We read with interest the article by Chakravarty and coworkers.1 Although the concept of salutary effects of running is appealing, there are some features of this study that require the findings to be interpreted with caution.
The Health Assessment Questionnaire Disability Index2 (HAQ-DI) comprises 8 categories, each of which has at least 2 component questions, totaling 20 items. There are 4 possible responses for each item, scored 0 to 3, and a category score is determined from the highest item score in that category. The sum of the 8 category scores is divided by 8, yielding a single disability index from 0 to 3. Thus, the HAQ-DI will tend to inflate when a subject has answered more than 1 item within a category with a nonzero score. Furthermore, since the categories are correlated, a person having difficulties within one category will often have difficulties in other categories. Both these properties make it likely that the HAQ-DI should exhibit nonlinear behavior. This was indeed the findings of a study on HAQ-DI in 1530 adults from the general Finnish population,3 demonstrating that the HAQ-DI increased exponentially with age older than 50 years for both men and women and at a faster rate for women. In the study by Chakravarty et al,1 the group of runners comprises 19% women, whereas the control group comprises 44% women. In addition, the control group is older than the runners, and the longer the study lasts, the greater will be the impact of age.4 The sex difference is even more striking when the subjects are categorized into “ever runners” (20% women) vs “never runners” (60% women). The linear model used by Chakravarty and coworkers1 will not handle the nonlinear properties of data correctly.