Worked out last Friday, including a 35 min run...everything good. My left knee tightened up that evening...never have had any problems with either knee. Since then, when I get up in the morning, knee feels OK but as the day goes on, it tightens up and is painful. When I sit for a long period, it is painful to stand and strertch out my leg. Eventually it stretches out and feels better but still somewhat painful. I am 63 years old and in excellent health. Never had any problems with my legs.