Aplicabilidade das equações de referência para o teste de caminhada de seis minutos em adultos e idosos saudáveis de um município do estado de São Paulo

El test de marcha de seis minutos (TM6) es considerado simple, seguro y de facil administracion, ademas de entregar resultados representativos sobre actividades habituales del dia a dia. Los objetivos del estudio fueron evaluar y comparar la distancia recorrida en el TM6 con las distancias previstas por ecuaciones disponibles en la literatura cientifica en adultos y adultos mayores saludables del municipio de Sao Carlos/SP, y verificar la aplicabilidad de esas ecuaciones en esta poblacion. Fueron evaluados 43 individuos (23 hombres), entre los 55 a los 78 anos, por medio de evaluacion fisica, espirometria y del TM6. Observamos que la distancia recorrida en el TM6 fue significativamente (test t-pareado: p<0,05) mayor que los valores previstos por las ecuaciones de Enright y Sherril, Masmoudi et al., Alameri, Al-Majed y Al-Howaikan y Dourado, Vidotto y Guerra, y los mismos fueron significativamente menores que los previstos por Troosters, Gosselink y Decramer, Gibbons et al., Enright et al., Camarri et al., Ben Saad et al. y Soares y Pereira. No fueron observadas diferencias significativas entre la distancia recorrida en el TM6 y los valores previstos por las ecuaciones de Chetta et al., y Iwama et al. La mayoria de las ecuaciones de referencias utilizadas en el presente estudio subestima o superestima los valores obtenidos en el TM6, excepto las propuestas por Chetta et al. y Iwama et al. que se observaron aceptables para la poblacion estudiada. Existe diferencia entre las distancias, incluso cuando el TM6 es realizado con una similar metodologia y estandarizacion rigurosa, ademas se hace hincapie en la necesidad de ecuaciones especificas para cada poblacion.

equations were proposed for Americans between 20 and 80 years old 19 and over 68 years of age 20 , for healthy Italians 21 , for Australians 22 , and for sedentary Tunisians who are between 40 and 80 years old 23 .In 2009, predictive equations were devised for Arabs 24 , Tunisians 25 , and Brazilians 26 .Finally, other equations, different from one another 29 , were proposed for Brazilians 27,28 in 2011.
Considering the number of equations predicted for the distance accomplished in the 6MWT available in scientific literature, the conduction of this study is justified; it aims at evaluating the applicability of these equations to apparently healthy adults and elderly people of the city of São Carlos (SP) and region, in addition to suggesting the most adequate ones to be used with patients in the clinical practice of this area.Therefore, this study aimed at: 1) evaluating and comparing the distance accomplished in the 6MWT by apparently healthy adults and elders of the city of Sao Carlos (SP) and region to the distances predicted by several equations available in scientific literature; 2) verifying the applicability of these equations to this population.

INTRODUCTION
The 6-minute walk test (6MWT) is regarded as one of the alternatives to maximal tests (cardiopulmonary test) because of its good reliability and reproducibility [1][2][3] , and for being a low-cost option of easy conduction.In addition to being well tolerated by patients [4][5][6][7][8] , it is also simple, safe, and it simulates a customary daily activity 9,10 , enabling the patient to determine his/her own speed and the need for pausing, which is an additional advantage to elderly people [4][5][6][7][8] .
Moreover, the 6MWT is considered a predictor of morbimortality 11 , and it presents great applicability in clinical practice, because it reflects the exercising capability of individuals with chronic obstructive pulmonary disease (COPD) 12 .It is also a method for evaluating the necessity of prescribing oxygen therapy, since it detects oxygen desaturation in patients with COPD 13 .
Recently, the 6MWT has not been regarded solely as a specific meter of cardiovascular capacity, but also as an indicator of global physical capacity in elderly people 14 , considering that it evaluates the responses provided by the cardiovascular, respiratory, and peripheral vascular systems 15 .
In this sense, some equations have been proposed with the purpose of predicting the distance accomplished in the 6MWT, but the predicted values are influenced by sex, height, age, and weight [16][17][18] , as well as by physiological and clinical factors 17 .Among the best known equations, we highlight those proposed by Enright and Sherril 16 for Americans who are over 40 years of age, and by Troosters, Gosselink, and Decramer 18  RESUMEN | El test de marcha de seis minutos (TM6) es considerado simple, seguro y de fácil administración, además de entregar resultados representativos sobre actividades habituales del día a día.Los objetivos del estudio fueron evaluar y comparar la distancia recorrida en el TM6 con las distancias previstas por ecuaciones disponibles en la literatura científica en adultos y adultos mayores saludables del municipio de Sao Carlos/SP, y verificar la aplicabilidad de esas ecuaciones en esta población.Fueron evaluados 43 individuos (23 hombres), entre los 55 a los 78 años, por medio de evaluación física, espirometría y del TM6.Observamos que la distancia recorrida en el TM6 fue significativamente (test t-pareado: p<0,05) mayor que los valores previstos por las ecuaciones de  Existe diferencia entre las distancias, incluso cuando el TM6 es realizado con una similar metodología y estandarización rigurosa, además se hace hincapié en la necesidad de ecuaciones específicas para cada población.

