When sitting in your chair, there should be 1” of space between the back of your knees and the edge of the seat pan. If there is more than 1” of space, your legs may not be getting the support they need resulting in fatigue. If there is less than 1” of space, the seat pan will put pressure on the back of your knee causing discomfort and possibly numbness. Also, this discomfort will likely cause you to move forward in your chair and stop using the backrest, causing additional fatigue and discomfort in your back. Find a chair that fits your legs – a chair that has a “seat pan depth” adjustment is ideal.