Currently, more than 15 million Americans are living with cardiovascular disease and roughly 660 000 new coronary events are occurring every year in the United States.1 For these patients with cardiovascular disease, cardiac rehabilitation (CR) is a class 1-A recommendation from the American College of Cardiology and the American Heart Association.2 It is a crucial step to recovering after heart events or for those with chronic cardiovascular disease. Upon entry to and exit from CR, submaximal exercise testing is used to determine exercise capacity and used initially to prescribe exercise. Submaximal exercise tests are cost-effective and can be performed easily in almost any setting. The protocol used varies on the basis of facility equipment, space, staffing, and patient abilities.
Commonly used submaximal test protocols in CR utilize multiple modalities and various protocols that either utilize predetermined workloads or allow patients to set their own pace. Self-paced protocols allow each patient to choose the workload he or she feels appropriate that translates well into exercise prescription in a CR setting. Commonly used self-paced tests in CR include the 6-min walk test (6MWT) and the North Carolina 6-min cycle test, which have been validated against standardized treadmill testing.3,4 Current recommendations for the 6MWT instruct patients to walk as far as possible in 6 min, but this can sometimes prove difficult for patients with other comorbidities such as orthopedic issues, obesity, diabetic neuropathy, and peripheral artery disease.
Recumbent stepping machines have been widely used for exercise training in rehabilitation settings due to their ability to accommodate those with limitations. These machines easily accommodate patients with strokes, heart disease, and various musculoskeletal issues. Prediction equations for oxygen uptake have previously been established for the recumbent stepper to provide health care and exercise professionals specific feedback for exercise testing and training.5 Further validation of the recumbent stepper utilized the YMCA protocol, adapted for the recumbent stepper, and demonstrated that oxygen consumption can be accurately predicted during submaximal exercise.6 Upon entry to CR, patients can be extremely deconditioned and generally have low exercise tolerance, with average aerobic capacities in men 5.5 metabolic equivalents of task (METs), and in women, 4.1 METs,7 making a recumbent stepper the ideal choice.
The aim of this study was to produce a self-paced, 6-min submaximal exercise test protocol that could be performed on a recumbent stepper. In addition, this study defines the response to exercise during the 6-min recumbent stepper test (6MRST) and provides initial values as reference standards for the 6MRST.
This study was approved by the institutional review board at the University of Kansas Health System. Patients who participated in CR at the University of Kansas Health System were given the opportunity to participate in the present study. Those who did not speak English as a primary language or who were unable to complete either a 6MWT or a 6MRST for any reason were not able to participate in the study. Furthermore, patients who voluntarily terminated their CR or were dismissed from the program due to lack of adherence or medical issues were excluded from the study. If a patient completed ≥12 of 36 prescribed sessions, he or she was asked to complete a post-CR test.
The 6MWT followed the most current ATS guidelines utilizing the test protocol and standardized encouragement every minute throughout the test.8 The 6MWT course was demarcated with 2 orange cones spaced 80 ft apart in a flat hallway, and distance walked was measured in feet.9,10 A NuStep T5XR (NuStep) was used as the modality for the 6MRST. Patients were familiarized with the recumbent stepper and allowed to practice for a few seconds, if necessary. Seat and arm lengths were adjusted to the point just before maximum extension to optimize performance and comfort. These settings were recorded and identical during pre- and post-CR testing. If patients were under upper-body restriction due to recent open-heart surgery, the use of arms was not allowed during pre- or post-CR testing. Patients were then instructed to attempt to cover as much distance as possible in 6 min. Furthermore, patients were informed that they could stop and take a break, if necessary. The timer was visible to patients throughout the test and resistance was set to level 3 for all patients. Average watts, total steps, and distance were recorded immediately following the test. Distance was recorded to the closest hundredth of a mile, as this is the only output from the NuStep, and then converted to feet. A continuous, single-lead electrocardiogram recorded resting and peak heart rates using a telemetry system (Scott Care). Blood pressures were measured manually using a handheld sphygmomanometer prior to and immediately after exercise. Peak heart rate was defined as the highest recorded heart rate during testing. A Borg 1 to 10 rating of perceived exertion scale was used to assess patient effort.11
All data were stored and managed within a secure online database. Statistical analyses were performed using the Statistical Package for the Social Sciences software (SPSS v.23.0; IBM). Data analysis included Pearson product-moment correlations to determine the relationship at both testing points between distances exercised, hemodynamic responses to exercise, and rating of perceived exertion for the 6MWT and the 6MRST. One-way analyses of variance were used to examine whether differences exist between genders relative to performance and overall response to testing.
