Functional Walk Tests in Individuals With Stroke
Relation to Perceived Exertion and Myocardial Exertion
Background and Purpose— Functional walk tests such as the 6- and 12-Minute Walk Test (ie, 6MWT and 12MWT, respectively) are submaximal measures used to determine functional capacity in individuals with compromised ability. The purpose of this study was to determine the relationship between these walk tests and measures of exertion (perceived and myocardial), in addition to impairment in individuals with stroke. The relationship among the 6MWT, 12MWT, and the more traditionally assessed measure of self-paced gait speed (generally assessed over a short distance, eg, 10 m) was also evaluated.
Methods— Twenty-five community-dwelling individuals with stroke were evaluated for the following: 12MWT distance, 6MWT distance, self-paced gait speed over 8 m, plantarflexion strength, Berg Balance Scale, Ashworth Scale of Spasticity, and Chedoke-McMaster Stroke Assessment. Heart rate (HR), rate-pressure product (RPP), and perceived exertion were assessed during the functional walk tests. Correlational analysis quantified the relationship between gait, impairment measures, and physiological responses during the functional walk tests.
Results— HR reached a steady state after 6 minutes and reflected a moderate exercise intensity of 63% of age-predicted maximum HR. The 6MWT, 12MWT, and self-paced gait speed were all highly correlated with one another (r>0.90) and were all also related to the severity of impairments. The functional walk distances did not relate either to perceived exertion or actual exertion (increase in the myocardial oxygen demand as measured by RPP).
Conclusions— Stroke-specific impairments are the major limitations to the distance walked in individuals with stroke. If the functional walk test is used to assess performance of an individual over time (eg, in response to an intervention), we recommend that both exertion (eg, increase in RPP or HR) and distance be measured.
Functional walk tests such as the 6- and 12-Minute Walk Test (ie, 6MWT and 12MWT, respectively) are submaximal measures that are often used to determine functional capacity in individuals with compromised ability. Functional capacity has been defined as the extent to which a person can increase exercise intensities and maintain those increased levels.1 Thus, functional walk tests differ from the standard self-paced gait speed evaluation (eg, over 8 m) in their requirement for sustained walking activity over extended periods of time.
There has been a recent trend toward more intensive exercise programs (eg, treadmill walking or circuit training) in acute and chronic stroke populations,2–6⇓⇓⇓⇓ and it would be timely to reexamine some of the current outcome measures used to evaluate these programs. Although measures such as maximum oxygen consumption provide important measures of cardiovascular fitness, functional walk tests have also been used to evaluate intensive programs2,3⇓ because they require the individual to sustain a submaximal intensity at an intensity and duration that might better reflect activities of daily living in this population. In fact, Poulin et al7 assessed the effects of a 9-week endurance training program in older men and found that a 280% improvement in endurance time at a submaximal level was accompanied by only a 10% increase in maximum oxygen consumption. In addition, not all subjects may be able to tolerate maximal exercise testing; Peeters and Mets8 found that 22% of their elderly patients with chronic heart failure were unable to complete a standard treadmill test.
Functional walk tests were originally developed for cardiorespiratory and cardiovascular populations.9,10⇓ In these populations, it has been shown that walk distances of both the 6MWT and 12MWT were related to gait speed measured over short distances.11 However, Duncan et al12 pointed out that one of the problems with current stroke outcome measures is that these measures were not always developed specifically for stroke. For example, there are a number of stroke-specific impairments that could potentially alter the outcome of the functional walk tests. Individuals with stroke may be limited by cardiovascular performance; however, factors such as muscle weakness (from peripheral and central origin),13–16⇓⇓⇓ balance impairment,14,16⇓ and spasticity17 could potentially influence the distance walked. Given these differences, even commonly held assumptions regarding the use of functional walk tests (eg, the 6MWT has predictive value similar to that of the 12MWT) should be verified for individuals with stroke.
The purpose of the present study was to determine the relationship among the 6MWT, 12MWT, and the more traditionally assessed measure of self-paced gait speed (assessed over a short distance, eg, 8 m). In addition, the relationship between these walk tests and measures of impairment and exertion were assessed. The following questions were addressed for individuals with stroke: (1) How do functional walk tests relate to perceived exertion as measured by the Borg Scale18 and myocardial exertion as indicated by the rate-pressure product (RPP)? (2) How does the heart rate (HR) response to functional walk tests relate to maximum age-predicted exertion levels and to HR levels predicted by ratings of perceived exertion? (3) Do the functional walk tests relate to measures of physical impairment, including balance, muscle strength, and spasticity? (4) What is the relationship between self-paced gait speed, 6MWT, and 12MWT, and do functional walk tests provide additional information compared with the standard self-paced gait speed?
