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Reliability of the two-minute walk test in individuals with transtibial amputation

  • Dina Brooks
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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  • Judith P. Hunter
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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  • Janet Parsons
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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  • Emma Livsey
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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  • Janice Quirt
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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  • Michael Devlin
    Affiliations
    Clinical Evaluation and Research Unit, West Park Healthcare Centre (Brooks, Hunter, Parsons); St. John's Rehabilitation Hospital (Hunter); Toronto Rehabilitation Institute (Parsons); and Department of Physical Therapy (Brooks, Hunter, Parsons, Livsey, Quirt) and Division of Physiatry (Devlin), Faculty of Medicine, University of Toronto, Toronto, Ont, Canada
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      Abstract

      Brooks D, Hunter JP, Parsons J, Livsey E, Quirt J, Devlin M. Reliability of the two-minute walk test in individuals with transtibial amputation. Arch Phys Med Rehabil 2002;83:1562-5. Objective: To determine inter- and intrarater reliability of the two-minute walk test (2MWT) in individuals with transtibial amputation. Design: Prospective; test-retest method by a pair of trained physical therapists. Setting: Two regional amputee rehabilitation centers in Canada. Participants: Thirty-three subjects (23 men, 10 women; mean age ± standard error, 63.6±2.0y) with transtibial amputation; 6 in outpatient rehabilitation, 27 in inpatient rehabilitation. The most common primary diagnoses were peripheral vascular disease (n=15) and diabetes (n=11). Interventions: Each subject performed a total of four 2MWTs, 1 test for each rater, on 2 consecutive days at approximately the same time of day. Subjects were given at least a 20-minute rest between tests. The order of raters was randomized on the first day and reversed for the next day. The walk tests were performed in the same enclosed corridors with the same starting point for all tests. The subjects were familiar with the test or were given 1 or more practice tests at least 1 day before testing. Subjects were allowed to walk with a mobility aid of their choice. Raters used a digital stopwatch to time the tests and a calibrated wheel with a counter to measure the distance walked in meters. The raters were blinded to each other's scores. Main Outcome Measure: Distance walked in 2 minutes (in meters). Results: Within-rater reliability was high (intraclass correlation coefficient [ICC],.90–.96). Between rater reliability was also high (ICC.98–.99). Analysis of variance (ANOVA) showed a significant effect for day of test (P<.001) in the inpatient group but no effect for therapist (P=.098) or for interaction of day and therapist (P=.710). Similarly, in the outpatient group, ANOVA showed a significant effect for day (P=.013) but no effect for therapist (P=.259) or interaction of day and therapist (P=.923). Conclusion: Although the 2MWT showed evidence of inter- and intrarater reliability in individuals with unilateral below-knee amputation, the distance walked in 2 minutes continued to improve over time. This improvement was not solely the result of a training and learning effect. © 2002 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation

