Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 9 , Pages 1782-1787, September 2008

Estimating Maximum Work Rate During Incremental Cycle Ergometry Testing From Six-Minute Walk Distance in Patients With Chronic Obstructive Pulmonary Disease

  • Kylie Hill, PhD

      Affiliations

    • Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • Department of Physiotherapy, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • School of Physiotherapy, Curtin University of Technology, Bentley, Western Australia, Australia
    • Corresponding Author InformationReprint requests to Kylie Hill, PhD, Respiratory Medicine, West Park Healthcare Centre, 82 Buttonwood Ave, Toronto, ON M6M 2J5, Canada
    • ,
  • Sue C. Jenkins, PhD

      Affiliations

    • Department of Physiotherapy, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • School of Physiotherapy, Curtin University of Technology, Bentley, Western Australia, Australia
    • Lung Institute of Western Australia, Western Australia, Australia
    • ,
  • Nola Cecins, MSc

      Affiliations

    • Department of Physiotherapy, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • School of Physiotherapy, Curtin University of Technology, Bentley, Western Australia, Australia
    • Lung Institute of Western Australia, Western Australia, Australia
    • ,
  • Danielle L. Philippe, BSc

      Affiliations

    • Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • ,
  • David R. Hillman, MD

      Affiliations

    • Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • ,
  • Peter R. Eastwood, PhD

      Affiliations

    • Department of Pulmonary Physiology, Sir Charles Gairdner Hospital, Nedlands, Western Australia, Australia
    • School of Physiotherapy, Curtin University of Technology, Bentley, Western Australia, Australia
    • School of Anatomy and Human Biology, University of Western Australia, Nedlands, Western Australia, Australia

Article Outline

Abstract 

Hill K, Jenkins SC, Cecins N, Philippe DL, Hillman DR, Eastwood PR. Estimating maximum work rate during incremental cycle ergometry testing from six-minute walk distance in patients with chronic obstructive pulmonary disease.

Objective

To develop a predictive equation to permit estimation of the maximum work rate (Wmax) achieved during an incremental cycle ergometry test from the measurement of 6-minute walk distance (6MWD) and its derivative, 6-minute walk work, which is the product of 6MWD and body weight.

Design

Cross-sectional observational study.

Setting

Outpatient physiotherapy and pulmonary physiology clinics in a tertiary hospital.

Participants

Patients (N=50; 36 men) with chronic obstructive pulmonary disease (forced expiratory volume in 1 second [FEV1]=37%±11% of predicted).

Interventions

Not applicable.

Main Outcome Measures

Measurements were obtained of 6MWD and Wmax achieved during a laboratory-based, symptom-limited incremental cycle ergometry test. Linear regression analyses were performed using 6MWD, height, weight, and FEV1 and using 6-minute walk work, height, and FEV1 to determine their contribution to Wmax and to develop predictive equations for estimating Wmax.

Results

The equations derived to estimate Wmax using 6MWD and 6-minute walk work, respectively, were as follows: Wmax (W)=(0.122×6MWD)+(72.683×height [m])–117.109 (r2=.67, standard error of the estimate [SEE]=10.8W) and Wmax (W)=17.393+(1.442×6-minute walk work) (r2=.60, SEE=11.8W).

Conclusions

Wmax can be estimated from equations based on measurements of 6MWD or 6-minute walk work. The estimate of Wmax derived from either equation may provide a basis on which to prescribe cycle ergometry training work rates that comply with the current guidelines for pulmonary rehabilitation.

Key Words: Exercise test, Pulmonary disease, chronic obstructive, Rehabilitation

List of Abbreviations: ANOVA, analysis of variance, COPD, chronic obstructive pulmonary disease, FEV1, forced expiratory volume in 1 second, 6MWD, six-minute walk distance, 6MWT, six-minute walk test, Spo2, oxygen saturation measured by pulse oximetry, 12MWT, twelve-minute walk test, Vo2peak, peak rate of oxygen uptake, Wmax, maximum work rate

 

