The worldwide population aging and hence the higher risk of agerelated diseases facilitates interest in robotic devices for rehabilitation and movement assistance. A major factor of a persons functional independence is the ability to perform the movement task of standing up from a chair, also known as sittostand (STS) movement. This work focuses on the development of a passive, wearable knee orthosis that supports the extension of the knee joint. As the knee flexes while sitting down a spring mechanism is loaded due to a cable wrapping around a deflection disc, thus generating a moment that assists standing up again. In order to prevent unintended knee extension this assistance moment should be minimized when the person is fully seated. To this end the geometry of the deflection disc, parametrized as Bspline curve, is optimized in order to control the knee extension moment with respect to the knee angle. Initial tests with a first prototype show promising results.