A total of 70 patients (aged 32-89 yr, mean age = 63 ± 13 yr) participating in CR completed pre- and post-CR testing. The population was predominantly men (n = 55), and the most prevalent diagnoses were percutaneous coronary interventions, coronary artery bypass grafts, and valve repair surgical procedures (34%, 33%, and 21%, respectively). Other diagnoses for the present study included heart transplant, stable angina, heart failure, and myocardial infarction. The average number of CR sessions completed was 34, and 74% of patients completed all 36 visits. During pre-CR testing, the average distance achieved for the 6MWT was 1274 ft and 1599 ft for the 6MRST. The average distance achieved during post-CR testing was 1588 ft for the 6MWT and 2101 ft for the 6MRST. Significant correlations were found between the distance exercised in the 6MWT and the 6MRST during pre- and post-CR testing (r = 0.540 and r = 0.700, respectively; P < .001). For pre- and post-CR testing, the average distance achieved was 1431 ft for the 6MWT and 1850 ft for the 6MRST (Figure). Significant correlations were also found between heart rate response to exercise and rating of perceived exertion between both tests during pre- and post-CR testing, meaning effort was similar between testing modalities (Table).
On average, men achieved a greater distance than women on the 6MRST during pre- and post-CR testing (P < .05). Pre-CR testing distance for the stepper was 1662 ft for men and 1369 ft for women. Post-CR testing distance was 2159 ft for men and 1887 ft for women. On the 6MWT, men walked a nonsignificant further distance than women at pre-CR testing (1304 vs 1164 ft, respectively). However, no significant differences between genders were found relative to heart rate response or rating of perceived exertion (P > .05).
The 6MRST was found to produce a similar response to exercise as the 6MWT in a CR setting when testing 70 patients upon entry and exit from a CR program. Significant relationships in distance achieved, heart rate response, and rating of perceived exertion were found. This is the first study to examine such a test in CR and not only reported that both the 6MRST and the 6MWT required similar levels of effort but also established initial reference values for the 6MRST.
On average, heart rate increased 23 to 33 beats/min from baseline to peak during pre- and post-CR testing. These values are consistent with the recommended target heart rate of 20 to 30 beats/min above the resting heart rate with moderate-intensity exercise recommended by the American Association of Cardiovascular and Pulmonary Rehabilitation and the American College of Sports Medicine.12,13 Rating of perceived exertion was found to be similar between the 6MRST and the 6MWT at both testing time points. As heart rate and rating of perceived exertion are measured on a regular basis in CR, the similarities between the tests are extremely important when measuring a patient's effort. In addition, no patient found it necessary to take a break during the 6MRST and no adverse events occurred. The recumbent stepper test was well tolerated by the patients in this study.
Additional variables collected included average power output (watts) and steps recorded during the 6MRST, and the data also showed significant correlations to 6MWT distance (P < .001). During pre-CR testing, the average power output was 41 W (range, 9-183 W) with an average of 549 steps (range, 301-930) achieved. Throughout post-CR testing, the average power output was 90 W (range, 19-325 W), with an average of 721 steps (range, 442-1169) achieved. These values serve as an initial reference for the 6MRST since no other published studies have examined using the test in CR patients.
Further validation for the 6MRST should include comparison with the gold standard of symptom-limited exercise testing. In addition, the scope of the 6MRST could be expanded by establishing the relationship between performance and health status in clinical populations. The 6MRST protocol used the same level of resistance for all patients for standardization. Ideally, resistance could be individualized for each patient or adjusted on the basis of the speed of the patient to truly allow patients to pace themselves and determine their individual workload, as is used with the 6MWT or the North Carolina Bike Test. The 6MWT was performed on an 80-ft (24.4-m) track, yet recommendations for the 6MWT state the walkway should be at least 98 ft (30 m) in length. While the 6MWT has served as a comparison for other new tests as well,4 comparison of the 6MRST with standardized treadmill testing or the gold standard of symptom-limited exercise testing would provide further validation for this test. Men represented a much larger proportion of the patients than women in the present study (79% vs 21%); however, this is a common trend in CR, as other studies have reported that women typically compose 27% to 29% of CR patient populations.14,15 As distance served as the primary outcome measure in this study, this test would apply best to other recumbent steppers that provide distance. However, other variables such as steps or watts could be investigated to determine whether they provide similar relationships to performance with standardized tests.
The 6MRST appears to be a viable alternative to the 6MWT in a CR setting based on the strong relationship between distances achieved, heart rate response, and rating of perceived exertion. The 6MRST could be used in lieu of a 6MWT in patients with disabilities that inhibit walking, those who are unsteady, or considered at a fall risk or to provide a specific measure of functional capacity relative to an exercise prescription.
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Keywords:Copyright © 2019 Wolters Kluwer Health, Inc. All rights reserved.
cardiac rehabilitation; exercise testing; recumbent stepper; 6-min walk test