In this work, we describe the conceptualization and development of a digital tool to support patients in adhering to prescription of physical exercise training during cardiac rehabilitation. To meet the need for an individual and trackable heart-healthy training plan – which became even more evident during the Coronavirus pandemic – the Activity Planning Tool was developed within a user-centered design process. Tackling the relationship between rehabilitation professional and patient, it allows to jointly set up, monitor, and regularly review a personalized physical activity plan.