Severe aortic valve stenosis is a common heart valve disease that can be treated with transcatheter aortic valve implantation (TAVI), which is the preferred treatment option for older and frailer patients. Physical performance is a crucial outcome for many of these patients. This thesis aimed to describe the physical performance, dietary intake, and physical activity of TAVI patients as a first step towards developing an intervention to enhance physical performance. The thesis findings indicate that, on average, most patients improve their physical performance after TAVI, but one in three patients experiences a clinically significant decline. Muscle strength also typically decreases in most patients after TAVI. Furthermore, pre-procedural low physical performance and muscle quality are independent predictors of mortality after TAVI. In the second part of the thesis, it was highlighted that most TAVI patients fail to meet the required protein intake, do not adhere to most dietary guidelines, and do not meet the guidelines for minimal required physical activity. Finally, the thesis examined the barriers and facilitators of behaviour change and found that many TAVI patients are ambivalent about changing their behaviour. Additionally, adherence to home-based cardiac rehabilitation among older patients was studied, and we observed adherence in two-thirds of these patients. In conclusion, the thesis emphasizes the need for interventions focused on protein intake, diet quality, and physical activity. The thesis proposes an intervention that should be tested for feasibility and effectiveness in future research.