Six-Minute Walk Test in Persons With Transtibial Amputation. 4 arm-leg ergometry, 5 and treadmill exercise. 5 During rehabilitation,

2354 ORIGINAL ARTICLE Six-Minute Walk Test in Persons With Transtibial Amputation Suh-Jen Lin, PT, PhD, Nisha Hathi Bose, PT, MS ABSTRACT. Lin S-J, Bose NH. Six-minute walk test in persons with transtibial amputation. Arch Phys Med Rehabil 2008;89:2354-9. Objective: This study was to report the within-day testretest reliability and the measurement properties of the sixminute walk test (6MWT) in persons with lower-limb (transtibial) amputation. Design: Test-retest study design. Setting: University research laboratory. Participants: Subjects (N 13) with transtibial amputation (9 men and 4 women; mean age, 46y). Interventions: Three trials of the 6MWT were conducted within 1 day with 20 to 30 minutes of rest between consecutive trials. Timed Up & Go (TUG) test and timed one-leg balance tests were conducted on another day. Main Outcome Measures: (1) Distance, heart rate, symptoms and signs of exercise intolerance during the walk test, (2) times of the TUG test and the one-leg balance test. Results: The intraclass correlation coefficient (ICC 3,1 ) value was.94. Bland and Altman graphs showed no systemic variations between trials and a small learning effect. The peak heart rate approximated 72% to 78% of the age-predicted maximal heart rate. Moderate degrees of correlation were observed in: (1) the 6MWT versus the TUG test (r.76, P.05), and (2) the 6MWT versus the timed prosthetic-leg stance (with eyes open: r.63, P.05; with eyes closed: r.61, P.05). Conclusions: These findings suggest that the 6MWT could be considered as a reliable measure of functional capacity, involves a moderate degree of exercise intensity, and is related in a moderate degree to postural control abilities in persons with transtibial amputation. Key Words: Amputees; Rehabilitation. 2008 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation THE REPORTED PREVALENCE of falling in communitydwelling amputees is about 52.4%, 1 which is higher than that reported among the elderly; additionally, 62% of these amputees are reported to have more than 2 common health problems. 1 With a lower-limb amputation, a person tends to live a sedentary lifestyle, possibly due to a fear of falling and the increased energy expenditure associated with prosthetic ambulation, particularly in the presence of multiple chronic diseases. To design a fitness training program for amputees, a reliable and feasible exercise test is important for quantifying exercise capacity, identifying symptoms and signs of exercise intolerance, and prescribing safe exercise programs in a community setting. Determination of exercise capacity in persons with lower-limb amputation has been a challenge. Different modes of exercise testing have been reported, such as one-leg cycling, 2,3 arm ergometry, 4 arm-leg ergometry, 5 and treadmill exercise. 5 During rehabilitation, arm ergometry might be a good choice due to weakness of the amputated leg, yet increased cardiac demand is a concern. The combined arm and leg ergometer exercise could challenge an amputee to a higher capacity than that of the arm ergometer, 5 but the equipment is expensive. When able to walk independently in the community with a prosthesis, an amputee has a better aerobic capacity than those still in rehabilitation, but may not have the skillful balance to walk comfortably on a treadmill. Therefore, a field exercise test (eg, walk tests) might better reflect an amputee s exercise capacity in the community than the aforementioned traditional exercise tests. Walk tests measure how far a person can walk within a certain limit of time. They are related to day-to-day activities, are easy to administer, and yield valuable information. Originally, a 12-minute running test was reported as a reliable field test of exercise capacity in healthy adults and it correlated modestly with a person s maximum oxygen consumption. 6 Later, this test was modified to a walk test in patients with chronic obstructive pulmonary disease. 7 The 12-minute test was timeconsuming for clinicians, so 2- and 6- minute versions of walk tests were evolved, and the 6MWT was shown to adequately differentiate exercise capacities among subjects while saving time. 8 Many factors affect the performance in walk tests, such as verbal encouragement, 9 walkway layout and length, 10 and the number of practice trials. 