Mitchell et al. (2) have recently described the change in objectively measured sedentary time for a 4-yr follow-up period in adolescents. They concluded that sedentary time increased at a rate of 15.9 and 22.8 min-d~(-1) per follow-up year in boys and girls, respectively. This is a very interesting and well-conducted cohort study; however, there is an important issue that needs to be discussed.
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