Ask someone in the gym to point to their trapezius muscles, and they’ll probably gesture toward the small mounds nestled between their neck and shoulders—but that’s really just the tip of the iceberg.

If you’ve got a lump on top of your shoulder muscle, you have a good chance of it being a trapezius knot. This type of lump in the trapezius muscle — called a myofascial trigger point — is among the ...