Palabras clave | valores de referencia; marcha; fisioterapia
The inclusion criteria were: presenting spirometric values within normal standards 30 , being healthy and over 55 years of age and considered insufficiently active or sedentary by the Physical Activity International Questionnaire (short version) 31 .Individuals who were smokers, alcoholic, had non controlled arterial hypertension and presented cardiovascular, metabolic, neurological, rheumatic and/or musculoskeletal diseases that prevented participation in the study were excluded.This study was approved by UFSCar's Ethics Research Committee (approval report number 074/2007), and all participants signed the free and informed consent form.
The individuals were submitted to a general physical assessment (anamnesis, weight, height, presence of diseases, and information on smoking, workout, medication, type of thorax and respiratory pattern, presence of cough and dyspnea, lung auscultation, blood pressure, cardiac and respiratory frequency, and peripheral oxygen saturation), in addition to spirometry and the 6MWT.Six-minute walk test: Performed according to ATS's rules 1 .The free cadence 6MWT was conducted twice on the same day, with 30-minute intervals, using the highest distance values for analysis.The individuals were instructed to walk as fast as possible during 6 minutes, and were encouraged each minute 1 .

Statistical analysis
The calculation of the sample size was performed by the software Ene version 2.0 (GlaxoSmithKline España S.A., Madrid, Spain, and Universidad Autónoma de Barcelona, Barcelona, Spain), based on the deviation standard (69.5 cm), and on the average of the distance variable, considering a minimum difference of 35 m 34 The results of this study were analyzed by the program Statistical Package for Social Sciences for Windows, version 18.0 (SPSS Inc., Chicago, Illinois, USA).We verified data distribution through Shapiro-Wilk's normality test.We used descriptive statistics for sample characterization, with data expressed in averages±standard deviations.For the analysis of the distance obtained in the 6MWT and the predicted distances we used paired t-test.A significance level of 5% was adopted.

