Chemistry/physics of Thermacare Continuous Low-level Heatwrap Therapy (CLHT)?

My Chemistry and physics courses were 50 years ago and I am pondering how Thermacare Continuous Low-level Heatwrap Therapy (CLHT) stay warm for so long. I only find medical references as to how the heat works. I haven’t found chemical/physics references as to how the heat is produced. Could you give us a simple explanation?

Holds heat due to excellent insulation most likely.

Or high specific heat…

Rather than a random guess, here is a link to a FAQ on the thermacare website that has some helpful information under “How it works”.

http://www.thermacare.com/Faq.aspx