- Email: [email protected]
Effect of stroke on step characteristics of obstacle crossing
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Effect of Stroke on Step Characteristics of Obstacle Crossing Catherine M. Said, BAppSc, Patricia A. Goldie, PhD, Aftab E. Patla, PhD, William A. Sparrow, PhD ABSTRACT. Said CM, Goldie PA, Patla AE, Sparrow WA. Effect of stroke on step characteristics of obstacle crossing. Arch Phys Med Rehabil 2001;82:1712-9.
cine and the American Academy of Physical Medicine and Rehabilitation
Objective: To compare spatial and temporal measures during lead limb obstacle crossing between subjects with stroke and healthy subjects. Design: Experimental, observational, with matched controls. Setting: Geriatric rehabilitation unit in a tertiary referral hospital. Participants: Distance data were available for 19 subjects with stroke and 19 able-bodied subjects. Temporal data were available for 16 subjects with stroke and 16 able-bodied subjects. Subjects with stroke were inpatients and had to be able to walk 10 meters without assistance or gait aid. Intervention: Subjects were required to step over high and wide obstacles, ranging from 1 to 8cm, and trials were videotaped. Main Outcome Measures: Toe clearance, preobstacle distance, postobstacle distance, step length, proportion of step length preobstacle, step time, preobstacle step time, postobstacle step time, and proportion of step time preobstacle were measured. Results: Mann-Whitney U tests were performed to determine differences between the 2 groups. Subjects with stroke had significantly higher toe clearance, smaller postobstacle distances, and greater step times than healthy subjects. Subjects with stroke did not demonstrate a significant reduction in preobstacle distance. Conclusion: By modifying their lead limb trajectory during obstacle crossing, persons with stroke reduce the risk of a trip due to toe contact, but the modification may expose them to other safety risks. Key Words: Cerebrovascular accident; Gait; Rehabilitation; Walking. © 2001 by the American Congress of Rehabilitation Medi-
NE AIM OF PHYSIOTHERAPY after stroke is to rehaO bilitate gait disorders to maximize the patient’s function in the home and community. Obstacle crossing is one of many
From the Physiotherapy Department, Austin and Repatriation Medical Centre, West Heidelberg, Victoria, Australia (Said); School of Physiotherapy, La Trobe University, Bundoora, Victoria, Australia (Said, Goldie); University of Waterloo, Ont, Canada (Patla); and School of Health Sciences, Deakin University, Victoria, Australia (Sparrow). Accepted in revised form December 26, 2000. Supported in part by a postgraduate studentship from the Faculty of Health Sciences, La Trobe University, a scholarship from the Felice Rosemary Lloyd Trust Fund, a grant from the Research and Professional Development Committee, School of Physiotherapy, La Trobe University, and a grant from the La Trobe University Faculty of Health Sciences Research Committee. Presented as a poster at the International Symposium on Gait Disorders, September 1999, Prague; and as an oral presentation at the Sixth International Physiotherapy Congress of the Australian Physiotherapy Association, 2000, Canberra, Australia. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Correspondence to Catherine M. Said, BAppSc, Physiotherapy Dept, Austin and Repatriation Medical Center, Locked Bag No. 1, West Heidelberg, Victoria, 3081, Australia, e-mail: [email protected]. Reprints will not be available. 0003-9993/01/8212-6507$35.00/0 doi:10.1053/apmr.2001.26247
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complex tasks associated with ambulation in the everyday environment. Evidence suggests that the ability to negotiate obstacles safely is compromised after stroke, with 1 study3 revealing that 10% of falls in subjects with stroke after discharge from hospital were related to an obstacle. It is important to understand the extent and nature of obstacle-crossing deficits after stroke to identify persons at risk of trips and to guide the development of treatment strategies. Little is known about the obstacle-crossing deficits that follow stroke. A previous study4 showed that 13 of 24 subjects with stroke who were ambulant without physical assistance or the use of a gait aid had difficulty stepping over a small obstacle. The reason for difficulties appeared to be related to the nature of the obstacle. Difficulty crossing a high obstacle (1– 8cm) usually occurred because the subject needed assistance or a rail to maintain balance. Difficulty crossing a wide obstacle was often due to lead limb (limb that crosses the obstacle first) contact with the obstacle. Subjects with stroke did not show a preference to lead with the affected or the unaffected limb, with most patients alternating the choice of leading limb between trials. Obstacle-crossing performance was inconsistent over 2 testing sessions; performance on the initial test did not predict performance on the second test. The poor reliability suggests that a simple pass or fail scoring system may not be the most appropriate measurement tool for people with stroke and that other measures of obstaclecrossing performance should be sought. As yet, the spatial and temporal characteristics of obstacle crossing have not been documented for this population. The present study undertook to investigate foot placement relative to the obstacle, obstacle clearance, and the timing of obstacle crossing after stroke in an effort to describe further the deficit and identify features that may impact on obstacle crossing safety. Successful clearance of the obstacle by both limbs is an important feature of obstacle crossing. The risk of a trip or fall is greater if the lead limb is obstructed, because the body’s center of mass (COM) is anterior to the base of support. It follows that lead limb clearances have been widely reported.5-11 Evidence suggests that people with pathology modulate lead foot clearance to provide safe obstacle crossing. Compared with healthy subjects, people with age-related maculopathy use higher toe clearances over obstacles with a low visual contrast to the supporting surface.10 It was hypothesized that it was more difficult for people with age-related maculopathy to judge obstacle height accurately under low-contrast conditions. It is not known whether persons with stroke have a similar increase in foot clearance to improve safety, or whether their limb elevation is limited by neurologic deficits. Various methods have been used to quantify foot clearance over an obstacle, including measurement from the lowest point of the lead foot,7 the toe,5,9 and the heel.11 Because the consequences of toe
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contact represent the greatest threat to stability, we elected to measure lead limb toe clearance during obstacle crossing (fig 1) to identify any modifications that may impact on obstacle crossing safety. The position of the obstacle within the crossing step or stride (fig 1) is also frequently investigated. Elderly subjects tend to cross obstacles further into the step than younger subjects. They achieve this modification by starting the task with the trail limb further from the obstacle and by placing the lead limb closer to the obstacle on landing.7,12 It has been hypothesized12 that this strategy reduces the risk of obstacle contact by increasing the time for online modification of the lead limb during the swing phase. The consequences of a trip are also reduced, as the COM is placed further posterior to the foot contact or pivot point.7 Crossing the obstacle further into the
step, however, increases the risk that the heel of the lead limb will contact the obstacle on landing. As illustrated in figure 1, we measured preobstacle distance, postobstacle distance, and the proportion of step length before obstacle crossing in the present study. The aim was to determine obstacle crossing strategies that either compromise or enhance safety after stroke. Various temporal factors of obstacle crossing have been investigated, including swing time of the lead limb,9 lead and trail limb stride time, and proportion of lead and trail limb stride time before the obstacle.11 Because people with stroke have longer step times than healthy subjects during unobstructed gait,2 it was expected that step times would be increased during obstacle crossing. We also expected that preobstacle step time and postobstacle step time would be longer. The percentage of time preobstacle crossing provides information about the proportion of time spent preparing for obstacle crossing, irrespective of total time. Whether people with stroke spend the same proportion of step time preobstacle as healthy subjects was unknown. By examining temporal variables in the present study we hoped to gain information about timing changes related to obstacle crossing after stroke. The present study investigated selected spatial and temporal characteristics of the gait pattern to define further the problems in obstacle crossing following stroke. The following variables were identified as important: toe clearance, preobstacle distance, postobstacle distance, step length, preobstacle time, postobstacle time, step time, and the proportion of step length and step time preobstacle. To quantify the deficit, we compared results from a group of subjects with stroke with a group of healthy subjects matched for age, gender, and height. METHOD
Fig 1. Measurements obtained from video over (A) high obstacles and (B) wide obstacles. Solid line represents the lead limb. Abbreviations: TCl, toe clearance, distance between lead limb toe and the obstacle’s top; PrD, preobstacle distance, distance between heel of the trail limb and the obstacle’s edge; PoD, postobstacle distance, distance between heel of the lead limb and the obstacle’s edge; Step Length, distance between heel of the trail limb and heel of the lead limb.
Subjects Consecutive admissions of stroke patients to a geriatric rehabilitation ward at a major tertiary referral hospital were screened for specific inclusion criteria. Subjects had to be receiving treatment from a physiotherapist for gait or balance disorders after a recent stroke, and be capable of walking a minimum of 10 meters 8 times with a rest but without a walking aid, orthosis, or assistance. They also had to be able to follow simple commands. Candidates were excluded if they had any medical conditions that could affect their mobility, including previous history of visual deficits. They were not excluded if they had previous strokes, provided they met other inclusion criteria. Healthy subjects with no medical conditions that might affect mobility, including visual deficits, were also recruited. Twenty-four subjects with stroke and 22 healthy subjects were tested. Features of performance including failure rate, circumstances of fails, consistency of performance, and choice of lead limb have been previously reported.4 Because of technical difficulties with the videocamera, video data on the distance parameters were not available for 5 subjects. Complete data for the distance parameters were available for 19 subjects with stroke (mean ⫾ standard deviation: age, 76.6 ⫾ 8.5yr; height, 163.5 ⫾ 9.9cm; leg length, 83.1 ⫾ 7.2cm; time since stroke, 29.2 ⫾ 12.4d) and 19 healthy subjects matched for age, gender, and height (11 women, 8 men; age, 76.1 ⫾ 8.0yr; height, 164.7 ⫾ 9.7cm; leg length, 82.6 ⫾ 5.6cm). The characteristics of the subset of subjects with stroke are in table 1. From the FIM™ instrument scores, note that some subjects may have used a walking aid or required supervision for their daily walking, but still met the inclusion criteria in the study.
