Comparing the Roles of Physical Activity and Fitness in Arterial Stiffness: How Important Is Exposure Measurement Error?

Abstract

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 …