Twenty-five community-dwelling individuals with stroke and residual unilateral weakness were recruited on a volunteer basis. Inclusion criteria were as follows: (1) a history of cerebrovascular accident of at least 1 year after stroke, (2) independent ambulation with or without an assistive device, and (3) written permission to participate in the study from a primary care physician. Participants were excluded if they had any of the following: (1) comprehensive aphasia, (2) medical instability (ie, uncontrolled hypertension, arrhythmia, or unstable cardiovascular status), (3) significant musculoskeletal problems from other than stroke, or (4) a score of <24 on the Folstein Mini-Mental State Test.19 Participants provided their informed consent, and approval was obtained from the university and hospital ethics committees.
To minimize practice effects,20,21⇓ one practice trial of (1) 12MWT, (2) 6MWT, (3) self-paced gait speed over 8 m, (4) plantarflexion strength, and (5) Berg Balance Scale was undertaken for 3 separate practice days before the test sessions.
Test sessions were undertaken for 3 days for the 5 tasks: 12MWT, 6MWT, self-paced gait speed over 8 m, plantarflexion strength, and Berg Balance Scale. The 12MWT and 6MWT were performed on separate days with one half of the subjects performing the 12MWT first and the other half performing the 6MWT first. In addition, on 1 of the 3 days, the lower extremity component of the Chedoke-McMaster Stroke Assessment was measured to determine the presence and severity of lower limb physical impairments; this measure has been shown to be valid and reliable in individuals with stroke.22 As well, the Ashworth Scale for the leg and foot was used to measure one component of spasticity, ie, the resistance to passive movement.23
All walking tests were completed with subjects wearing their shoes and usual assistive devices (eg, cane, ankle-foot orthosis). Self-paced gait speed was calculated from the mean of 3 walking trials. The cumulative distance and time of consecutive strides (ie, from foot contact with one leg to the next foot contact with the same leg) were recorded by infrared-emitting diodes (Northern Digital) attached to the foot during the middle section (ie, an ≈4-m section representative of constant gait speed) of the 8-m walkway.
For the functional walk tests, subjects were instructed to “walk as far as possible around a 42-m rectangular path within the given time (6 or 12 minutes) and not to stop unless they needed to,”24 and the total distance was measured. HR (Polar Electro Inc) was recorded every 2 minutes during the functional walk tests, and blood pressure (BP) was recorded (A&D Engineering Inc) before and at the end of the functional walk tests. RPP, a measure of myocardial oxygen demand, was calculated as the product of HR and systolic BP. Subjects were asked how hard they perceived that they were working during the test on the 16-point Borg Rating of Perceived Exertion (RPE) scale17 (1) before starting the functional walk test, (2) during the last 10 seconds of the 6MWT, and (3) at the 6-minute point of the 12MWT and during the last 10 seconds of the 12MWT.
All gait variables (self-paced gait speed and functional walk distances) were normalized to leg length. We have previously evaluated the test-retest reliability (separate days) for 22 individuals with stroke and found the intraclass correlation to be 0.99 for the 12MWT distance and 0.95 for self-paced gait speed.
Plantarflexion strength was selected to measure peripheral strength because the plantarflexion push-off phase is the single largest mechanical power phase during normal gait.25 Three trials of isokinetic plantarflexion strength (average torque normalized to body mass) of the paretic and nonparetic ankle were assessed at 30°/s on a KinCom strength dynamometer (Chattanooga Inc Corp). This protocol has been reported previously and has been found to be reliable in individuals with stroke.21
Balance was assessed by using the Berg Balance Scale. This test, composed of 14 tasks of sitting and standing activities, has been shown to be a valid and reliable measure of balance in stroke.26,27⇓ Of the 25 subjects, 10 walked with a cane, and 9 subjects used an ankle-foot orthosis.
Descriptive statistics were performed for all variables measured. Pearson product moment correlations quantified the relationship between (1) gait (self-paced, 6MWT, and 12MWT) and impairment measures (Chedoke-McMaster lower extremity impairment score, Berg Balance Scale, plantarflexion strength, and spasticity) and (2) gait and exertion measures (RPE and RPP). In addition, Pearson product moment correlations among the 3 gait measures (6MWT, 12MWT, and gait speed) were undertaken. HR at the end of the functional walk tests was compared with the predicted HR based on perceived exertion (RPE multiplied by 10) by paired t test and Pearson product moment correlations. Statistical analyses were performed with SPSS 9.0 (SPSS Inc), with a significance level set at P<0.05 (2-tailed).
Subject characteristics, impairment measures and gait data, and physiological responses during walk tests are presented in Tables 1, 2, and 3⇓⇓, respectively. Subjects were currently taking prescribed medications to control for hypertension (ie, peripheral vasodilators, n=6; diuretics, n=6; and ACE inhibitors, n=15) and preexisting cardiac conditions (ie, β-blockers, n=3; antiarrhythmic agents, n=1). Seven participants were on antidepressants.