      Keywords

      WALK TESTS ARE QUANTITATIVE measures of speed and distance that provide information about functional exercise capacity. In cardiorespiratory populations, some of the timed walk tests, such as the 12-, 6-, and 2-minute walk tests, are valid measures of exercise capacity.
      • Solway S
      • Brooks D
      • Lacasse Y
      • Thomas S
      A qualitative systematic review of the measurement properties of functional walk tests used in the cardiorespiratory domain.
      These tests correlate in differing degrees with volume of oxygen uptake (V̇O2/kg), ventilation, some measures of pulmonary function, and morbidity and mortality.
      • Solway S
      • Brooks D
      • Lacasse Y
      • Thomas S
      A qualitative systematic review of the measurement properties of functional walk tests used in the cardiorespiratory domain.
      An informal unpublished Canadian survey of amputee programs in 1998 reported the two-minute walk test (2MWT) as the second most commonly used outcome measure after the FIM™ instrument. Recently, we evaluated the construct validity and responsiveness of the 2MWT in subjects with lower-limb amputation.
      • Brooks D
      • Parsons J
      • Hunter JP
      • Devlin M
      • Walker J
      The 2-minute walk test as a measure of functional improvement in persons with lower limb amputation.
      We concluded that the 2MWT was responsive to change with rehabilitation and that it correlated with measures of physical function and prosthetic use in this population. To date, no investigators have reported the reliability of the 2MWT in amputee populations.
      The reliability of walk tests has been reported in other populations. Larson et al
      • Larson JL
      • Covey MK
      • Vitalo CA
      • Alex AG
      • Patel M
      • Kim MJ
      Reliability and validity of the 12-minute distance walk in patients with chronic obstructive pulmonary disease.
      reported high intrarater reliability of the 12-minute walk (r=.93–.98) in individuals with chronic obstructive pulmonary disease (COPD). Harada et al
      • Harada ND
      • Chiu V
      • Stewart AL
      Mobility-related function in older adults: assessment with a 6-minute walk test.
      and Montgomery and Gardner
      • Montgomery PS
      • Gardner AW
      The clinical utility of a six-minute walk test in peripheral arterial occlusive disease patients.
      reported high test-retest reliability of the 6-minute walk (r=.95, r=.94, respectively) in older adults and individuals with peripheral arterial occlusive disease. Upton et al
      • Upton CJ
      • Tyrrell JC
      • Hiller EJ
      Two minute walking distance in cystic fibrosis.
      evaluated the 2MWT's test-retest reliability in children with cystic fibrosis and reported no significant difference with repeated testing as evidenced by a low mean coefficient of variation (2.6%). Connelly et al
      • Connelly DM
      • Stevenson TJ
      • Vandervoort AA
      Between- and within-rater reliability of walking tests in a frail elderly population.
      reported the 2MWT's intra- and interrater reliability to be good to high (intraclass correlation coefficient [ICC],.82–.95) in a normal frail elderly population. In addition, the test-retest reliability of both comfortable and fast walking speeds of both amputees and normal subjects has been reported to be high (ICC,.83–.98).
      • Boonstra AM
      • Fidler V
      • Eisma WH
      Walking speed of normal subjects: aspects of validity of gait analysis.
      The purpose of the present study was to examine the inter- and intrarater reliability of the 2MWT in individuals with transtibial amputation.

      Methods

      The methods for the present study were based on the study by Connelly
      • Connelly DM
      • Stevenson TJ
      • Vandervoort AA
      Between- and within-rater reliability of walking tests in a frail elderly population.
      who examined the reliability of the 2MWT in the frail elderly. Our participants were physical therapists and clients from 2 regional amputee rehabilitation programs in the province of Ontario. Ethics approval was obtained from the research ethics boards of the University of Toronto and the 2 centers involved. At each site, reliability was evaluated by using the test-retest method with 2 physical therapists. Each subject performed a total of 4 walks: on 2 consecutive treatment days, the subject completed 2 walks, 1 test for each rater, at approximately the same time of day. Subjects were given at least a 20-minute rest between each 2MWT. The order of raters was randomized on the first day and reversed for the next day.

      Participants

      Study inclusion required each individual to have a limb prosthesis, to have completed a minimum of 2 weeks of rehabilitation, to tolerate 2 minutes of walking, to have no prosthetic modifications planned, and to have no other medical restrictions preventing them from participating in the test. Subjects were excluded if they were cognitively impaired or unable to give consent, poorly motivated to cooperate with the procedure, or unable to participate on 2 consecutive treatment days. All subjects gave informed consent before testing commenced.
      A total of 33 individuals with transtibial amputation (23 men, 10 women) were recruited: 6 were undergoing outpatient rehabilitation and 27 were enrolled in inpatient rehabilitation at 1 of 2 regional rehabilitation sites. Twelve of the subjects had undergone amputation on the left lower extremity, and 21 had undergone amputation on the right side. Sixteen subjects (10 inpatients, 6 outpatients) were from 1 regional rehabilitation site, and 17 were from the other site. The majority of the subjects (n=31) required a mobility aid as follows: a walker or rollator (n=24), 1 or 2 canes (n=5), or crutches (n=2).
      The subjects' primary diagnoses were peripheral vascular disease (n=20), diabetes (n=11), osteomyelitis (n=1), and sarcoma (n=1). Thirty subjects presented with at least 1 additional comorbid diagnosis that included either diabetes, coronary artery disease, congestive heart failure, or chronic lung disease. The individuals typically used patellar tendon-bearing plastic laminate sockets, suspended by waist belt and fork strap, and a solid ankle cushioned heel (SACH) foot. Although no prosthesis changes were made after the study commenced, changes may have been made 1 day before the first day of testing.

