Background
Standards specifying the amount of moderate-to-vigorous physical activity (MVPA) children should accumulate have been introduced in settings where children spend large amounts of time (e.g., recess, physical education, afterschool programs, summer day camps). Often these settings schedule a finite amount of amount of time for physical activity (PA) opportunities (e.g., 20min recess, 45min lesson, 60min during an afterschool program), and fail to maximize the amount of MVPA children accumulate during. Therefore, many settings that care for children struggle to meet MVPA standards and policies. Strategies which maximize the amount of time children spend in MVPA during scheduled PA opportunities are needed. Targeting staff behaviors and the structure of PA opportunities are two promising strategies for increasing children’s engagement in MVPA. The LET US Play principles, which stands for lines, elimination, team size, uninvolved staff/kids, and space, equipment, and rules, were designed to optimize MVPA levels by modifying games and were established from elements identified as primary barriers to maximizing children’s MVPA during commonly played games. Experimental evidence on the effectiveness of LET US Play to elicit higher levels of MVPA has yet to be established.

Objectives
The objective of this study was to compare the amount of MVPA children accumulate during 6 commonly played games delivered in their traditional format versus the games modified according to the LET US Play principles.

Methods
Children (K-5th grade) were recruited from a single summer day camp. Over 8 weeks, four 1-hour PA sessions were delivered daily on non-consecutive days each week. To avoid issues with temperature, the first session occurred outdoors and included kickball and soccer. The remaining three sessions occurred indoors and included free play, dodgeball, tag games, and relay races. Each 1-hour session was conducted as follows: 5 minutes at the beginning and end for accelerometer placement/removal and demographic data collection, and two 20 minute gameplay segments split by a 10 minute water break. During the 20 minute gameplay segment one of the six games mentioned above were played with children using traditional rules in one 20 minute segment and modifying the rules to adhere to the LET US Play principles in the other 20 minute segment (see Table 1). Each game, modified and traditional versions, was played an equal number of times during the first or second 20 minute segments using a random counterbalanced design. A complete description of the traditional and modified version of each game is presented in Table 1. Approximately 20 children participated in each activity session. Participants wore ActiGraph GT3X+ accelerometers on the right hip for the duration of each 1-hour activity session with only the two 20 minute segments used for analysis. All statistical analyses were performed using Stata (v.13.1, College Station, TX). Repeated measures mixed effects models were used to estimate differences in the percent of time children spent in MVPA during LET US Play versus traditional PA opportunities. Models were run separately for girls and boys and controlled for age and race.

Results
Over the 8 weeks, 267 children (mean age 7.5 yrs.; 43% girls; 29% African American) participated in 50, 1-hour activity sessions representing 100, 20 minute modified or traditional segments of the activities. The average number of 1-hour sessions was 8.33 (range; 7 kickball – 9 dodgeball, tag games, and relay races). The differences in the number of sessions was due to scheduling conflicts, slow transitions (i.e. a group arriving late to an activity session), and poor weather. For boys, statistically significant increases were found in the percent of time spent in MVPA across all six games incorporating the LET US play principles in comparison to traditional sessions (see Table 2). The largest and smallest MVPA differences for boys were during tag games (20.2%), and free play (8.6%), respectively. For girls, statistical significance was found in 4 out of the 6 games for differences in MVPA (Table 2). The largest was during tag games (20.4%), and smallest during free play (8.3%). Overall, the percentage of children meeting 50% time spent in MVPA increased to 53.1% during dodgeball (largest difference), and to 2.0% during kickball (smallest difference).

Conclusions
The LET US Play principles 1) lead to greater accumulation of MVPA for boys and girls across all 6 types of commonly played games and 2) can increase the percent of children attaining the 50% of time in MVPA standard widely adopted in many settings that care for children.

Implications
Based on these findings, despite best efforts, having all children attain the 50% time in MVPA standard may not be a realistic goal. Thus, the language used in MVPA policies for settings that care for children needs revisiting to identify the most appropriate MVPA goals. Additionally, across settings that care for children, training and support for adults in the LET US Play principles, is needed.