My husband has a very large knot on his left bicep in the back, the size of a grapefruit and has no pain, what is this ? he has no problem moving it and has no knot anywhere else on his body.

Is : Is the "knot" hard, or more soft? Can you move it around or does it stay put? Is it warm? Was there trauma? I am concerned about the size being that of a grapefruit. With that said, this should be seen by a physician well versed in musculoskeletal problems. This could be a hematoma, lipoma or a ruptured muscle. Good luck.

Agree : Agree with the questions asked by drs. Fowler and rothmeir- need more info. Having said that, it could be anything from a benign fatty tumor i.E lipoma to something more serious. So if it is "very large" and especially if it has recently been getting larger, it should be checked out. An MRI is a good test for something like that. If there is still a question it should be biopsied by someone familiar with this type of issue.