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OBSTACLE-CROSSING STEP CHARACTERISTICS, Said Table 1: Stroke Subject Profile
Subject
Age (yr)
Gender
Neglect
Visual Deficit
FIM Walk
Days Poststroke
Site of Lesion
2 3 4* 5* 7 8 9 10 12 13* 14 20 21 22 23 24 25 26 27
83 74 72 80 82 84 65 85 62 71 76 84 73 82 59 70 81 90 83
F F F M F M M F F M F M M M F M F F F
Nil Nil Left Nil Nil Nil Left Left Nil Right Nil Left Nil Nil Nil Right Nil Left Left
Nil Nil Left hemianopia Vertical gaze palsy Nil Nil Nil Nil Nil Nil Nil Left hemianopia Nil Nil Nil Nil Nil Nil Nil
5 7 5 5 6 7 5 6 7 7 7 7 6 7 7 6 7 5 5
27 22 19 39 32 20 57 26 34 44 24 15 15 49 23 45 22 27 17
Brainstem Left stroke Right parieto-occipital infarct Brainstem Left stroke Right frontoparietal Right frontoparietal Right frontal Right anterior cerebral artery infarct Left thalamic Right stroke Right posterior parietal Right cerebellar Left cortical Left internal capsule Left cortical Left subcortical Right cortical Right cortical
Abbreviations: M, male; F, female. * Subjects were excluded from analysis of temporal data.
Additional data were missing on the temporal parameters for 1 subject with stroke, because heel contact was obscured by the contralateral limb, and for 3 healthy subjects because heel contact occurred out of the camera’s field of view. Complete temporal data were available for 16 subjects with stroke (age, 77.1 ⫾ 9.1yr; height, 162.1 ⫾ 9.9cm; leg length, 81.9 ⫾ 7.0cm; time since stroke, 28.3 ⫾ 12.5d) and 16 healthy subjects matched for age, gender, and height (age, 76.3 ⫾ 8.5yr; height, 162.2 ⫾ 8.4cm; leg length, 81.0 ⫾ 4.4cm). Apparatus The test was conducted on a linoleum-surfaced, 10-meter walkway that had a set of parallel bars positioned alongside its middle portion. Red balsa wood obstacles (1.5mm thick ⫻ 60cm long) were in 3 sizes; 1.0cm, 4.0cm, and 8.0cm. Before each trial, a single obstacle was placed in the middle of the walkway at 5 meters. To provide a high obstacle, it was secured vertically to the walkway with a small amount of adhesive gum, and to present a wide obstacle it was placed flat on the walkway. These sizes were selected because they were representative of obstacles likely to be encountered in the home and community. Because previous research9,13 indicated that subjects used different strategies over obstacles in different orientations, we analyzed separately the results from high crossings and wide crossings. If an obstacle was contacted by a foot, it either broke or fell flat to the ground, minimizing the risk of a trip. A Philips Explorer VHS videocameraa was positioned perpendicular to the walkway, in line with the obstacle, to record the crossing step. The camera was positioned 3 meters from the obstacle, and captured a 1.6-meter section of the walkway. For safety, all subjects wore a lightweight safety belt around the waist throughout testing. A video distance and angle measurerb (VDAM) was used to measure the various distance and temporal measures. The system was calibrated using an object of known height and length. Distance parameters were obtained by manually digitizing the heel, toe, and obstacle. The VDAM calculated distance between points at the various phases of the gait cycle by counting
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the pixels between the 2 points and scaling with respect to the calibration object after correcting for squareness. To determine the validity of these measures with respect to a commercially available system, results from 2 subjects were compared with those obtained by the PEAK motion analysis system.c A minimum of a week elapsed between the 2 measurements. Nine trials were included in the analysis. High correlations were demonstrated for toe clearance (r ⫽ .96, ICC3,1 ⫽ .95) and postobstacle distance (r ⫽ .99, ICC3,1 ⫽ .99). Temporal measures were obtained by counting the frames between events (25 frames/s). The retest reliability of this method was examined by comparing the frames counted for the variable postobstacle step time on 2 occasions. A minimum of 1 week separated the 2 occasions. Twenty-three trials, from 6 subjects, were included in the analysis. The retest reliability was high (r ⫽ .96, ICC3,1 ⫽ .94). Procedure The study was approved by the institutions’ human research ethics committees, and informed consent was obtained from all subjects before testing. Before starting the testing session, a brief (approximately 30s) video recording was obtained. This video was used to calibrate the VDAM. The complete testing protocol has been described previously.4 In brief, all subjects were required to negotiate 3 obstacle heights (1cm, 4cm, 8cm), 3 obstacle widths (1cm, 4cm, 8cm), and perform 2 unobstructed trials, for a total of 8 trials. The tests were performed in comfortable, well-fitting shoes and without use of any walking aid or orthosis. Subjects wore corrective lenses if these were normally worn during ambulation. The order of presentation of obstacle size and orientation (height or width) was counterbalanced across subjects. Subjects performed the test twice. However, because more subjects were available for test 2, only the data from the second trial has been reported. The data therefore represent performance after a familiarization session. All subjects were advised not to attempt any obstacle that they felt posed a risk to their safety. The instructions were:
OBSTACLE-CROSSING STEP CHARACTERISTICS, Said
“The purpose of the study is to see how safely you can step over an obstacle, without using a rail. Walk from the start of the walkway to the chair at the end at a comfortable speed, and step over the obstacle in the middle without touching it. If you think that any obstacle is too difficult for you, I want you to stop. The rail is there, but only use it if you need it to keep your balance. Do you have any questions?” For the unobstructed trials, subjects were instructed to walk to the chair at the end of the walkway at a comfortable speed. All subjects were allowed to inspect visually and manually the obstacle before each walk on each day. The therapist then demonstrated the first obstructed trial. Subjects were free to start walking and step over the obstacle with either limb. The tester, a physiotherapist, walked beside the subject to provide assistance if needed. To minimize fatigue for the subjects with stroke, a wheelchair was provided to return them to the start of the walkway. Subjects rested for at least 1 minute between trials, while the obstacle was changed, and were allowed to rest for as long as needed. The trial was scored as either a pass or a fail. A pass was recorded if the subject ambulated the entire walkway and cleared the obstacle with both limbs without requiring assistance or the use of the rail. A fail was recorded if the subject required assistance from the tester or the rail, contacted the obstacle, or failed to step over the obstacle. The testing procedure took from 20 to 40 minutes and the total distance covered was 80 meters, with rests every 10 meters. The VDAM provided data on the following variables: toe clearance, preobstacle distance, postobstacle distance, and step time, as illustrated in figure 1. Step time (from trail limb heel contact until lead limb heel contact), preobstacle step time (trail limb heel contact to lead limb toe clearance) and postobstacle step time (lead limb toe clearance to lead limb heel contact) were also calculated. Preobstacle step length or time was then expressed as a proportion of the total step length or time. Statistical Analysis Data for some variables differed significantly from a normal distribution, as determined by the Shapiro-Wilks statistic. Nonparametric statistical methods were therefore used throughout the analysis to maintain a consistent approach. Because people use different strategies when crossing obstacles in different orientations, data for high and wide obstacles were analyzed separately. Because subjects were allowed to lead with either limb, we could not analyze data for the affected and unaffected limb separately. Multiple Mann-Whitney U tests were performed using SPSS softwared to determine the differences between subjects with stroke and healthy subjects for the variables of toe clearance, preobstacle distance, postobstacle distance, step length, proportion of crossing distance preobstacle, preobstacle time, postobstacle time, step time, and proportion of step time preobstacle. To correct the type I error rate for the 3 comparisons (1cm, 4cm, 8cm) at each orientation, a Bonferroni adjustment was performed, and a significance level of p less than .017 was used. It could be argued that the number of comparisons should demand a more rigorous approach and a more stringent correction for the familywise error. However, because the present study was largely exploratory, and the consequences of a type I error are not hazardous, we considered that this approach was appropriate.14 RESULTS No fails were recorded for any of the healthy subjects. Of the 19 subjects with stroke included in the analysis of the spatial