      Study protocol

      At each site, the walk tests were performed in an enclosed corridor. The same corridor was used for each test. The corridors were level ground, relatively free from distractions, and longer than 40m. The starting point was the same for all tests and was clearly marked. The subjects were instructed to walk as far as they could in the 2 minutes. To control for learning and practice effects, the subjects were familiar with the test or were given 1 or more practice tests at least 1 day before testing, usually in a different corridor than that used for testing. Subjects were allowed to walk with a mobility aid of their choice and were given a rolling start of 2 or 3 steps. No talking was permitted by raters or subjects during the tests. Raters used a digital stopwatch to time each test and a calibrated wheel with a counter to measure the distance walked in meters. The raters were blinded to each other's score.

      Statistical analysis

      Reliability was determined by calculating the ICC. The ICC provides a measure for evaluating reliability because it takes into account both the between- and within-subjects components of variance as well as the heterogeneity of the sample.
      • Norman GR
      • Streiner DL
      Biostatistics: the bare essentials.
      Values greater than.60 are considered acceptable reliability.
      • Masanic CA
      • Bayley MT
      Interrater reliability of neurologic soft signs in an acquired brain injury population.
      We examined the consistency of the test over time by repeated-measures 2-way analysis of variance (ANOVA) to determine the difference over the 2 days and between therapists. Post hoc analysis was performed by using the Student-Newman-Keuls procedure.

      Results

      The characteristics of the subjects in the total sample and in the inpatient and outpatient subgroups are in table 1.
      Table 1Subjects' characteristics, outpatients, inpatients, and total sample
      OutpatientsInpatientsAll
      N62733
      Age (y)55.3±4.4 (44–69)65.4±2.1 (42–80)63.6±2.0 (42–80)
      No. of prior walk tests7.0±1.5 (1–10)1.5±0.2 (1–6)2.6±0.5 (1–10)
      Days since prosthesis fitted105.2±53.0 (28–365)30.7±9.4 (7–270)44.2±12.8 (7–365)
      Days of rehabilitation134.0±46.7 (56–365)31.5±3.5 (8–90)50.2±10.9 (8–365)
      Days since amputation188.7±46.8 (63–365)89.9±15.0 (25–365)107.8±16.1 (25–365)
      NOTE. Values are mean ± standard error (SE) and range.
      Individuals undergoing outpatient rehabilitation were generally younger. They had undergone significantly more walk tests before testing than the individuals undergoing inpatient rehabilitation (P<.05). Furthermore, days since amputation, days of rehabilitation, and days since prosthetic fitting were greater in the outpatient group (all P<.03).
      Tables 2 and 3 portray the mean distance walked on the 2 days for the 2 raters in the inpatient and outpatient subgroups, respectively.
      Table 2Distance walked in 2 minutes and inter- and intrarater reliability (ICCs) of 2 raters recording 2MWTs (m) of inpatient transtibial amputees on 2 days
      Rater 1Rater 2ICC*
      Day 150.1±4.651.5±4.2.98
      Day 257.0±5.057.5±5.2.98
      ICC†.90.94
      *The reliability on any single day, reflecting interrater reliability. †The reliability of any single rater, reflecting intrarater reliability.
      NOTE. Values are mean ± SE.
      Table 3Distance walked in 2 minutes and inter- and intrarater reliability (ICCs) of 2 raters recording 2MWTs (m) of outpatient transtibial amputees on 2 days
      Rater 1Rater 2ICC*
      Day 1121.1±18.6123.2±15.5.98
      Day 2137.9±14.7140.7±15.9.99
      ICC†.95.96
      *The reliability on any single day. †The reliability of any single rater.
      NOTE. Values are mean ± SE.
      Within-rater reliability was high, with ICCs ranging from.90 to.96. Between-rater reliability was also high, with ICCs ranging from.98 to.99. A scatterplot of the distance walked for both days is in figure 1.
      Figure thumbnail gr1
      Fig. 1Distance walked by (A) inpatients and (B) outpatients in 2 minutes for 2 raters on 2 different days. The solid line indicates the line of identity.
      In the inpatient group, a 2-way repeated-measures ANOVA showed a significant effect of day of test (P<.001) but no effect for therapist (P=.098) or interaction of day and therapist (P=.710). Similarly, in the outpatient group, a 2-way repeated-measures ANOVA showed a significant effect of day (P=.013) but no effect for therapist (P=.259) or interaction of day and therapist (P=.923). Regardless of the tester, distances walked on day 2 were greater than on day 1 for both groups. Figure 2 shows the changes in the distance walked for tests 1 and 2 (on day 1) and tests 3 and 4 (on day 2).
      Figure thumbnail gr2
      Fig. 2Mean distance ± SE walked by (A) inpatients and (B) outpatients in 2 minutes for each test regardless of the rater on 2 consecutive days.