11 With advances in technology, a variety of physiologic monitoring can be incorporated into walk tests, such as pulse oximetry, metabolic analysis, and telemetry electrocardiography. Not only could functional capacity be quantified, but also the limiting factors of exercise intolerance, such as arrhythmia, oxygen desaturation, and anaerobic threshold could be identified. 12-17 Initially, the 6MWT was primarily applied in patients with cardiac or respiratory diagnoses. 11,18,19 Later, it was applied to the elderly, 17,20 to children, 21 and to patients with renal failure, 22 fibromyalgia, 23 and cerebral palsy. 24 In the above stud- From the School of Physical Therapy, Texas Woman s University, (Lin) and the Department of Physical Medicine and Rehabilitation, Baylor University Medical Center, (Hathi), Dallas, TX. Presented to the Texas Physical Therapy Association, October 27 29, 2006, Ft Worth, TX, and to the American Physical Therapy Association, February 14 17, 2007, Boston, MA. Supported by the Texas Physical Therapy Foundation. No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Reprint requests to Suh-Jen Lin, PT, PhD, 8194 Walnut Hill Ln, Dallas, TX 75231, e-mail: slin@twu.edu. Published online November 1, 2008 at www.archives-pmr.org. 0003-9993/08/8912-00039$34.00/0 doi:10.1016/j.apmr.2008.05.021 ANOVA BMI CI ICC LOA LSD RPE 6MWT TUG List of Abbreviations analysis of variance body mass index confidence interval intraclass correlation coefficient limit of agreement least significant difference rating of perceived exertion six-minute walk test Timed Up & Go

SIX-MINUTE WALK TEST IN PERSONS WITH TRANSTIBIAL AMPUTATION, Lin 2355 ies, the test-retest reliability values were high, with ICC values ranging from.82 to.99, except for patients with fibromyalgia (ICC.73). Taking learning effect into consideration, 1 practice trial of the 6MWT is generally recommended. 11 For the criterion-related validity of the 6MWT, the distance ambulated has been consistently shown to correlate moderately to a person s peak oxygen consumption. 11,18,19,21 Besides aerobic capacity, other factors such as one-leg balance and postural control might affect performance in walk tests. Timed one-leg balance was shown to have moderate to high reliability in healthy adults and could predict falls in the elderly. 25 Gait asymmetry and reduced prosthetic leg stance time are characteristics of amputee gait. 26 A good one-leg balance should enable an amputee to walk a longer distance because walking is a series of alternating cycles between the stance phase and the swing phase. The TUG test is another measure of physical mobility. The test involves asking a subject to stand up from an armchair, walk 3m, turn, walk back to the chair, and sit down. The TUG test is similar to many daily activities involving transfer, balance, walking speed, and turning around. The TUG test was shown to have high intrarater and interrater reliability values, 27 and good content validity and concurrent validity with other balance measures and walking speed. 28 The postural control of turning around at the end of 3-m walking seems to be similar to the turning around movement at the end of each lap of walking during walk tests. Hence, it is likely that an amputee with a better walking speed and efficiency in turning around during the TUG test could also walk a longer distance in the 6MWT. The distance of the 6MWT reflects whether a person could practically ambulate in the community. The requirements of typical community ambulation such as walking in a supermarket or a department store involve a distance of about 132m to 342m for communities of different sizes. 29 Previously, the 2-minute walk test in amputees showed good reliability, was capable of predicting rehabilitation potential, 30 and correlated well to other physical functions. 