of the lead and the trail foot during step negotiation were therefore also of interest, and we used a dual-force-platform apparatus to measure simultaneously the force/time characteristics of the trail foot before the raised surface and the lead-foot kinetics to show the forces exerted on the platform itself. Patla and colleagues ⁽¹³⁾ proposed that during obstructed gait a higher-order parameter such as impulse is modulated in adapting the gait pattern. To present a concise picture of the force/time characteristics in raised surface negotiation, we also selected foot–ground force impulses to highlight possible falls-risk–related differences between the young and the older individuals. Impulse is the integral of the force/time curve and represents both the magnitude of force and the duration of force application. The capacity to propel the body vertically and horizontally (in the anterior-posterior direction) in order to negotiate an obstacle is dependent on the impulses generated in these two directions of motion. This fundamental observation has been recognized in previous work ^(8)(13). It was hypothesized that obstacle-crossing trajectories and associated impulses would highlight age-related declines in gait that might predispose older individuals to falls in the raised surface task.

	Method

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Subjects
Six healthy young women (age 21.2 years, standard deviation [SD] 1.3 years; height: 172.2 cm, SD 8.9 cm; body mass: 73.1 kg, SD 14.2 kg; leg length: 87.7 cm, SD 5.0 cm), and six healthy older women (age: 67.6 years, SD 4.8 years; height: 158.0 cm, SD 4.7 cm; body mass: 66.1 kg, SD 10.1 kg; leg length: 80.3 cm, SD 2.8 cm) volunteered to participate in the experiment. The young adults were recruited from the academic community of Victoria University and the older subjects were community-living individuals drawn from our subject pool. An in-house questionnaire was used to screen for musculoskeletal and visual impairments that might affect normal locomotion. All participants completed informed consent procedures approved by the Victoria University Research Ethics Committee.

Equipment and Experimental Setup
The experimental setup is shown in the top panel of Fig. 1. Lower limb movement was recorded with a PEAK (Peak Technologies Inc.) two-dimensional motion analysis system interfaced to a personal computer, which also ran the data smoothing, and analysis programs. The raised surface was a solid wooden construction 0.15 m high, 1 m wide, and 5 m long. The Peak video camera was positioned approximately 8 m from the platform edge perpendicular to the plane of motion. The experimental setup also incorporated a dual-force-platform system with which to simultaneously record the lead- and the trail-foot ground reaction forces.

View larger version (20K): [in this window] [in a new window]   Figure 1. A, Experimental setup illustrating the distance/time and foot clearance variables as subjects were stepping on a raised surface, and B, Typical force–time curves (vertical and anterior-posterior) during gait obtained by the force platforms. The area under the curve represents impulse. The total impulse has been divided into braking and propulsive components.

Data Analysis
For each of the conditions, raw horizontal and vertical marker coordinates were automatically digitized and smoothed with a low-pass digital filter with a cutoff frequency of 6 Hz for one complete lead- and trail-foot stride for 5 trials randomly selected in each condition. The marker trajectories were calibrated with the Peak software and calculated with respect to absolute time and as a percentage of the step cycle, i.e., from preobstacle toe off to postobstacle foot contact of the lead and the trail feet. Key kinematic variables including vertical and horizontal clearances and time/distance parameters of the crossing step were calculated from the smoothed marker displacement data for both lead and trail feet. These dependent variables are also illustrated in Fig. 1.

In addition to the kinematic variables, impulse variables extracted from the ground reaction-force data for both the lead and the trail feet were used in the analysis. Vertical and anterior-posterior impulses during the braking (Brake) and propulsive (Prop) phases of the gait cycle were calculated from the corresponding force–time curves, as shown at the bottom of Fig. 1. To eliminate between-subject differences, the impulse values were normalized by body mass (in Newton seconds per kilogram). The values of the above kinematic and kinetic variables for the five trials for individual subjects in each condition (unobstructed walking, stepping on, and stepping off) were computed for statistical analysis. The effect of age on the dependent variables was determined with one-way analysis of variance (SPSS Inc.) procedures. Because the condition effect on the dependent variables is not a major issue for this report, aging effects were determined for each of the conditions separately. For those variables specific to the raised surface, such as foot clearance, the analysis was run for two conditions, on and off. The .05 probability level was used as the criterion for statistically significant differences between group means for each of the dependent measures.

