Heat is commonly used "loosen" muscles, but what exactly is happening to relax the muscle? It wouldn't make sense that increased blood flow alone would relax muscle.
Read this it was very helpful to me as a Reiki Practitioner who wants to know how the heat generated by my hands on therapy with my patients for Hospice brings such relaxation to them.
There may not be one because it may not have any effect.
It appears to be a placebo effect. There is not a lot of clinical evidence heat does anything beneficial, in fact cold may actually be more effective. Evidence of an effect appears to be mostly anecdotal or unscientific "studies".