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 Table of Contents  
Year : 2016  |  Volume : 11  |  Issue : 4  |  Page : 192-196

Test–retest reliability and correlates of 6-minute walk test in patients with primary osteoarthritis of knees

1 Department of Physiotherapy, Majmaah University, Al Majmaah, Kingdom of Saudi Arabia
2 Department of Physiotherapy, GJUST, Hisar, Haryana, India

Date of Web Publication8-Nov-2016

Correspondence Address:
Dr. Sivachidambaram Kulandaivelan
Department of Physiotherapy, GJUST, Hisar - 125 001, Haryana
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-3698.192668

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Background: In the assessment of primary osteoarthritis (OA) of knees several clinical tool including 6 minutes walk test (6 MWT) are used. The objective of this study was to to analyze the test-retest reliability of 6 MWT and its correlation with various parameters.
Methods: Eighty patients (age, 56-79 years) with OA of knees met the inclusion criteria. Demographic and clinical characteristics including radiological severity of OA (by X-ray K/L grading) were recorded. Patients filled in the knee injury and osteoarthritis outcome score (KOOS) questionnaire. All patients performed 6 MWT twice with at least 48 h gap in-between.
Results: Test–retest reliability of 6 MWT in primary OA knee patients was excellent with ICC 0.991 (95% confidence interval was 0.986–0.994). 6 MWT had a weak correlation with KOOS-symptom and KOOS-activities of daily living (rho = 0.397 and 0.364 respectively), a strong correlation with KOOS-pain and KOOS-sports (rho = 0.605 and 0.521 respectively), and a very strong correlation with KOOS-quality of life (rho = 0.758). It had a weak correlation with age and height (r = 0.497 and 0.302), a strong correlation with VAS, weight, and BMI (rho = −0.655, r = −0.510, and − 0.691, respectively), and a very strong correlation with disease severity (rho = −0. 849).
Conclusion: 6 MWT was a reliable test and positively correlated with all KOOS subscales and negatively correlated with other parameters except height in primary OA knee.

Keywords: Intraclass correlation coefficient, knee injury and osteoarthritis outcome score, knee outcome measure

How to cite this article:
Ateef M, Kulandaivelan S, Tahseen S. Test–retest reliability and correlates of 6-minute walk test in patients with primary osteoarthritis of knees. Indian J Rheumatol 2016;11:192-6

How to cite this URL:
Ateef M, Kulandaivelan S, Tahseen S. Test–retest reliability and correlates of 6-minute walk test in patients with primary osteoarthritis of knees. Indian J Rheumatol [serial online] 2016 [cited 2023 Feb 5];11:192-6. Available from:

  Introduction Top

Osteoarthritis (OA), is one of the most common articular conditions that affect older adults. Old age and increased weight cause OA knee that results in pain and weakened muscles around the joint. It has a major impact on both physical (objective) and psychological (subjective) health leading to more disability. Weak muscles decrease aerobic capacity and reduce tolerance for activities of daily living (ADL), including walking. Walking is the activity most commonly reported as difficult by those with knee OA.[1] It can also be reduced as an indirect consequence to mechanical complications.[2] Compared to healthy adults, people with knee OA walks more slowly due to a shorter stride length, decreased cadence, and pain.[3],[4]

There are two types of mobility outcome measures to be considered: The individual's assessment of his/her own physical function (self-reported) and observed physical performance (performance). Self-reported measures, subjective, are sometimes regarded as less quantifiable than objective measures. Some of the commonly used self-reported measures in OA knee are Knee injury and Osteoarthritis Outcome Score (KOSS), Short Form-36 (SF-36), etc., Clinicians most often measure performance directly using performance-based measurements such as the 6-min walk test (6 MWT) and timed up and go test, and stair climb test. All these tests, both subjective as well as objective, are inexpensive and easy to administer. They give valuable information such as disease severity and disease prognosis to the clinician.

6 MWT not only assess the patient's walking ability, but also gives an indication of the endurance level of the individual. Hence, it is an excellent functional outcome measure for OA knee that is recommended by American College of Rheumatology (ACR). Its reliability was determined in elderly population, postsurgical intervention of hip and knee joint in Western population,[5],[6] but not yet been appraised in the Indian population.

Thus, the primary objective of the present study was to assess the test–retest reliability of 6 MWT. Secondary objectives were to assess the correlation of 6 MWT with anthropometric variables (age, height, weight, and body mass index [BMI]), subscales of self-reported questionnaire (KOOS), visual analog scale (VAS), and disease severity in Indian OA knee patients.

