Measures

At baseline clinic visit (prior to randomization), participants completed a self-administered questionnaire which included sections on demographic variables (i.e. age, gender, education level, and ethnicity), text messaging use, previous web or app use to monitor PA (yes or no), and confidence in their ability to increase their PA levels (very confident or confident/somewhat confident). We also collected weight and height measurements to calculate baseline body mass index (BMI). A detailed description of these items and response options are published elsewhere.¹⁵

A brief follow-up questionnaire was conducted over the telephone at the end of the 6-week study period. Items asked participants about their experience with the Fitbit One (i.e., tracker, website, and mobile app), and in the intervention group only, the SMS-based PA prompts. Specifically, these items assessed participants’ attitudes on the usability of the Fitbit One and SMS text messages in addition to whether they thought these intervention components were useful in helping them to increase their activity levels. Questionnaire items on the Fitbit One were: “The Fitbit tracker was easy to use,” “Overall, the Fitbit tracker helped me to be more physically active,” “The Fitbit website was easy to use,” and “Overall, the Fitbit website helped me to be more physically active” [Ratings: 5 = strongly agree, 4 = agree, 3 = neutral, 2 = disagree, or 1 = strongly disagree]. Items on the SMS-based PA prompts were: “Daily text messages that prompted me to be physically active helped me to be more physically active” [Ratings: 5 = strongly agree, 4 = agree, 3 = neutral, 2 = disagree, or 1 = strongly disagree] and “The 3 daily text messages that prompted me to be physically active were. . .” [Response options: 3 = too many, 2 = just right, or 1 = too few].

Participants were asked to assess their actual use and responses, or their level of engagement, with each intervention component. Items on using the Fitbit tracker were: “On a typical day, I checked the Fitbit tracker to see. . . (1) how many steps I've taken, (2) how much distance I’ve travelled, and (3) if the flower grew taller (for PA intensity) [Ratings: 5 = very often, 4 = often, 3 = sometimes, 2 = rarely, or 1 never]. Item on the Fitbit website (fitbit.com) was: “In a typical week, I logged onto my Fitbit.com account. . .” [Ratings: 5 = every day (7 days/week), 4 = most days (5 days/week), 3 = some days (3–5 days/week), 2 = rarely (1–2 days/week), and 1 = never (0 days/week)]. Items on the Fitbit mobile app were: “Did you use the Fitbit mobile app?” [Ratings: 1 = yes or 0 = no]. If yes, “How often did you use the Fitbit mobile app?” [Ratings: 6 = more than once a day, 5 = about once a day, 4 = few times per week, 3 = couple times per week, 2 = about once per week, or 1 = less than once per week]. Items on SMS-based PA prompts were: “Overall, did you engage in at least a 10-minute bout of physical activity after receiving a text message from the study? Would you say. . .” [Ratings: 5 = always, 4 = usually, 3 = about half the time, 2 = rarely = 2, or 1 = never] and “How soon after receiving a text message did you engage in at least a 10-minute bout of physical activity? On average, would you say. . .” [Ratings: 7 = 1–30 minutes, 6 = 31–59 minutes, 5 = 1–2 hours, 4 = 3–6 hours, 3 = 7–9 hours, 2= 10–12 hours, or 1 = more than 12 hours]. Higher response scores indicated a higher level of engagement.

The primary dependent variable for measure of physical activity was steps that was objectively measured using a tri-axial accelerometer (Actigraph GT3X+), which is a valid and reliable measure of PA in adults, at baseline and 6-week follow-up.^16,17 ActiLife 6.10 software was used to process Acti-graph GT3X+ data to flag invalid data or those collected when the device was not worn for exclusion in the final analysis. Details on processing Actigraph GT3X+ accelerometer data using ActiLife 6 Data Analysis Software including how to conduct wear time validation is available elsewhere.¹⁸ In brief, the Troiano technique¹⁹ was used to set parameters to detect non-wear periods for wear time validation, specifically: a “minimum length” of 90 minutes of consecutive zeros to define a “non-wear” period; a “spike tolerance” (or “spurious count” tolerance) of 2 minutes to continue counting non-wear periods as non-wear until this threshold was met; “wear periods” of less than 10 minutes were ignored; “minimum wear time per day” was set to 600 minutes per day; and “minimum days of valid wear time” was at least 5 days per week including a weekend day.¹⁸