The six-minute walk test in healthy children: reliability and validity

The aim of this study was to assess the reliability and validity of the 6-min walk test (6MWT) in healthy children. Chinese secondary school students were randomly recruited. They attended the current authors' unit on two occasions, separated by 2 weeks. Physical examination and standardised maximum incremental exercise testing on a treadmill were performed on the first visit. Spirometry and 6MWT were carried out on the second visit. A randomly selected subgroup was invited to return for repeat 6MWT at an interval of 2–4 weeks. Seventy-eight subjects were recruited; however, four failed to achieve maximal effort on exercise test. The final group included 43 young females and the mean±sd age of the subjects was 14.2±1.2 yrs. Physical examination was unremarkable in all cases. The mean±sd per cent predicted forced expiratory volume in one second was 91.4±10.2%. Concurrent validity was demonstrated by good correlation between the 6-min walking distance and maximum oxygen uptake determined on the exercise treadmill. Test–retest reliability was undertaken in 52 subjects, and the intraclass correlation coefficient (95% confidence interval) was calculated as 0.94 (0.89–0.96). In addition, Bland and Altman plots demonstrated a high degree of repeatability. In healthy children, the 6-min walk test is a reliable and valid functional test for assessing exercise tolerance and endurance.

A n individual's response to exercise is an important clinical assessment tool, since it provides a composite assessment of their respiratory, cardiac and metabolic systems. The current gold standard for assessing a person's aerobic exercise response is the maximum incremental cardiopulmonary exercise test. However, most daily activities are performed at submaximal levels of exertion and, therefore, it has been proposed that submaximal functional tests are a better reflection of physical capability [1]. The ability to walk a set distance is a quick, easy and inexpensive way to assess physical function. It is also an important component of quality of life, since it reflects the capacity to undertake day-today activities. BALKE [2] developed a simple test for examining functional capacity: measuring the distance walked during a defined period of time. A 12-min performance test was then developed to evaluate the physical fitness of healthy individuals [3]. This test was subsequently modified for use in patients with chronic bronchitis [4]. In order to allow the test to be used in patients with respiratory diseases, for whom walking 12 min was too demanding, a shortened version, the 6-min walk, was developed, and found to perform equally as well [5]. A recent review of functional walking tests concluded that the 6-min walk test (6MWT) is easier to carry out, more acceptable and provides a better reflection of activities of daily living than other walk tests [1].
The 6MWT has been frequently used to measure outcomes before and after treatment in patients with moderate-to-severe heart and lung disease. It has also been used to measure functional status and for epidemiological research purposes. The distance covered in 6 min (6MWD) has been shown to accurately predict morbidity and mortality from cardiopulmonary diseases [6]. Previous studies involving children using the 6MWT have been performed in groups of diseased patients with physical dysfunction, but have not included healthy controls [7][8][9]. Therefore, the aim of this study was to establish the reliability and validity of the 6MWT performed in a cohort of healthy Chinese children.

Study population
The study was carried out over the summer (July-September) in 2004. The subjects were healthy students aged 12-16 yrs, who were randomly selected from local secondary schools. Each child underwent a thorough physical examination. Their weight and standing height were measured with a calibrated weighing scale and stadiometer using standard anthropometric methods [10]. Any subjects with underlying cardiopulmonary disease or conditions that could interfere with the walk test, or subjects who were taking any medications were excluded. All subjects underwent a standardised maximum incremental treadmill exercise test at their first visit and returned within 2 weeks for the 6MWT. Spirometry was performed during the second visit before the subject underwent the walk test. A randomly selected group was asked to return 2-4 weeks after the initial 6MWT to have a supplementary walk test in order to assess its repeatability. Approval was obtained for the study from the Ethics Committee of the Chinese University of Hong Kong (Shatin, Hong Kong), and informed consent was obtained from parents and the subjects.
Treadmill exercise test Aerobic capacity was assessed by running on an electromechanical treadmill (GE Marquette Medical Systems with centre ETT system; Advanced BioSystems, Hollywood, FL, USA). The treadmill was programmed according to the Bruce protocol. Breath-by-breath gas samples were collected via a comfortably fitted facemask (paediatric large size, 8950 series; Hans Rudolph, Kansas City, MS, USA) and analysed throughout the test by open-circuit calorimetry using the Medgraphics System CPX/DTM metabolic cart (Medical Graphics Corporation, St Paul, MN, USA). After a steady resting period of 1 min, the subject was asked to exercise on the treadmill at a maintained speed. Care was taken to ensure complete safety and a physician was present during the whole procedure. Heart rate was monitored continuously during the test. Peak oxygen consumption (V9O 2 ,max), expressed as L?min -1 , was determined when two of the following three conditions were reached: 1) respiratory exchange ratio .1.0; 2) heart rate .85% of age-predicted maximum; or 3) the child was exhausted and refused to carry on despite strong verbal encouragement.
Pulmonary assessment Spirometry (Spirolab II; Medical International Research, Rome, Italy), using standard techniques, was used to measure forced expiratory volume in one second (FEV1) and forced vital capacity. The best of three efforts was compared with local ageand sex-matched reference values [11]. This measurement was taken before the 6MWT.

