Quote Originally Posted by Kalidurga View Post
If you watch pro bike races, you'll notice that most of the pros ride that way, too. I haven't really researched it so I can't explain it, but I assume it has to do with which thigh muscles are used depending on how the leg is positioned. Generally, the adductors on the inside of the thigh are weaker than the abductors along the outside. Pedaling with the knees pointing out, which I see fairly frequently on the bike trail, pulls the adductors into play and puts the knee in a stressful position. It's an inefficient position. Pointing the knees slightly in toward the top tube, though, brings all of the more powerful quad and abductor muscles into play and keeps the knees in a safer position. Much more efficient and powerful for pedaling. Plus, pulling the thighs toward each other that way will eventually help to strengthen the weaker adductors a bit.

That's all my own speculation based on what little I know of anatomy, though. I could be totally off-base.
Knee-in position actually works the adductors more, not the abductors, as the abductors work to pull the knee in outward rotation from the hip. My inner thighs are infinitely stronger than my outer, from riding horses, and in a hard effort, I'll enlist those muscles more and my knees might come a little closer to the top tube, toes floating out slightly. I also know this is the case because my right leg has a bit of a curve in it--slightly bowed. Even with body geometry help, it still tracks more knee-in than the other. And if I'm not careful, I'll strain one of the longer groin muscles--the one that runs right next to the inner ham. Clearly overusing the adductors, not the abductors. Knee-in might pull on a tight ITB, but it's not enlisting those abductors more than normal.