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

OBJECTIVE

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 to maintain a consistent total physical activity level (TPA). The purpose of this study was to identify the evidence of compensation in middle-school girls.

SUBJECTS

Participants were 6916, 8th 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 (CI): -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 min less of inactivity on the same day (95% CI: -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.