Six-minute walk test
The 6MWT was conducted according to standardised protocol [12]. A light meal was acceptable before the test, but the subject was told to avoid vigorous exercise within 2 h of beginning the test. No ''warm-up'' period before the test was allowed and the subject sat at rest in a chair, located near the starting position, for at least 10 min before the test started. During this time, pulse, oxygen saturation and blood pressure were measured. The subject was then instructed to walk up and down a measured corridor, covering as much ground as possible over a 6-min period. The test was self-paced and the patient was allowed to rest if desired, although the clock continued to run. The wording of encouragement during the testing was standardised (''keep going'', ''you are doing fine'', ''everything is going well'') and given by the same person at set times during the test. The 6MWD was also recorded. Pulse and oxygen saturation were recorded at the end of the test. The 6MWT was performed by the same investigator who was unaware of the subjects' performance on the treadmill test.

Statistical analysis
Results are expressed as mean¡SD, unless otherwise indicated. Concurrent validity was assessed by performing Pearson's correlation on the association between 6MWD and V9O 2 ,max obtained from the treadmill exercise test. Inter-observer reliability of the 6MWT was analysed using Bland-Altman plots and by determination of the intraclass correlation coefficient. The level of significance was set at 5%.

RESULTS
A total of 78 subjects were recruited; however, four failed to achieve maximal effort on the treadmill exercise test and were, therefore, excluded from the final analysis. The mean¡SD age of the subjects was 14.2¡1.2 yrs, and included 43 young females and 31 young males. None of the subjects was smokers and physical examination was unremarkable in all cases. Demographic data, lung function and characteristics of the 6MWT and treadmill test are shown in table 1.

6-MIN WALK TEST
A.M. LI ET AL.

DISCUSSION
In this study, it has been shown that the 6MWT in healthy children is reliable. There was no significant difference in the 6MWD reached at two time points separated by a mean of 18 days and the intraclass correlation coefficient was excellent.
In addition, it was demonstrated that the test is valid, as a significant correlation was established between the 6MWD and V9O 2 ,max obtained during the treadmill exercise stress test.
The assessment of an individual's response to exercise is an important clinical tool, as it provides a global examination of the respiratory, cardiac and metabolic systems. The current gold standard for assessing the aerobic exercise response is the maximum incremental cardiopulmonary exercise test. This allows objective determination of the appropriate intensity needed to perform prolonged exercise, the quantification of factors limiting exercise and a definition of the underlying pathophysiological mechanisms [12]. However, as most daily activities are performed at submaximal levels of exertion, submaximal functional tests are more appropriate and allow a better reflection of physical capability. The 6MWT, which is easy to perform and cost-effective, has been proposed as the best indicator of functional capacity among all submaximal exercise tests [1]. Previous studies involving children using the 6MWT were performed in groups of diseased patients and did not include healthy controls [7][8][9]. GULMANS et al. [7] studied 23 children with cystic fibrosis (CF) and found the 6MWT to be valid and useful in assessing the exercise tolerance and endurance of children with mild-to-moderate symptoms of CF. They also found significant negative correlation with pulmonary hyperinflation. NIXON et al. [8] recruited 17 patients and performed 6MWT and the incremental exercise test as part of their pre-heart, lung or combined heart-lung transplantation work-up. They found the 6MWT to be a better indicator of functional capacity in severely ill children. This current study is the first to assess the reliability and validity of the 6MWT involving normal healthy children.
A significant correlation was found between the 6MWD and FEV1. This correlation has also been demonstrated in other studies involving patients with chronic pulmonary disease and, specifically, CF [7][8][9]. Height is another important determinant of the 6MWD. This is not surprising, as taller people have, in theory, a larger stride length and, thus, a greater 6MWD. Young males are also found to have greater exercise capacity and 6MWD than young females, probably as a result of their greater muscle mass.
There are certain limitations to the present study. First, only 74 subjects were recruited. This sample size was inadequate for the establishment of reference equations for prediction of the total distance walked over 6 min for healthy children. Nevertheless, that was not the main objective of the study, as the aim was to demonstrated reliability and validity of 6MWT in healthy children. Secondly, this study involved only Chinese children aged 12-16 yrs. Similar assessment of the 6MWT would need to be repeated in other age groups and populations to ascertain its reliability and validity, as age, sex and height have all been shown to affect the 6MWD [12].