I have this really annoying mass on the back of my knee that pops out and occasionally leads to strains on the site of the mass. It bulges out when I fire the hamstring/calf (When you push back the leg like you were doing a hamstring curl). I have been told it is the plantaris muscle that could be herniated. I had doppler imaging done it to show that it is not fluid at all and to make sure it was not a baker's cyst. I have had MRI on it and the doc said it comes up totally normal but again it is only really pronounced and bulges out when the surrounding muscles are fired, which is impossoble to do for the forty minutes it takes to do an MRI. It seems to almost tuck under the lower hamstring that is in back of the knee when the muscle is not being fired. It is really quite nasty looking; about golfball width and a little bigger than the size of half a golf ball pushing out. All the people I have seen have never seen it before and don't really know what to make of it. Any PT's or anatomy experts out there? Any help from anybody who has an idea what to make of this is greatly appreciated.