When there's pressure on the back of the thighs, the front edge of the chair may be too high. On many office chairs the seat can be tilted. I have a "The Gypsy Sit Upon"
www.thegypsyquilter.com from a LQS. It's something like the exercise balls but shaped like a round pillow. It takes the core muscles to keep balanced, so you do get some excercise. I take it to retreats when I can't take my office chair, it lifts me up about 2 inches.