Objectives: In this article, we describe the design approach of a game-based, mobile coaching application for older adults (65-75 years) that aimed to stimulate physical activity (PA) in daily life. We evaluated older adults' experiences using this application compared with a standard coaching application in terms of engagement, motivation to be physically active, and in relation to the applied design features. Development and Design: An iterative design approach was followed to develop the game-based coaching application, called WordFit. Step count data (FitBit) were used for the crossword-inspired game. The standard coaching application (the ActivityCoach) displayed FitBit data. Materials and Methods: Participants were asked to use the ActivityCoach for 1 week first and then use WordFit for up to 3 weeks. Engagement was determined by logging actual use and motivation for PA by a daily in-app motivation question and step count data. Afterward, a semistructured interview was conducted. Results: Twenty older adults participated (71.0 ± 5.0 years). Of these, 16/20 actively used the ActivityCoach (13.1 ± 10.0 days) and 8/20 used WordFit (14.8 ± 9.5 days). For the latter, mean PA before WordFit use was 5852 ± 3652 steps/day, while mean PA using WordFit was 7236 ± 3335 steps/day. The interviews (n = 12) showed that step data and feedback on performance through the ActivityCoach/FitBit were prominent motivators for PA. WordFit was generally played as a stand-alone brain trainer. Cognitive challenge, in-game challenges, and challenges through emergent gameplay were important for engagement to use the game. Older adults did not play WordFit together. Discussion: WordFit was not seen as a tool to help improve PA behavior. Enjoyment of the game concept could be improved by providing older adults with new and nontraditional gaming concepts before assessing game preferences. Follow-up studies should be conducted in a target group that is known for low adherence rates and that can benefit from the intervention provided.

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