The expected growth of the population of aging adults is associated with an increase in the incidence of chronic conditions that may affect functional capacity. Accurate and valid tests and measures are needed to correctly examine the effects of impaired mobility and document change in response to intervention. Such tests should accurately measure the limitations that older individuals experience including assessment of their mobility and function.1,2 Tests that measure maximum oxygen consumption (VO2max) are considered a gold standard for assessing aerobic capacity. However, the inability to achieve VO2max due to fatigue, neuromuscular or musculoskeletal impairments, and motivation may limit the utilization of maximal exercise testing.3 Ambulation is essential to maintaining independence and quality of life. Ambulation can be negatively affected by age-related changes in the cardiovascular, pulmonary, and musculoskeletal systems.4 The 6-minute walk test (6MWT) has been used by clinicians and researchers to measure changes in aerobic capacity, functional status, and disability.5–7 In addition, the 6MWT has been used as a marker for functional decline, to assess responses to therapeutic and pharmacologic interventions as well as to predict morbidity and mortality.2,5,7,8
Changes in heart rate (HR) represent a significant clinical measure of response to exercise. An individual's HR response will normally increase with the intensity of an exercise program and is frequently used to assess level of fitness of healthy individuals and patients.
The increase in HR during exercise is a product of increased sympathetic and decreased parasympathetic tone.9 Chronotropic incompetence, or the inability of HR to appropriately change during exercise, can be assessed by measurement of the peak HR, proportion of age-predicted maximal HR achieved (chronotropic index), or proportion of HR reserve (HRR) at peak exercise.9 The chronotropic index allows for a description of the normal response to exercise independent of age and resting HR.10 These measures have good prognostic value. During maximal exercise testing, normal chronotropic index values range from 0.8 to 1.3.10 Diller et al10 defined chronotropic incompetence as failure to achieve a chronotropic index of 0.8. In addition, Azarbal et al11 found that the proportion of HRR used atpeak exercise was strongly correlated with risk of cardiac death and all-cause mortality. The clinical utility of chronotropic incompetence has been emphasized by several studies in individuals who were asymptomatic and both at risk or have cardiac disease.9,10,12 Some studies found a high correlation between HRR and maximum oxygen consumption and VO2 reserve.9,11 Swain et al12 reported that a percent of the HRR can be considered a good predictor of the percent of maximum oxygen consumption reserve (VO2R) when prescribing treadmills (TM) exercise.
Walk tests are cost-effective methods that are used to determine the ability of a person to participate in demanding physical activities with reasonable technical expertise and equipment.13 The 6MWT is typically performed in a long and straight hallway that is free of obstacles and traffic. However, many clinics do not have the space to perform the 6MWT. Treadmills can be an efficient, and space-saving alternative to performing the standard 6MWT. The purpose of this study was to compare the results from a level ground (LG) 6MWT to a TM 6MWT in healthy adults. In addition, we examined differences between age groups in their ability to complete the 6MWT.
A convenience sample of 39 healthy individuals recruited from a local community met the inclusion criteria and agreed to participate. The inclusion criteria were (a) healthy men and women, (b) between the ages of 45 and 75 years, (c) who were able to ambulate independently, and (d) not currently participating in a vigorous exercise program. To investigate age-related changes between middle age through older age, participants were recruited to 1 of 3 age groups: 45–54, 55–64, and 65–74 years old. Individuals were excluded if they (a) reported major lower extremity musculoskeletal, neuromuscular, cardiopulmonary, or cognitive disorders; (b) used an assistive device for ambulation; or (c) reported using medications that influence HR response to exercise. One participant was excluded because of severe knee arthritis and another did not complete LG 6MWT test as instructed. Two additional participants were unable to walk on the TM without holding on to the rail.
The study procedure was approved by the institutional review board. Each participant performed the 6MWT on LG or TM over 2 consecutive testing days. The order of testing was randomly selected by flipping a coin. Participants completed each test condition twice on the same day. Only data from the second trial were used for the analysis.5 Heart rate was measured before and immediately after the test in a seated position using a pulse oximeter (GE TuffSat, Helsinki, Finland). In addition, blood pressure (BP) measures were recorded before and immediately after each trial. Heart rate reserve was calculated as the difference between age-predicted maximum and resting HR. To describe the level or work performed during each test, the ratio of HR measured after TM and LG trials to HRR (chronotropic index) was calculated using the following equation14,15:
Standardized instructions were provided before each TM and LG trial, and every 2 minutes as follows: “walk as far as possible in 6 minutes without running or jogging.” The LG 6MWT was performed using a 100-ft corridor. Participants were taught how to rate their levels of exertion using the Borg rating of perceived exertion (RPE) prior to testing. To monitor changes in levels of exertion during and immediately after each test, participants were asked to complete the Borg RPE scale.
