I have been riding a mountain bike recreationally about 3-4 nites per week. About 2-3 hours each nite. No idea how many miles im logging, maybe guess between 20-30? I ride mostly on pavement. But i like to do a lot of wheelies as i am getting pretty darn good. I do jumps wherever i see an opportunity or obstacle that is accomodating. Yesterday I had my longest ride ever of about 4 hours. I could tell my legs were done on the way home. Today they are just screaming at me. Very specifically BEHIND the knees. When i stretch them or stand up it's like the tendons or whatevers are stretching to their limit. When my legs are bent, theres no pain. I use a 26" Trek, i have the seat up fairly high, my feet JUST touch the ground. I push off on my wheelies with my right leg and thats the most painful of the two. You think i did any permanent damage or is it just fatigue and a "good 'ol workout"? Thanks everyone. Anymore questions let me know.