The current Coronavirus Disease 2019 (COVID-19) pandemic has resulted in extraordinary public health orders of social distancing and self-isolation, leading to widespread disruption and discontinuation of cardiac rehabilitation programmes and other social opportunities for cardiovascular disease (CVD) patients to exercise. In Austria, the government initiated drastic public health measures (national lockdown) on March 16, 2020, leading to closure of all outpatient cardiac rehabilitation facilities and restriction of inpatient rehabilitation to patients with urgent medical indications only. This study aims to explore the impact of COVID-19-related national lockdown and public health restrictions on cardiac rehabilitation patients, with respect to maintenance of physical activity for secondary CVD prevention. The study poses three research questions, which will be addressed in a mixed-methods study with sequential quantitative-qualitative (QUANT-QUAL) design: 1. What was the impact of the COVID-19-related lockdown on patients' physical activity and physical fitness levels? (QUANT stage) 2. What was the patient experience of the closure of group-based cardiac rehabilitation training due to COVID-19 public health restrictions? (QUAL stage) 3. Which insights and learning points may be drawn from patients' experiences during COVID-19 public health restrictions with respect to the provision of home-based digital support for physical activity? (QUAL stage) The study will recruit a cohort of up to 40 cardiac rehabilitation patients from one outpatient cardiac rehabilitation centre in Salzburg, Austria, whose rehabilitation programme was interrupted by COVID-19 public health orders, including "lockdown". Patients will undergo re-assessment of physical fitness in cycle ergometry test and re-assessment of cardiovascular risk profile. This will be compared with patients' most recent available test results from before the COVID-19 lockdown (i.e. prior to mid-March 2020) from patient records. Additionally, patients will take part in a semi-structured qualitative interview in which they will be invited to reflect on their personal experiences during the COVID-19 lockdown and thereafter.