Your problem most likely lies in fore shortening then. You've got to remember that our perspective of people changes as their limbs move out. So you're probably making the thighs of your person too long in comparison to the actual view.
Find a good reference picture for someone sitting, and then take a measurement with a ruler to measure from their hip joint to their knee; and then from their knee to their ankle and compare the two. The hip-knee should be shorter (by a considerable bit) in comparison to the knee to ankle.
http://www.geocities.com/raviwartooth/sitting.jpg
Take this picture for example. Even if you measure with your fingers you'll realise that foreshortening is taking place because the guys upper to lower leg ration is different. In a standing person they should be roughly the same (though there can be some variation from person to person; and if you're trying to draw anime, there's a lot of variation. *hisses at anime*)
Evisceration is a sign of respect.