What Does A Lump On The Thigh Muscles Indicate?

Question: Last night I noticed a lump/knot in/on one of my thigh muscle (on back of right leg) while I had it in a very tightened position. I can only feel it when I flex my muscle a certain way and at a certain angle (with knee bent at 45 or more angle with pressure on the toes) and it immediately goes away as soon as I let up and can not be felt again until I get the muscle in the same tightened position and angle. In fact, it is difficult to feel unless I am in a certain position with the thigh muscle flexed tight. I can not feel it while standing or sitting normally - even if I press deep into my muscle region. Its feels about the size of the end of your thumb and feels the same hardness as the muscle itself . I had my wife feel it and she said it feels as if it is at the end of the muscle. Not painful and doesn't affect mobility, etc. I am 57, 6'0, 180 and in excellent physically condition (I am a high school basketball official so I do a lot of running). I haven't experience any injury to that area lately - although I did have a bad hamstring injury while in college but otherwise I don't recall any trauma tot he area.

Ok, thank you. I felt for it again tonight when I got home and it is not as "large" as it seemed last night. In fact, it feels more now as if my muscle has a small indenture in the middle of it and what I am am feeling is the part after the indenture because it feels the same on both sides of the "indenture" . (Hope that makes sense). It dois set feel like a knot tonight as it did last night and was much hard to locate.

I'm going to schedule an appointment with my GP. I did notice that at the same place on my left leg is a slight bump - just not as pronounced as the one in the right leg but in the same location as the other i inquired about. Must admit it eases my mind some.