RESULTS
Table 1 shows the anthropometric and spirometric characteristics of the population studied.
Table 2 shows the distance accomplished in the 6MWT and the distance predicted by several equations available in the literature on the sample studied.
We found that the distance accomplished in the 6MWT was significantly longer than those predicted by the equations of Enright and Sherrill 16 , Masmoudi et al. 23 , Alameri, Al-Majed and Al-Howaikan 24 , and Soares and Pereira 28 , and the values were significantly lower than those predicted by the equations of Troosters, Gosselink, Decramer 18 , Gibbons et al. 19 , Enright et al. 20 , Camarri et al. 22 , Ben Saad et al. 25 , and Dourado,  38 , reported that the predicted distance 16 tends to underestimate the one accomplished, considering that they did not find significant difference in patients with COPD, healthy women, and patients with miastenia gravis, respectively.We found that Trooster, Gosselink, and Decramer's equation 18 overestimated the distance accomplished.We believe this is due to a methodological difference, given that these authors 18 adopted intervals of 2.5 hours between the 6MWTs, and 30-minute intervals were set for this study.In agreement, Barata et al. 15 , found overestimated predicted values 18 for both sexes.It is worth highlighting that Enright and Sherrill 16 , and Troosters, Gosselink, and Decramer 18 did not abide by ATS 1 to conduct the 6MWT.
In this study, we verified that the equations of Gibbons et al. 19 , and Camarri et al. 22 , overestimated the distance accomplished in the 6MWT for individuals of both sexes.The cause may be multifactorial, among them test standardization and different ethnicities 26 .Gibbons et al. 19 , used a sample that ranged from 20 to 80 years, and ours varied from 55 to 78 years of age.Studies report that the shorter distance walked by elderly people 15 is due to a decrease in strength, muscle mass, and lung functioning 39 .
Even though Camarri et al. 22 , conducted the 6MWT using a methodology similar to the one adopted here, the predicted values overestimated the ones obtained, a finding that corroborates with Iwama et al. 26 This is attributed to Brazilians' multiracial profile, which elicited the expectation of values that were lower than those reached by Caucasians 15 .
The equations proposed by Masmoudi et al. 23 , and Alameri, Al-Majed and Al-Howaikan 24 underestimated the distance obtained in the 6MWT.This is due to a difference between the populations, since the capacity for exercising is relatively preserved in Brazilians 40 .Butland et al. 41 , inferred that this difference may be associated with lifestyles.In relation to Alameri, Al-Majed, and Al-Howaikan's equation 24 , the difference may be explained by the conduction of only one 6MWT.Studies suggest that in order to establish the longest distance accomplished in the 6MWT with certainty, the test should be performed three times 42 .
The values predicted by Chetta et al.'s equation 21 did not differ significantly from the values obtained in our conduction of the 6MWT, because both used similar methodology 1 , even though these authors 21 analyzed a younger sample than the one in this study.Contrary to our results, Iwama et al. 26 , found a slight overestimation 26 using this equation 21 .
Iwama et al. 26 , were the first to investigate predicted values and potential demographic and anthropometric factors that are determining for the 6MWT in Brazilians.Our results did not differ from the predicted Vidotto and Guerra 27 .However, we did not observe significant differences between the distance accomplished in the 6MWT and that predicted by the equations of Chetta et al. 21, and Iwama et al. 26 Therefore, the latter equations are the most applicable to Brazilians in São Carlos (SP) and region (Table 2).

DISCUSSION
The 6MWT is considered a great indicator of functional capacity in elderly people 15 .Studies show correlation between the distance accomplished in the 6MWT and age, sex, height, weight, and body mass index 17 .In a Brazilian study, only age and sex were found to be significant determinants of the distance accomplished in the 6MWT 26 .Geographical heterogeneity was revealed as another important factor in determining the distance obtained in the 6MWT.A study performed at ten centers of seven countries showed that the distance accomplished in the 6MWT by individuals who were over 40 years of age was considered a determined factor 35 , thus affirming the necessity of specific equations for each country 35 .
In this study, the values predicted by Enright and Sherrill's equation 16 underestimated the distance achieved in the 6MWT.Corroborating this, Moreira, Moraes and Tannus 36 , Soares et al. 37 , and Resqueti values, since the methodology used for the 6MWT and the populations presented similar characteristics.
The values predicted by Dourado, Vidotto and Guerra's equations 27 , in turn, overestimated those obtained in the 6MWT.We suggest that this is due to age and the number of individuals included in the sample, since the 6MWT methodology and the populations' nationalities are similar.On the other hand, Soares and Pereira's equations 28 underestimated the values obtained in the 6MWT.This is attributed to the number of individuals evaluated, race, and also the conduction of three 6MWTs.

CONCLUSION
Most of the 6MWT reference equations used in this study either underestimate or overestimate the values obtained in the 6MWT, which indicates their inadequacy to the population studied here.Moreover, we verified that the equations proposed by Chetta et al. 21, and Iwama et al. 26 , were applicable to the population analyzed.It is important to emphasize that there is need for other studies that evaluate the equations with a larger number of participants and in other regions of the country with the purpose of affirming the applicability of these equations to Brazilian people.

Table 1 .
Anthropometric and spirometric characteristics of the population studied Data expressed in averages (±standard deviation).BMI: body mass index; FEV 1 : expiratory volume forced in the first second; FVC: forced vital capa city; Relation FEV 1 /FVC: relation FEV 1 by FVC; MVV: maximal voluntary ventilation; pred: predicted

Table 2 .
Values of the walked distance obtained in the 6MWT, and values of predicted distance for the 6MWT according to several equations available in scientific literature Data expressed in average (±standard deviation).Paired t-test: *p<0.05 in the 6MWT≠reference equations.WD: distance; 6MWT: six-minute walk test et al.