This study evaluated usability of the PAI Health application in 10 cardiac rehabilitation patients (attending rehabilitation ³3 days/week). Following an initial maximal exercise test, participants wore a PA monitor for 14 days continuously and completed a 19-item usability questionnaire to describe satisfaction with the application. Participants attended a focus group visit to discuss their experiences, allowing participants to give open-ended feedback. Participant VO2max values averaged 22.74±5.33 ml/kg/min and exercise volumes of 207.4±59.2 minutes/week. Overarching themes emerged describing usability of the PAI Health application for cardiac patients. Theme 1: Participants were comfortable wearing the monitor but had issues interpreting the application. Theme 2: Participants were encouraged to perform more PA. Theme 3: The HR monitor was inaccurate. Theme 4: Participants wanted more focus on other aspects of the application/more compatibility with other applications. Generally, the PAI Health application was received favorably. However, exercise volume did not relate to increased VO2max values.