Exercise capacity plays an essential role in determining quality of life and prognosis, both in disease1–4 and health.5 The assessment of exercise capacity may bring important information regarding the health of a population, and yet, it is under used in primary care. The gold standard assessment protocol is the graded exercise test, which is performed on a treadmill or ergometric bicycle and the patient is monitored with a gas analyzer. However, it is expensive and requires a specialized team, which might be an obstacle in screening larger populations.
In clinical settings, attention has been given to field tests, which could objectively assess exercise capacity, are easier to perform, and do not require expensive equipment.6,7 The 6-minute walk test (6MWT) is commonly used in clinical practice, where the individual is asked to walk as far as they can during 6 minutes. The 6MWT has an important role in assessing patients with chronic conditions,1,3,8 and it is considered to be more tolerable than the graded exercise test, since it is self-paced and similar to day-to-day walking. This test has been validated and its reliability has already been assessed in different populations.9–14 Six-minute walk test's reliability studies have shown that this test presents a difference between a first and a second test due to a learning effect, when applied in an individual with a cardiorespiratory disease,12,14,15 or in healthy older people.9 Six-minute walk test has already been validated by some studies, which have found correlation between the covered distance in the 6MWT and the oxygen consumption in a graded exercise test; furthermore, 6MWT presented correlation with quality of life and predicts hospitalization and mortality in cardiorespiratory diseases.1,3,16–24
The 6MWT can be interpreted by comparing its results with reference values,6 which are expected results in a test for a certain individual calculated using demographic and anthropometric parameters, based in a series of tests performed in a healthy population. Six-minute walk test's reference values have already been studied for Brazilian25,26 and other populations.27–30 Variables usually included in the equations to predict 6MWT distance are gender,25–32 age,25–32 body mass index (BMI),26,31 height,26–29,32 and weight.27–29,32
Nevertheless, the American Thoracic Society's6 recommendations state that a 30-m corridor is necessary for a reliable execution of the 6MWT, which, sometimes, is not available in a small facility. Furthermore, before performing a 6MWT, a practice test must be conducted, which increases the time for its execution. These characteristics may reduce 6MWT applicability in primary care, where a large population is usually screened and the space and time are usually more restricted.
The 6-minute step test (6MST) requires less time and space to be performed and might be used as an alternative to assess exercise capacity in primary care. Step tests usually require only a room to be performed and are relatively as low cost as other field tests.33
The 6MST presents others characteristics, which points it as a better suitable test for screening low exercise capacity in the community. Similarly as the 6MWT, 6MST is self-paced and the height of the step is similar to a step from a regular stair, leading to a better tolerability of 6MST when compared with other step tests.33 A possible advantage of the 6MST over the walk test is the absence of a familiarization test requirement34,35; thereby the time to conduct the test is reduced. However, the 6MST's test–retest reliability has been assessed in patients with pulmonary diseases.34,35 Six-minute step test's physiological responses have already been studied in patients with chronic pulmonary diseases,35–38 and it was capable of identifying abnormal responses to exercise.
However, the assessment of the exercise capacity by the 6MST in a community has not still been done, and the reasons might be related to the lack of reliability and validity studies in a healthy population, and the absence of reference values to predict the test performance. To address this research gap, the purpose of this study was to test the 6MST's reliability and validity in healthy subjects and to establish reference values for exercise capacity using the number of steps in this test. The hypothesis of the authors was that 6MST may be a reliable and valid test. In addition, we expect that demographic and anthropometric measures could predict exercise capacity assessed by the number of steps in this test.
Participants were enrolled from June 2011 until June 2012, and they were invited through placed posters in the university and its neighborhood, and advertised on local radio, television, and in newspapers. Potential participants were asked over the telephone if they met the inclusion criteria to participate in the study. The inclusion criteria were any person without any known diseases and older than 18 years. Participants were defined as healthy if during a telephone call they could confirm that (1) they were able to manage their activities of daily living and (2) without the presence of any diagnosed cardiac, orthopedic, pulmonary, oncologic, or neurologic condition that could decrease their functional capacity, with the exception of controlled hypertension without the use of beta-blockers.
To be included in the analysis, the participants needed to attend all days of the assessment and to have a BMI value under 35 kg/m2, normal pulmonary function assessed by spirometry [forced vital capacity (FVC) >80% of the predicted normal, forced expiratory volume in the first second (FEV1) >80%, and FEV1/FVC >0.7], and a normal exercise capacity verified by a covered distance (in meters) in the 6MWT higher than 75% of the predicted normal value.29
One hundred ten volunteers were assessed for the study. Four were excluded as they had FEV1/FVC ratio under 0.7, 8 were excluded because they had a BMI above 35 kg/m2, 6 were excluded as they did not complete all baseline assessment, and 1 because she presented a covered distance (in meters) in the 6MWT under 75% of the predicted performance. Of the 91 participants included in the analysis, 49 were women and 42 were men, the age range was between 18 and 86 years.
