Performing knowledge work often involves prolonged sitting in a limited number of positions with little physical effort. This can lead to discomfort in the neck and shoulder region, and health problems in the back and neck. Subsequently, it can affect job performance adversely. In this PhD project is the influence of different seat design variables investigated, combined with posture and movement behavior in several tasks, on comfort and discomfort. The research was carried out in various office environments and in the train. In addition, a prototype lounge chair has been studied, whereby watching a monitor screen was central. The results of these studies contribute to the scientific debate on optimal seating support for knowledge work, but also to practical applications for functional chair design and evaluation.