Outcomes
Minutes of MVPA are assessed via ActiGraph GT3X-plus, a small, lightweight accelerometer (
http://www.theActiGraph.com) [
37] that is reliable and valid for assessing MVPA [
38,
39]. It records acceleration counts from which minutes of MVPA will be estimated [
39]. A trained staff member provides and reviews instructions on wearing the accelerometer with each girl and plays a two-minute video, created by the research team in collaboration with a local production company, for the purpose of reiterating the importance of wearing the monitor. Girls are told to wear the monitor attached to an elastic belt on their right hip from the time getting out of bed in AM to the time returning to bed to sleep at night for seven consecutive days, but not when bathing or swimming. To remind them to wear it, girls receive an automated phone call to their homes every morning before school and at 11 A.M. on Saturday and Sunday until the monitor is returned to school. Monitors are initialized and set to begin data collection at 5:00 A.M. on the day after they are distributed to girls at school. Data are collected and stored in raw format, as this new ActiGraph model has high storage capacity and allows choice of time sampling interval (or epoch length) after data are collected and downloaded. This way, the research team may apply pattern recognition techniques to data if desired.
Although different epoch levels can be selected, initial analysis will be conducted using count cut-points (15-second epochs) created by Evenson and colleagues [
40]. Prior research by one of the study authors showed these cut-points have the best sensitivity and specificity when compared to others [
41]. Count thresholds will be used for activity intensities: moderate (574–1002 counts/15 seconds) and vigorous PA (≥1003 counts/15 seconds). Fifteen-second increments with counts at or above 574 will be summed from 6 A.M. to midnight to determine minutes of MVPA. Data will be excluded if the monitor is taken off during waking hours (noted by 20 or more consecutive minutes of continuous zeros - not observed in an awake child wearing an accelerometer) [
42]. Guided by the 80% rule, we plan to use data on participants who provide at least 4 days of data (a significant increase in reliability occurs at 4 days). On weekdays and weekends, we expect monitors to be worn 11.2 hours and 7.2 hours, respectively (adolescents are expected to be awake 14 hours/day on weekdays and 9 hours/day on weekends so 80% corresponds to 11.2 and 7.2 on weekdays and weekends, respectively) [
43].
Cardiovascular fitness is determined using the Progressive Aerobic CV Endurance Run (PACER) [
44], a 15-meter or 20-meter shuttle run, that estimates aerobic capacity and CV endurance. No more than six participants at a time run from one line to another on a flat surface, according to audio cues. Each time participants complete a run in one direction, it is considered completion of one lap. The distance (15 versus 20 meters) of one lap is determined by space available. The pace of the audio cues increases with time until participants can no longer complete the laps in the time allotted. Staff members record number of laps completed for use in analysis (higher number of laps = greater CV fitness) [
44]. Students are considered finished with the test when they have not completed two laps within the allotted time. Number of laps completed is converted to estimated VO
2 for analysis. PACER has been used with overweight adolescent girls [
45]. It avoids singling out lesser fit individuals by allowing them to finish first rather than last with everyone watching.
Body mass index and percent body fat are measured behind a privacy screen. Height without shoes is measured to the nearest 0.1 cm with a Shorr Board (Shorr Productions, Olney, MD). Weight is assessed to the nearest 0.1 kg and percent body fat is measured to the nearest 0.1% using a foot-to-foot bioelectric impedance scale (Tanita Corporation, Tokyo, Japan). BMI raw scores (weight in kg/height in meters squared) are calculated; z-scores and percentiles for age are determined using a SAS program for CDC Growth Charts, available online from the National Center for Chronic Disease Prevention and Health Promotion.
Cognitive and affective variables
Perceived benefits of and barriers to PA are measured with two scales: a 12-item Perceived Benefits Scale and a 17-item Perceived Barriers Scale. Cronbach’s alphas for a 10-item Perceived Benefits Scale and a 9-item Perceived Barriers Scale developed by the first author in prior research with adolescents were .80 and .78, respectively [
46]. Face, content, and construct validity has also been reported for each scale [
46]. Both have response choices ranging from
not at all true (coded 0) to
very true (coded 3). Prior to the pilot and this current study, the investigators added new items to the existing instruments based on recommendations that were received from 25 6
th- through 8
th- grade girls participating in focus groups conducted by the first author. In the pilot study, the additional items resulted in Cronbach’s alphas of .85 and .88 at baseline and .84 and .86 at post-intervention for the Perceived Benefits and Barriers Scales, respectively [
25]. Individually tailored motivational and feedback messages delivered via the iPad are based on each girl’s responses to scale items related to both mediating variables. Each girl’s top benefits and barriers are also reported on the one-page printout provided to the nurse for use during the motivational interviewing sessions.