Participants practiced walking on the TM at least once to familiarize themselves with the TM protocol, emergency pull cord and learn how to signal for a change in TM speed. During practice, participants were also asked to self-select a TM start speed they could maintain for 6 minutes. Practice trials continued until each participant felt comfortable with the protocol. Participants began each trial at 50% of their self- selected start speed, which was increased quickly to 100%. They were encouraged to signal whether to increase speed, decrease speed, or stop the TM as needed. All TM trials were completed at 0% incline on a Landice 8700 (Randolph, NJ).
The TM 6MWT distance walked was recorded using a measuring wheel held against the belt. The wheel was used to record distance in feet rather than 1/10 mile increments displayed on the TM. Also the measuring wheel could be quickly removed from the belt if a participant needed to stop the TM during the test. A researcher guarded participants and adjusted speed per participants' request. To ensure consistency with LG trials, all TM controls were covered. A seated rest was provided between trials for at least 10 minutes or when participants indicated that they were ready for the second trial and HR and BP recovered to resting values. Participants were informed that they could stop the test at any time; however, the 6-minutes would continue to elapse.
Frequency distribution, descriptive statistics, bivariate scatter plots, skewness, and kurtosis were used to screen data and to test for parametric and multivariate assumptions. Data were analyzed using mixed-model multiple analysis of variance (MANOVA) with 2 factors (group and test condition). The grouping variable included 3 levels (age groups) and the test condition included 2 levels (TM and LG). The dependent variables included Chronotropic Index (change in HR as a proportion of HRR) and distance walked on TM and LG.
A total of 35 participants (24 women and 11 men) between 48 and 75 years old were included in this study (Tables 1 and 2). More women participated in the first and second age groups (80% and 72%, respectively) and more men participated in the third group (56%). The average BMI of all groups was found to be overweight (26.1–28.1 kg/m2). Normal resting HR of all subjects was 73/min (95% CI: 69–78 bpm), and BP 131/79 mm Hg (95% CI: 125–136/75–81 mm Hg).
There was no significant main effect of test condition (P = .06) or group (P = .25) and no interaction between test conditions and group (P = .78) using Wilks' lambda multivariate test. There was no difference between 6MWT on TM and LG in the distance walked (P = .07) or HRR (P = .39). Similarly, during the 6MWT there was no difference among the 3 age groups in HRR (P = .98, .62, .73) or distance walked (P = .64, .14, .57). Table 3 summarizes descriptive statistics for group and test conditions.
On average, the 6MWT resulted in a moderate increase in HR to 113 bpm (95% CI: 106–121/min) and systolic BP to 146 mm Hg (95% CI: 140–152 mm Hg) and only minimal change in diastolic BP to 82 mm Hg (95% CI: 79–85 mm Hg). No participants in this group of healthy adults required a break during the 6MWT and they completed the 6MWT with a Borg RPE of 13.1 (2.1) and 14.1 (1.6) on LG and TM, respectively. There was a statistically significant difference between the Borg RPE on TM and LG (P = .001).
In this healthy group of adults aged 48 to 75 years, the 6MWT led to chronotropic indices (% HRR) of 45.8 (95% CI: 34.1–46.9) on TM and 42.7 (95% CI: 35.5–48.8) on LG. To examine relationships between performance on LG and TM, Pearson product-moment correlation coefficients were calculated. There were significant and high correlation between LG and TM distance (r = 0.85, P < .001),%HRR (r = 0.7, P < .001) and moderate correlation with systolic BP measurements (r = 0.59, P < .001).
The 6MWT elicited a modest increase in HR values among healthy middle-aged and older adults. This response to exercise correlated highly with the work performed.2 We found that the 6MWT can be performed on LG or on the TM quickly and safely with no adverse effects, similar to findings by Enright et al.16 The 6MWT is easier to complete than maximal TM exercise tests for older adults and could be used in specific instances as an indicator of exercise capacity.1,17
Our results indicated that distance walked and HR responses during the 6MWT on the TM and LG were not different. Since our sample consisted of healthy individuals younger than 75 years, the distance they walked on TM and on LG was similar to or slightly farther than reported in other studies (Table 4).2,8,18 In addition, using a 100-ft hallway reduced the number of turns participants needed to make and possibly increased in the distance they were able to complete in 6 minutes. Contrary to our results, only 1 study reported a significant difference between LG and TM 6MWT.19 Camargo et al7 report that the TM 6MWT distance was less than LG 6MWT distance. The authors attributed the difference between LG and TM 6MWT to subject unfamiliarity with TM walking, coordination of their extremities according to a speed not established by them and a limited maximum speed. Comparison of 6MWT results to prior studies carries a risk of misinterpretation due to the lack of standardization of the 6MWT protocols,3 especially the use of encouragement during the test or number of trials performed.2,5,7,8,19,21 Some investigators used the first, second, or longest 6MWT distance.5,7 In addition, there was no standardization for the length of the hallway that may influence 6MWT distance recorded with shorter distances as it will require more turns.2,20
Achievement of similar 6MWT distances for healthy older adults and those who were engaged in stage II cardiac rehabilitation on LG were reported by some investigators17 (Table 4). Lower 6MWT distance was reported in the older adults,22 individuals with or without chronic conditions, and patients with chronic obstructive pulmonary disease.2,16,19 Although the distance walked in our study was not statistically different among the 3 age groups, the difference between the distance walked by the youngest and oldest participants in our study demonstrated a clinically relevant difference with moderate to high effect sizes.23 Similar conclusions have been drawn by other authors who reported a trend toward age-related decline in performance on the 6MWT and 400-m walk test.8,22,24 Therefore, it is recommended to use age-related data when interpreting older adults' test results.
