My strength increased consistently and I put on maybe 5-10 pounds of muscle over 5 months. I was thrilled, but recently my knees began to ache. Sometimes the pain occurs while I am doing the deadlift or squat, but not usually. The pain will typically strike at random later in the day, while I am walking to lunch, or walking home from work. It's not a sharp pain, but deep and sometimes intense - like I have to sit down because I can't walk intense. It soon passes and I hobble on, then I'm walking normally five minutes later. I consult Beginning Strength regularly and try to be rigorous about my form, but I think I must be doing something wrong. What is the most likely cause here? Am I just getting old? make sure your knees aren't buckling inwards. push your knees outwards during the concentric movement.