Epsom salts are used for many things. Deep muscle strains, may require more heat to increase circulation to the muscles.

Heat for PNers is generally a trigger for more pain, hence we don't use hot water to soak. Our problems are nerve generated.

I don't know why the directions on the Epsom salts says "hot" water anyway. Some people would not do well with "hot". Many OTC products including supplements have
"iffy" directions IMO. Either incomplete, or out of date, or whatever, is what I've seen on some things.

In general heat makes nerves fire MORE. My Chiro told me this, and I limit heating pads now. My old MP can still be activated by heat. (steam rooms, heating pads, or hot tubs).