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data, 5 fails occurred; 1 fail was because of contact with the obstacle, 3 fails were because of the need for assistance, and 1 fail was because of use of the rail. Subjects led with the affected limb in 48% of the trials. Of the 16 subjects with stroke included in the analysis of the temporal data, only 2 fails occurred, both because of the need for assistance. Subjects led with the affected limb in 54% of the trials. Spatial Measures of Obstacle Crossing Comparing the stroke group and their healthy counterparts on the spatial measures (fig 2), we found a consistent trend for toe clearance to be greater among the subjects with stroke. Statistical analysis determined that toe clearance was significantly greater in the stroke group on the 1-cm high (U ⫽ 66, p ⬍ .017), 8-cm high (U ⫽ 79, p ⬍ .017), and the 4-cm wide obstacles (U ⫽ 97, p ⬍ .017). There was also a consistent trend for the postobstacle distance to be reduced in the stroke group. The difference was significant for 5 of the 6 conditions, including the 1-cm high (U ⫽ 92, p ⬍ .017), 4-cm high (U ⫽ 69, p ⬍ .017), 8-cm high (U ⫽ 86, p ⬍ .017), 4-cm wide (U ⫽ 96, p ⬍ .017), and the 8-cm wide (U ⫽ 83, p ⬍ .017) obstacles. In contrast, the preobstacle distance did not differ significantly between the 2 groups for any condition (U ⬎ 130, p ⬎ .017). Although step length tended to be smaller in the stroke group, the difference was not significant over any condition (p ⬎ .017). Subjects with stroke had a larger proportion of step length preobstacle, therefore crossing the obstacle further into the step. This difference, however, was only significant over the 4-cm high (U ⫽ 89, p ⬍ .017) and 8-cm high obstacles (U ⫽ 94, p ⬍ .017). In summary, during obstacle crossing subjects with stroke had increased toe clearance, reduced postobstacle distance, and they crossed the obstacle further into the step when compared with healthy subjects. Temporal Measures of Obstacle Crossing The temporal data for subjects with stroke and healthy subjects (fig 3) showed that subjects with stroke had significantly increased step time during crossing of all obstacles (1-cm high, U ⫽ 17, p ⬍ .017; 4-cm high, U ⫽ 22.5, p ⬍ .017; 8-cm high, U ⫽ 11, p ⬍ .017; 1-cm wide, U ⫽ 21, p ⬍ .017; 4-cm wide, U ⫽ 49.5, p ⬍ .017; 8-cm wide, U ⫽ 24, p ⬍ .017). Similarly, their postobstacle step times were increased over all conditions (U ⬍ 51.5, p ⬍ .017). Preobstacle step times were also significantly larger in the subjects with stroke over all obstacles (U ⬍ 46.5, p ⬍ .017) except the 4-cm wide obstacle. Although the proportion of preobstacle step time varied more in the stroke group, no statistically significant difference existed between the 2 groups over any condition (U ⬎ 77, p ⬎ .017). In summary, while subjects with stroke had longer step times than healthy subjects, both groups spent the same proportion of step time in the preobstacle phase of crossing. DISCUSSION In the present study, subjects with stroke who were able to ambulate without assistance or a gait aid used a different lead limb trajectory during obstacle crossing than healthy subjects. The differences and similarities between the obstacle crossing characteristics of this group of subjects with stroke and healthy subjects are discussed below, emphasizing the potential impact of adaptations on safety. Possible explanations for the modifications to gait during obstacle crossing following stroke are then considered. The higher toe clearances can be seen as a safety adaptation, because they minimize the risk of a trip due to inadvertent toe
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Fig 2. Notched boxplots representing toe clearance, postobstacle distance, and preobstacle distance over high and wide obstacles for subjects with stroke and healthy subjects. The notch in the boxplot represents the median; upper and lower edges of the box represent the 25th and 75th percentiles, respectively; whiskers represent the 90th percentile; dots represent the outliers. * Significant difference between stroke and healthy subjects.
Fig 3. Notched boxplots representing step time and the proportion of step time preobstacle over high and wide obstacles for subjects with stroke and healthy subjects. The notch in the boxplot represents the median; upper and lower edges of the box represent the 25th and 75th percentiles, respectively; whiskers represent the 90th percentile; dots represent the outliers. * Significant difference between stroke and healthy subjects.
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contact. In comparison with healthy subjects, subjects with stroke had significantly higher median toe clearances for 3 of the 6 obstacle conditions, and a trend toward higher clearances on the remaining 3 conditions. Toe clearances in the stroke group over the 1-cm high obstacle and all the wide obstacles were positively skewed, with some individuals using extremely large clearances. Great variability also existed between subjects with stroke over these obstacles as illustrated in figures 2 and 3. In some instances, the increased toe clearance used by the subjects with stroke may have had a negative effect on obstacle crossing safety by increasing stability demands. As the lead limb steps over the obstacle, stability demands are met by the trail (stance) limb. We did not investigate how the increase in lead limb toe clearance was achieved by subjects in our stroke group, but previous studies5,9,10 on healthy subjects showed that it is obtained by a combination of increased hip and knee flexion and hip elevation. These strategies all increase the stability requirements of the task,9 and may be reflected by the increased moments at the hip and ankle of the trail (stance) limb as obstacle height increases.15 Considering that total limb elevation includes toe clearance and the height of the obstacle, it follows that stability demands are substantially greater over higher obstacles. If stability requirements are not met, loss of balance may occur. It was previously reported4 that persons with stroke required assistance or the use of a rail in 6.9% of the trials when crossing 1- to 8-cm high obstacles, compared with 1.4% when crossing 1- to 8-cm wide obstacles. The significantly greater toe clearances used over high obstacles by the group of stroke subjects may have partially contributed to their increased reliance on external supports to maintain balance. The factors contributing to increased toe clearance poststroke require consideration. Persons with stroke may feel greater concern for safety during obstacle crossing, which may lead to greater caution during crossing and a resultant increase in toe clearance. It is also possible that the variety of deficits following stroke may contribute to their limb trajectories being more variable and inconsistent. This variability would be reflected in measures such as toe clearance. Increased variability in obstacle clearance would create an undesirable increase in the risk of toe contact, unless it was accompanied by an increase in median clearance. The increased median toe clearance exhibited by stroke subjects in the present study may be an attempt to compensate for increased individual variability in toe clearance. Subjects in the stroke group maintained the preobstacle distance while reducing the postobstacle distance. As a consequence, they had a larger proportion of the step length preobstacle, and crossed the obstacle further into the step. This strategy may be safer because it minimizes the risk of toe contact earlier in swing phase, which represents a serious threat to stability.7 It also allows greater time for visually mediated online modifications to the trajectory before obstacle crossing.10,12 Maintaining the preobstacle distance following stroke may optimize safety by other mechanisms. The trail limb plays an important role during single limb stance by maintaining stability while the lead limb steps over the obstacle. Chou and Draganich15 showed that adduction and internal rotation moments at the ankle increase on the stance limb as the preobstacle distance between the trail limb and the obstacle is reduced. This finding suggests that greater muscle activity around the ankle would be required to control the movement, particularly in the evertors. This aspect of control may prove to
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be difficult after stroke. Once the lead limb has safely landed, the trail limb is then required to cross the obstacle. Recent research16 in young healthy adults showed that placing the trail limb closer to the obstacle and decreasing the preobstacle distance is associated with reductions in hip, knee, and ankle flexion, with a resultant decrease in toe clearance.16 This placement increases the risk of trail limb foot contact. Trail limb trajectory cannot be guided by vision; therefore, people with a stroke may elect to position the trail limb for optimal obstacle clearance. Further investigation into the influence of trail limb preobstacle position on stability and trail limb trajectory during obstacle crossing is warranted. The reduced postobstacle distance found in the stroke group represents the most obvious threat to obstacle-crossing safety. It suggests that postobstacle foot positioning was compromised. The small postobstacle distances increase the chances of lead limb contact on landing, particularly while stepping across a wide obstacle. It has been previously reported4 that nearly half of poststroke fails were from lead limb contact, with 83% occurring while crossing a 1- to 8-cm wide obstacle. This finding suggests that inappropriate placement of the lead limb after crossing may contribute to failures in obstacle crossing in people with stroke, and therefore reduce safety. Several factors may have contributed to the smaller postobstacle distance found in the present study. The reduction may reflect a modified trajectory due to reduced knee extension after clearing the obstacle, particularly for the affected limb. Stroke subjects may have difficulty generating sufficient rectus femoris activity to extend the knee after obstacle crossing,9 or be unable to activate their biceps femoris to control the horizontal velocity for limb landing. It is possible that people with stroke may require even greater forces to extend the knee than healthy subjects, given that there may be more limb flexion at obstacle crossing to achieve toe clearance. Other musculoskeletal factors, such as reduced hamstring length, may also affect leg trajectory. Alternatively, people with stroke may bring the heel backward at landing as has been documented in patients with age related maculopathy.10 This strategy minimizes the risk of the foot slipping forward at heel contact. The smaller postobstacle distance may also be related to the stability requirements of the task. While stepping over the obstacle, the stance (trail) limb maintains stability until initial contact of the lead limb. After stroke, subjects may maintain the whole body COM closer to the stance foot, until the lead limb lands safely. This action would effectively limit the postobstacle distance obtained by the lead limb, but would also reduce the consequences of a trip or slip of the lead limb. Finally, step time during obstacle crossing was significantly increased in subjects with stroke than in healthy subjects. This may be a safety strategy because it would increase the time available for modification of the swing limb trajectory. However, increased step time can challenge stability by increasing the time spent in single limb support. Single-limb stance times were not measured; however, because postobstacle (swing) time also increased significantly, it is unlikely that the increased step time is entirely attributable to an increase in double limb support. The increase in step time may have contributed to the difficulties maintaining balance in some subjects with stroke. Although not directly related to safety, the efficiency of the strategies chosen should be considered. Other authors17 have shown that in healthy subjects an increase in foot clearance is associated with an increase in energy requirements. It is likely that the high toe clearances used by the subjects with stroke represent an inefficient strategy. Use of inefficient strategies