31 However, the distance covered in 2 minutes is too short compared with typical community ambulation. In this study, we explored the within-day test-retest reliability and other measurement properties of the 6MWT in persons with lower-limb amputations. We hypothesized that the first trial would be as reliable as the second and the third trials (ie, there will be good agreement among the 3 trials of the 6MWT). Secondly, we quantified the exercise intensity and described possible symptoms and signs of exercise intolerance associated with the 6MWT in amputees, such as shortness of breath, chest pain, fatigue, leg claudication, oxygen desaturation, or arrhythmia. Thirdly, we examined its clinical measurement properties by correlating the distance of walking in the 6MWT to 2 postural control measures: the TUG test and the timed one-leg balance test at eyes-open and eyes-closed conditions. We hypothesized that subjects who had a better postural control, indicated by a longer one-leg balance time and a quicker (shorter time in the) TUG test, would walk a longer distance in the 6MWT. METHODS Subjects Thirteen subjects (9 men and 4 women; 1 Hispanic, 1 black, and 11 whites) with unilateral below-knee (transtibial) amputation participated in this study. Subjects were recruited through local advertisement as well as a support group for people with amputations. The causes of amputation included trauma (in 9 subjects) and vascular problems (in 4 subjects). The inclusion criteria included the following: with transtibial amputation, walking independently with a prosthesis in the community, absence of skin breakdown of the residual limb in the past 3 months, and having well-controlled medical conditions. All prospective subjects who fulfilled the inclusion criteria were recruited as subjects. In addition, participants in previously mentioned studies of the 6MWT walked independently; therefore, we chose to use similar criteria and to exclude those amputees who used assistive devices, had recent illness, or had hospital admissions. Informed consent was obtained. The experimental protocol was approved by the Institutional Review Board of the Texas Woman s University. Study Protocol Two test sessions were conducted. The first session consisted of informed consent, basic data collection, and 3 trials of the 6MWT. The second session scheduled within 2 weeks of the first session was for the TUG test and the timed one-leg balance test. In order to minimize the drop-out rate and to reduce the number of testing sessions, we conducted the 3 trials of 6MWT within 1 day, but provided participants with 20 to 30 minutes of rest between trials to minimize the influence of fatigue. Procedure Basic data included medical history, age, height, weight, and the amputee Day activity questionnaire. 32 The Day activity questionnaire is a brief scoring form which evaluates an amputee s ability in the following categories: putting on and taking off the prosthesis, the length of time using the prosthesis, stairs climbing, details of employment, ambulation aids used, domestic responsibilities, regular walking habits, and social activities. Based on the assessment of clinical records of the amputee participants by experienced clinicians, the activity levels of participants were classified into 5 categories. It was found that the 5 activity levels corresponded to specific ranges of the Day activity scores (very high, 30; high, 10 29; average, 9 to 9; restricted, 40 to 10; inactive, 40). In addition, the activity score was shown to correlate linearly to annual step counts. 32 Six-minute walk test. The 6MWT was conducted according to published standards. 11 Heart rhythms were monitored continuously with a modified chest lead V 5 by the ECG telemetry system. a Blood pressure, oxygen saturation through a pulse oximeter, b and RPE of the 10-point Borg scale 33 were recorded prior to and immediately at the completion of each trial. Subjects had at least 20 minutes of rest between consecutive trials to allow the heart rate to return to baseline. Prior to each trial, subjects were instructed to walk in an extended stretch (45.72m or 150ft) of hallway with cones marking the distance according to the following standardized written instructions: The object of this test is to walk as far as possible for 6 minutes. You are to walk back and forth in this corridor. Six minutes is a long time to walk, so you will be exerting yourself. You will probably get out of breath or become exhausted. You can speed up, slow down, or take a rest. You may lean against the wall while resting, but resume walking as soon as you are able... 11 To partially account for a possible memory effect, each subject began the 3 trials at 3 different starting points along the path. Standardized words of encouragement were called out to each subject at each minute mark ( Keep up the good work, you have x minutes left! ). One investigator provided the encouragement, monitored the signs and symptoms of exercise intolerance when a subject passed by, and recorded the number of laps walked.

