In 21 years of selling saddles, the most common complaint I've heard is "My knees hurt!" As a matter of fact, about the only time that I haven't heard this complaint is when I have an English rider come in. Their knees seem to be just fine! I've heard several theories on why this is, but this week I've given it a lot of thought, and tested a few saddles, and had sore knees. (of course, I am getting a little older, and about everything else hurt just as much...)
The most obvious reason for sore knees in an average Western saddle is the sheer amount of leather that is under your leg. It's often several layers of thick leather that you're constantly fighting to turn back toward you, which is exhausting. This can be somewhat alleviated by overturning your stirrups or having them professionally and permanently turned with a Nevada Twist. Taking it several steps further, Circle Y's "Softee" leather has greatly alleviated the problem. The leather is soft, supple, and just not as bulky under your leg. Over half of my customers with bad knees have been made more comfortable by choosing a Circle Y Flex Lite trail saddle.
However, there are still complaints of knee pain, usually from people with prior knee injuries or short legs. After recalling a conversation I had years ago with a stirrup maker and doing some studying myself, I found another inherent problem with the Western Saddle. As described above, the fenders and stirrup leathers are designed to hang straight, then asked to turn so that you can place your foot in the stirrup. This causes another problem; the stirrup is now turned, but is positioned at an angle. Your foot is now at an angle, with your instep lower than the outside of your foot. This can put an unbelievable amount of stress on your ankle and knees during a long ride, especially if you tend to ride with a lot of weight in your stirrups.
That explains why English riders rarely feel as much discomfort. The English stirrup leather is hung so that the stirrup is flat and already facing the rider. For those of you that have terrible knees and are looking for a solution that does not switch you to an English saddle, I can't recommend the Tucker Equitation Endurance saddle enough. It offers a lot of security, a Western seat and all the attachment points you need for traditional trail riding gear. However ,it has English rigging and stirrup leathers to almost eliminate both of the problems outlined above. My wife (with terrible knees and an even worse back) rides this saddle, and she flat out won't ride another saddle anymore. She's content and comfortable enough to go all day in the saddle now.
There have been a few attempts by stirrup makers over the years to address the angle problem, but it doesn't seem like any have had the marketing to back up the idea, so they've all fizzled. Not a bad idea for the future, though! For the time being, the Tucker Trail Glide stirrup is about as good as you can get for helping to distribute the pressure evenly over the sole of your foot.