CURRENT GUIDELINES FOR pulmonary rehabilitation state that the optimal training regimen to produce physiologic benefits in patients with COPD consists of relatively long training sessions at an intensity exceeding 60% of a patient's Wmax.1 These guidelines do not state a preference for walking or cycle-based exercise in the assessment of exercise capacity or the rehabilitation of patients with COPD. It may be argued that, compared with cycle-based exercise, walking-based exercise is of greater functional relevance because it is a common activity of daily living. Nevertheless, the use of a cycle ergometer is frequently described in the assessment of exercise capacity and the rehabilitation of patients with COPD.2, 3, 4 Compared with walking as a modality to assess exercise capacity, the use of a cycle ergometry protocol may be preferred because it has the following advantages: (1) it allows more accurate quantification of the external work rate applied, (2) it requires less space, and (3) it is less prone to introducing inaccurate measurements of heart rate, blood pressure, and arterial Spo2 as a consequence of motion artifact.5 Compared with walking-based exercise in the rehabilitation of patients with COPD, cycle-based exercise training may be preferred because this modality has been shown to place greater load on the muscles of locomotion6 and therefore is likely to elicit greater specific conditioning of the quadriceps muscle.7, 8

To apply the current guidelines regarding training intensity to cycle-based exercise training, a measurement of the Wmax achieved during an incremental cycle ergometry test is required. However, the resources available to conduct pulmonary rehabilitation programs are limited, and staff are often precluded from accessing the equipment and expertise necessary to conduct an incremental cycle ergometry test.4, 9 Consequently, many clinicians involved in providing pulmonary rehabilitation programs do not have access to a measurement of Wmax, and exercise capacity is often measured using field-based walking tests.4, 9 Compared with an incremental cycle ergometry test, field tests require minimal equipment and are simple and inexpensive to perform. If it were possible to estimate Wmax from the results of a field walking test, initial cycle ergometry training work rates could be prescribed in accordance with the current guidelines for pulmonary rehabilitation in programs, without access to the results of an incremental cycle ergometry test.1

The most commonly applied field tests to measure exercise capacity in COPD are self-paced timed walking tests such as the 12MWT,10 the 6MWT,11 and the externally paced incremental shuttle walk test.12 Methods have been published that allow Wmax to be estimated from the results of the 12MWT13 and the incremental shuttle walk test.14 Although previous studies have reported a strong relationship between distance walked during the 6MWT (6MWD) and Wmax measured during an incremental cycle ergometry test,15, 16 their analyses have not resulted in a method that enables estimation of Wmax from the 6MWT results.

The primary aim of this study was to develop a regression equation to estimate the Wmax achieved during a laboratory-based, symptom-limited incremental cycle ergometry test from the measurement of 6MWD. Based on the findings of other researchers, which showed that the association between Wmax and 6MWD is strengthened when 6MWD is multiplied by body weight (6-minute walk work),15 a secondary aim of this study was to determine the proportion of variance in Wmax that is explained using a regression equation that included 6-minute walk work.

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Methods 

A prospective cross-sectional study was undertaken. During a 2-week period, measurements were made of body anthropometrics, resting lung function, 6MWD, and Wmax achieved during an incremental cycle ergometry test.

Participants 

The subjects for this study were recruited from referrals to pulmonary rehabilitation programs and from respondents to local advertising to gather participants for 1 of 2 studies that were designed to investigate the effects of inspiratory muscle training in COPD subjects. Inclusion criteria comprised a diagnosis of COPD, a smoking history in excess of 10 pack-years, and an FEV1 between 15% and 70% of the predicted normative value.17 Exclusion criteria comprised a history of lung surgery or spontaneous pneumothorax, use of gait aids or long-term oxygen therapy, any comorbid condition thought to adversely affect exercise performance (eg, musculoskeletal conditions, symptomatic ischemic heart disease, neurologic, or cognitive impairment), a body mass index greater than 35kg/m2, or tapering doses of corticosteroids or methylxanthines. The studies were approved by the appropriate human research ethics committees, and written informed consent was obtained from each subject prior to participation.

Measurements: Resting Lung Function 

Measurements were obtained of lung volumes (Medgraphics Elite Series DX plethysmograph),a FEV1, forced vital capacity (digital pneumotachograph),b single-breath diffusing capacity for carbon monoxide (transfer test),c and concentration of exhaled carboxyhemoglobin (carbon monoxide monitor).d To more completely describe the study population, lung function results were compared with normative data.17, 18, 19 Subjects were classified as current smokers if they stated that they were still smoking or their expired carboxyhemoglobin concentration exceeded 10 parts per million.20

Measurements: Exercise Capacity 

Exercise capacity was measured using a 6MWT and a laboratory-based, symptom-limited incremental cycle ergometry test; each test was separated by a minimum of 24 hours. For both tests, the Borg category ratio scale21 was used to measure dyspnea prior to commencing the test, each minute during the test, and on test completion. The same scale was used to obtain a score for leg fatigue on completion of both tests.