      Discussion

      The 2MWT exhibits good within- and between-rater reliability in individuals with transtibial amputation. However, the distance walked in 2 minutes was not constant over time, but increased over the 2 days in individuals undergoing either out- or inpatient rehabilitation.
      Reliability is a fundamental measurement property that is relatively easy to determine. It is quantified in terms of degree of consistency and repeatability when properly administered under similar circumstances. Clinically, this property is important because it allows the clinician to determine the amount of noise or random error in the tool.
      • Upton CJ
      • Tyrrell JC
      • Hiller EJ
      Two minute walking distance in cystic fibrosis.
      The clinician, when interpreting the results of outcome measures, must be able to determine how much of the measured change is due to real change in the client's health status and how much is due to measurement error. Reliability is population specific
      • McDowell I
      • Newell C
      Measuring health: a guide to treating scales and questionnaires.
      and should not be seen as a property that a particular test does or does not possess. Instead, any measure will have certain reliability when applied by certain clinicians, to a specific population, and under specific conditions. In individuals with transtibial amputation, the 2MWT is a reliable measure.
      Some of the other psychometric properties of the 2MWT, mainly validity and responsiveness, have been reported in other populations including the elderly, individuals with respiratory disease, and individuals with amputation. Butland et al
      • Butland RJ
      • Pang J
      • Gross ER
      • Woodcock AA
      • Geddes DM
      Two-, six-, and 12-minute walking tests in respiratory disease.
      and Bernstein et al
      • Bernstein M
      • Despars J
      • Singh N
      Reanalysis of the 12-minute walk in patients with chronic obstructive pulmonary disease.
      both evaluated concurrent validity in individuals with respiratory disease and reported that the distance walked in the 2MWT strongly correlated with the distances walked in both the 6- and the 12-minute walk tests (r=.892–.955). Bernstein
      • Bernstein M
      • Despars J
      • Singh N
      Reanalysis of the 12-minute walk in patients with chronic obstructive pulmonary disease.
      examined construct validity in individuals with COPD and reported that the distance walked in 2 minutes moderately correlated to maximal and strongly correlated to submaximal oxygen consumption (r=.45, r=.55, respectively). Furthermore, Upton et al
      • Upton CJ
      • Tyrrell JC
      • Hiller EJ
      Two minute walking distance in cystic fibrosis.
      concluded that, in children with cystic fibrosis who had near-normal respiratory function, the 2MWT was a more discriminative measure than peak expiratory flow rate. In individuals with lower-extremity amputation, the 2MWT distance correlated with measures of physical function and prosthetic use.
      • Brooks D
      • Parsons J
      • Hunter JP
      • Devlin M
      • Walker J
      The 2-minute walk test as a measure of functional improvement in persons with lower limb amputation.
      With respect to responsiveness, Guyatt et al
      • Guyatt GH
      • Pugsley SO
      • Sullivan MT
      • et al.
      Effect of encouragement on walking test performance.
      showed that responsiveness to treatment, measured by the within-individual standard deviation (SD) of distance walked in 2 minutes, was less in the 2-minute test than in the 6-minute test (SDs,.90 vs.74) in individuals with chronic airflow limitation, chronic heart failure, or both.
      • Guyatt GH
      • Pugsley SO
      • Sullivan MT
      • et al.
      Effect of encouragement on walking test performance.
      However, in a sample of elderly men with COPD, changes in 2MWTs were strongly correlated with changes in V̇O2/kg (r=.53) and maximal oxygen uptake (V̇O2max) (r=.53).
      • Bernstein M
      • Despars J
      • Singh N
      Reanalysis of the 12-minute walk in patients with chronic obstructive pulmonary disease.
      In individuals with lower-extremity amputation, we
      • Solway S
      • Brooks D
      • Lacasse Y
      • Thomas S
      A qualitative systematic review of the measurement properties of functional walk tests used in the cardiorespiratory domain.
      have recently shown that the 2MWT was responsive to change with rehabilitation.
      • Brooks D
      • Parsons J
      • Hunter JP
      • Devlin M
      • Walker J
      The 2-minute walk test as a measure of functional improvement in persons with lower limb amputation.