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may limit the amount of activity that persons with stroke are able to do. Resulting fatigue also may place subjects at greater risk of falls at a later point in time. Another factor that may have impacted on the trajectory of the lead limb in this stroke group is their overall slower gait speed. Although the relationship between the spatial and temporal characteristics of obstacle crossing and speed has not been explored, it is reasonable to expect that these variables are related. Intersegmental dynamics play an important role in obstacle crossing in healthy subjects.18 For example, the increased hip flexion obtained in healthy subjects while crossing large obstacles is the result of an increase in knee flexor activity, not an increase in hip flexor activity.13 Motion-dependent moments are influenced by segment angular acceleration and velocity and joint linear acceleration. Although not measured in the present study, it is likely that these variables were all lower in the stroke subjects, and may have limited the use of intersegmental dynamics. This may have influenced the trajectory of their lead limb and, as a result, its landing position. The slower gait speeds may reflect an inability to activate appropriate muscle groups sufficiently, or it may be a safety modification to minimize the adverse consequences of contact with the obstacle. It would be of interest to examine the obstacle-crossing characteristics over a variety of gait speeds in healthy subjects, in order to determine whether preobstacle distance, postobstacle distance, and step length vary with gait speed. These studies would give us a fuller understanding of the deficits in obstacle crossing exhibited by people with stroke. Biomechanical, kinematic, and kinetic factors obviously play an important role in the control of limb trajectory. Alone, however, they may not explain all the trajectory modifications people make after stroke. Obstacle crossing involves a complex interaction between a person’s visual perception of the obstacle, and the implementation of an appropriate avoidance strategy. Although the neural pathways underlying perception and action in obstacle crossing in humans are not fully understood, animal studies provide some insight into the structures that may be involved. Visual information about the object size and location is mapped onto the primary visual cortex in the occipital lobe. Information is then processed via the ventral pathway (occipitotemporal) and dorsal pathway (occipitoparietal). It has been hypothesized19,20 that the dorsal pathway is of importance in the control of visually mediated movements, whereas a lesion in the ventral pathway does not compromise the ability to cross obstacles or reach for objects. Studies21 examining obstacle crossing in cats showed that the motor cortex plays a critical role in the motor control of obstacle crossing, particularly in regulating limb elevation and foot placement. The cerebellum also appears to play a role in visually mediated gait modifications.22 Lesions in these areas may result in an inability to modify the trajectory of the limb to clear the obstacle. Lesions that compromise balance, such as cerebellar lesions, may also impact on ability to complete the task. Because of the small sample size in the current study, we did not correlate step characteristics with site of lesion. It is interesting to note that, despite the heterogenous nature of subjects and lesions included in the present study, sufficient similarities existed in the strategies used by the subjects with stroke to differentiate them from healthy subjects. Study Limitations and Future Development The present results show that lead limb trajectory during obstacle crossing is significantly altered after stroke, when compared with healthy age-matched subjects. The study has
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several limitations. First, to be included, the subjects with stroke had to be capable of ambulating without physical assistance, a gait aid, or orthosis. The results can therefore only be generalized to subjects with similar characteristics. Second, because subjects were allowed to step over the obstacle with the preferred limb, we did not compare separately the affected and unaffected limbs of the subjects in the stroke group with the healthy subjects. Because stroke is typically a unilateral disorder, the characteristics of each limb need to be quantified to understand the nature of the poststroke deficit. An understanding of the kinematics of obstacle crossing will also provide an insight into some of the potential causes of the altered spatial and temporal characteristics. When the deficit in obstacle crossing after stroke is finally understood, training strategies designed to remediate the deficit can be explored. CONCLUSION In a select group of subjects with stroke, capable of ambulating a short distance without gait aids or assistance, obstaclecrossing deficits were not attributable to subjects’ inability to elevate the lead limb sufficiently. The reduced postobstacle distance suggests that the deficit may be related to difficulty placing the limb at landing. In addition, aspects of the strategies selected by the subjects with stroke may have further stressed an already compromised balance system. Acknowledgments: The authors thank the staff at the Austin and Repatriation Medical Centre for assisting in the study, and Kerri Martin for assisting with vision assessments. We also acknowledge the technical assistance from La Trobe University Technical Services, Faculty of Health Sciences, La Trobe University. References 1. Goldie PA, Matyas TA, Evans OM. Deficit and change in gait velocity during rehabilitation after stroke. Arch Phys Med Rehabil 1996;77:1074-82. 2. Olney SJ, Richards C. Hemiparetic gait following stroke. Part I: characteristics. Gait Posture 1996;4:136-48. 3. Forster A, Young J. Incidence and consequences of falls due to stroke: a systematic inquiry. Br Med J 1995;311:83-6. 4. Said CM, Goldie PA, Patla AE, Sparrow WA, Martin KE. Obstacle crossing in subjects with stroke. Arch Phys Med Rehabil 1999;80:1054-9. 5. Patla AE, Rietdyk S, Martin C, Prentice S. Locomotor patterns of the leading and the trailing limbs as solid and fragile obstacles are stepped over: some insights into the role of vision during locomotion. J Mot Behav 1996;28:35-47. 6. Alexander NB, Mollo JM, Giordani B, Ashton-Miller JA, Schultz AB, Grunawalt JA, et al. Maintenance of balance, gait patterns and obstacle clearance in Alzheimer’s disease. Neurology 1995; 45:908-14. 7. Chen H-C, Ashton-Miller JA, Alexander NB, Schultz AB. Stepping over obstacles: gait patterns of healthy young and old adults. J Gerontol A Biol Sci Med Sci 1991;46:M196-203. 8. Chen H-C, Schultz AB, Ashton-Miller JA, Giordani B, Alexander NB, Guire KE. Stepping over obstacles: dividing attention impairs performance of old more than young adults. J Gerontol A Biol Sci Med Sci 1996;51:M116-22. 9. Patla AE, Rietdyk S. Visual control of limb trajectory over obstacles during locomotion: effect of obstacle height and width. Gait Posture 1993;1:45-60. 10. Patla AE, Elliott DB, Flanagan J, Rietdyk S, Spaulding S. Effects of age-related maculopathy on strategies for going over obstacles of different heights and contrast [asbstract]. Gait Posture 1995;3: 105. 11. Sparrow WA, Shinkfield AJ, Chow S, Begg RK. Characteristics of gait in stepping over obstacles. Hum Mov Sci 1996;15:605-22. 12. Patla AE, Prentice SD, Gobbi LT. Visual control of obstacle avoidance during locomotion: strategies in young children, young
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13. 14. 15.
16. 17. 18. 19.
and older adults. In: Fernandez AM, Teasdale N, editors. Changes in sensorimotor behavior in aging. Amsterdam: Elsevier Science; 1996. p 257-77. McFadyen BJ, Winter DA. Anticipatory locomotor adjustments during obstructed human walking. Neurosci Res Commun 1991; 9:37-44. Keppel G. Design and analysis: a researcher’s handbook. 3rd ed. Englewood Cliffs (NJ): Prentice-Hall; 1991. Chou L-S, Draganich LF. Increasing obstacle height and decreasing toe obstacle distance affect the joint moments of the stance limb differently when stepping over an obstacle. Gait Posture 1998;8:186-204. Chou L-S, Draganich LF. Placing the trailing foot closer to an obstacle reduces flexion of the hip, knee, and ankle to increase the risk of tripping. J Biomech 1998;31:685-91. Chou L-S, Draganich LF, Song S-M. Minimum energy trajectories of the swing ankle when stepping over obstacles of different heights. J Biomech 1997;30:115-20. Patla AE, Prentice SD. The role of active forces and intersegmental dynamics in the control of limb trajectory over obstacles during locomotion in humans. Brain Res 1995;106:499-504. Goodale MA, Milner AD, Jakobson LS, Carey DP. A neurological
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dissociation between perceiving objects and grasping them. Nature 1991;349:154-6. 20. Patla AE. The neural control of locomotion. In: Spivack BS, editor. Evaluation and management of gait disorders. New York: Marcel Dekker; 1995. p 53-78. 21. Drew T, Jiang W, Kably B, Lavoie S. Role of the motor cortex in the control of visually triggered gait modifications. Can J Physiol Pharmacol 1996;74:426-42. 22. Armstrong DM, Marple-Horvat DE. Role of the cerebellum and motor cortex in the regulation of visually controlled locomotion. Can J Physiol Pharmacol 1996;74:443-55. Suppliers a. Philips Electronics Australia Ltd, 15 Blue St, North Sydney, New South Wales 2060, Australia. b. Brian Sleenman. La Trobe University Technical Services, Faculty of Health Sciences. Latrobe Univ, Bundoora, Victoria, 3086, Australia. c. Peak Performance Technologies Inc, 7388 S Revere Pkwy, Ste 603, Englewood, CO 80112. d. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.