Six-minute walk test 

The 6MWT was performed over a 45-m straight and level course within an enclosed corridor according to a protocol adapted from the American Thoracic Society.11 Details of the instructions and encouragement given to the subjects have been described elsewhere.22 Two tests were performed, separated by a 30-minute seated rest period. Heart rate (heart rate monitor)e and arterial Spo2 (finger probe and pulse oximeter)f were monitored continuously throughout both tests. To more completely describe the study population, the 6MWD was compared with normative data.23 Six-minute walk work was calculated as the product of the greatest 6MWD for each subject (in kilometers) and their body weight (in kilograms).24 In accordance with our local clinical guidelines, any subject that showed a decrease in Spo2 to less than 80% during the 6MWT was instructed by the investigator supervising the test to stop and rest.5 Because the 6MWT was terminated prematurely in these subjects, data related to their participation in the study were excluded from all analyses.

Incremental cycle ergometry test 

A symptom-limited incremental cycle ergometry test was performed on an electronically braked bicycle ergometer.g Subjects were required to remain seated for a minimum of 2 minutes and then cycle (between 40–50rpm) against an initial work rate of 10W. Thereafter, work rate was increased by 10W each minute until voluntary cessation. Standardized instructions and encouragement were provided to facilitate a maximum performance. Wmax was defined as the highest work rate achieved for a minimum of 30 seconds. Throughout the test, breath-by-breath measurements of ventilation, breathing pattern, and gas exchange were collected using an automated exercise metabolic system (Medgraphics CardiO2).a Spo2 was monitored continuously through an ear probe and pulse oximeterh and 12-lead electrocardiography was used throughout the test. To more completely describe the study population, Vo2peak achieved during the incremental cycle ergometry test was compared with normative data.25

Statistical Analyses 

The greatest 6MWD from the 2 tests for each subject was used in the following analyses. Baseline (pre-exercise) measurements of heart rate, dyspnea, and Spo2 were compared between the 6MWT and incremental cycle ergometry test using paired t tests. Peak values for heart rate and dyspnea and the lowest measure of Spo2 and leg fatigue reported on test completion during the incremental cycle ergometry test and 6MWT were compared using paired t tests or, when necessary, analysis of covariance using the pre-exercise measurements as the covariant measures. Paired t tests were used to compare the following variables during each test: (1) pre-exercise and peak measurements of heart rate and dyspnea, and (2) pre-exercise and lowest measurements of Spo2. Associations between Wmax and body anthropometrics, resting lung function, and Vo2peak were examined using Pearson product-moment correlation coefficients (r). Linear regression analysis was performed using 6MWD, height (in meters), weight (in kilograms), and FEV1 (in liters) and using 6-minute walk work (in km·kg), height (in meters), and FEV1 (in liters) to determine their contribution to Wmax and to develop the predictive equations for estimating Wmax. The assumption of linearity between these variables was assessed by plotting the standardized residuals against the predicted values for each regression equation. ANOVA of the regression equations was performed to determine if the relationship between the variables was greater than could be expected by chance.

Based on the results of an earlier study,16 we estimated that a sample size of 46 subjects would yield adequate statistical power (β=1.0) for the 95% confidence interval around the correlation coefficient (r) between measurements of Wmax and 6MWD to range between 0.7 and 0.9. This sample size was inflated by 10% to account for withdrawals.

All analyses were performed using statistical software.i The distribution of data was examined, and no transformation of data was necessary. An α value (P) of less than .05 was considered to be significant. All data are expressed as mean ± SD unless otherwise stated.

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Results 

A total of 50 subjects (36 men) participated in this study. Data from an additional 4 subjects were excluded because they showed a decrease in Spo2 to less than 80% during the 6MWT. As a result, the supervising investigator terminated their tests prematurely. Anthropometry and lung function data for the 50 subjects are shown in table 1. Of the 50 subjects, 7 (14%) were classified as current smokers based on either their self-reported smoking status or measured carboxyhemoglobin levels.

Table 1. Anthropometric and Lung Function Data for Subjects
VariableMean ± SDRange
Age (y)68±851–83
Height (m)1.7±0.11.4–1.9
Weight (kg)67.4±13.542.5–104.0
BMI (kg/m2)23.4±3.516.6–30.9
FEV1 (L)1.0±0.40.5–2.3
FEV1 % of predicted37±1119–59
FEV1/FVC (%)37±922–58
TLC % of predicted118±1577–144
FRC % of predicted158±2999–214
Dlco % of predicted34±1011–55

NOTE. Of the 50 subjects, 36 were men.

