RESPONSE

Dear Editor-in-Chief: We thank Drs. Guinhouya and Hubert for their comments and interest in this important area of research. Using similar methods to Trost et al. (4), we investigated whether the observation that girls are less active than boys is confounded by gender differences in physical maturity. We aligned the accelerometer data on chronological and biological age and found that when aligned on biological age the gender differences in physical activity disappeared (3). Drs. Guinhouya and Hubert express concern about our use of Trost et al.'s (4) moderate to vigorous physical activity (MVPA) cut points because they felt these intensity thresholds were too low. Drs. Guinhouya and Hubert advocate the use of the Actigraph MVPA cut points of ≥ 3000 counts per minute. They suggest that if we were to use a more "discriminative" MVPA cut point that the gender differences in physical activity would persist despite any biological age adjustment. We assume their rationale stems from studies that show the gap in physical activity between genders increases as intensity increases (2,5). However, our data do not support this as we showed that when aligned on biological age, gender differences in vigorous physical activity only (counts per minute ranging from 3311 to 4381) also disappeared; therefore, we believe our conclusions remain justified. We agree that there needs to be a consensus on accelerometer cut points used to delineate physical activity intensity levels. Unfortunately, many questions remain that preclude definitive cut-point selection for children and/or adults (see accelerometry best practices paper by Ward et al. (6) for a complete discussion). We caution against the selection of cut points that produce physical activity guideline compliance rates with face validity. For example, we have shown that a prima facie assessment of physical activity using average day accelerometry data can yield markedly different "meeting the guidelines" results compared with more rigorous assessments using daily, bouts-only, and a combination of both daily and bouts-only physical activity (see Figs. 3 and 5 in Esliger and Tremblay 2007 (1)). In this example, the percentage of children meeting the 60-min physical activity guideline ranges from 97% to 3% even though Trost et al. (4) cut points were used. It is important to remember that accelerometers measure minutes (or indeed seconds) of physical activity that in the past could/would not have been captured with questionnaires. As such, we should not be surprised that children and even some adults may accumulate many minutes of MVPA. Our challenge going forward is to develop evidence-based cut points to determine the amount, intensity, and pattern of accelerometer assessed physical activity that provides health benefits. Lauren Sherar, MSc Adam Baxter-Jones PhD College of Kinesiology University of Saskatchewan Saskatoon, Canada Dale Esliger, MSc University of Exeter School of Sport and Health Sciences Exeter, United Kingdom Mark Tremblay, PhD Research Institute, Children's Hospital of Eastern Ontario Ottawa, Canada