PuzzleWalk incorporates behavior change techniques (BCTs), a theory-based method of promoting healthy behavior change by leveraging psychological determinants. The example techniques included in PuzzleWalk are a comprehensive, visualized user guide, self-monitoring of target performance, contingent rewards, and goal setting.

It is a spot the difference puzzle game comprising 660 major city images around the world. This format was chosen because it is easy to understand the purpose of the game, and it can quickly capture the user's interests without a complex comprehension process. Moreover, this visual image-based game facilitates visual interaction, which is a unique strength of individuals with ASD. The most unique design element of PuzzleWalk is the conversion algorithm between steps and game-solving time. Specifically, the user's accumulated steps are directly converted to game-solving time to motivate PA participation.

Google Fit (Google LLC) is a PA-tracking platform developed by Google for Android and Apple iOS. The app uses a sensor built into a smartphone device to automatically track PA, including steps and active minutes. It also allows users to journal and record a variety of forms of PA (eg, cycling, weightlifting, and yoga) by manually setting the activity tracking mode. Google Fit uses a heart point-based reward system as a gamification strategy to provide users with individualized exercise tips incorporated with PA recommendations outlined by the American Heart Association. The number of heart points received based on active minutes is the app's primary gamification strategy.

All participants received visualized step-by-step instructions (eg, search and download on Google Play or App Store, user registration, goal setting, and PA behavior tracking) on the assigned PA app (PuzzleWalk or Google Fit) and used it from the beginning of the intervention start (fourth week) until the end of the intervention (eighth week). Both the PuzzleWalk and Google Fit groups received reminders regarding the use of the PA app during the first week of the intervention period and autonomously continued to use the app until the intervention ended.

Accelerometer-Measured Objective Physical Activity Levels

Beck Anxiety Inventory (BAI; Beck, Epstein, Brown, & Steer, 1988) was used to assess participants' prolonged state of anxiety. The Beck Anxiety Inventory is a self-report scale comprising 21 items that measure the severity of anxiety symptoms during the past week. The score range is 0-63, with higher scores indicating greater degrees of anxiety.