  Methodology Top

The study was conducted in different hospitals in Hyderabad, India, during April and May 2013.

Data were taken from the OA knee individuals who were diagnosed based on the ACR criteria [7] which were supported by radiographic evidence. Individuals with any evidence of secondary OA, inflammatory arthritis, any lower extremity muscle strain, ligament sprain, and those with neurologic conditions including low Mini-Mental examination score were excluded from the study. Other exclusion criteria were modified from Focht et al.[8] All exclusion criteria were asked verbally except Mini–Mental examination score.

The BMI (in kg/m 2) was calculated from measured height and weight. Pain was measured by visual analogue scale (VAS) 100 mm.

Radiographs were assessed for the presence of joint space narrowing, marginal spur formation, and subchondral cyst formation. The radiographs were evaluated by a blinded experienced radiologist using the Kellgren and Lawrence (KL) grading, in which 0 refers no radiographic OA and 4 refers to severe OA. The KL method of classification was used where grade 0 = normal, 1 = doubtful osteophyte, 2 = definite osteophyte, 3 = moderate joint space narrowing, and 4 = severe joint space narrowing. The intrarater reliability of KL grading has been established to be high.[9]

Knee injury and osteoarthritis outcome score (KOOS) is a most commonly used outcome measure tool. It is an extension of the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Initially developed for the young and/or physically active patients with knee injury and/or OA,[10] it is also valid for elderly patients.[11]

6 MWT was conducted in a well-ventilated, quiet, hard, and flat surface hallway with at least 20 m distance in between. Two cones were placed 20 m apart, and the patients were instructed to walk at their own pace to cover maximum distance in 6 min. Participants were not allowed to carry a watch and were not provided any feedback during the test. Verbal encouragement was avoided. The test was terminated if the person reported chest pain, intolerable shortness of breath, leg cramps, and appeared diaphoresis or pale/ashen appearance.[12] 6 MWT was administered twice with at least 48 h gap in-between.

Data were presented in mean ± standard deviation (SD), standard error of mean, and 95% confidence interval (95% CI) as descriptive analysis. Intraclass correlation coefficient (ICC) was used to see the test–retest reliability of 6 MWT. Unpaired t-test was used to observe gender difference. One-way ANNOVA was used to see the content validity of 6 MWT with KL grading (disease severity). Pearson's correlation (r) test and Spearman correlation (rho) test were used for analyzing continuous and ordinal/interval scale variables, respectively, with 6 MWT. All correlation coefficients were classified as 0–0.3 - none; 0.31–0.5 - weak; 0.51–0.7 - strong; 0.71–0.9 - very strong; and >0.9 - excellent.[13]

  Results Top

A consecutive sample of 80 (29 males and 51 females) was taken out of 96 eligible patients. Sixteen patients refused to give consent, making it 83% acceptance rate. All refused patients were males, and the reason given was lack of time and/or long distance of testing center from home. Basic characteristics of the sample (mean ± SD) were 50.68 ± 15.51 years, 1.59 ± 0.11 m, 72.72 ± 14.98 kg, and 28.81 ± 4.86 kg/m 2 for age, height, weight, and BMI, respectively.

[Table 1] shows that there was a significant sex difference in 6 MWT. Males covered 20% more distance than females (95% CI: 30.67 m and − 104.30 m, P < 0.001). There was an excellent test–retest reliability of 6 MWT. Compared to the 1st test, there was only <3% improvement in the 2nd test.
Table 1: Gender difference in 6-min walk test and test-retest reliability of 6-min walk test in primary osteoarthritis knee patients (n=80)

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[Table 2] shows ANNOVA along with post hoc analysis of 6 MWT with KL grading (disease severity) (content validity). It is expected that as the disease progress, that is evident from X-ray, 6 MWT should decrease. The result confirmed this hypothesis and there was a significant difference between disease severity grading. All the subsequent grading showed a significant reduction in 6 MWT except Grade 1 and 2. An average of 54.42 m difference was found between the grading (range: 30.65 m to 72.46 m).
Table 2: Analysis of variance table for disease severity with post hoc analysis (content validity) of 6-min walk test in primary osteoarthritis knee patients (n=80)

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6 MWT had a weak-to-strong correlation with subscales of self-reported questionnaire KOOS [Table 3] (concurrent validity). It has a weak negative correlation with symptom and ADL subscales, strong negative correlation with pain and sports subscales, and very strong negative correlation with QOL subscales.
Table 3: Correlation of 6-min walk test with knee injury and osteoarthritis outcome score subscales (concurrent validity) in primary osteoarthritis knee patients

