Here are a couple of very informative studies on the effect of heat on tissues.  This information goes a long way towards explaining how applied heat (in warm ups and warm downs) is an important part of your training.  Heat is essential for preparing for PE exercises, increasing tissue response from the exercises themselves, and promoting greater blood flow and speeding recovery.

http://www.ncbi.nlm.nih.gov/pubmed/15827938?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=1

http://www.ncbi.nlm.nih.gov/pubmed/11380276?itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVDocSum&ordinalpos=3