The seated knee extension machine is a great way to isolate the knee extensors, mainly the #rectusfemoris and #vastimuscles. However, caution must be taken for any individuals with knee osteoarthritis, meniscal injuries, or any other pathology that is sensitive to high shearing forces and loads.
From a biomechanics approach, when the knee flexion angle is 90 degrees, the external moment arm goes directly through the axis of rotation, meaning there is little external torque at the knee joint. As you extend you knee (ie the knee flexion angle decreases), the external moment arm gets further away from the axis of rotation, meaning much higher external torques are placed on the knee.
Thus, for those with knee osteoarthritis or previous meniscal injuries, it is best to limit using this machine ONLY FROM 90 TO 45 DEGREES, NEVER to full extension. Past 45 degrees of knee extension, there is considerable more shearing and compressive forces at the knee. Or better yet, why not incorporate functional, bodyweight, closed chain exercises like #squats