I've read a study that showed heating to 45 degrees celcius (~113 degrees farenheit) was the temp to shoot for. The study used far infrared light for heating. This is due to the fact that this type of light can penetrate the body up to about 4 inches, I believe. When heated to this temp. permanent stretch was realized and it caused far less damage than stretching w/o heat. This is the reason that many yoga studios use this same temp and type of heat. It does a number on your fertility though, so tread carefully and avoid this if you are trying to impregnate someone.
UPDATE: I had this saved on my computer for some reason. LOL
The following information has been summarized from Chapter 9 of Therapeutic Heat and Cold, Fourth Edition, Editors Justus F. LeHydromaxann, M.D., Williams, and Wilkin, or concluded from data gathered there.
Generally it is accepted that heat produces the following desirable therapeutic effects:
1. Infrared heat increases the extensibility of collagen tissues.
Tissues heated to 45 degrees Celsius and then stretched exhibit a non-elastic residual elongation of about 0.5 to 0.9 percent that persists after the stretch is removed. This does not occur in these same tissues when stretched at normal tissue temperatures. Therefore 20 stretching sessions can produce a 10 to 18 percentage increase in length of tissues heated and stretched.
Stretching of tissue in the presence of heat would be especially valuable in working with ligaments, joint capsules, tendons, fasciae, and synoviurn that have become scarred, thickened, or contracted. Such stretching at 45 degrees Celsius caused much less weakening in stretched tissues for a given elongation than a similar elongation produced at normal tissue temperatures.
Experiments cited clearly showed low-force stretching could produce significant residual elongation when heat is applied together with stretching or range-of-motion exercises. This is safer than stretching tissues at normal tissue temperatures