During my trail ride, I found it a lot harder to keep my toes up, heels down while riding. Part of it is needing to get back in the habit. But, as I was riding, I remember that part of it might also be that I need to strengthen up those muscles in my ankles again after not riding for long periods for some time.
As I was riding, I was getting some knee pain when I put my left heel down as well. I had a hip injury some years ago as a young kid, and I'm not as flexible in my left hip as I am in my right. So I don't know if that has anything to do with it. I had minor knee ache in my right, but nothing compared to my left.