All included patients signed a consent form, which had been approved by the human ethics committee of the university (decision number 009/2011).
Trained physiotherapists performed all the measurements, which were organized over 2 days with a 48-hour interval between the first and second day.
- First day: the participants underwent an assessment for information such as gender, age, medications, symptoms, smoking, and alcohol habits. They undertook a physical examination to obtain height, weight, abdominal circumference, and leg length. A pulmonary function assessment (spirometry) was performed. Participants also underwent two 6MSTs or two 6MWTs; the test conducted on this day was decided by randomization using sealed envelopes. There was a 30-minute interval between the first and second test, when the patient was asked to rest.
- Second day: the participants underwent a body composition analysis and performed twice the test, which had not been performed on the first day.
Anthropometric and Body Composition Analysis
The individuals were asked to remove their shoes and to wear light clothing before they were weighed and had their height measured (model 110FF; Welmy, São Paulo, Brazil). Body mass index was calculated (Table 1). Lower limb length (umbilicus until medial malleolus) and the abdominal circumference (using the umbilicus as reference) were assessed using a measuring tape.
Fat-free mass in kilograms and fat mass in percentage of the total weight (fat%) were obtained using a body composition analyzer (model BC-553; Tanita, Arlington, Texas).39 Fat-free mass index (FFMI) was calculated (Table 1).40 To undertake this analysis, the participants were asked not to eat or drink for 4 hours before it. Before being submitted to an exercise test, a light meal was offered to them.
A calibrated spirometer (Microquark; Cosmed, Rome, Italy) was used following Brazilian guidelines.41 Forced vital capacity, FEV1, and the ratio between FEV1/FVC were obtained and used in the analysis. The FVC and FEV1 values were expressed as a percentage of the predicted normal value42; moreover, a spirometry was considered without abnormalities when FEV1 and FVC >80%, and FEV1/FVC >0.7.43
Six-Minute Step Test
Two nonblinded physiotherapists were involved in each step test, 1 to conduct the test and the other to count the amount of steps the participant climbed up and down (1 cycle of climbing up and down was counted as 1 step). A wooden step with height of 20 cm was used.35 The participants were instructed to climb up and down at a speed they thought would allow them to perform the maximum amount of steps they could do during 6 minutes. They could begin the climbing with whichever lower limb they felt more comfortable with, and they could change it at anytime during the test, thus, the test would be more similar with an activity of daily living. The standardized instructions for patients before the test were published elsewhere.34
For better reliability, the test followed the same principles for the 6MWT,6 thereby the same standardized verbal incentives each minute were used and the participant chose their own pace. Before and after the test, heart rate (HR), blood pressure, and SpO2 were assessed; dyspnea and lower limb fatigue were asked using a modified Borg scale (0–10).
Throughout the test, HR (Polar Vantage NVTM, model 1901001, Oulu, Finland) and SpO2 (model 2500; Nonin, Minneapolis, Minnesota) were monitored, and if the individual's HR presented higher than the submaximal HR (Table 1) or the SpO2 was under 85%, the assessor stopped the test until the HR was reduced by 10 beats/min under the submaximal HR or until the SpO2 increased to 88% or more. The participant could also chose to stop the test to rest, but in either case, the timer was not stopped during the interruption. The choice of interrupting the test in those situations was done to increase the safety of the test.
Six-Minute Walk Test
Exercise capacity was assessed as a control in all participants by a well-accepted test, the 6MWT.9,24,44,45 The test was performed following the directions of the American Thoracic Society,6 after a familiarization test, the values used in the analysis were taken from the test with the best performance (highest performance between the first and second test). The covered distance was expressed in absolute values (in meters) and as a percentage of the predicted value (Table 1) from a reference equation for Brazilian individuals.26
Statistical analysis was performed by a statistician, and in all analyses, an alpha level was set at 0.05. The data had been verified using Shapiro–Wilk test, and they were parametric. Mean and SD were used to describe the data.