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Effect of Stroke on Step Characteristics of Obstacle Crossing Catherine M. Said, BAppSc, Patricia A. Goldie, PhD, Aftab E. Patla, PhD, William A. Sparrow, PhD ABSTRACT. Said CM, Goldie PA, Patla AE, Sparrow WA. Effect of stroke on step characteristics of obstacle crossing. Arch Phys Med Rehabil 2001;82:1712-9.
cine and the American Academy of Physical Medicine and Rehabilitation
Objective: To compare spatial and temporal measures during lead limb obstacle crossing between subjects with stroke and healthy subjects. Design: Experimental, observational, with matched controls. Setting: Geriatric rehabilitation unit in a tertiary referral hospital. Participants: Distance data were available for 19 subjects with stroke and 19 able-bodied subjects. Temporal data were available for 16 subjects with stroke and 16 able-bodied subjects. Subjects with stroke were inpatients and had to be able to walk 10 meters without assistance or gait aid. Intervention: Subjects were required to step over high and wide obstacles, ranging from 1 to 8cm, and trials were videotaped. Main Outcome Measures: Toe clearance, preobstacle distance, postobstacle distance, step length, proportion of step length preobstacle, step time, preobstacle step time, postobstacle step time, and proportion of step time preobstacle were measured. Results: Mann-Whitney U tests were performed to determine differences between the 2 groups. Subjects with stroke had significantly higher toe clearance, smaller postobstacle distances, and greater step times than healthy subjects. Subjects with stroke did not demonstrate a significant reduction in preobstacle distance. Conclusion: By modifying their lead limb trajectory during obstacle crossing, persons with stroke reduce the risk of a trip due to toe contact, but the modification may expose them to other safety risks. Key Words: Cerebrovascular accident; Gait; Rehabilitation; Walking. © 2001 by the American Congress of Rehabilitation Medi-
NE AIM OF PHYSIOTHERAPY after stroke is to rehaO bilitate gait disorders to maximize the patient’s function in the home and community. Obstacle crossing is one of many
From the Physiotherapy Department, Austin and Repatriation Medical Centre, West Heidelberg, Victoria, Australia (Said); School of Physiotherapy, La Trobe University, Bundoora, Victoria, Australia (Said, Goldie); University of Waterloo, Ont, Canada (Patla); and School of Health Sciences, Deakin University, Victoria, Australia (Sparrow). Accepted in revised form December 26, 2000. Supported in part by a postgraduate studentship from the Faculty of Health Sciences, La Trobe University, a scholarship from the Felice Rosemary Lloyd Trust Fund, a grant from the Research and Professional Development Committee, School of Physiotherapy, La Trobe University, and a grant from the La Trobe University Faculty of Health Sciences Research Committee. Presented as a poster at the International Symposium on Gait Disorders, September 1999, Prague; and as an oral presentation at the Sixth International Physiotherapy Congress of the Australian Physiotherapy Association, 2000, Canberra, Australia. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit upon the author(s) or upon any organization with which the author(s) is/are associated. Correspondence to Catherine M. Said, BAppSc, Physiotherapy Dept, Austin and Repatriation Medical Center, Locked Bag No. 1, West Heidelberg, Victoria, 3081, Australia, e-mail: [email protected]. Reprints will not be available. 0003-9993/01/8212-6507$35.00/0 doi:10.1053/apmr.2001.26247
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1,2
complex tasks associated with ambulation in the everyday environment. Evidence suggests that the ability to negotiate obstacles safely is compromised after stroke, with 1 study3 revealing that 10% of falls in subjects with stroke after discharge from hospital were related to an obstacle. It is important to understand the extent and nature of obstacle-crossing deficits after stroke to identify persons at risk of trips and to guide the development of treatment strategies. Little is known about the obstacle-crossing deficits that follow stroke. A previous study4 showed that 13 of 24 subjects with stroke who were ambulant without physical assistance or the use of a gait aid had difficulty stepping over a small obstacle. The reason for difficulties appeared to be related to the nature of the obstacle. Difficulty crossing a high obstacle (1– 8cm) usually occurred because the subject needed assistance or a rail to maintain balance. Difficulty crossing a wide obstacle was often due to lead limb (limb that crosses the obstacle first) contact with the obstacle. Subjects with stroke did not show a preference to lead with the affected or the unaffected limb, with most patients alternating the choice of leading limb between trials. Obstacle-crossing performance was inconsistent over 2 testing sessions; performance on the initial test did not predict performance on the second test. The poor reliability suggests that a simple pass or fail scoring system may not be the most appropriate measurement tool for people with stroke and that other measures of obstaclecrossing performance should be sought. As yet, the spatial and temporal characteristics of obstacle crossing have not been documented for this population. The present study undertook to investigate foot placement relative to the obstacle, obstacle clearance, and the timing of obstacle crossing after stroke in an effort to describe further the deficit and identify features that may impact on obstacle crossing safety. Successful clearance of the obstacle by both limbs is an important feature of obstacle crossing. The risk of a trip or fall is greater if the lead limb is obstructed, because the body’s center of mass (COM) is anterior to the base of support. It follows that lead limb clearances have been widely reported.5-11 Evidence suggests that people with pathology modulate lead foot clearance to provide safe obstacle crossing. Compared with healthy subjects, people with age-related maculopathy use higher toe clearances over obstacles with a low visual contrast to the supporting surface.10 It was hypothesized that it was more difficult for people with age-related maculopathy to judge obstacle height accurately under low-contrast conditions. It is not known whether persons with stroke have a similar increase in foot clearance to improve safety, or whether their limb elevation is limited by neurologic deficits. Various methods have been used to quantify foot clearance over an obstacle, including measurement from the lowest point of the lead foot,7 the toe,5,9 and the heel.11 Because the consequences of toe
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contact represent the greatest threat to stability, we elected to measure lead limb toe clearance during obstacle crossing (fig 1) to identify any modifications that may impact on obstacle crossing safety. The position of the obstacle within the crossing step or stride (fig 1) is also frequently investigated. Elderly subjects tend to cross obstacles further into the step than younger subjects. They achieve this modification by starting the task with the trail limb further from the obstacle and by placing the lead limb closer to the obstacle on landing.7,12 It has been hypothesized12 that this strategy reduces the risk of obstacle contact by increasing the time for online modification of the lead limb during the swing phase. The consequences of a trip are also reduced, as the COM is placed further posterior to the foot contact or pivot point.7 Crossing the obstacle further into the
step, however, increases the risk that the heel of the lead limb will contact the obstacle on landing. As illustrated in figure 1, we measured preobstacle distance, postobstacle distance, and the proportion of step length before obstacle crossing in the present study. The aim was to determine obstacle crossing strategies that either compromise or enhance safety after stroke. Various temporal factors of obstacle crossing have been investigated, including swing time of the lead limb,9 lead and trail limb stride time, and proportion of lead and trail limb stride time before the obstacle.11 Because people with stroke have longer step times than healthy subjects during unobstructed gait,2 it was expected that step times would be increased during obstacle crossing. We also expected that preobstacle step time and postobstacle step time would be longer. The percentage of time preobstacle crossing provides information about the proportion of time spent preparing for obstacle crossing, irrespective of total time. Whether people with stroke spend the same proportion of step time preobstacle as healthy subjects was unknown. By examining temporal variables in the present study we hoped to gain information about timing changes related to obstacle crossing after stroke. The present study investigated selected spatial and temporal characteristics of the gait pattern to define further the problems in obstacle crossing following stroke. The following variables were identified as important: toe clearance, preobstacle distance, postobstacle distance, step length, preobstacle time, postobstacle time, step time, and the proportion of step length and step time preobstacle. To quantify the deficit, we compared results from a group of subjects with stroke with a group of healthy subjects matched for age, gender, and height. METHOD
Fig 1. Measurements obtained from video over (A) high obstacles and (B) wide obstacles. Solid line represents the lead limb. Abbreviations: TCl, toe clearance, distance between lead limb toe and the obstacle’s top; PrD, preobstacle distance, distance between heel of the trail limb and the obstacle’s edge; PoD, postobstacle distance, distance between heel of the lead limb and the obstacle’s edge; Step Length, distance between heel of the trail limb and heel of the lead limb.
Subjects Consecutive admissions of stroke patients to a geriatric rehabilitation ward at a major tertiary referral hospital were screened for specific inclusion criteria. Subjects had to be receiving treatment from a physiotherapist for gait or balance disorders after a recent stroke, and be capable of walking a minimum of 10 meters 8 times with a rest but without a walking aid, orthosis, or assistance. They also had to be able to follow simple commands. Candidates were excluded if they had any medical conditions that could affect their mobility, including previous history of visual deficits. They were not excluded if they had previous strokes, provided they met other inclusion criteria. Healthy subjects with no medical conditions that might affect mobility, including visual deficits, were also recruited. Twenty-four subjects with stroke and 22 healthy subjects were tested. Features of performance including failure rate, circumstances of fails, consistency of performance, and choice of lead limb have been previously reported.4 Because of technical difficulties with the videocamera, video data on the distance parameters were not available for 5 subjects. Complete data for the distance parameters were available for 19 subjects with stroke (mean ⫾ standard deviation: age, 76.6 ⫾ 8.5yr; height, 163.5 ⫾ 9.9cm; leg length, 83.1 ⫾ 7.2cm; time since stroke, 29.2 ⫾ 12.4d) and 19 healthy subjects matched for age, gender, and height (11 women, 8 men; age, 76.1 ⫾ 8.0yr; height, 164.7 ⫾ 9.7cm; leg length, 82.6 ⫾ 5.6cm). The characteristics of the subset of subjects with stroke are in table 1. From the FIM™ instrument scores, note that some subjects may have used a walking aid or required supervision for their daily walking, but still met the inclusion criteria in the study.