This study indicates that the 6MWT is a submaximal exercise test that requires a moderate increase in HR in healthy middle-aged and older adults. 6MWT can be used by clinicians when evaluating exercise capacity at a level comparable to efforts performed during the activities of daily living.25 Turner et al2 examined submaximal and maximal physiologic responses during exercise testing in 20 patients with moderate-to-severe chronic obstructive pulmonary conditions. They reported no significant differences between the 6MWT and cycle ergometer maximal exercise test performed for peak HR. They reported 6MWT peak HR, which was consistent with our findings.
Data from this study also showed that the use of TM is a valid alternative to LG 6MWT with moderate to a high correlation with HR responses and distance walked. Peeters and Mets17 reported a moderate correlation (r = 0.69) between the 6MWT distance and the symptom-limited maximum TM walking exercise test distance, among a group of older adult subjects with chronic heart failure. Previous studies established the LG and TM 6MWT as valid8 and reliable1,5,19 tests in several asymptomatic and patient populations. Some studies found good test-retest reliability in patients with cardiovascular disease, with ICCs from 0.94 to 0.96.26,27 Other studies have shown construct validity between distance walked in 6 minutes and peak oxygen consumption in patients with heart failure26,28 and chronic obstructive pulmonary disease.2 Camargo et al7 reported a significant but low to moderate correlations between the TM 6MWT and hemodynamic values, most notably cardiac output in subjects with pulmonary arterial hypertension.7
Studies showed that the distance walked on LG during a 6MWT was correlated (r = 0.73 —0.93) to the maximum power (watt/kg) achieved during the graded cycle ergometer test,2,18 lower body strength using a chair stand test (r = 0.67), tandem stance (r = 0.52), and gait speed (r = — 0.73).1 TM 6MWT predicted Vo2max using a graded cycle ergometer test in healthy young adults.5 LG 6MWT distance was found to be a good predictor of peak oxygen uptake in patients with advanced heart failure26 and those with chronic obstructive pulmonary disease.2 Moreover, the distance walked in 6 minutes was a significant predictor of an increased likelihood of death (walked > 300 m) among subjects with left ventricular dysfunction29 and increased survival in subjects with pulmonary arterial hypertension who covered greater than 377 m.7
Although a TM 6MWT is easy to perform and allows ongoing monitoring of vital signs, 2 individuals could not walk on the TM without holding the rails and were subsequently excluded from the results. Four participants indicated that they had to work harder on the TM than on LG. Only 5 (14%) participants requested to decrease the TM speed during trials. Conversely, on LG the pace is easier to adjust without the use of controls when walking. Analysis of Borg RPE scores revealed that the TM 6MWT elicited significantly higher perceived exertion to perform than LG 6MWT. The difference of 1 point on Borg RPE scale, however, is clinically insignificant. Borg RPE indicated that participants were working at somewhat hard to hard levels. Peeters and Mets17 also found that 22% of older adult subjects with congestive heart failure could not complete a modified maximal TM exercise testing. They cited TM speed and fear of falling influenced patients' decision to discontinue.
Limitations to this study include the use of convenience sampling, small and unequal group sizes, and unbalanced distribution of male and female participants. Such limitations may have influenced the results of the study. Further investigation of the utility of TM 6MWT is needed to determine the validity of TM 6MWT in a variety of conditions to further reveal its diagnostic validity.
The TM 6MWT can be used as a submaximal exercise test and a safe alternative to LG 6MWT in community-living middle-aged and older adults. Both LG and TM 6MWT generate moderate levels of cardiopulmonary stress with a low risk of cardiovascular events or extreme fatigue and fear of overexertion.
The authors thank David Schilling, DPT, and Jim Smith, DPT, for their assistance in editorial support and manuscript preparation.
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Keywords:Copyright © 2012 the Section on Geriatrics of the American Physical Therapy Association
elderly; heart rate reserve; 6-minute walk test; treadmill