2356 SIX-MINUTE WALK TEST IN PERSONS WITH TRANSTIBIAL AMPUTATION, Lin Difference of Trial 2 Trial 1 70 60 50 40 30 20 10 0 10 20 30 40 50 60 Bland-Altman Plot LOA 63.5 95%CI 9.45 mean 95%CI LOA 44.6 70 400 450 500 550 600 650 700 750 Average of Trial 1 and Trial 2 (meter) Fig 1. Bland-Altman graph with LOA. The differences between trial 2 and 1 (trial 2 minus trial 1) plotted against their mean for each subject, together with the 95% CI and the 95% LOA. Timed Up & Go test. Each subject was instructed to rise from a standard armchair, walk a 3-m short distance, turn around, walk back, and return to sit in the chair according to the original guideline. 28 The time to complete the TUG test was recorded, and the average of 3 trials was used for data analysis. Timed one-leg balance test. We demonstrated the procedures to subjects first. Initially, each subject stood on both legs with eyes open. Then, one researcher counted down backward from 3 seconds to 1 second, and called out go, and the subject was expected to maintain one-leg balance on one leg while raising the other leg. The test was terminated when either the nonsupporting leg touched the ground or the subject maintained the balance for 30 seconds 25 (the ceiling of the test). Subjects started with the sound leg, then the amputated leg. After 3 trials on each leg, they repeated the procedures with eyes closed. One research assistant stood in front of the subject timing the test, and another assistant stood behind the subject for protection from falls. The variability of the 3 trials for each condition was analyzed and the average of the 3 trials for each condition was used for final data analysis. Data Analysis Data were analyzed using SPSS version 12.0. c Descriptive statistics were used for basic data analysis. The relative reliability among the 3 trials was analyzed with the ICC (model 3) because this model is appropriate for test-retest reliability. We also used the Bland-Altman graphs with limits of agreement to express the actual units of measurement, as well as the paired t test with a 95% CI to assess whether there was a systemic change in the mean (absolute reliability). A separate 1-way repeated measures ANOVA was conducted to test for differences on peak heart rate and mean arterial blood pressure. If significant effect existed, the post hoc Fisher s LSD pairwise comparison was conducted. The Friedman 1-way repeated measures ANOVA by ranks was used for the RPE scale for the 3 trials. Fisher s LSD pairwise comparisons were calculated for significant effects. Reliability of the 3 trials of the timed one-leg balance test was examined with ICC 3,1. The Pearson correlation analysis was used to analyze the relationships between the distance ambulated and: the mean time of the TUG test, and the mean times of the timed one-leg balance test on the prosthetic leg and on the sound leg under eyes-open and eyes-closed conditions. The level of significance ( ) was set at.05. RESULTS The subject characteristics were as follows: age, 46.0 14.8 years; height, 174.3 10.8cm; weight, 85.9 18.6kg; BMI, 28.0 3.6kg/m 2. The average number of years of prosthesis experience was 7.61 9.25, ranging from 6 months to 32 years for the 4 vascular amputees and from 3 months to 18 years for the 9 traumatic amputees. Four traumatic amputees did not have any medical comorbidities: 3 had used prosthesis for 1 month to 1.5 years, and 1 who had amputation in childhood had used prosthesis for over 15 years. Nine of the 13 amputees had multiple cardiovascular comorbidities, where 8 had hypertension, 7 were overweight (BMI, 25 29.9 kg/m 2 ), 6 had metabolic syndrome, 5 reported high cholesterol, 4 were class I obese (BMI, 30 34.5kg/m 2 ), 3 had arrhythmia, 3 had diabetes, 2 had peripheral vascular disease, and 1 had undergone coronary artery bypass. Nine of 13 subjects had more than 2 common health problems. The average Day activity score was 31 11. The subjects were either part-time employed or fulltime homemakers, except for 1 subject who was temporarily unemployed. The ICC for the 3 trials of the 6MWT was.94. Figure 1 shows the Bland-Altman graph on the difference of distance ambulated between trials 2 and 1 (trial 2 minus trial 1) plotted against their means for each subject, with 95% CI and 95% LOA. There are no systemic errors and no statistically significant difference between trial 2 and trial 1 (zero was included in the 95% CI). In figure 2, the Bland-Altman graph also shows no systemic variation but with a significant difference between trial 3 and trial 2 (zero was not included in the 95% CI). All 13 subjects completed the 3 trials without the need of rest during the 6MWT. One subject had occasional abnormal heart rhythm of premature ventricular contraction at rest, but no further abnormal rhythms were observed during the test and no other abnormal symptoms or signs of exercise intolerance were elicited. No subjects experienced intermittent claudication, chest pain, or oxygen desaturation. Three subjects with hypertension had a higher blood pressure response (up to 180 mmhg), but it did not reach the level required for termination of the walk test. The detailed results of peak heart rate, mean arterial blood pressure, RPE, and distance of the 6MWT are shown in table 1. Difference of Trial 3 Trial 2 80 70 60 50 40 30 20 10 0 10 20 30 40 Bland-Altman Plot LOA 57.8 95% CI mean 16 95% CI LOA 25.8 50 400 450 500 550 600 650 700 750 Average of Trial 2 and 3 (meter) Fig 2. Bland-Altman graph with LOA. The differences between trial 3 and 2 (trial 3 minus trial 2) plotted against their mean for each subject, together with the 95% CI and the 95% LOA.