A comparison between male and female groups for all studied characteristics was performed using Student t test. The Pearson correlation coefficient was used to assess the correlation between the performance on the first 6MST and other variables (age, weight, height, lower limb length, abdominal circumference, BMI, fat%, and FFMI). Correlations between 6MST's and 6MWT's performances were performed to verify criterion validity, using the criterion of r > 0.7 as a validity marker.46
Reliability was verified comparing the performance (number of steps) and physiological responses in the first and second 6MST and 6MWT. Reliability was analyzed using the intraclass correlation coefficient (ICC), classifying the values as low (ICC <0.4), good (ICC >0.4 and <0.75), and excellent (ICC >0.75).47 Reliability was also assessed by the comparison between the mean number of steps in the first and second test (Paired t test). Moreover, the analysis of the error between the first and second tests was conducted using the mean bias and agreement limits presented in a Bland-Altman plots, the minimum detectable difference (MDD), the standard error of measurement (SEM), and the 95% confidence interval of the mean 6MST performance (steps).48 Calculation for these error values are shown in Table 1.
Equations to obtain reference values of performance on 6MST were obtained through 3 multiple linear regressions using the stepwise method, the dependent variables of each equation was the first, second, or the best performance on the 6MST. The independent variables to predict the performance were gender, age, weight, height, lower limb length, BMI, and abdominal circumference. Tests to verify collinearity, interactions, and the normality of the residues among the independent variables were performed.
The characteristics of the sample are described in Table 2. Sample size was considered sufficient as a minimum of 84 volunteers were included. We calculated the sample size in the same way as a previous study with similar objectives.49 During the first or second 6MST, the researchers asked the participants to stop each test 2 ± 2 times (minimum of 0 and maximum of 8 times) due to an increase in the HR higher than the submaximum HR.
The number of steps in the first 6MST presented correlation with age (r = −0.51; P < 0.001), height (r = 0.38; P = 0.004), lower limb length (r = 0.43; P < 0.001), abdominal circumference (r = −0.23; P = 0.025), fat% (r = -0.60; P < 0.001), and FFMI (r = 0.34; P = 0.001). There was a significant correlation between the number of steps in the first 6MST and the largest covered distance between the first and second 6MWT (r = 0.72; P < 0.001). In addition, all the correlations performed using the number of steps in the first 6MST were also performed using the second 6MST and the highest number of steps between the first and second 6MST, with similar results (difference between r values not higher than 0.03).
The 6MST was reliable in the measurement of performance (number of steps) and some physiological measures were reliably assessed when the 6MST initial test and retest data were compared (Table 3). Intraclass correlation coefficient values higher than 0.75 (P < 0.001) were found between the initial 6MST and the retest for number of steps, HR, dyspnea, and systolic blood pressure, which reflects the existence of excellent reliability for these measures.47 Moreover, ICC values higher than 0.4 and lower than 0.75 (P < 0.001) was found for SpO2, lower limb fatigue, and diastolic blood pressure. Considering other aspects of the reliability, there was no difference (P = 0.65; paired t test) between the first and second performance in the 6MST (Table 2); and the error values were MDD = 27.26 steps, SEM = 11.75 steps, which allowed us to calculate the 95% CI for the mean performance (number of steps) for the first 6MST (95% CI: 125.96–172.04 steps). Bland–Altman plot is displayed in Figure 1, the mean error (steps) was 0.74 (95% CI: −31.1 to 32.6). There were 6 participants with error values outside the 95% CI, 5 of them presented an increase in the second test, which might be due a familiarization with the test. One participant presented a decrease in the second test probably due to a high performance in the first 6MST, causing more fatigue in the second test.
The multiple linear regression presented a problem with collinearity, which was corrected by excluding BMI from the list of independent variables. Body mass index was excluded instead of other independent variables since it also did not present normality of the residues. No interactions were verified among the independent variables on the proposed equations.
Only gender and age were chosen as predictors of the performance in the first 6MST by the stepwise model. Gender, age, and abdominal circumference were included in the model to predict the number of steps in the second test or in the best performance between first and second 6MST. The 3 equations and their respective R2 values and residual standard errors are shown in Table 4.
Six-minute step test performance (number of steps) was reliable in the test–retest comparison, and some physiological measures were reliably assessed when 6MST initial test and retest data were compared. Additionally, 6MST performance was strongly correlated with 6MWT, which may suggest 6MST is valid for assessing exercise capacity. The expected performance (number of steps) on the first 6MST may be predicted for each individual by using gender and age, thereby facilitating the identification of people whose exercise capacity (assessed by number of steps) is lower than expected. Those characteristics allow clinicians and researchers to perform this low-cost exercise test to screen larger populations using less space and in a shorter period of time compared with other commonly used exercise tests.
