How can you tell the difference between a muscle knot and a tumor?

A muscle knot is painful, but should go away with time. A tumor probably won't. If you have any doubt which you have, you should have it checked by your doctor.

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A muscle knot (muscle spasm) is usually an acute process that can litterally occur within hours or shorter. It can persist and become chronic. It's generally painful or at least not comfortable.

A tumor is generally slow growing taking months to years to develop. Many times, you may not even be bothered by it other than the fact that it's there.