Comparing the roles of physical activity and fitness in arterial stiffness: how important is exposure measurement error?

Comparing the Roles of Physical Activity and Fitness in Arterial Stiffness: How Important Is Exposure Measurement Error? To the Editor: The article by Boreham et al1 in the November 2004 issue of Hypertension reports the associations between physical activity and cardiorespiratory fitness with pulse-wave velocity (PWV), a measure of arterial stiffness. One of the objectives of the study was to compare the relative relationships of physical activity and fitness with the outcome PWV. Indeed, the authors’ interpretation of their data leads them to state that “arterial stiffness related benefits of exercise are most likely to accrue if exercise prescription in young adults targets improvements in cardiorespiratory fitness”, implying that fitness is more important than physical activity in preventing stiffening of the arteries. Clearly, understanding the manner in which modifiable factors such as low physical activity and poor fitness relate with cardiovascular health is important for disease prevention. However, the conclusions and recommendations in the present study are based on data in which differential exposure measurement error is probably a pivotal factor; this possibility receives little attention in the article. Cardiorespiratory fitness, as assessed through a maximal oxygen consumption test, is not a simple construct, because fitness is determined by both modifiable factors, such as habitual physical activity (volume, intensity, and mode), smoking, and cardiovascular pathology, as well as nonmodifiable factors, such as genotype and intrauterine environment. Therefore, in considering the risk predisposed by low fitness and physical inactivity, one should preferably measure both exposures to a similar level of precision and then assess them in combination, thereby accounting for any shared variance. However, this is difficult to achieve. Even when physical activity is measured objectively, differences in measurement precision persist between activity and fitness.2,3 Only by measuring activity and fitness objectively and by undertaking multiple measures of both exposures combined with the appropriate statistical analyses can this problem be adequately addressed. In the present study, the correlation between the estimate of physical activity derived using a questionnaire and the true level of physical activity is likely to be r 0.30;4 whereas the correlation between the estimate of cardiorespiratory fitness and true fitness is likely to exceed r 0.80.5 Thus, measurement error will be much greater in the estimate of physical activity compared with the estimate of fitness and will inappropriately decrease the probability of detecting a significant association for physical activity. Moreover, an exercise stress test using respiratory gas exchange to determine fitness is an objective measure, whereas physical activity questionnaires are subjective instruments and are thus prone to response bias. If, for example, the young adults who had stiffer arteries in the present study over-reported their sports physical activity, this would cause the magnitude of effect ( coefficient) for the association between activity and PVW to appear less than it truly is. By contrast, if under-reporting of occupational activity occurred in those with high arterial tonicity, this could cause overestimation of the magnitude of the relationship with PVW. The nature of this reporting bias is known to be modified, among other factors, by gender and by level of obesity,6 and simply adjusting for these factors may not be adequate. However, it is not possible to know the extent or direction of such biases without a concurrent objective criterion measure. In conclusion, whereas the association between cardiorespiratory fitness and PVW in the present study is highly plausible, the associations, or lack thereof, between the physical activity subdomains and PVW are likely to be far less reliable, owing to the high degree of measurement error and response bias characteristic of physical activity estimated via questionnaire.