Enjoyment, another mediating variable, is assessed with the PA Enjoyment Scale [
47], which has demonstrated factorial and construct validity when used with adolescents [
48,
49]. To decrease the overall response burden and length of the original 16-item instrument, create an equal balance between negatively and positively worded items, and exclude double negatives found to be misunderstood by girls in the prior pilot work [
25], three positively worded and three negatively worded items were selected for inclusion in this study. Negatively worded items (e.g., I feel bored) are reverse scored. Four response choices range from
not at all true to
very true. In the pilot study, Cronbach’s alphas were .78 and .81 at baseline and post-intervention, respectively, for the 6-item scale [
25]. Each girl’s responses to the scale items were provided for the nurse.
Social support received for PA is measured via an 8-item Social Support Scale. One item example is:
Someone encourages me to exercise. A 5-item scale was used in the past by the first author [
50]. The items in the 5-item scale were based on two scales developed by other researchers to assess parent and peer support [
51,
52]. Prior to the pilot study, the first author modified the scale to increase item clarity and focus on support received from people in general or as a whole in the participant’s life. To increase the comprehensiveness of the scale, three items were added. The additional three items and refinements were also based on evaluative feedback from girls in the focus groups conducted by the first author. Four response choices included:
Never (coded 0),
Rarely (coded 1)
, Sometimes (coded 2), and
Often (coded 3). Higher scores indicate greater social support and vice versa. In prior work, scale items assessing encouragement and provision of transportation from others, were found to be related to PA [
50]. In the pilot study, the 8-item scale had a Cronbach’s alpha of .93 [
25]. Responses related to this mediating variable, such as sources and forms of social support, were used to tailor the iPad-delivered messages and also shared with the nurse via the one-page printout.
PA self-efficacy is measured with two instruments in order to capture different dimensions of the mediating construct. One instrument is used to determine how much girls agree that they can be active in their free time when facing barriers or not. The instrument was originally developed by Saunders and colleagues [
53] and was later modified to include 8-items with five response choices ranging from “disagree a lot” to “agree a lot” [
48]. Test-retest reliability in 6
th and 8
th grade girls was .84 [
48,
54]. In this study, because social support was already being assessed, two items focusing on the girl’s ability to ask others to be active with them were deleted. Also, the middle response choice, “neither agree nor disagree” included in the 8-item measure was deleted in the remaining 6 items.
The second measure of self-efficacy assesses how sure girls are of their ability to adhere to exercising whether or not barriers exist. The items of the resisting relapse factor of the Self-Efficacy for Exercise Behaviors Scale [
55] developed for adults were modified by other researchers for the Active by Choice Today (ACT) trial [
56]. Because two of the original 12 items were deemed irrelevant, they were deleted, and only 10 items were used in the ACT trial [
56]. Reliability and construct validity are reported elsewhere [
57,
58].
Motivation is measured with the Behavioral Regulation in Exercise Questionnaire – 2 (BREQ-2). The 19-item scale for measuring this mediator was modified to delete six redundant items and three items that were similar to those included in the enjoyment scale. Reducing the number of redundant items was essential to reduce the overall survey length and response burden for this young age group. Some minor wording changes were made to aid in comprehension. Response choices range from “not true” to “very true.” In one study with adolescents, Cronbach alpha coefficients for the five subscales associated with the BREQ-2, including amotivation, external regulation, introjected regulation, identified regulation, and intrinsic motivation, were 0.82, 0.76, 0.74, 0.74 and 0.87, respectively [
59]. A more recent study showed adequate factorial validity and moderate to high internal consistency reliability for the five subscales with Cronbach alpha values ranging between 0.61 and 0.88 [
60].
Personal factors and behaviors
Single items are used to assess the following personal factors and behaviors: number of days attending PE or gym class in a typical week in the current semester or quarter; participation on a sports team or in classes/lessons (dance, martial arts, gymnastics, tennis) at school or outside school; number of hours watching TV or movies, playing video games or using the Internet for something that is not schoolwork, and talking on the phone or sending text messages. SES is determined by an item listed on the consent form that asks parents whether or not their child participates in the free and reduced price lunch program. Age, academic grade, race and ethnicity are obtained from items listed on the consent form or screening tool.
Each girl’s stage of development is assessed with the Pubertal Development Scale [
61]. Alpha coefficients are .68 - .83 [
61]. Pender et al. found agreement between observations of participants' developmental characteristics and self-report to be >90%. In other studies, correlation coefficients comparing observed developmental characteristics and self-reported scores were as high as .79 [
61]. Girls rate themselves on growth spurt, body hair (underarm), skin changes, and overall development as compared to other girls of similar age. Response choices will be: (1) no; (2) yes, barely; (3) yes, definitely; or (4) development complete. Girls respond to questions about breast growth and menstruation. Scores for body hair, breast growth, and menarche (1 = no menstruation yet; 2 = yes, menstruation started) are combined to determine developmental stage (early, middle, or late puberty).