BACKGROUND: Schools provide settings for physical activity (PA), but objective tools for measuring PA are lacking. We assessed an instrument to directly observe group PA and measured the leisure-time PA of adolescents throughout the school day. METHODS: Leisure-time PA was studied by direct observation in 24 middle schools in Southern California using SOPLAY (System for Observing Play and Leisure Activity in Youth). SOPLAY uses momentary time sampling to record the activity of each individual as sedentary, walking, or very active. RESULTS: The largest proportion of students visited activity areas at lunch time (19.5%), followed by before (4. 1%) and after school (2.1%). More boys than girls visited activity areas before school (33.4 vs 7.7) and at lunch time (148.8 vs 36.7). Boys in areas engaged in more moderate to vigorous PA than girls before school (59.6 vs 40%) and at lunch time (67.7 vs 51.7%). CONCLUSIONS: Few students use opportunities to be physically active during lesiure time at school. Policies and environmental manipulations (e.g., supervision, equipment, structured programs) are needed to attract more adolescents, especially girls, to existing activity areas. The feasibility of SOPLAY for measuring group PA was established.