Abbreviations: BMI, body mass index; Dlco, diffusing capacity of lung for carbon monoxide; FRC, functional residual capacity; FVC, forced vital capacity; TLC, total lung capacity.

Exercise Tests 

The mean 6MWD for the 50 subjects was equal to 73%±17% of the predicted normal values.23 The mean 6-minute walk work was 31.3±9.9km·kg (range, 9.8–50.3km·kg). Of the 50 subjects, 9 (18%) required between 1 and 3 transient rests during the 6MWT due to intolerable dyspnea; the rest time ranged from 17 to 125 seconds. Heart rate, Spo2, symptom scores, and the distance achieved in meters during the 6MWT are shown in table 2.

Table 2. Exercise Test Results in Subjects
Variable6MWTICET
Distance (m)464±110
Wmax (W) 63±18
V̇o2 (mL/min) 874±243
V̇o2 (% predicted) 38.5±13.1
V̇co2 (mL/min) 901±264
V̇e (L/min) 36.6±10.8
V̇e/MVV (%) 99.3±18.0
Pre-exercise heart rate (bpm)87±1184±12
Peak heart rate during test (bpm)119±15⁎123±17⁎
Pre-exercise Spo2 (%)95±198±1†
Lowest Spo2 during test (%)87±3⁎93±4⁎†
Pre-exercise dyspnea score0.5±0.80.4±0.5
Peak dyspnea score during test5.2±2.4⁎6.1±2.0⁎†
Leg fatigue score on test completion2.7±2.25.0±2.6†

NOTE. Values are mean ± SD. Of the 50 subjects, 36 were men.

Abbreviations: ICET, incremental cycle ergometry test; MVV, maximum voluntary ventilation; V̇co2, carbon dioxide production; V̇e, minute ventilation.

⁎P<.05 vs pre-exercise measurements.

†P<.05 vs 6MWT.

The Wmax and Vo2peak achieved during the incremental cycle ergometry test were 63±18W and 874±243mL/min, respectively (38.5%±13.1% of predicted).25 Cardiorespiratory responses and symptom scores during the incremental cycle ergometry test are shown in table 2.

Comparing Responses to the 6MWT and Incremental Cycle Ergometry Test 

Compared with the 6MWT, pre-exercise Spo2 was 3%±2% higher prior to the commencement of the incremental cycle ergometry test (P<.001). The decrease in Spo2 was greater during the 6MWT than the incremental cycle ergometry test (8%±3% vs 4%±4%; P<.001). A total of 37 subjects decreased their Spo2 to less than 90% during the 6MWT compared with 10 subjects during the incremental cycle ergometry test. Peak dyspnea scores were greater during the incremental cycle ergometry test than the 6MWT (6.1±2.0 vs 5.2±2.4; P=.021). The incremental cycle ergometry test elicited greater leg fatigue than the 6MWT (5.0±2.6 vs 2.7±2.2; P<.001) (see table 2).

Associations Between 6MWD, 6-Minute Walk Work, and Wmax 

Wmax was associated with height (r=.36, P=.009), weight (r=.30, P=.032), FEV1 expressed in liters (r=.51, P<.001), 6MWD (r=.75, P<.001) (fig 1A), and 6-minute walk work (r=.77, P<.001) (fig 2A). The Vo2peak achieved during the incremental cycle ergometry test was associated with 6MWD (r=.63, P<.001) and 6-minute walk work (r=.83, P<.001). The predictive equations derived from 6MWD and 6-minute walk work that explained the highest proportion of the variance in the measurement Wmax were, respectively, as follows:

and

  • View full-size image.
  • Fig 1. 

    (A) Regression line and 95% confidence interval for the association between 6MWD and Wmax measured during an incremental cycle ergometry test. Wmax (W)=(0.122×6MWD)+(72.683×height [m])–117.109 (r=.82, standard error of the estimate, 10.8W). (B) Scatter plot of the standardized residuals and values of Wmax (W) estimated using 6MWD and height according to the equation presented in (A). Regression line indicates that the assumption of linearity was met.

  • View full-size image.
  • Fig 2. 