Distance for 6MWT and 12MWT
For both test distances (6MWT and 12MWT), subjects covered similar distances over each 2-minute segment, with only a 2% and 3% reduction in distance from the first to the last 2-minute segment for the 6MWT and 12MWT, respectively (Figure). Despite the knowledge of a longer exercise duration required for the 12MWT, subjects paced themselves identically, with a mean distance of 268 m in the first 6 minutes of both functional walk tests. The distances calculated by using the self-paced gait speed for the 6MWT and 12MWT (gait speed multiplied by 6 or 12 minutes) were 287.4 and 574.9 m, respectively. These values overestimated the actual distance covered for the 6MWT (by 7%) and for the 12MWT (by 8%). Given that the gait speed during the walk test was relatively constant, subjects paced themselves more slowly during the functional walk tests.
For both functional walk tests, a large increase in HR (36% and 30% for the 6MWT and 12MWT, respectively) occurred during the first 2 minutes. Then, for the next 4 minutes (from time 2 to 6 minutes), there was a small increase (<7%) in HR for both walk tests. During the last 6 minutes of the 12MWT, there was virtually a plateau in HR, with only a 1% increase (Figure).
HR climbed to a mean maximum of 100.4 bpm for the 6MWT and 101.5 bpm for the 12MWT. This HR reflects an exercise intensity of 63±9.0% (range 50% to 86%) and 64±10.8% (range 49% to 93%) of age-predicted maximal HR (220 bpm minus age) for the 6MWT and 12MWT, respectively.
Actual Exertion Versus Perceived Exertion
Actual exertion, assessed by using the RPP (ie, HR×SBP), which is an indication of the myocardial oxygen demand, increased 57% and 56% from the start to the end of the 6MWT and 12MWT, respectively. This increase of RPP (in addition to the increase in HR and increase in systolic BP) was not correlated with the functional walk distance (Table 4).
The mean perceived exertion at the 6-minute point for both tests were similar (RPE 11), whereas the RPE rose an additional 2 points over the last 6 minutes of the 12MWT. The RPE scale was developed such that the corresponding HR can be predicted by multiplying the RPE by 10.28 The predicted HR based on the RPE at the end of the functional walk test was significantly higher (P<0.05) than the actual HR (15% and 32% higher for the 6MWT and 12MWT, respectively). In addition to the discrepancy in magnitude between the predicted HR (based on RPE) and the actual HR, the correlation between these 2 measures was not significant for either functional walk test.
Correlations of Gait Measures With Subject Characteristics and Impairments
All walking measures (functional walk distances and self-paced speed) were correlated with balance function (r=0.78 to 0.80), Chedoke-McMaster impairment score (r=0.69 to 0.76), plantarflexion strength of the paretic side (r=0.42 to 0.54), and spasticity (−0.42 to −0.53) (Table 4). Furthermore, all walking measures were highly correlated with one another (Table 4), with a 0.97 correlation between the 6MWT and 12MWT. The functional walk distances could be predicted by the following equations: (1) 6MWT distance (m)=320.9×(gait speed in m/s)+11.7; (2) 12MWT distance (m)=655.4×(gait speed in m/s)+7.2; and (3) 12MWT distance (m)=2.02×(6MWT distance in meters)−10.6.
Functional walk distances were very low and were ≈60% of the values reported for chronic respiratory subjects10 and only 42% to 50%29–31⇓⇓ of those reported for older adults. The very slow pace of these subjects serves as a caution when attempting to understand the mechanisms underlying their functional walk performance compared with established healthy normative values or with results from other pathological groups.
Physiological Workload During the Walk Tests
All but 1 subject in the present study remained within the 85% of age-predicted maximum HR that is recommended as the upper limit for submaximal exercise testing.32 The individual who reached levels closest to his maximum HR was one of the most impaired subjects and had the lowest balance score, greatest spasticity, and measures reflecting the slowest gait speeds.
Endurance has been defined as the time limit of a person’s ability to sustain a particular level of physical effort,1,33⇓ and a reduction in speed over the test would have indicated that endurance was challenged during this test. The intensity of the exercise can be classified as moderate according to the HR32 (mean 63% of age-predicted maximum HR), and subjects sustained their levels of physical effort with only a 2% to 3% decrease in gait speed over the test with a steady state in HR attained at 6 minutes. Given the minimal reduction in speed over the test, it is not possible to determine to what extent that endurance was challenged. However, Dean et al34 suggested that individuals with stroke slowed down during the 6MWT because they found that the self-paced gait speed multiplied by a 6-minute time interval overestimated the actual 6MWT distance covered. In contrast, our cohort did not slow down but, in fact, paced themselves at a slower gait speed, which was maintained throughout the functional walk test. Although some investigators have used the 6MWT to provide an outcome measure of endurance in individuals with stroke,2,3⇓ we believe that these walk tests better represent a measure of functional capacity.