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[Table 4] shows a correlation between 6 MWT and some OA determinants. 6 MWT has a weak correlation with age and height; a strong negative correlation with weight, BMI, and pain; and a very strong negative correlation with disease severity (KL grading).
Table 4: Correlation between 6-min walk test and some osteoarthritis knee determinants

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  Discussion Top

The primary objective of the present study was to assess the test–retest reliability of 6 MWT in Indian primary OA knee patients. The result shows an excellent reliability in this population. These results supports observations noted in two previous studies.[5],[6] Long-term excellent reliability (ICC 0.94) was reported in end-stage hip and knee OA.[6] Rejeski et al.[5] reported two weekly test–retest reliability of 0.87 for patients with knee OA.

There was <3% improvement in 2nd time over 1st time in this study. Previous studies show <4%–<10% improvement in 2nd time.[14],[15],[16] The reason for this improvement is familiarization of procedure, learning effect, and self-motivation to do better than previous one.

Incidence of OA knee is more common in females than males.[17] The result reveals that females covered 20-25% less distance than their male counterparts. This was also noted in the studies by Ateef et al.[18] and Longerstedt et al.[19] who presented similar sex difference in walking distance among moderate-to-severe OA knee patients. The reasons for this discrepancy are weaker knee muscles, shorter stride length, higher BMI, and lower gait speed (slower walking) in female OA knee patients.

6 MWT had a favorable correlation with all subscales of KOOS in our previous study.[20] The results of the present studies are also similar to that study.

6 MWT strongly correlated with KOOS-Pain subscale and VAS pain scale. Similar findings were also noted in a study by Stratford et al.[21] who found 0.74 correlation between VAS and 6MWT in OA patients. However, literature shows a weak correlation of 6 MWT with pain subscales at −0.39[22] and −0.205.[23] Pain is the cardinal sign of OA knee and inversely affects the walking distance. People with knee pain walk slowly which in turn increases joint loading that deteriorate the condition further.[24],[25]

6 MWT is strongly correlated with KOOS-Sports subscale but weakly correlated with KOOS-ADL subscale. Literature supports a strong correlation of 6 MWT with WOMAC physical function subscale in OA knee patients (r = −0.54[22] and r = −0.646[23]). As factors other than simple walking alone influence physical function, the relationship between self-reported physical function and performance measures, such as the 6-min walk, is low.[26]

There was a negative correlation between age and 6 MWT. Age negatively correlated walking distance (r = −0.41[27] and r = −0.249[28]) in knee OA patients. It is also supported by healthy elder population (r = −0.51[14] and r =−0.42[29]), adult population (r − 0.36[15] and r = −0.66[30]), and women [31] (r = −0.27). OA is a degenerative condition that worsens as the age progress; hence, there was a negative correlation between age and 6 MWT. Other reasons include muscular atrophy, slow reaction time, slow gait speed, and muscle weakness, associated with age.

There was a weak positive correlation between height and 6 MWT. This is supported in healthy older adults (r = 0.54[14]) and healthy adult population (r = 0.35[15] and r = 0.42[30]). The reason for positive correlation between height and 6 MWT is that as the height increases, stride length of the individual also increases. This will increase the overall walking distance.

There was a strong negative correlation between weight and 6 MWT. This is supported in OA knee patients [32] (r = −0.528) and healthy women [31] (r = −0.68). The reason for negative correlation is as weight increases gait speed decreases and joint loading also increases, causing more damage to the joint.[24],[25]

There was a strong negative correlation between BMI and 6 MWT. This is supported in Indian OA knee patients [33] (r = −0.69), Indian older adults [34] (r = −0.81), and healthy women [31] (r = −-0.77). Maly et al.[35] reported a negative correlation between 6MWT and BMI (r = −-0.39). The possible reason for less value than the present study may be explained by age, i.e. mean age reported by Maly et al.[35] was 68.3 whereas the mean age of the present study was 50.68. Our previous studies [28],[33] explained that as the age progresses, even patients with less BMI get OA easily that might affect the r value.

  Conclusion Top

6 MWT is a reliable objective measurement of OA knee in Indian population. It has good content and concurrent validity. Hence, it can be used as an outcome measure in rehabilitation protocols.

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Conflicts of interest

There are no conflicts of interest.

  References Top

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  [Table 1], [Table 2], [Table 3], [Table 4]

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