I get stiffness and soreness behind my knees when sitting in my office chair or car for more than an hour. what causes this and how do i fix it?

Prolonged: flexion and pressure on the popliteal space should not normally be a problem unless associated with prior injury, obesity, vascular insufficiency or other pathology. Try to uncover and correct these first. If none of those are in play then stretch your legs regularly to avoid.

Knee pain: Agree with Dr. Burns, best to get evaluated to get to the underlying causes. Best wishes.