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OBSTACLE-CROSSING STEP CHARACTERISTICS, Said Table 1: Stroke Subject Profile
Subject
Age (yr)
Gender
Neglect
Visual Deficit
FIM Walk
Days Poststroke
Site of Lesion
2 3 4* 5* 7 8 9 10 12 13* 14 20 21 22 23 24 25 26 27
83 74 72 80 82 84 65 85 62 71 76 84 73 82 59 70 81 90 83
F F F M F M M F F M F M M M F M F F F
Nil Nil Left Nil Nil Nil Left Left Nil Right Nil Left Nil Nil Nil Right Nil Left Left
Nil Nil Left hemianopia Vertical gaze palsy Nil Nil Nil Nil Nil Nil Nil Left hemianopia Nil Nil Nil Nil Nil Nil Nil
5 7 5 5 6 7 5 6 7 7 7 7 6 7 7 6 7 5 5
27 22 19 39 32 20 57 26 34 44 24 15 15 49 23 45 22 27 17
Brainstem Left stroke Right parieto-occipital infarct Brainstem Left stroke Right frontoparietal Right frontoparietal Right frontal Right anterior cerebral artery infarct Left thalamic Right stroke Right posterior parietal Right cerebellar Left cortical Left internal capsule Left cortical Left subcortical Right cortical Right cortical
Abbreviations: M, male; F, female. * Subjects were excluded from analysis of temporal data.
Additional data were missing on the temporal parameters for 1 subject with stroke, because heel contact was obscured by the contralateral limb, and for 3 healthy subjects because heel contact occurred out of the camera’s field of view. Complete temporal data were available for 16 subjects with stroke (age, 77.1 ⫾ 9.1yr; height, 162.1 ⫾ 9.9cm; leg length, 81.9 ⫾ 7.0cm; time since stroke, 28.3 ⫾ 12.5d) and 16 healthy subjects matched for age, gender, and height (age, 76.3 ⫾ 8.5yr; height, 162.2 ⫾ 8.4cm; leg length, 81.0 ⫾ 4.4cm). Apparatus The test was conducted on a linoleum-surfaced, 10-meter walkway that had a set of parallel bars positioned alongside its middle portion. Red balsa wood obstacles (1.5mm thick ⫻ 60cm long) were in 3 sizes; 1.0cm, 4.0cm, and 8.0cm. Before each trial, a single obstacle was placed in the middle of the walkway at 5 meters. To provide a high obstacle, it was secured vertically to the walkway with a small amount of adhesive gum, and to present a wide obstacle it was placed flat on the walkway. These sizes were selected because they were representative of obstacles likely to be encountered in the home and community. Because previous research9,13 indicated that subjects used different strategies over obstacles in different orientations, we analyzed separately the results from high crossings and wide crossings. If an obstacle was contacted by a foot, it either broke or fell flat to the ground, minimizing the risk of a trip. A Philips Explorer VHS videocameraa was positioned perpendicular to the walkway, in line with the obstacle, to record the crossing step. The camera was positioned 3 meters from the obstacle, and captured a 1.6-meter section of the walkway. For safety, all subjects wore a lightweight safety belt around the waist throughout testing. A video distance and angle measurerb (VDAM) was used to measure the various distance and temporal measures. The system was calibrated using an object of known height and length. Distance parameters were obtained by manually digitizing the heel, toe, and obstacle. The VDAM calculated distance between points at the various phases of the gait cycle by counting
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the pixels between the 2 points and scaling with respect to the calibration object after correcting for squareness. To determine the validity of these measures with respect to a commercially available system, results from 2 subjects were compared with those obtained by the PEAK motion analysis system.c A minimum of a week elapsed between the 2 measurements. Nine trials were included in the analysis. High correlations were demonstrated for toe clearance (r ⫽ .96, ICC3,1 ⫽ .95) and postobstacle distance (r ⫽ .99, ICC3,1 ⫽ .99). Temporal measures were obtained by counting the frames between events (25 frames/s). The retest reliability of this method was examined by comparing the frames counted for the variable postobstacle step time on 2 occasions. A minimum of 1 week separated the 2 occasions. Twenty-three trials, from 6 subjects, were included in the analysis. The retest reliability was high (r ⫽ .96, ICC3,1 ⫽ .94). Procedure The study was approved by the institutions’ human research ethics committees, and informed consent was obtained from all subjects before testing. Before starting the testing session, a brief (approximately 30s) video recording was obtained. This video was used to calibrate the VDAM. The complete testing protocol has been described previously.4 In brief, all subjects were required to negotiate 3 obstacle heights (1cm, 4cm, 8cm), 3 obstacle widths (1cm, 4cm, 8cm), and perform 2 unobstructed trials, for a total of 8 trials. The tests were performed in comfortable, well-fitting shoes and without use of any walking aid or orthosis. Subjects wore corrective lenses if these were normally worn during ambulation. The order of presentation of obstacle size and orientation (height or width) was counterbalanced across subjects. Subjects performed the test twice. However, because more subjects were available for test 2, only the data from the second trial has been reported. The data therefore represent performance after a familiarization session. All subjects were advised not to attempt any obstacle that they felt posed a risk to their safety. The instructions were:
OBSTACLE-CROSSING STEP CHARACTERISTICS, Said
“The purpose of the study is to see how safely you can step over an obstacle, without using a rail. Walk from the start of the walkway to the chair at the end at a comfortable speed, and step over the obstacle in the middle without touching it. If you think that any obstacle is too difficult for you, I want you to stop. The rail is there, but only use it if you need it to keep your balance. Do you have any questions?” For the unobstructed trials, subjects were instructed to walk to the chair at the end of the walkway at a comfortable speed. All subjects were allowed to inspect visually and manually the obstacle before each walk on each day. The therapist then demonstrated the first obstructed trial. Subjects were free to start walking and step over the obstacle with either limb. The tester, a physiotherapist, walked beside the subject to provide assistance if needed. To minimize fatigue for the subjects with stroke, a wheelchair was provided to return them to the start of the walkway. Subjects rested for at least 1 minute between trials, while the obstacle was changed, and were allowed to rest for as long as needed. The trial was scored as either a pass or a fail. A pass was recorded if the subject ambulated the entire walkway and cleared the obstacle with both limbs without requiring assistance or the use of the rail. A fail was recorded if the subject required assistance from the tester or the rail, contacted the obstacle, or failed to step over the obstacle. The testing procedure took from 20 to 40 minutes and the total distance covered was 80 meters, with rests every 10 meters. The VDAM provided data on the following variables: toe clearance, preobstacle distance, postobstacle distance, and step time, as illustrated in figure 1. Step time (from trail limb heel contact until lead limb heel contact), preobstacle step time (trail limb heel contact to lead limb toe clearance) and postobstacle step time (lead limb toe clearance to lead limb heel contact) were also calculated. Preobstacle step length or time was then expressed as a proportion of the total step length or time. Statistical Analysis Data for some variables differed significantly from a normal distribution, as determined by the Shapiro-Wilks statistic. Nonparametric statistical methods were therefore used throughout the analysis to maintain a consistent approach. Because people use different strategies when crossing obstacles in different orientations, data for high and wide obstacles were analyzed separately. Because subjects were allowed to lead with either limb, we could not analyze data for the affected and unaffected limb separately. Multiple Mann-Whitney U tests were performed using SPSS softwared to determine the differences between subjects with stroke and healthy subjects for the variables of toe clearance, preobstacle distance, postobstacle distance, step length, proportion of crossing distance preobstacle, preobstacle time, postobstacle time, step time, and proportion of step time preobstacle. To correct the type I error rate for the 3 comparisons (1cm, 4cm, 8cm) at each orientation, a Bonferroni adjustment was performed, and a significance level of p less than .017 was used. It could be argued that the number of comparisons should demand a more rigorous approach and a more stringent correction for the familywise error. However, because the present study was largely exploratory, and the consequences of a type I error are not hazardous, we considered that this approach was appropriate.14 RESULTS No fails were recorded for any of the healthy subjects. Of the 19 subjects with stroke included in the analysis of the spatial