      One surprising finding from the present study was that the distance walked in 2 minutes improved over the 2 days of testing. This improvement was statistically significant in each of the 2 subgroups (inpatients, outpatients). One possible explanation is that subjects experienced a training or learning effect. Guyatt
      • Guyatt GH
      • Pugsley SO
      • Sullivan MT
      • et al.
      Effect of encouragement on walking test performance.
      established the presence of a learning and training effect by performing 6 repeated tests on the 2-minute walk. The distance walked improved on the first 2 walks compared with the last 4 walks in adults with chronic airflow limitation or chronic heart failure or both. Furthermore, training and learning effects with repeated testing have been identified in several studies on the 6-minute walk test (6MWT).
      • Solway S
      • Brooks D
      • Lacasse Y
      • Thomas S
      A qualitative systematic review of the measurement properties of functional walk tests used in the cardiorespiratory domain.
      For instance, 1 study
      • Guyatt GH
      • Sullivan MJ
      • Thompson PJ
      • et al.
      The 6-minute walk: a new measure of exercise capacity in patients with chronic heart failure.
      showed that 6MWT distances could improve 60m after 3 repeated walk tests in individuals with COPD. However, in these studies, this effect was attenuated by the third test, which was not the case in our present study. Although a learning and training effect may have influenced our findings, we do not believe that this was the main reason for the improvement observed. The change in distance walked over the 2 days was highly significant in both subgroups of individuals (inpatients, outpatients); however, the majority (5/7) of outpatient subjects had undergone more than 5 training walks before testing. All subjects had at least 1 training walk, and the majority had at least 2. We speculate that 2 other factors may have contributed to the improvement over time. First, the same corridor and starting point were used for all tests. Considering that the distances walked by the subjects were relatively short, it is possible that they remembered their performance and were encouraged through external cues to “try harder” on subsequent tests. Most other studies on the 2-minute walk looked at performance in the first 2 minutes of a 6- or a 12-minute walk test, thus minimizing the effect of memory on performance. Figure 2 shows that, regardless of the tester, performance improved between tests on the first and second day. This continued improvement supports the notion of a potential effect of memory and the individual desire to show improved performance. Second, because these subjects were undergoing rehabilitation, it is possible that their walking capacity may have improved overnight. Alternatively, repeated walking as performed during the tests may be therapeutic, and therefore the changes may reflect a treatment effect of the test itself. Future studies could examine this effect by studying individuals several months after termination of rehabilitation or by using a different corridor or different starting point for repeated testing.
      As mentioned, most other studies evaluating the 2MWT have evaluated different populations, and McDowell and Newell
      • McDowell I
      • Newell C
      Measuring health: a guide to treating scales and questionnaires.
      caution that reliability is population specific. We suspect that the continuous improvement seen in our present study reflects the specific activity limitations of individuals recovering from lower-extremity amputation. Further research on the learning effect in this population is needed. Further validation of the 2MWT in these individuals should be undertaken, including different time frames for testing, comparisons with other measures of functional mobility (eg, the timed-up-and-go), balance tests, and formal gait analysis.

      Conclusion

      The 2MWT is practical, simple, quick, and easy to administer. In the present prospective study, we found evidence of good inter- and intrarater reliability of the 2MWT in for the population of persons with unilateral below-knee amputation. However, the distance walked in 2 minutes was not constant, and it improved over time. The improvement was not solely the result of a training and learning effect and does not appear to plateau with repetition. The explanation for the improved performance remains unclear.

      Acknowledgements

      We thank Janet Campbell, Debbie Albuquerque, Ian Lowe, Maria Laskowski, and Rosalie Chan for their assistance with data collection.

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