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Cut-point adjustment issue

Posted by RicRosenkranz on 17 Nov 2011 at 05:56 GMT

Since these cut points were developed using the
older Actigraph 7164, which registers activity about 9% higher in
children than the newer GT1M model used in this study [27], we decided that a cut-off
of >4000 counts per minute would adequately capture MVPA in this group.

http://plosone.org/article/info:doi/10.1371/journal.pone.0018576#article1.body1.sec2.sec4.p1

Although determination of moderate-to-vigorous physical activity (MVPA) from accelerometer data is almost always a complicated issue, the authors appear to have made an error in the conversion of cut-points that were originally derived from studies using the 7164 model. Corder et al (2007) showed that the GT1M-derived counts per minute were approximately 9% lower than those of the 7164. Given that scenario, the current authors should have subtracted 9% from the 7164-derived values (between 3425 and 3581 cpm). If they had done so, they would have ended up with cut-points for MVPA closer to 3200 cpm, which aligns perfectly with those of Puyau et al (2002). Instead, it appears that the 9% was added, and then additionally inflated to a cut-point of 4000 cpm. Using such a high cut-point for MVPA likely yields a measure with high specificity and low sensitivity, and this probably explains the very low estimates of mean daily MVPA minutes observed in these children. Even 3200 is arguably conservative, as Trost and colleagues (2011) have recently shown that the Evenson (2008) or Freedson (2005) equations provide the best classification accuracy, and both of those equations would yield cut-points nearly 1000 cpm lower than those of Puyau et al (2002).

Regardless, the present authors have done commendable work here, and have contributed to the evidence base with a much-needed longitudinal study addressing the relationship between higher intensity physical activity and weight gain.

Corder K, Brage S, Ramachandran A, Snehalathan C, Wareham N, et al. (2007) Comparison of two Actigraph models for assessing free living physical activity in Indian adolescents. J Sports Sci 25: 1607–1611.

Evenson KR, Cattellier D, Gill K, Ondrak K, McMurray RG. (2008) Calibration of two objective measures of physical activity for children. J Sports Sci 26:1557–65.

Freedson PS, Pober D, Janz KF. (2005). Calibration of accelerometer output for children. Med Sci Sports Exerc. 37(11 suppl): S523–30.

Puyau MR, Adolph AL, Vohra FA, Butte NF. (2002). Validation and Calibration of Physical Activity Monitors in Children, Research Obesity 10(3): 150–7.

Trost SG, Loprinzi PD, Moore R, Pfeiffer KA. (2011). Comparison of Accelerometer Cut Points for Predicting Activity Intensity in Youth. Med. Sci. Sports Exerc., 43 (7):1360–1368.

No competing interests declared.