SIX-MINUTE WALK TEST IN PERSONS WITH TRANSTIBIAL AMPUTATION, Lin 2357 Table 1: Results of the 3 Trials of the 6MWT Categories Trial 1 Trial 2 Trial 3 Mean SD Mean SD Mean SD Distance (m) 544.6 64.5 554.0 71.4 570.0 80.1 Average speed (m/s) 1.51 0.18 1.54 0.20 1.58 0.22 Peak heart rate (% age-predicted maximal*) 72.2 7.0 76.4 8.0 77.8 10.0 Mean arterial pressure (mmhg) 110.0 10.0 109.0 8.0 109.0 8.0 RPE 3.5 1.6 3.5 1.5 4.0 2.2 *(Peak heart rate/[220 age]) 100%. (Diastolic pressure 1/3 [systolic pressure diastolic pressure]). Paired t test showed significant differences between trial 3 and trial 1, and between trial 3 and trial 2 (P.05). 1-way repeated measures ANOVA (P.05). Pairwise comparisons showed significant differences between trial 1 and trial 2, and between trial 1 and trial 3 (P.05). Results showed a significant main effect for peak heart rate (F 2,24 5.96, P.02). Results of mean arterial blood pressure did not show a significant main effect (F 2,24.23, P.74), indicating that mean arterial pressure did not significantly change among the 3 trials. Results of RPE also did not show a significant main effect (F 2,24 2.04, P.16), indicating that RPE did not significantly change among the 3 trials. Twelve subjects performed the TUG test and the timed one-leg balance test. One man dropped out for the second test session. The relationship between the walk test and the TUG test was shown in figure 3 (P.004). The reliability of the 3 trials of the timed one-leg balance test was high, except for the 2 conditions of the prosthetic leg (table 2). There were moderate correlations between the distance ambulated and the times of the mean timed one-leg balance on the prosthetic leg at eyes-open and eyes-closed conditions (r.63 and r.61, P.05, respectively). The correlation coefficients between the distance ambulated and the times of the timed one-leg balance test on the sound leg were low (eyes open, r.00; eyes closed, r.42). DISCUSSION The prevalence of cardiovascular comorbidity in our subjects was alarming. This prevalence was close to that reported for community-dwelling amputees (62%). 1 Five of the 9 traumatic amputee subjects seemed to have developed multiple cardiovascular comorbidities over the years, which was possibly due to physical inactivity. One limitation of our study is that we had a mixed group of amputees. It could not be determined whether the comorbidities in vascular amputees were due simply to a sedentary lifestyle or to a combination of vascular problems and sedentary lifestyle. Although our subjects average activity level was classified as high based on the amputee Day activity questionnaire, 32 it only reflected that they could carry out daily activities most of the time and did not indicate that they actively participated in regular exercises. The high prevalence of comorbidity reflected the urgent need of secondary prevention in persons with lower-limb amputation. With the standardized protocol, the learning effect of the 6MWT was likely minimized. The high ICC value for the distance of walking among the 3 trials indicated excellent test-retest reliability, which was close to those reported in the literature. 11,18,19 Although we found a statistically significant learning effect from trial 2 to trial 3, the difference of 16m was small when compared with the suggested clinically significant distance of improvement for the 6MWT after interventions (ie, 54 80m). 11,18,34 Based on the high ICC value and the absence of systemic errors from trial to trial, we consider that the 6MWT has a good within-day test-retest reliability of functional capacity in persons with lower-limb amputation, but 2 practice trials are recommended. However, whether the 6MWT is sensitive to change after exercise training in this population would await further studies. Although there were no systemic variations in the distance of walking, we observed 2 outliers on the Bland-Altman graphs. One subject walked a much shorter distance on the second trial. Because similar rest periods between trials were provided to all subjects, and this subject s third trial was actually close to his first trial, it was unclear whether it was due to fatigue. Further studies involving between-day tests could help elucidate the possible role of fatigue. Another subject walked a much longer distance on the third trial. We speculate that this subject might have perceived the third trial as his last trial, and thus wanted to do his best. Future studies incorporating feedback from participants on how they accomplished those distances in the 6MWT, on whether they used their usual speed or tried their best for each trial, might provide us with more insights on this issue. The peak heart rate responses seemed to have a different trend. Our amputee subjects might have worked harder for trial 2 and trial 3. However, neither the mean arterial pressure nor the RPE scale was significantly different. Because we have only examined the within-day reliability of the 6MWT, further studies involving the 6MWT on different days might rule out the possible effects of any cumulative physiologic effects. Incorporation of metabolic analysis during the 6MWT might yield more objective information on exercise capacity than what was available with the measurement of heart rate alone, because factors other than exercise could affect the heart rate response. Our subjects appeared to have achieved their best performance. As shown in table 1, their average walking speeds ranged from 1.51 to 1.58m/s (3.4 3.6mph), which were faster than the common comfortable walking speeds of healthy adults aged 20 to 60 years (1.29m/s to 1.46m/s). 35 In addition, their relative exercise intensities were about 72% to 78% of their age-predicted maximal heart rates, which were slightly lower than those reported in the literature (about 78% 85% of the Timed Up and Go Test (sec) 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 r = 0.76 0.0 300 350 400 450 500 550 600 650 700 Distance of Trial 1 (meter) Fig 3. Relationship between the distance of walking and time of the TUG test.

