Aerobic capacity in patients with rheumatoid arthritis: a comparison of two submaximal test methods

In a clinical setting it is important to evaluate aerobic capacity in individuals with rheumatoid arthritis (RA) and to have a choice between tests, owing to disability of varying severity. Two submaximal tests, a bicycle ergometer test and a treadmill walking test, are commonly used. Despite expected differences in the results, these tests have been used interchangeably. The aim of the current study was to compare the results of the two tests, the size of the difference and factors expected to influence the results.

Fifty-two outpatients with RA performed the two tests. Agreement and correlations between the results of the tests were calculated. Multivariate analysis was used to study the relationships between gender, weight, health assessment questionnaire, global health assessment and the difference between the tests.

Sixty per cent of the subjects exhibited a higher estimated value of maximum oxygen uptake (VO(2max)) in the treadmill test. The limits of agreement for the estimated VO(2max) values between the two methods ranged from -13.4 to +18.4 ml x min(-1) x kg(-1), and the intraclass correlation coefficient (ICC(Two-way mixed)) was 0.34 (95% confidence interval [CI] 0.07, 0.56). Body weight was independently associated with the difference between the two tests (regression coefficient 0.3; 95% CI 0.14, 0.42). A higher body weight had a greater impact on the results of the bicycle test (R(2) = 0.28; regression coefficient -0.3; 95% CI -0.47, -0.17) than on the treadmill walking test (R(2) = 0.02; regression coefficient -0.06; 95% CI -0.14, 0.03).

It is not advisable to use the two submaximal methods interchangeably. Weight influenced the difference between the two tests, and to a higher degree in the bicycle test than the treadmill walking test.