The nonsignificant correlation between the RPP and distance walked is in contrast with the results from Fitts and Guthrie,35 who reported a strong positive correlation between the increase in HR and 6MWT distance in individuals with chronic renal failure (r=0.81). This discrepancy is likely due to the mechanical inefficiency of gait in persons with stroke, which could vary depending on the severity and the combination of impairments. For example, the metabolic cost of walking has been found to be related to the degree of spasticity.17 It is also possible that the subjects’ medications could affect the myocardial response to exercise; however, ACE inhibitors and β-blockers have been reported to have minimal effect on the change in HR and systolic BP36 during submaximal exercise.
Perceived Exertion and Relation to HR During Walk Tests
Ratings of perceived exertion are generally believed to be valid and reliable markers of physiological intensity during exercise37 and are recommended to monitor exercise intensity.32 A higher RPE has been reported to be related to a shorter 12MWT distance in individuals with respiratory disease (r=−0.59),10 suggesting that more severely impaired subjects perceive greater exertion. However, the lack of correlation between RPE and distance walked or between predicted HR based on RPE and actual HR suggests that further studies to identify factors influencing perceived exertion in the stroke population are needed. Given the lack of correlation, in addition to the overestimation of exercise HR based on RPE, individuals with stroke should not rely solely on RPE when trying to gauge exercise intensity in relation to HR.
A variety of physiological, pharmacological, psychological, and performance factors could potentially elevate the perception of physical exertion. Seventeen of 25 subjects were taking ACE inhibitors and β-blockers, which have been shown to increase perceived exertion during submaximal exercise, possibly because of their effect on contractile muscle function.36 We postulate that the increased RPE can also be attributed to the presence of stroke-specific impairments (eg, muscle weakness or spasticity), which may increase peripheral muscular discomfort and fatigue and be perceived as requiring more exertion. Bard17 found that the metabolic cost of self-paced walking was higher in persons with stroke than in healthy individuals and was related to the severity of impairment.
Interestingly, the major difference between the 6MWT and 12MWT was in the perceived exertion, despite a stable HR and constant velocity. This concurs with other studies which have shown that cumulative effect contributes to increases in perceived exertion.38
Which Walk Test Should Be Used?
The 0.97 correlation between the 6MWT and 12MWT distance, in addition to the finding that HR did not increase substantially after 6 minutes, suggests that the 6MWT can be used in place of the longer 12MWT for individuals with stroke (Equation 3). More controversially, one could suggest the simple use of self-paced gait speed to predict functional walk test performance, given that correlations were >0.90 between self-paced gait speed and the functional walk tests (Equation 2). Although we found that gait speed and 6MWT provide similar distance information, the functional walk test does have important value when HR and BP are monitored. The functional walk test can be described as a measure of functional capacity that evaluates the ability of an individual to maintain a moderate level of physical activity over a time period that may be reflective of the activities of daily living. Measures of RPP and RPE, together with the distance walked, may provide an indication of an individual’s physiological tolerance to submaximal activity. One of the limitations of the functional walk test is that subjects can vary either or both the distance and the exertion. If the functional walk test is used to assess performance of an individual over time (eg, in response to an intervention), we recommend that both exertion (eg, increase in RPP or HR) and distance be measured. A ratio of exertion and distance walked may be an ideal measurement, although the linearity and sensitivity of such a measure need to be explored in the future. Such measures are similar to the energy expenditure index39 and physiological cost index,40 which have been used in pediatric and polio populations, respectively, to calculate the change in HR per meter walked.
Performance in the functional walk tests depends on factors including motivation, respiratory function, cardiovascular fitness, neuromuscular function, and peripheral muscle strength. The high correlations between the functional walk tests and impairments suggest that stroke-specific impairments are major limitations to the distance walked during these tests. The contribution of cardiovascular fitness (eg, maximum oxygen consumption) to the functional walk test performance was not assessed, although others have reported no or low correlations with oxygen consumption during maximal exercise testing in cardiovascular and respiratory populations.4,9,23⇓⇓ The physiological demands of functional walk tests are distinct from those of cycle ergometer or treadmill tests and may be a better indicator of functional capacity required for normal daily activities.9,35⇓
Limitations of the Study
One limitation of the present study is that a multivariate analysis was not undertaken because of the small sample size, although this would be a useful future avenue of research.
This study was supported by the BC Health Research Foundation and a grant-in-aid from the Heart and Stroke Foundation of British Columbia and the Yukon.
- Received October 18, 2001.
- Revision received December 3, 2001.
- Accepted December 6, 2001.
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