2358 SIX-MINUTE WALK TEST IN PERSONS WITH TRANSTIBIAL AMPUTATION, Lin Table 2: Three Trials of the Timed One-Leg Balance at Eyes-Open and Eyes-Closed Conditions and the Corresponding ICC Groups Trial Sound Leg (s) ICC 3,1 Prosthetic Leg (s) ICC 3,1 Eyes open (n 12) Eyes closed (n 12) 1 24.52 10.66 0.93 1.43 0.77 0.40 2 23.60 11.61 2.31 1.25 3 26.19 9.01 3.00 2.29 Mean 24.77 9.82 2.26 1.06 1 8.47 8.63 0.70 0.99 0.52 0.48 2 10.07 10.13 1.18 0.40 3 9.42 10.09 1.13 0.62 Mean 9.33 0.42 1.10 0.37 NOTE. Values are mean SD unless otherwise noted. peak heart rate). 12,13,15-17 A few of our subjects took beta blockers, which could blunt heart rate response. Some subjects had multiple cardiovascular comorbidities. These factors limit the accuracy of predicting maximal heart rate from the agepredicted equation. Hence, we could have overestimated their maximal heart rate. On the relationships with clinical measurement properties, the 6MWT seems to examine similar aspects of postural control as the TUG test. With a small sample size, we observed a moderately negative linear relationship between the distance walked and the time of the TUG test the better the performance in the TUG test (ie, quicker, shorter time), the greater the distance an amputee can walk in the 6MWT. Our TUG test results were close to those of elderly people in their sixties. 36 Previously, the 2-minute walk test in amputees also showed a negative correlation between the distance ambulated and the time of the L-Test (a modified version of TUG test), 31 but it was in a curvilinear fashion. That was probably due to the ceiling effect of the 2-minute walk test, which was not observed in this study. Furthermore, the loss of voluntary ankle control after transtibial amputation has likely contributed to the impaired prosthetic-leg balance and the shorter distance of walking, as shown by the moderately positive correlations between the prosthetic-leg balance and the distance ambulated. Additionally, the redundancy of postural control from visual and proprioceptive inputs also seemed to have been reduced for the sound leg. These findings point to the need to incorporate balance training into traditional fitness programs for persons with lower-limb amputation. An improved balance could potentially reduce the fear of falling and increase participation in physical activities. This hypothesis would require further studies. CONCLUSIONS Overall, the current study suggests that the 6MWT can be considered as a reliable and practical exercise test to be used in health promotion programs for community-dwelling amputees, because it showed excellent within-day test-retest reliability and seemed to adequately challenge an amputee s functional capacity, balance, and postural control abilities, as required in community ambulation. However, the findings of this study should be substantiated by further studies with a larger sample size and between-day trials. Acknowledgments: We thank Lori Hall, PT, MS, for her assistance in funding procurement at the initial stage of the project, and the following physical therapists for their assistance in data collection and data analysis: Erin George, MS, Jeff Hogan, MS, Paul Walters, MS, Cassie Sommerlad, MS, Tracy Dingman, MS, and Lola Elder, MS. References 1. Miller WC, Speechley M, Deathe B. 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