	Results

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Stride Length and Duration and Gait Cycle Phase Modulations
The most straightforward measures of gait adaptations associated with negotiating a raised surface are the modifications to length and duration of the lead-foot and the trail-foot stride. The data presented in Table 1 show the gait adaptations by young and older individuals in stride length and duration for unobstructed walking and stepping on and off the raised surface. For unobstructed walking the older individuals had significantly shorter stride length for both the lead (F[1,10] = 14.3, p < .01) and trail (F[1,10] = 12.2, p < .01) foot. Stride length is influenced by many factors including age, stature, and disability. The older group was significantly shorter (p < .01) in stature and also had shorter (p < .01) leg length than their younger counterparts. These differences in anthropometric characteristics may have contributed to some of the between-group differences in stride length. In normal unobstructed gait there should be no distinction between lead and trail foot, such that the stride parameters for both feet would be expected to be approximately equal. For the older subjects the trail-foot stride was, however, 5 cm longer than that of the lead foot, suggesting an asymmetry associated with visually guiding the lead foot onto the further force platform and, in so doing, shortening the lead foot stride.

Vertical Clearance
Vertical clearances (as illustrated in Fig. 1) for stepping on and off are shown in Fig. 2. The older participants had significantly lower lead-foot (heel) clearance (F[1,10] = 7.9, p < .05) whereas the toe of the trail foot showed no significant clearance difference. For both groups clearance during stepping off was low compared with the stepping-on condition, ranging from 1.0 cm to 3.3 cm. It is interesting that, in stepping off, the older individuals cleared the step by a significantly greater margin with the lead (F[1,10] = 6.6, p < .05) foot; the trail-foot clearance was also greater, but not significantly.

View larger version (14K): [in this window] [in a new window]   Figure 2. Mean (± standard deviation) lead-foot (heel) and trail-foot (toe) vertical clearances during stepping on and stepping off a 15-cm raised surface for young and older individuals. Significant differences between the young and the older groups are starred, *p < .05.

View larger version (11K): [in this window] [in a new window]   Figure 3. Plan view of lead- and trail-foot placement prestep and poststep negotiation. Significant young–old differences are starred, *p < .01, **p < .001.

View larger version (21K): [in this window] [in a new window]   Figure 4. Lead- and trail-foot prestep and poststep crossing percentage distances and times.

View larger version (23K): [in this window] [in a new window]   Figure 5. Force impulse ratios (relative to unobstructed condition) under the trail and the lead feet during stepping-on and stepping-off conditions. The data in the bar graphs indicate group mean ± standard deviation. These impulse ratios have been shown during the braking and the propulsive phases of the support phase (see Fig. 1). Note different scaling in anterior-posterior impulse values for the stepping-off condition that are due to large mean and SD values.

Stepping Off
Fig. 5 also shows (bottom panels) that in stepping off, the trail-foot braking vertical impulse for the older individuals was considerably greater than for the young subjects by approximately 13%. The anterior-posterior braking impulse was, again, also considerably higher in the older group compared with that of their younger counterparts. Both the the vertical braking and vertical propulsive impulses were relatively lower in the older group under the lead foot. It is also interesting to note that in the anterior-posterior direction the young subjects decreased braking impulse, whereas the older individuals increased the braking impulse relative to the unobstructed impulses under the lead foot. The propulsive impulse under the lead foot increased in both groups, but changes were greater for the young.

	Discussion

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Vertical foot clearance over a step is clearly an important parameter for safe transit. For stepping onto a raised surface of 15 cm, we found clearances of 8.8 to 10.6 cm, approximately the same amplitude as reported previously for stepping over an obstacle of approximately the same height ^(9)(12). This suggests some similarity in gait adaptation for stepping on the raised surface and for stepping over obstacles. For stepping off a raised surface, foot clearance was low compared with that of stepping on, less than 3 cm, and was particularly low for the trail foot. Such low clearances would appear to demand very precise foot trajectory control (particularly by the trail foot) during stepping off a raised surface. Relatively higher clearances for the older participants may reflect a safety strategy in stepping off a raised surface.

