This, from Wikipedia: (Reference:
http://en.wikipedia.org/wiki/Heat_therapy):
Topic: Heat Therapy; Mechanism of Action - "Heat creates higher tissue temperatures, which produces vasodilation that increases the supply of oxygen and nutrients and the elimination of carbon dioxide and metabolic waste.[4]
Heat therapy is useful for muscle spasms, myalgia, fibromyalgia, contracture, bursitis.[4]
Because heat is a vasodilator, it should be avoided in tissues with inadequate vascular supply, in case of acute injury, in bleeding disorders (because heat would increase bleeding), in tissues with a severe lack of sensitivity, in scars.[4]"
Wikipedia reference [4]: Raj, P. Prithvi,
Practical Management of Pain, 3rd Edtion, Mosby, 2000; ISBN 978-0-8151-2569-3
Hope this helps.