I have Arthritis in my knees and I take Enbrel(injection into thigh, once a week). I feel like I have lost my muscular endurance in my legs. I live a very active life, but I can't help notice that I feel(in my legs) weak. I don't feel tired, just my legs do. Does anyone have an answer for this? Is this what happens with Arthritis, or is it the medicine, or do I just need to work out harder???