The most important finding here was that in approaching the raised surface the older individuals appear to have used a nonoptimal foot placement strategy in which, compared with that of young subjects, the trail foot was placed farther away from the edge of the step as a proportion of stride distance. One consequence of trail-foot placement far from the surface edge is that, in the event of disturbance to the lead-foot trajectory, the older subjects would have very little correction time (5% of step time) and little latitude in foot placement beyond the step edge. This observation implies that foot placement on the approach to a raised surface may be a critical determinant of safety. Previous findings ⁽⁴⁾⁽⁵⁾ have emphasized muscular strength and declines in neuromuscular function as dominant factors in the capacity to negotiate obstructions safely. In light of the present findings, however, it appears that older people's foot placement strategy is nonoptimal and, on first consideration, such a strategy would not appear to be directly associated with declines in neuromuscular capacities such as strength and flexibility.

In stepping on and off raised surfaces and in other obstacle avoidance tasks, the trail foot is positioned first, and visual guidance of foot position relative to the obstacle is the critical process underlying this ability. The general principle of gait control that we advance here is that optimal placement of the trail foot relative to obstructions is essential for safe traverse. Once the trail limb has been positioned, the intrinsic dynamics of the lead limb play a major role in determining lead-foot trajectory over obstacles or obstructions.

The pattern of impulse modulations seen in Fig. 5 is consistent with the above observation concerning the lower horizontal foot clearance beyond the surface edge by the lead foot. As shown in Fig. 3 and Fig. 4, horizontal step clearance (both absolute and as a percentage of step length) for the older participants when stepping on was significantly lower. This feature of raised surface negotiation was highlighted as potentially hazardous because of the older person's relatively small (approximately 6-cm) horizontal clearance. The low anterior-posterior trail-foot propulsive impulse combined with increased braking impulse associated with the older walkers did not therefore assist the lead foot in traversing the step edge. Rather, the older individuals stepped onto the surface with a short lead-foot clearance that was not assisted by impulses from the trail foot. An increase in vertical impulse would assist in elevating the whole body and consequently facilitate vertical foot clearance. In this task, the significantly greater lead-foot clearance of older individuals in stepping off, presented in Fig. 2, may therefore have been engendered by their appreciably greater vertical trail-foot impulses.

All the participants in the study were screened for musculoskeletal or visual impairments by use of an in-house questionnaire. Subjects reporting any balance or locomotor problems were not included in the study and there was no evidence during testing of participants having difficulties in maintaining balance. The between-group differences reported herein can therefore reasonably be considered as primarily due to age-related declines in neuromuscular function.

In the raised surface task investigated here there are a number of risk-related consequences of nonoptimal trail-foot placement by older individuals. First, when the trail foot was positioned farther from the step, the lead-foot contacted the raised surface later in the stride. As suggested above, in the event of either obstacle contact or other disturbance to stability there would be less correction time or "available response time" ⁽¹⁴⁾ in which to arrest the forward motion of the whole body center of mass before it moves forward beyond the base of support provided by the feet. Second, trail-foot position before the step is associated with vertical clearance (between the heel and the surface edge), and in this experiment in stepping on the significantly lower lead-foot clearance by the older individuals provided less margin for error in the vertical direction. Third, the narrow margin between the edge of the step and the lead-foot heel shown by the older individuals implies reduced capacity to lengthen the step to achieve stable foot placement in the event of any imbalance. The observation that the young subjects made heel contact on average 17 cm from the step edge compared with a 6-cm horizontal clearance for the older individuals indicates that lead-foot placement was much less constrained in young people.

To investigate further the effects of lead-foot constraint we calculated the mean within-subject step-to-heel SD beyond the step edge, in other words the variability associated with the lead-foot horizontal clearance. The low SD of lead-foot horizontal clearance in older individuals (2.9 compared with 9.1 cm for the young) suggests that, in contrast to older individuals who place the trail foot closer to the step, lead-foot placement by young people is permitted, as it were, to be considerably more variable because of the greater distance from the step edge. A foot placement strategy in negotiating raised surfaces that allows greater variability in foot placement, within safe bounds, would impose less constraint on lower limb targeting and presumably allow attention to be diverted to other features of the environment. In road crossing, for example, attention could be diverted to oncoming vehicles. The general implication of the findings reported here is that falls and perhaps other accidents in older individuals may, in part, be due to increased attention demands associated with the primary task (negotiating obstacles), leaving less time to monitor critical and in some cases hazardous features of the external environment.

	Acknowledgments

 
This research was supported by Grant ARC SGS19/95 from the Australian Research Council. The authors thank Kate Kloot and Daniel Halliday for assistance with the data analysis and figures.

Received April 6, 1999

Accepted August 6, 1999

	References

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