    (A) Regression line and 95% confidence interval for the association between 6-minute walk work (6MWW) and Wmax measured during an incremental cycle ergometry test. Wmax (W)=17.393+(1.442×6MWW) (r=.77, standard error of the estimate, 11.8W). (B) Scatter plot of the standardized residuals and values of Wmax (W) estimated using 6MWW according to the equation presented in (A). Regression line indicates that the assumption of linearity was met.

Plots of the standardized residuals against the predicted values derived using these regression equations are shown in figures 1B and 2B, respectively. Compared with equations derived using only measures of anthropometry and lung function, a greater proportion of the variance in the measurement of Wmax was explained using equations that included either 6MWD (coefficient of determination, 67%) or 6-minute walk work (coefficient of determination, 60%). The standard error of the estimate for the equation that included 6MWD and 6-minute walk work was 10.8 and 11.8W, respectively. The results of ANOVA were significant for both equations (P<.001).

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Discussion 

To our knowledge, this study is the first to develop a regression equation that can estimate the Wmax achieved during an incremental cycle ergometry test using the results of a 6MWT in subjects with moderate-to-severe COPD. Regression analyses met the assumption of linearity. In addition, a similar proportion of variance in Wmax was explained whether 6MWD or 6-minute walk work was used in the equation. In the absence of a direct measurement of Wmax, either equation could be used to estimate Wmax and thereby calculate the initial cycle ergometry training loads to comply with the current pulmonary rehabilitation guidelines.1

The association between Wmax and 6MWD reported in this study (r=.75) is stronger than that reported by some13, 15, 24 but not all16, 26 previous studies that have compared measures of exercise capacity achieved during laboratory and self-paced field walking tests. The strength of the association between these variables reported in this study is likely the result of the high 6MWD achieved by our subjects (464±110m) relative to the severity of their airflow obstruction (FEV1=37%±11% of predicted). This high 6MWD is probably related to our 6MWT protocol, which includes the use of standardized encouragement to continue walking during any rest period due to intolerable symptoms, consistent reporting of the greater of two 6MWDs, and the use of a limited number of experienced staff to supervise all 6MWTs. It might also be related to our exclusion of patients prescribed long-term oxygen therapy.

Our findings are consistent with those of another cohort of Australian COPD patients who showed a high 6MWD relative to the severity of their lung disease.27 These findings support the hypothesis that local environmental and lifestyle factors may have contributed to the high 6MWD in the COPD population that we observed in this study. Regardless of the mechanism, the equations reported to estimate Wmax will be most accurate when the 6MWT protocol described in this study is adopted.

Our observation of correlations that tend to increase in strength between the measurement of Wmax and weight (r=.30), height (r=.36), and FEV1 (r=.51) is in agreement with a previous study28 that examined the relationships among measurements of Wmax, anthropometric, and lung function variables in patients with COPD. Our finding of a stronger association between Wmax and 6MWD (r=.75) and 6-minute walk work (r=.77) than measures of weight, height, and FEV1 is probably because Wmax and 6MWD are different measures of the same construct, that is, exercise capacity. Nevertheless, our finding of correlations of similar strength between Wmax and both 6MWD and 6-minute walk work contrasts with a previous study by Carter et al.15 They noted a stronger association between Wmax and 6MWD when 6MWD was multiplied by body weight to yield 6-minute walk work (r=.59 vs r=.79).15 This disparity is most likely due to differences in subject characteristics and test protocols between the 2 studies. Specifically, compared with the subjects of this study, the subjects studied by Carter had a greater range of body weights (67.4±13.5kg vs 81.0±17.2kg),15 resulting in a greater range of 6-minute walk work values and thereby potentially strengthening the association between Wmax and 6-minute walk work. Furthermore, in contrast with this study, Carter's study15 failed to optimize 6MWD by including a practice 6MWT29, 30 and used a ramp protocol with varying magnitudes of incremental change during the incremental cycle ergometry test (between 10–30W). These differences are likely to have yielded a submaximal 6MWD but greater Wmax in their subjects,31, 32 thus reducing the strength of the association between these measures but allowing for a substantial increase in the strength of this association when work was accounted for by multiplying by body weight. The contention that differences in cycle ergometry protocols are due, at least in part, for the disparate findings between this study and that by Carter15 is supported by our finding of a substantial increase in the strength of the association between Vo2peak measured during the incremental cycle ergometry test and 6MWD (r=.63) versus 6-minute walk work (r=.83). Unlike Wmax, the measurement of Vo2peak is independent of the magnitude of incremental change used during the incremental cycle ergometry test.31