The inverse correlation found among the performance on 6MST and age was expected, since the aging process causes muscle loss23 and lower oxygen consumption, which leads to a diminished functional capacity.16,24 This correlation was also found in other studies, which analyzed the correlation between age and exercise capacity, assessed by different exercise tests.16,17,25 A proportional correlation with body fat percentage and the number of steps on 6MST was found, since women present both more body fat and lower exercise capacity.26
These characteristics might allow 1 to consider the 6MST to assess the loss of exercise capacity due to aging; as well as to verify the difference in exercise capacity among men and women, since those groups had statistically distinct exercise capacity assessed by this test. Although 6MST showed correlations with other demographic and anthropometric variables, correlation results were weak.
The strong correlation found between 6MWT and 6MST demonstrated that the 6MST performance may be valid in assessing exercise capacity in the studied population. Their similar characteristics may explain this correlation, such as being a self-paced test and both being related to an activity of daily living.28 The correlation might not have been higher because the 6MST presents work against gravity, which does not happen during the 6MWT. Other studies verified the correlation between the 6MWT and 6MST, and in both studies, the correlations were very similar (r = 0.7038 and 0.7550P < 0.05) to the results of this study; however, they were performed in patients with chronic obstructive pulmonary disease (COPD). In addition, in patients with interstitial lung disease, the validity of the 6MST was assessed comparing it with the graded exercise test, and the authors found a moderate correlation (r = 0.52; P < 0.05) between the number of steps and the oxygen consumption.25
The number of steps in the 6MST showed excellent test–retest reliability assessed by the ICC, and there was no significant difference between the means on the first and second test, which may imply it is a reliable test. However, error values were higher (mean bias 3 steps with 95% CI of 20 steps) than the ones found by Dal Corso et al in patients with interstitial lung disease (mean bias 1 step with 95% CI of 1.5 steps),35 and by a previous study from our group conducted with patients with COPD (mean bias 5 steps with 95% CI of 13 steps).34 The higher error in this study may be explained by a high SD, caused by a wide range of participants' age, which reflects in a large variability in 6MST performance. The 6MST may have lower error values in specific groups, as seen previously in other studies in patients with chronic diseases.34,35,51 Moreover, since the test–retest was conducted in the same day, future studies should verify if this test would be applicable and reliable to verify change over time and due to an intervention.
Heart rate, dyspnea, and systolic blood pressure measures demonstrated excellent reliability in the test–retest analysis of 6MST. Moreover, lower limb fatigue has almost reached the same criteria. Although ICC values for SpO2 and diastolic blood pressure were under the excellence reliability criteria (ICC <0.75), error rates for these variables were low. This may have happened due to an expected low variability during exercise in healthy participants, which usually drops down the correlation coefficients and are better assessed by other reliability analysis, such as error rates.
The equation to predict reference values for the performance in the first test included age and gender; In addition, the model to predict the performance in the second test or in the choice of the highest number of steps between the first and second test also included abdominal circumference. The correlation between both age and gender with exercise capacity has already been explained, and they also have been included in prediction equations of 6MWT performance.25–28 Although the inclusion of abdominal circumference in the equation instead of weight or BMI was not expected, the insertion of that variable might be due to the inverse relationship of abdominal adiposity and functional capacity.52
When screening a community for low exercise capacity, clinicians should consider the error values of the test while using the reference values obtained with these equations; therefore, they should investigate patients with a performance of 30 steps lower than the predicted number, since they probably have some condition that limits their exercise capacity. This study had some limitations worthy of being mentioned. First, a prospective validation of the reference values was not performed. Second, the validity analysis was not done comparing 6MST with the gold standard graded exercise test; nevertheless, the comparison with 6MWT may also bring important information, since it seems to be a better test to assess overall mobility and physical functioning in older people. Third, the wide age range increased the error rates, which might imply in limiting the age range in future studies with this exercise test. The lack of blindness of participant and assessors is a limitation as well; however, the use of a standardized protocol, with the same instructions and incentive phrases, minimizes this limitation.
This study brings an important contribution, since health professionals, especially those working in primary care, can use the reference values to find individuals with poor exercise capacity, that might imply in the presence of some condition or risk factor affecting their health. Although the safety of the test was not an objective of the study, there were no participants who suffered an adverse event.
The 6MST might also be suitable for being used in a domiciliary environment, since the step is portable and the test is self-paced. Moreover, 6MST was designed to decrease the risk of an adverse event, since the test is interrupted temporally whenever any physiological variables indicate higher stress. However, future studies need to verify 6MST's safety and applicability in this environment.
Six-minute step test is a reliable and valid test. Moreover, the number of steps may be predicted by demographic and anthropometric variables with moderate strength of prediction.
The authors are grateful for all the volunteers who kindly accepted to participate in this study. In addition, they are thankful for all the help with the article provided by Joyce N. F. da Costa, Simone F. Davi, and Dr. Nancy Preston.
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