Compensation or displacement of physical activity in middle school girls: The Trial of Activity for Adolescent Girls

The “activitystat” hypothesis suggests that increases in moderate-to-vigorous physical activity (MVPA) are accompanied by a compensatory reduction in light physical activity (LPA) and/or an increase in inactivity in order to maintain a consistent total physical activity level (TPA). OBJECTIVE: The purpose of this study was to identify evidence of compensation in middle school girls. SUBJECTS: Participants were 6,916, 8(th) grade girls from the Trial of Activity for Adolescent Girls (TAAG). DESIGN: Inactivity and physical activity were measured over 6- consecutive days using accelerometry (MTI Actigraph). A within-girl, repeated measures design was used to assess associations between physical activity and inactivity using general linear mixed models. RESULTS: Within a given day, for every one MET-minute more of inactivity, there was 3.18 MET-minutes (95% confidence interval: −3.19, −3.17) less of TPA (activity > 2 METS) on the same day. Daily inactivity was also negatively associated with TPA on the following day. Each additional minute of MVPA was associated with 1.85 minutes less of inactivity on the same day (95% confidence interval: −1.89, −1.82). Daily MVPA was also negatively associated with inactivity the following day. CONCLUSION: Our results, based on 6-days of observational data, were not consistent with the “activitystat” hypothesis, and instead indicated that physical activity displaced inactivity, at least in the short term. Longer intervention trials are needed, nevertheless our findings support the use of interventions to increase physical activity over discrete periods of time in middle school girls.