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data, 5 fails occurred; 1 fail was because of contact with the obstacle, 3 fails were because of the need for assistance, and 1 fail was because of use of the rail. Subjects led with the affected limb in 48% of the trials. Of the 16 subjects with stroke included in the analysis of the temporal data, only 2 fails occurred, both because of the need for assistance. Subjects led with the affected limb in 54% of the trials. Spatial Measures of Obstacle Crossing Comparing the stroke group and their healthy counterparts on the spatial measures (fig 2), we found a consistent trend for toe clearance to be greater among the subjects with stroke. Statistical analysis determined that toe clearance was significantly greater in the stroke group on the 1-cm high (U ⫽ 66, p ⬍ .017), 8-cm high (U ⫽ 79, p ⬍ .017), and the 4-cm wide obstacles (U ⫽ 97, p ⬍ .017). There was also a consistent trend for the postobstacle distance to be reduced in the stroke group. The difference was significant for 5 of the 6 conditions, including the 1-cm high (U ⫽ 92, p ⬍ .017), 4-cm high (U ⫽ 69, p ⬍ .017), 8-cm high (U ⫽ 86, p ⬍ .017), 4-cm wide (U ⫽ 96, p ⬍ .017), and the 8-cm wide (U ⫽ 83, p ⬍ .017) obstacles. In contrast, the preobstacle distance did not differ significantly between the 2 groups for any condition (U ⬎ 130, p ⬎ .017). Although step length tended to be smaller in the stroke group, the difference was not significant over any condition (p ⬎ .017). Subjects with stroke had a larger proportion of step length preobstacle, therefore crossing the obstacle further into the step. This difference, however, was only significant over the 4-cm high (U ⫽ 89, p ⬍ .017) and 8-cm high obstacles (U ⫽ 94, p ⬍ .017). In summary, during obstacle crossing subjects with stroke had increased toe clearance, reduced postobstacle distance, and they crossed the obstacle further into the step when compared with healthy subjects. Temporal Measures of Obstacle Crossing The temporal data for subjects with stroke and healthy subjects (fig 3) showed that subjects with stroke had significantly increased step time during crossing of all obstacles (1-cm high, U ⫽ 17, p ⬍ .017; 4-cm high, U ⫽ 22.5, p ⬍ .017; 8-cm high, U ⫽ 11, p ⬍ .017; 1-cm wide, U ⫽ 21, p ⬍ .017; 4-cm wide, U ⫽ 49.5, p ⬍ .017; 8-cm wide, U ⫽ 24, p ⬍ .017). Similarly, their postobstacle step times were increased over all conditions (U ⬍ 51.5, p ⬍ .017). Preobstacle step times were also significantly larger in the subjects with stroke over all obstacles (U ⬍ 46.5, p ⬍ .017) except the 4-cm wide obstacle. Although the proportion of preobstacle step time varied more in the stroke group, no statistically significant difference existed between the 2 groups over any condition (U ⬎ 77, p ⬎ .017). In summary, while subjects with stroke had longer step times than healthy subjects, both groups spent the same proportion of step time in the preobstacle phase of crossing. DISCUSSION In the present study, subjects with stroke who were able to ambulate without assistance or a gait aid used a different lead limb trajectory during obstacle crossing than healthy subjects. The differences and similarities between the obstacle crossing characteristics of this group of subjects with stroke and healthy subjects are discussed below, emphasizing the potential impact of adaptations on safety. Possible explanations for the modifications to gait during obstacle crossing following stroke are then considered. The higher toe clearances can be seen as a safety adaptation, because they minimize the risk of a trip due to inadvertent toe
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OBSTACLE-CROSSING STEP CHARACTERISTICS, Said
Fig 2. Notched boxplots representing toe clearance, postobstacle distance, and preobstacle distance over high and wide obstacles for subjects with stroke and healthy subjects. The notch in the boxplot represents the median; upper and lower edges of the box represent the 25th and 75th percentiles, respectively; whiskers represent the 90th percentile; dots represent the outliers. * Significant difference between stroke and healthy subjects.
Fig 3. Notched boxplots representing step time and the proportion of step time preobstacle over high and wide obstacles for subjects with stroke and healthy subjects. The notch in the boxplot represents the median; upper and lower edges of the box represent the 25th and 75th percentiles, respectively; whiskers represent the 90th percentile; dots represent the outliers. * Significant difference between stroke and healthy subjects.
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OBSTACLE-CROSSING STEP CHARACTERISTICS, Said
contact. In comparison with healthy subjects, subjects with stroke had significantly higher median toe clearances for 3 of the 6 obstacle conditions, and a trend toward higher clearances on the remaining 3 conditions. Toe clearances in the stroke group over the 1-cm high obstacle and all the wide obstacles were positively skewed, with some individuals using extremely large clearances. Great variability also existed between subjects with stroke over these obstacles as illustrated in figures 2 and 3. In some instances, the increased toe clearance used by the subjects with stroke may have had a negative effect on obstacle crossing safety by increasing stability demands. As the lead limb steps over the obstacle, stability demands are met by the trail (stance) limb. We did not investigate how the increase in lead limb toe clearance was achieved by subjects in our stroke group, but previous studies5,9,10 on healthy subjects showed that it is obtained by a combination of increased hip and knee flexion and hip elevation. These strategies all increase the stability requirements of the task,9 and may be reflected by the increased moments at the hip and ankle of the trail (stance) limb as obstacle height increases.15 Considering that total limb elevation includes toe clearance and the height of the obstacle, it follows that stability demands are substantially greater over higher obstacles. If stability requirements are not met, loss of balance may occur. It was previously reported4 that persons with stroke required assistance or the use of a rail in 6.9% of the trials when crossing 1- to 8-cm high obstacles, compared with 1.4% when crossing 1- to 8-cm wide obstacles. The significantly greater toe clearances used over high obstacles by the group of stroke subjects may have partially contributed to their increased reliance on external supports to maintain balance. The factors contributing to increased toe clearance poststroke require consideration. Persons with stroke may feel greater concern for safety during obstacle crossing, which may lead to greater caution during crossing and a resultant increase in toe clearance. It is also possible that the variety of deficits following stroke may contribute to their limb trajectories being more variable and inconsistent. This variability would be reflected in measures such as toe clearance. Increased variability in obstacle clearance would create an undesirable increase in the risk of toe contact, unless it was accompanied by an increase in median clearance. The increased median toe clearance exhibited by stroke subjects in the present study may be an attempt to compensate for increased individual variability in toe clearance. Subjects in the stroke group maintained the preobstacle distance while reducing the postobstacle distance. As a consequence, they had a larger proportion of the step length preobstacle, and crossed the obstacle further into the step. This strategy may be safer because it minimizes the risk of toe contact earlier in swing phase, which represents a serious threat to stability.7 It also allows greater time for visually mediated online modifications to the trajectory before obstacle crossing.10,12 Maintaining the preobstacle distance following stroke may optimize safety by other mechanisms. The trail limb plays an important role during single limb stance by maintaining stability while the lead limb steps over the obstacle. Chou and Draganich15 showed that adduction and internal rotation moments at the ankle increase on the stance limb as the preobstacle distance between the trail limb and the obstacle is reduced. This finding suggests that greater muscle activity around the ankle would be required to control the movement, particularly in the evertors. This aspect of control may prove to
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be difficult after stroke. Once the lead limb has safely landed, the trail limb is then required to cross the obstacle. Recent research16 in young healthy adults showed that placing the trail limb closer to the obstacle and decreasing the preobstacle distance is associated with reductions in hip, knee, and ankle flexion, with a resultant decrease in toe clearance.16 This placement increases the risk of trail limb foot contact. Trail limb trajectory cannot be guided by vision; therefore, people with a stroke may elect to position the trail limb for optimal obstacle clearance. Further investigation into the influence of trail limb preobstacle position on stability and trail limb trajectory during obstacle crossing is warranted. The reduced postobstacle distance found in the stroke group represents the most obvious threat to obstacle-crossing safety. It suggests that postobstacle foot positioning was compromised. The small postobstacle distances increase the chances of lead limb contact on landing, particularly while stepping across a wide obstacle. It has been previously reported4 that nearly half of poststroke fails were from lead limb contact, with 83% occurring while crossing a 1- to 8-cm wide obstacle. This finding suggests that inappropriate placement of the lead limb after crossing may contribute to failures in obstacle crossing in people with stroke, and therefore reduce safety. Several factors may have contributed to the smaller postobstacle distance found in the present study. The reduction may reflect a modified trajectory due to reduced knee extension after clearing the obstacle, particularly for the affected limb. Stroke subjects may have difficulty generating sufficient rectus femoris activity to extend the knee after obstacle crossing,9 or be unable to activate their biceps femoris to control the horizontal velocity for limb landing. It is possible that people with stroke may require even greater forces to extend the knee than healthy subjects, given that there may be more limb flexion at obstacle crossing to achieve toe clearance. Other musculoskeletal factors, such as reduced hamstring length, may also affect leg trajectory. Alternatively, people with stroke may bring the heel backward at landing as has been documented in patients with age related maculopathy.10 This strategy minimizes the risk of the foot slipping forward at heel contact. The smaller postobstacle distance may also be related to the stability requirements of the task. While stepping over the obstacle, the stance (trail) limb maintains stability until initial contact of the lead limb. After stroke, subjects may maintain the whole body COM closer to the stance foot, until the lead limb lands safely. This action would effectively limit the postobstacle distance obtained by the lead limb, but would also reduce the consequences of a trip or slip of the lead limb. Finally, step time during obstacle crossing was significantly increased in subjects with stroke than in healthy subjects. This may be a safety strategy because it would increase the time available for modification of the swing limb trajectory. However, increased step time can challenge stability by increasing the time spent in single limb support. Single-limb stance times were not measured; however, because postobstacle (swing) time also increased significantly, it is unlikely that the increased step time is entirely attributable to an increase in double limb support. The increase in step time may have contributed to the difficulties maintaining balance in some subjects with stroke. Although not directly related to safety, the efficiency of the strategies chosen should be considered. Other authors17 have shown that in healthy subjects an increase in foot clearance is associated with an increase in energy requirements. It is likely that the high toe clearances used by the subjects with stroke represent an inefficient strategy. Use of inefficient strategies