The equations presented in the current study should be considered only when a direct measurement of Wmax is not available. It is important to note that patients with similar 6MWDs demonstrated considerable variability in their performance during the incremental cycle ergometry test, achieving different maximum work rates. For example, examination of figure 1A reveals that those patients who walked approximately 500m in the 6MWT achieved maximum work rates that ranged between 40 and 90W. This disparity in Wmax for a given 6MWD may be explained, at least in part, by the differences in the severity of peripheral muscle dysfunction within this group of patients. Specifically, patients with more marked impairment in quadriceps function will probably achieve a lower Wmax, given the greater specific load on the quadriceps with cycle-based exercise compared with walking-based exercise.6 Thus, reducing the variability in this measurement to a single estimate using the equations presented in this study is likely to overestimate Wmax in some COPD patients and underestimate it in others. Therefore, the estimate of Wmax derived using either equation should be considered as a starting point from which initial cycle ergometry training work rates can be calculated. Thereafter, patients will require careful monitoring to ascertain their tolerance to the training loads, with adjustments made according to their symptoms.1

We observed a more pronounced decrease in arterial Spo2 but less dyspnea and leg fatigue during the 6MWT than during the incremental cycle ergometry test. The mechanisms for these findings probably reflect the different physiologic responses previously demonstrated between cycling and walking-based exercises in patients with COPD.33 Specifically, gas exchange inefficiency and dead space/tidal volume ratios are greater during walking-based exercise compared with cycle-based exercise.33 These factors, in combination with differences in body position and upper-limb activity during walking-based exercise, have been postulated to contribute, at least in part, to the greater decrease in arterial Spo2 observed during walking tests of exercise capacity in COPD patients.16, 33, 34 Compared with walking-based exercise, earlier work has showed that cycle-based exercise results in a greater concentration of arterial lactate33 and elicits neuromuscular fatigue in the quadriceps.6 These factors may explain the heightened sensations of dyspnea and leg fatigue that subjects reported during the incremental cycle ergometry test in our study.

Study Limitations 

The limitations of this study relate to a relatively small sample size (N=50), the high proportion of men (72%) in the study sample, and the need to validate the equations in a new COPD cohort. Further study is needed in COPD patients to determine the following: (1) the accuracy with which these equations estimate Wmax and (2) the capacity of the patient to train at the loads derived from the estimated Wmax.

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Conclusions 

The estimate of Wmax derived from either 6MWD or 6-minute walk work and the strength of these associations provide a basis on which to prescribe initial cycle ergometry training work rates that comply with the current guidelines for pulmonary rehabilitation for those programs without access to the results of an incremental cycle ergometry test.1 It is important to note that, although the estimate derived using the equations in this study will be most accurate when our 6MWT protocol is adopted, the equations are unlikely to provide a valid estimate of Wmax in COPD patients who have been prescribed long-term oxygen therapy, experienced a decrease in arterial Spo2 to less than 80% during the 6MWT, and/or used a rollator during ambulation, because patients with these characteristics were excluded from our cohort.

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  • a Medical Graphics Corp, 350 Oak Grove Pkwy, St Paul, MN 55127.
  • b Vertek Series; Hewlett Packard, 3000 Hanover St, Palo Alto, CA 94304-1185.
  • c Transfer Test Pulmonary Function System, Model 1182, P.K. Morgan Ltd, 4 Bloors Ln, Rainham, Gillingham, Kent, ME8 7ED, UK.
  • d MicroCO Meter; Cardinal Health Ltd, Quayside, Chatham Maritime, Chatham, Kent, ME4 4QY, UK.
  • e Polar monitor a1 series, a1 wrist receiver with T31 transmitter; Polar Electro Oy, HQ Professorintie 5, FIN-90440 Kempele, Finland.
  • f Ohmeda 3700; BOC Healthcare, Louisville, CO 80027.
  • g ER 900; Erich Jaeger GmbH, Leibnizstr 7, 97204 Hoechberg, Germany.
  • h Mars Model 2001; Novametrix Medical Systems Inc, 5 Technology Dr, Wallingford, CT 06492.
  • i SSPS Version 15.0; SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.

 Supported by the National Health and Medical Research Council (Australia) (grant no. 212016).

 No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.

PII: S0003-9993(08)00391-2

doi:10.1016/j.apmr.2008.01.020

Archives of Physical Medicine and Rehabilitation
Volume 89, Issue 9 , Pages 1782-1787, September 2008

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