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OBSTACLE-CROSSING STEP CHARACTERISTICS, Said
may limit the amount of activity that persons with stroke are able to do. Resulting fatigue also may place subjects at greater risk of falls at a later point in time. Another factor that may have impacted on the trajectory of the lead limb in this stroke group is their overall slower gait speed. Although the relationship between the spatial and temporal characteristics of obstacle crossing and speed has not been explored, it is reasonable to expect that these variables are related. Intersegmental dynamics play an important role in obstacle crossing in healthy subjects.18 For example, the increased hip flexion obtained in healthy subjects while crossing large obstacles is the result of an increase in knee flexor activity, not an increase in hip flexor activity.13 Motion-dependent moments are influenced by segment angular acceleration and velocity and joint linear acceleration. Although not measured in the present study, it is likely that these variables were all lower in the stroke subjects, and may have limited the use of intersegmental dynamics. This may have influenced the trajectory of their lead limb and, as a result, its landing position. The slower gait speeds may reflect an inability to activate appropriate muscle groups sufficiently, or it may be a safety modification to minimize the adverse consequences of contact with the obstacle. It would be of interest to examine the obstacle-crossing characteristics over a variety of gait speeds in healthy subjects, in order to determine whether preobstacle distance, postobstacle distance, and step length vary with gait speed. These studies would give us a fuller understanding of the deficits in obstacle crossing exhibited by people with stroke. Biomechanical, kinematic, and kinetic factors obviously play an important role in the control of limb trajectory. Alone, however, they may not explain all the trajectory modifications people make after stroke. Obstacle crossing involves a complex interaction between a person’s visual perception of the obstacle, and the implementation of an appropriate avoidance strategy. Although the neural pathways underlying perception and action in obstacle crossing in humans are not fully understood, animal studies provide some insight into the structures that may be involved. Visual information about the object size and location is mapped onto the primary visual cortex in the occipital lobe. Information is then processed via the ventral pathway (occipitotemporal) and dorsal pathway (occipitoparietal). It has been hypothesized19,20 that the dorsal pathway is of importance in the control of visually mediated movements, whereas a lesion in the ventral pathway does not compromise the ability to cross obstacles or reach for objects. Studies21 examining obstacle crossing in cats showed that the motor cortex plays a critical role in the motor control of obstacle crossing, particularly in regulating limb elevation and foot placement. The cerebellum also appears to play a role in visually mediated gait modifications.22 Lesions in these areas may result in an inability to modify the trajectory of the limb to clear the obstacle. Lesions that compromise balance, such as cerebellar lesions, may also impact on ability to complete the task. Because of the small sample size in the current study, we did not correlate step characteristics with site of lesion. It is interesting to note that, despite the heterogenous nature of subjects and lesions included in the present study, sufficient similarities existed in the strategies used by the subjects with stroke to differentiate them from healthy subjects. Study Limitations and Future Development The present results show that lead limb trajectory during obstacle crossing is significantly altered after stroke, when compared with healthy age-matched subjects. The study has
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several limitations. First, to be included, the subjects with stroke had to be capable of ambulating without physical assistance, a gait aid, or orthosis. The results can therefore only be generalized to subjects with similar characteristics. Second, because subjects were allowed to step over the obstacle with the preferred limb, we did not compare separately the affected and unaffected limbs of the subjects in the stroke group with the healthy subjects. Because stroke is typically a unilateral disorder, the characteristics of each limb need to be quantified to understand the nature of the poststroke deficit. An understanding of the kinematics of obstacle crossing will also provide an insight into some of the potential causes of the altered spatial and temporal characteristics. When the deficit in obstacle crossing after stroke is finally understood, training strategies designed to remediate the deficit can be explored. CONCLUSION In a select group of subjects with stroke, capable of ambulating a short distance without gait aids or assistance, obstaclecrossing deficits were not attributable to subjects’ inability to elevate the lead limb sufficiently. The reduced postobstacle distance suggests that the deficit may be related to difficulty placing the limb at landing. In addition, aspects of the strategies selected by the subjects with stroke may have further stressed an already compromised balance system. Acknowledgments: The authors thank the staff at the Austin and Repatriation Medical Centre for assisting in the study, and Kerri Martin for assisting with vision assessments. We also acknowledge the technical assistance from La Trobe University Technical Services, Faculty of Health Sciences, La Trobe University. References 1. Goldie PA, Matyas TA, Evans OM. Deficit and change in gait velocity during rehabilitation after stroke. Arch Phys Med Rehabil 1996;77:1074-82. 2. Olney SJ, Richards C. Hemiparetic gait following stroke. Part I: characteristics. Gait Posture 1996;4:136-48. 3. Forster A, Young J. Incidence and consequences of falls due to stroke: a systematic inquiry. Br Med J 1995;311:83-6. 4. Said CM, Goldie PA, Patla AE, Sparrow WA, Martin KE. Obstacle crossing in subjects with stroke. Arch Phys Med Rehabil 1999;80:1054-9. 5. Patla AE, Rietdyk S, Martin C, Prentice S. Locomotor patterns of the leading and the trailing limbs as solid and fragile obstacles are stepped over: some insights into the role of vision during locomotion. J Mot Behav 1996;28:35-47. 6. Alexander NB, Mollo JM, Giordani B, Ashton-Miller JA, Schultz AB, Grunawalt JA, et al. Maintenance of balance, gait patterns and obstacle clearance in Alzheimer’s disease. Neurology 1995; 45:908-14. 7. Chen H-C, Ashton-Miller JA, Alexander NB, Schultz AB. Stepping over obstacles: gait patterns of healthy young and old adults. J Gerontol A Biol Sci Med Sci 1991;46:M196-203. 8. Chen H-C, Schultz AB, Ashton-Miller JA, Giordani B, Alexander NB, Guire KE. Stepping over obstacles: dividing attention impairs performance of old more than young adults. J Gerontol A Biol Sci Med Sci 1996;51:M116-22. 9. Patla AE, Rietdyk S. Visual control of limb trajectory over obstacles during locomotion: effect of obstacle height and width. Gait Posture 1993;1:45-60. 10. Patla AE, Elliott DB, Flanagan J, Rietdyk S, Spaulding S. Effects of age-related maculopathy on strategies for going over obstacles of different heights and contrast [asbstract]. Gait Posture 1995;3: 105. 11. Sparrow WA, Shinkfield AJ, Chow S, Begg RK. Characteristics of gait in stepping over obstacles. Hum Mov Sci 1996;15:605-22. 12. Patla AE, Prentice SD, Gobbi LT. Visual control of obstacle avoidance during locomotion: strategies in young children, young
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13. 14. 15.
16. 17. 18. 19.
and older adults. In: Fernandez AM, Teasdale N, editors. Changes in sensorimotor behavior in aging. Amsterdam: Elsevier Science; 1996. p 257-77. McFadyen BJ, Winter DA. Anticipatory locomotor adjustments during obstructed human walking. Neurosci Res Commun 1991; 9:37-44. Keppel G. Design and analysis: a researcher’s handbook. 3rd ed. Englewood Cliffs (NJ): Prentice-Hall; 1991. Chou L-S, Draganich LF. Increasing obstacle height and decreasing toe obstacle distance affect the joint moments of the stance limb differently when stepping over an obstacle. Gait Posture 1998;8:186-204. Chou L-S, Draganich LF. Placing the trailing foot closer to an obstacle reduces flexion of the hip, knee, and ankle to increase the risk of tripping. J Biomech 1998;31:685-91. Chou L-S, Draganich LF, Song S-M. Minimum energy trajectories of the swing ankle when stepping over obstacles of different heights. J Biomech 1997;30:115-20. Patla AE, Prentice SD. The role of active forces and intersegmental dynamics in the control of limb trajectory over obstacles during locomotion in humans. Brain Res 1995;106:499-504. Goodale MA, Milner AD, Jakobson LS, Carey DP. A neurological
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dissociation between perceiving objects and grasping them. Nature 1991;349:154-6. 20. Patla AE. The neural control of locomotion. In: Spivack BS, editor. Evaluation and management of gait disorders. New York: Marcel Dekker; 1995. p 53-78. 21. Drew T, Jiang W, Kably B, Lavoie S. Role of the motor cortex in the control of visually triggered gait modifications. Can J Physiol Pharmacol 1996;74:426-42. 22. Armstrong DM, Marple-Horvat DE. Role of the cerebellum and motor cortex in the regulation of visually controlled locomotion. Can J Physiol Pharmacol 1996;74:443-55. Suppliers a. Philips Electronics Australia Ltd, 15 Blue St, North Sydney, New South Wales 2060, Australia. b. Brian Sleenman. La Trobe University Technical Services, Faculty of Health Sciences. Latrobe Univ, Bundoora, Victoria, 3086, Australia. c. Peak Performance Technologies Inc, 7388 S Revere Pkwy, Ste 603, Englewood, CO 80112. d. SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.
Arch Phys Med Rehabil Vol 82, December 2001